Solar system a beginners guide1

The first satellites designed to observe the Sun were NASA’s Pioneers 5, 6, 7, 8 and 9,
which were launched between 1959 and 1968. These probes orbited the Sun at a distance
similar to that of the Earth’s orbit, and made the first detailed measurements of the solar
wind and the solar magnetic field. Pioneer 9 operated for a particularly long period of
time, transmitting data until 1987.

In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided scientists with
significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint
U.S.-German probe that studied the solar wind from an orbit carrying the spacecraft
inside Mercury’s orbit at perihelion. The Skylab space station, launched by NASA in
1973, included a solar observatory module called the Apollo Telescope Mount that was
operated by astronauts resident on the station. Skylab made the first time-resolved
observations of the solar transition region and of ultraviolet emissions from the solar
corona. Discoveries included the first observations of coronal mass ejections, then called
“coronal transients”, and of coronal holes, now known to be intimately associated with
the solar wind.

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was
designed to observe gamma rays, X-rays and UV radiation from solar flares during a time
of high solar activity. Just a few months after launch, however, an electronics failure
caused the probe to go into standby mode, and it spent the next three years in this inactive
state. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and
repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission
subsequently acquired thousands of images of the solar corona before re-entering the
Earth’s atmosphere in June 1989.

Japan’s Yohkoh (Sunbeam) satellite, launched in 1991, observed solar flares at X-ray
wavelengths. Mission data allowed scientists to identify several different types of flares,
and also demonstrated that the corona away from regions of peak activity was much more
dynamic and active than had previously been supposed. Yohkoh observed an entire solar
cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its
lock on the Sun. It was destroyed by atmospheric reentry in 2005.

One of the most important solar missions to date has been the Solar and Heliospheric
Observatory, jointly built by the European Space Agency and NASA and launched on
December 2, 1995. Originally a two-year mission, SOHO has now operated for over ten
years (as of 2006). It has proved so useful that a follow-on mission, the Solar Dynamics
Observatory, is planned for launch in 2008. Situated at the Lagrangian point between the
Earth and the Sun (at which the gravitational pull from both is equal), SOHO has
provided a constant view of the Sun at many wavelengths since its launch. In addition to
its direct solar observation, SOHO has enabled the discovery of large numbers of comets,
mostly very tiny sungrazing comets which incinerate as they pass the Sun.

All these satellites have observed the Sun from the plane of the ecliptic, and so have only
observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to
study the Sun’s polar regions. It first traveled to Jupiter, to ‘slingshot’ past the planet into
an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was
well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994.
Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic
field strength at high solar latitudes, finding that the solar wind from high latitudes was
moving at about 750 km/s (slower than expected), and that there were large magnetic
waves emerging from high latitudes which scattered galactic cosmic rays.

Elemental abundances in the photosphere are well known from spectroscopic studies, but
the composition of the interior of the Sun is more poorly understood. A solar wind
sample return mission, Genesis, was designed to allow astronomers to directly measure
the composition of solar material. Genesis returned to Earth in 2004 but was damaged by
a crash landing after its parachute failed to deploy on reentry into Earth’s atmosphere.
Despite severe damage, some usable samples have been recovered from the spacecraft’s
sample return module and are undergoing analysis.

In October 2006 NASA launched two nearly identical spacecraft which will film the Sun
from widely separated points in their orbits to produce the first 3D movies and images of
CME’s and other solar activity. The Stereo Mission spacecraft will circle the Sun at the
same distance as the Earth, with one slightly ahead of Earth and the other trailing. Their
separation will gradually increase so that after 4 years they will be almost diametrically
opposite each other in orbit. [34]

Sun observation and eye damage

Large solar flare recorded by the SOHO/EIT telescope using UV light from the He+
emission line at 30.4 nm.

Sunlight is very bright, and looking directly at the Sun with the naked eye for brief
periods can be painful, but is generally not hazardous. Looking directly at the Sun causes
phosphene visual artifacts and temporary partial blindness. It also delivers about
4 milliwatts of sunlight to the retina, slightly heating it and potentially (though not
normally) damaging it. UV exposure gradually yellows the lens of the eye over a period

of years and can cause cataracts, but those depend on general exposure to solar UV, not
on whether one looks directly at the Sun.

Viewing the Sun through light-concentrating optics such as binoculars is very hazardous
without an attenuating (ND) filter to dim the sunlight. Unfiltered binoculars can deliver
over 500 times more sunlight to the retina than does the naked eye, killing retinal cells
almost instantly. Even brief glances at the midday Sun through unfiltered binoculars can
cause permanent blindness.One way to view the Sun safely is by projecting an image onto
a screen using binoculars. This should only be done with a small refracting telescope
(or binoculars) with a clean eyepiece. Other kinds of telescope can be damaged by this
procedure.

Partial solar eclipses are hazardous to view because the eye’s pupil is not adapted to the
unusually high visual contrast: the pupil dilates according to the total amount of light in
the field of view, not by the brightest object in the field. During partial eclipses most
sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of
the photosphere have the same surface brightness as during a normal day. In the overall
gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the
solar image receives about ten times more light than it would looking at the non-eclipsed
sun. This can damage or kill those cells, resulting in small permanent blind spots for the
viewer.The hazard is insidious for inexperienced observers and for children, because
there is no perception of pain: it is not immediately obvious that one’s vision is being
destroyed.

During sunrise and sunset, sunlight is attenuated through rayleigh and mie scattering of
light by a particularly long passage through Earth’s atmosphere, and the direct Sun is
sometimes faint enough to be viewed directly without discomfort or safely with
binoculars (provided there is no risk of bright sunlight suddenly appearing in a break
between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this
atmospheric attenuation.

Attenuating filters to view the Sun should be specifically designed for that use: some
improvised filters pass UV or IR rays that can harm the eye at high brightness levels. In
general, filters on telescopes or binoculars should be on the objective lens or aperture
rather than on the eyepiece, because eyepiece filters can suddenly shatter due to high heat
loads from the absorbed sunlight. Welding glass is an acceptable solar filter, but “black”
exposed photographic film is not (it passes too much infrared).

Sun and culture

Many civilizations have viewed the Sun as a sacred body. In Hindu religious literature,

the Sun is notably mentioned as the visible form of God that one can see every day. In

Hinduism, Surya (Devanagari: , sūrya) is the chief solar deity, son of Dyaus

Pitar. The Sun was also worshiped in Inca, Aztec and Egyptian culture.[37]

Many Greek myths personify the Sun as a titan named Helios, who wore a shining crown
and rode a chariot across the sky, causing day. Over time, the sun became increasingly
associated with Apollo.

The Roman Empire adopted Helios into their own mythology as Sol. The title Sol
Invictus (“the undefeated Sun”) was applied to several solar deities, and depicted on
several types of Roman coins during the 3rd and 4th centuries.

Early Christian iconography reveals Jesus as reflecting several attributes of Sol Invictus,
such as a radiated crown or, occasionally, a solar chariot. It is also speculated that the
observation of Christmas on December 25th is derived from a pagan Sun holiday which
occurred on the same date.

Mercury (planet)

Mercury

Orbital characteristics (Epoch J2000)

Avg. distance from Sun 57,909,176 km
0.387 098 93 AU

Orbital circumference 360,000,000 km
(2.406 AU)

Eccentricity 0.205 630 69

Perihelion 46,001,272 km
0.307 499 51 AU

Aphelion 69,817,079 km
0.466 698 35 AU

Orbital period 87.969 34 d
(0.240 846 9 a)

Synodic period 115.8776 d

Avg. Orbital Speed 47.36 km/s

Max. Orbital Speed 58.98 km/s

Min. Orbital Speed 38.86 km/s

Inclination 7.004 87°
(3.38° to Sun’s equator)

Longitude of the 48.331 67°
ascending node

Argument of the 29.124 78°
perihelion

Number of satellites 0

Physical characteristics

Equatorial diameter 4879.4 km
Surface area (0.383 Earths)
Volume
Mass 7.5×107 km²
(0.147 Earths)

6.083×1010 km³
(0.056 Earths)

3.302×1023 kg
(0.055 Earths)

Mean density 5.427 g/cm³

Equatorial gravity 3.701 m/s²
(0.377 gee)

Escape velocity 4.435 km/s

Rotation period 58.6462 d (58 d 15.5088 h)
Rotation velocity 10.892 km/h (at the equator)
Axial tilt ~0.01°

Right ascension 281.01° (18 h 44 min 2 s) 1
of North pole

Declination 61.45°

Albedo 0.10-0.12

Surface temp. min mean max
90 K 440 K 700 K

Avg. Surface temp.: Day 623 K

Avg. Surface temp.: Night 103 K

Adjective Mercurian

Atmospheric characteristics

Atmospheric pressure trace
Potassium 31.7%
Sodium 24.9%
Atomic Oxygen 9.5%
Argon 7.0%

Helium 5.9%
Molecular Oxygen 5.6%
Nitrogen 5.2%
Carbon dioxide 3.6%
Water 3.4%
Hydrogen 3.2%

Mercury (IPA: / mɛːkjəri/) is the innermost and smallest planet in the solar system,
orbiting the Sun once every 88 days. It ranges in brightness from about −2.0 to 5.5 in
apparent magnitude, but is not easily seen — its greatest angular separation from the Sun
(greatest elongation) is only 28.3° (it can only be seen in twilight). Comparatively little is
known about the planet: the only spacecraft to approach Mercury was Mariner 10 from
1974 to 1975, which mapped only 40%–45% of the planet’s surface.

Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has
no natural satellites and no substantial atmosphere. The planet has a large iron core which
generates a magnetic field about 1% as strong as that of the Earth. Surface temperatures
on Mercury range from about 90 to 700 K (-180 to 430°C) , with the subsolar point being
the hottest and the bottoms of craters near the poles being the coldest.

The Romans named the planet after the fleet-footed messenger god Mercury, probably
for its fast apparent motion in the twilight sky. The astronomical symbol for Mercury,
displayed at the top of the infobox, is a stylized version of the god’s head and winged hat
atop his caduceus, an ancient astrological symbol. Before the 5th century BC, Greek
astronomers believed the planet to be two separate objects: one visible only at sunrise, the
other only at sunset. In India, the planet was named Budha ( ), after the son of
Chandra (the Moon). The Chinese, Korean, Japanese, and Vietnamese cultures refer to
the planet as the water star, based on the Five Elements. The Hebrews named it Kokhav
Hamah (‫)המח בכוכ‬, “the star of the hot one” (“the hot one” being the Sun).

Structure

Mercury is one of the four terrestrial planets, meaning that like the Earth it is a rocky
body. It is the smallest of the four, with a diameter of 4879 km at its equator. Mercury
consists of approximately 70% metallic and 30% silicate material. The density of the
planet is the second-highest in the solar system at 5430 kg/m³, only slightly less than
Earth’s density.

Internal structure: core, mantle and crust

Diagram showing Mercury’s large core

Mercury’s high density can be used to infer details of its inner structure. While the Earth’s
high density results partly from compression at the core, Mercury is much smaller and
its inner regions are not nearly so compressed. Therefore, for it to have such a high
density, its core must be large and rich in iron. Geologists estimate that Mercury’s core
occupies about 42% of its volume. (Earth’s core occupies about 17% of its volume.)

Surrounding the core is a 600 km mantle. It is generally thought that early in Mercury’s
history, a giant impact with a body several hundred kilometers across stripped the planet
of much of its original mantle material, resulting in the relatively thin mantle compared to
the sizable core (alternative theories are discussed below).

Mercury’s crust is thought to be about 100–200 km thick. One very distinctive feature of
Mercury’s surface is numerous ridges, some extending over several hundred kilometers.
It is believed that these were formed as Mercury’s core and mantle cooled and contracted
after the crust had solidified.

Mercury has a higher iron content than any other major planet in our solar system.
Several theories have been proposed to explain Mercury’s high metallicity. The most
widely accepted theory is that Mercury originally had a metal-silicate ratio similar to
common chondrite meteors and a mass approximately 2.25 times its current mass; but
that early in the solar system’s history, Mercury was struck by a planetesimal of
approximately 1/6 that mass. The impact would have stripped away much of the original
crust and mantle, leaving the core behind. A similar theory has been proposed to explain
the formation of Earth’s Moon.

Alternatively, Mercury may have formed from the solar nebula before the Sun’s energy
output had stabilized. The planet would initially have had twice its present mass. But as
the protosun contracted, temperatures near Mercury could have been between 2500 and
3500 K, and possibly even as high as 10000 K. Much of Mercury’s surface rock could
have vaporized at such temperatures, forming an atmosphere of “rock vapor” which
could have been carried away by the solar wind.

A third theory suggests that the solar nebula caused drag on the particles from which
Mercury was accreting, which meant that lighter particles were lost from the accreting
material. Each of these theories predicts a different surface composition, and two
upcoming space missions, MESSENGER and BepiColombo, both aim to take
observations that will allow the theories to be tested.

Surface

Mercury’s surface is very similar in appearance to that of the Moon, showing extensive
mare-like plains and heavy cratering, indicating that it has been geologically inactive for
billions of years. The small number of unmanned missions to Mercury means that its
geology is the least well understood of the terrestrial planets. Surface features are given
the following names:

• Albedo features — areas of markedly different reflectivity
• Dorsa — ridges (see List of ridges on Mercury)
• Montes — mountains (see List of geological features on Mercury#Mountains)
• Planitiae — plains (see List of plains on Mercury)
• Rupes — scarps (see List of scarps on Mercury)
• Valles — valleys (see List of valleys on Mercury)

During and shortly following the formation of Mercury, it was heavily bombarded by
comets and asteroids for a period that came to an end 3.8 billion years ago. During this
period of intense crater formation, the planet received impacts over its entire surface,
facilitated by the lack of any atmosphere to slow impactors down. During this time the
planet was volcanically active; basins such as the Caloris Basin were filled by magma
from within the planet, which produced smooth plains similar to the maria found on the
Moon.

Mercury’s Caloris Basin is one of the largest impact features in the Solar System.

Craters on Mercury range in diameter from a few meters to hundreds of kilometers
across. The largest known crater is the enormous Caloris Basin, with a diameter of
1300 km. The impact which created the Caloris Basin was so powerful that it caused lava
eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the
antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the
“Weird Terrain”. It is believed that shock waves from the impact traveled around the
planet, and when they converged on the antipodal point of the impact caused extensive
fracturing of the surface there.

The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal
point.
The plains of Mercury have two distinct ages: the younger plains are less heavily cratered
and probably formed when lava flows buried earlier terrain. One unusual feature of the
planet’s surface is the numerous compression folds which crisscross the plains. It is
thought that as the planet’s interior cooled, it contracted and its surface began to deform.
The folds can be seen on top of other features, such as craters and smoother plains,
indicating that they are more recent. Mercury’s surface is also flexed by significant tidal
bulges raised by the Sun—the Sun’s tides on Mercury are about 17% stronger than the
Moon’s on Earth.

Like the Moon, the surface of Mercury has likely incurred the effects of space weathering
processes. Solar wind and micrometeorite impacts can darken the albedo and alter the
reflectance properties of the surface.
The mean surface temperature of Mercury is 452 K (353.9°F, 178.9°C), but it ranges
from 90 K (-297.7°F, -183.2°C) to 700 K (800.3°F, 426.9°C); by comparison, the

temperature on Earth varies by only about 150 K. The sunlight on Mercury’s surface is
6.5 times as intense as it is on Earth, with a solar constant value of 9.13 kW/m².

Despite the generally extremely high temperature of its surface, observations strongly
suggest that ice exists on Mercury. The floors of some deep craters near the poles are
never exposed to direct sunlight, and temperatures there remain far lower than the global
average. Water ice strongly reflects radar, and observations reveal that there are patches
of very high radar reflection near the poles. While ice is not the only possible cause of
these reflective regions, astronomers believe it is the most likely.

The icy regions are believed to be covered to a depth of only a few meters, and contain
about 1014–1015 kg of ice. By comparison, the Antarctic ice sheet on Earth weighs about
4×1018 kg, and Mars’ south polar cap contains about 1016 kg of water. The origin of the
ice on Mercury is not yet known, but the two most likely sources are from outgassing of
water from the planet’s interior or deposition by impacts of comets.

Atmosphere

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

Mercury is too small for its gravity to retain any significant atmosphere over long periods
of time; it has a tenuous atmosphere containing hydrogen, helium, oxygen, sodium,
calcium and potassium. The atmosphere is not stable—atoms are continuously lost and
replenished, from a variety of sources. Hydrogen and helium atoms probably come from
the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into
space. Radioactive decay of elements within Mercury’s crust is another source of helium,
as well as sodium and potassium. Water vapor is probably present, being brought to
Mercury by comets impacting on its surface.

Magnetic field

Despite its slow rotation, Mercury has a relatively strong magnetic field, with a magnetic
field strength 1% as strong as the Earth’s. It is possible that this magnetic field is
generated in a manner similar to Earth’s, by a dynamo of circulating liquid core material.
However, scientists are unsure whether Mercury’s core could still be liquid, although it
could perhaps be kept liquid by tidal effects during periods of high orbital eccentricity. It
is also possible that Mercury’s magnetic field is a remnant of an earlier dynamo effect
that has now ceased, with the magnetic field becoming “frozen” in solidified magnetic
materials.

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet,
creating a magnetosphere inside which the solar wind does not penetrate. This is in
contrast to the situation on the Moon, which has a magnetic field too weak to stop the
solar wind impacting on its surface and so lacks a magnetosphere.

Orbit and rotation

The orbit of
Mercury is the
most eccentric
of the major
planets, with
the planet’s
distance from
the Sun ranging
from
46,000,000 to
70,000,000 kilo
meters. It takes
88 days to
complete the
orbit. The diagram on the left illustrates the effects of the eccentricity, showing
Mercury’s orbit with a circular orbit with the same semi-major axis. The higher velocity
of the planet when it is near perihelion is clear from the greater distance it covers in each
5-day interval. The size of the spheres, inversely proportional to their distance from the
Sun, illustrates the varying heliocentric distance. This varying distance to the Sun,
combined with a unique 2:3 resonance of the planet’s rotation around its axis, result in
complex variations of the surface temperature.

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in
the diagram on the left. As a result, transits of Mercury across the face of the Sun can
only occur when the planet is crossing the plane of the ecliptic at the time it lies between
the Earth and the Sun. This occurs about every seven years on average.

Mercury’s axial tilt is only 0.01 degrees. This is over 300 times smaller than that of
Jupiter, which is the second smallest axial tilt of all planets at 3.1 degrees. This means an
observer at Mercury’s equator during local noon would never see the sun more than
1/100 of one degree north or south of the zenith.

At certain points on Mercury’s surface, an observer would be able to see the Sun rise
about halfway, then reverse and set before rising again, all within the same Mercurian
day. This is because approximately four days prior to perihelion, Mercury’s angular
orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent
motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular
rotational velocity. Thus, the Sun appears to be retrograde. Four days after perihelion, the
Sun’s normal apparent motion resumes.

Advance of perihelion

When it was discovered, the slow precession of Mercury’s orbit around the Sun could not
be completely explained by Newtonian mechanics, and for many years it was
hypothesized that another planet might exist in an orbit even closer to the Sun to account
for this perturbation (other explanations considered included a slight oblateness of the
Sun). The success of the search for Neptune based on its perturbations of Uranus’ orbit
led astronomers to place great faith in this explanation, and the hypothetical planet was
even named Vulcan. However, in the early 20th century, Albert Einstein’s General Theory
of Relativity provided a full explanation for the observed precession. Mercury’s
precession showed the effects of mass dilation, providing a crucial observational
confirmation of one of Einstein’s theories—Mercury is slightly heavier at perihelion than
it is at aphelion because it is moving faster, and so it slightly “overshoots” the perihelion
position predicted by Newtonian gravity. The effect is very small: the Mercurian
relativistic perihelion advance excess is just 43 arcseconds per century. The effect is even
smaller for other planets, being 8.6 arcseconds per century for Venus, 3.8 for Earth, and
1.3 for Mars.

Research indicates that the eccentricity of Mercury’s orbit varies chaotically from 0
(circular) to a very high 0.47 over millions of years. This is thought to explain Mercury’s
3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to
arise during a period of high eccentricity.

Spin-orbit resonance

After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same
hemisphere is again illuminated.

For many years it was thought that Mercury was synchronously tidally locked with the
Sun, rotating once for each orbit and keeping the same face directed towards the Sun at
all times, in the same way that the same side of the Moon always faces the Earth.
However, radar observations in 1965 proved that the planet has a 3:2 spin-orbit

resonance, rotating three times for every two revolutions around the Sun; the eccentricity
of Mercury’s orbit makes this resonance stable. The original reason astronomers thought
it was synchronously locked was because whenever Mercury was best placed for
observation, it was always at the same point in its 3:2 resonance, hence showing the same
face. Due to Mercury’s 3:2 spin-orbit resonance, a solar day (the length between two
meridian transits of the Sun) lasts about 176 Earth days. A sidereal day (the period of
rotation) lasts about 58.7 Earth days.

Observation

Mercury’s apparent magnitude varies between about -2.0 - brighter than Sirius - and 5.5.
Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the
Sun’s glare for much of the time. Mercury can be observed for only a brief period during
either morning or evening twilight. The Hubble Space Telescope cannot observe Mercury
at all.

Mercury exhibits moonlike phases as seen from Earth, being “new” at inferior
conjunction and “full” at superior conjunction. The planet is rendered invisible on both of
these occasions by virtue of its rising and setting in concert with the Sun in each case.
The half-moon phase occurs at greatest elongation, when Mercury rises earliest before
the Sun when at greatest elongation west, and setting latest after the Sun when at greatest
elongation east (its separation from the Sun ranging from 18.5° if it is at perihelion at the
time of the greatest elongation to 28.3° if it is at aphelion).

Mercury attains inferior conjunction every 116 days on average, but this interval can
range from 111 days to 121 days due to the planet’s eccentric orbit. Its period of
retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior
conjunction. This large range also arises from the planet’s high degree of orbital
eccentricity.

View of Mercury from Mariner 10

Mercury is more often easily visible from Earth’s Southern Hemisphere than from its
Northern Hemisphere; this is because its maximum possible elongations west of the Sun
always occur when it is early autumn in the Southern Hemisphere, while its maximum
possible eastern elongations happen when the Southern Hemisphere is having its late
winter season. In both of these cases, the angle Mercury strikes with the ecliptic is
maximized, allowing it to rise several hours before the Sun in the former instance and not
set until several hours after sundown in the latter in countries located at South Temperate
Zone latitudes, such as Argentina and New Zealand. By contrast, at northern temperate
latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky.
Mercury can, like several other planets and the brightest stars, be seen during a total solar
eclipse.

Mercury is brightest as seen from Earth when it is at a gibbous phase, between half full
and full. Although the planet is further away from Earth when it is gibbous than when it
is a crescent, the greater illuminated area visible more than compensates for the greater
distance. The opposite is true for Venus, which appears brightest when it is a thin
crescent.

Studies of Mercury

Early astronomers

Mercury has been known since at least the 3rd millennium BC, when it was known to the
Sumerians of Mesopotamia as Ubu-idim-gud-ud, among other names. The Babylonians
(2000–1000 BC) succeeded the Sumerians, and early Babylonians may have recorded
observations of the planet: although no records have survived, late Babylonian records
from the 7th century BC refer to much earlier records. The Babylonians called the planet
Nabu or Nebu after the messenger to the Gods in their mythology.

The ancient Greeks gave the planet two names: Apollo when it was visible in the morning
sky and Hermes when visible in the evening. However, Greek astronomers came to
understand that the two names referred to the same body, with Pythagoras being the
first to propose the idea.

Ground-based telescopic research

This Mariner 10 view from 4.3 million km is similar to the very best views that can be
achieved telescopically from Earth

The first telescopic observations of Mercury were made by Galileo in the early 17th
century. Although he observed phases when he looked at Venus, his telescope was not
powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first
observations of the transit of a planet across the Sun when he saw a transit of Mercury
predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that
the planet had orbital phases similar to Venus and the Moon. The observation
demonstrated conclusively that Mercury orbited around the Sun.

A very rare event in astronomy is the passage of one planet in front of another
(occultation), as seen from Earth. Mercury and Venus occult each other every few
centuries, and the event of May 28, 1737 is the only one historically observed, having
been seen by John Bevis at the Royal Greenwich Observatory. The next occultation of
Mercury by Venus will be in 2133.

The difficulties inherent in observing Mercury mean that it has been far less studied than
the other planets. In 1800 Johann Schröter made observations of surface features, but
erroneously estimated the planet’s rotational period at about 24 hours. In the 1880s
Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s
rotational period was 88 days, the same as its orbital period due to tidal locking. This
phenomenon is known as synchronous rotation and is also shown by Earth’s Moon.

The theory that Mercury’s rotation was synchronous became widely held, and it was a
significant shock to astronomers when radio observations made in the 1960s questioned
this. If Mercury were tidally locked, its dark face would be extremely cold, but
measurements of radio emission revealed that it was much hotter than expected.
Astronomers were reluctant to drop the synchronous rotation theory and proposed
alternative mechanisms such as powerful heat-distributing winds to explain the
observations, but in 1965 radar observations showed conclusively that the planet’s
rotational period was about 59 days. Italian astronomer Giuseppe Colombo noted that this
value was about two-thirds of Mercury’s orbital period, and proposed that a different

form of tidal locking had occurred in which the planet’s orbital and rotational periods
were locked into a 3:2 rather than a 1:1 resonance. Data from space probes subsequently
confirmed this view.

Ground-based observations did not shed much further light on the innermost planet, and
it was not until space probes visited Mercury that many of its most fundamental
properties became known. However, recent technological advances have led to improved
ground-based observations: in 2000, high-resolution lucky imaging from the Mount
Wilson Observatory 60-inch telescope provided the first detailed views of the parts of
Mercury which were not imaged in the Mariner missions.

Research with space probes

Reaching Mercury from Earth poses significant technical challenges, since the planet
orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft
launched from Earth must travel over 91 million kilometers into the Sun’s gravitational
potential well. Starting from the Earth’s orbital speed of 30 km/s, the change in velocity
(delta-v) the spacecraft must make to enter into a Hohmann transfer orbit that passes near
Mercury is large compared to other planetary missions.

The potential energy liberated by moving down the Sun’s potential well becomes kinetic
energy; requiring another large delta-v to do anything other than rapidly pass by Mercury.
In order to land safely or enter a stable orbit since the planet has very little atmosphere, the
approaching spacecraft cannot use aerobraking and must rely on rocket motors. A trip to
Mercury actually requires more rocket fuel than that required to escape the solar system
completely. As a result, only one space probe has visited the planet so far.

Mariner 10

The Mariner 10 probe, the only probe yet to visit the innermost planet

Mariner 10

The only spacecraft to approach Mercury so far has been NASA’s Mariner 10 (1974–75).
The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could
approach Mercury—the first spacecraft to use this gravitational “slingshot” effect.

Mariner 10 provided the first close-up images of Mercury’s surface, which immediately
showed its heavily cratered nature, and also revealed many other types of geological
features, such as the giant scarps which were later ascribed to the effect of the planet
shrinking slightly early in its geological history. Unfortunately, the same face of the
planet was lit at each of Mariner 10’s close approaches, resulting in less than 45% of the
planet’s surface being mapped.

The spacecraft made three close approaches to Mercury, the closest of which took it to
within 327 km of the surface. At the first close approach, instruments detected a magnetic
field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be
much too slow to generate a significant dynamo effect. The second close approach was
primarily used for imaging, but at the third approach, extensive magnetic data were
obtained. The data revealed that the planet’s magnetic field is much like the Earth’s,
which deflects the solar wind around the planet. The Moon’s magnetic field, on the other
hand, is so weak that the solar wind reaches the surface. However, the origin of
Mercury’s magnetic field is still the subject of several competing theories.

Just a few days after its final close approach, Mariner 10 ran out of fuel, its orbit could no
longer be accurately controlled and mission controllers instructed the probe to shut itself
down. Mariner 10 is thought to be still orbiting the Sun, still passing close to Mercury
every few months.

MESSENGER

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space
ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the
Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The MESSENGER
spacecraft will make several close approaches to planets to place it onto the correct
trajectory to reach an orbit around Mercury. It made a close approach to the Earth in
February 2005, and to Venus in October 2006. Another Venusian encounter will follow
in 2007, followed by three close approaches to Mercury in 2008 and 2009, after which it
will enter orbit around the planet in March 2011.

The mission is designed to shed light on six key issues: Mercury’s high density, its
geological history, the nature of its magnetic field, the structure of its core, whether it
really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the
probe is carrying imaging devices which will gather much higher resolution images of
much more of the planet than Mariner 10, assorted spectrometers to determine
abundances of elements in the crust, and magnetometers and devices to measure
velocities of charged particles. Detailed measurements of tiny changes in the probe’s
velocity as it orbits will be used to infer details of the planet’s interior structure.

BepiColombo

Mercury as imaged by the Mariner 10 spacecraft

Japan is planning a joint mission with the European Space Agency called BepiColombo,
which will orbit Mercury with two probes: one to map the planet and the other to study
its magnetosphere. An original plan to include a lander has been shelved. Russian Soyuz
rockets will launch the probes in 2013. As with MESSENGER, the BepiColombo probes
will make close approaches to other planets en route to Mercury, passing the Moon and
Venus and making several approaches to Mercury before entering orbit. The probes will
reach Mercury in about 2019, orbiting and charting its surface and magnetosphere for a
year.

The probes will carry a similar array of spectrometers to those on MESSENGER, and
will study the planet at many different wavelengths including infrared, ultraviolet, X-ray
and gamma ray. Apart from intensively studying the planet itself, mission planners also
hope to use the probe’s proximity to the Sun to test the predictions of General Relativity
theory with improved accuracy.

The mission is named after Giuseppe (Bepi) Colombo, the scientist who first determined
the nature of Mercury’s orbital resonance with the Sun and who was also involved in the
planning of Mariner 10’s gravity-assisted trajectory to the planet in 1974.

Venus

Venus

Orbital characteristics (Epoch J2000)

Semi-major axis 108,208,926 km
0.723 331 99 AU

Orbital circumference 680,000,000 km
4.545 AU

Eccentricity 0.006 773 23

Perihelion 107,476,002 km
0.718 432 70 AU

Aphelion 108,941,849 km
0.728 231 28 AU

Orbital period 224.700 69 d
(0.615 197 0 a)

Synodic period 583.92 d

Avg. orbital speed 35.020 km/s

Max. orbital speed 35.259 km/s

Min. orbital speed 34.784 km/s

Inclination 3.394 71°

Longitude of the (3.86° to Sun’s equator)
ascending node 76.680 69°

Argument of the 54.852 29°
perihelion

Number of satellites 0

Physical characteristics

Equatorial diameter 12,103.7 km
(0.949 Earths)

Surface area 4.60×108 km2
(0.902 Earths)

Volume 9.28×1011 km³
(0.857 Earths)

Mass 4.8685×1024 kg
(0.815 Earths)

Mean density 5.204 g/cm3

Equatorial gravity 8.87 m/s2
(0.904 g)

Escape velocity 10.36 km/s
Rotation period −243.0185 d
Rotation velocity 6.52 km/h (at the equator)

Axial tilt 2.64°
272.76° (18 h 11 min 2 s) 1
Right ascension
of North pole

Declination 67.16°

Albedo 0.65

Surface* temp. min* mean max
228 K 737 K 773 K

Adjective Venusian or (rarely) Cytherean

(*min temperature refers to cloud tops only)

Atmospheric characteristics
Atmospheric pressure 9.2 MPa

Carbon dioxide ~96.5%
Nitrogen ~3.5%
Sulfur dioxide .015%
Argon .007%
Water vapor .002%
Carbon monoxide .0017%
.0012%
Helium .0007%
Neon
Carbonyl sulfide trace
Hydrogen chloride
Hydrogen fluoride

Venus (IPA: / vi nəs/) is the second-closest planet to the Sun, orbiting it every 224.7
Earth days. After Earth’s Moon, it is the brightest object in the night sky, reaching an
apparent magnitude of −4.6. As an inferior planet, from Earth it never appears to venture
far from the Sun, and its elongation reaches a maximum of 47.8°. Venus reaches its
maximum brightness shortly before sunrise or shortly after sunset, and is often referred to
as the Morning Star or as the Evening Star.

A terrestrial planet, it is sometimes called Earth’s “sister planet”, as the two are similar in
size and bulk composition. The planet is covered with an opaque layer of highly reflective
clouds and its surface cannot be seen from space in visible light, making it a subject
of great speculation until some of its secrets were revealed by planetary science in the
20th century. Venus has the densest atmosphere of the terrestrial planets, consisting mostly
of carbon dioxide, and the atmospheric pressure at the planet’s surface is 90 times
that of the Earth.

Venus’ surface has been mapped in detail only in the last 20 years. It shows evidence of
extensive volcanism, and some of its volcanoes may still be active today. In contrast to
the constant crustal movement seen on Earth, Venus is thought to undergo periodic
episodes of plate tectonics, in which the crust is subducted rapidly within a few million
years separated by stable periods of a few hundred million years.

The planet is named after Venus, the Roman goddess of love, and most of its surface
features are named after famous and mythological women. The adjective Venusian is
commonly used for items related to Venus, though the Latin adjective is the rarely used
Venereal; the now-archaic Cytherean is still occasionally encountered.

Structure

Venus is one of the four terrestrial planets, meaning that, like the Earth, it is a rocky
body. In size and mass, it is very similar to the Earth, and is often described as its ‘twin’.
The diameter of Venus is only 650 km less than the Earth’s, and its mass is 81.5% of the
Earth’s. However, conditions on the Venusian surface differ radically from those on
Earth, due to its dense carbon dioxide atmosphere.

Internal structure

Though there is little direct information about its internal structure, the similarity in size
and density between Venus and Earth suggests that it has a similar internal structure: a
core, mantle and crust. Like that of Earth, the Venusian core is at least partially liquid.
The slightly smaller size of Venus suggests that pressures are significantly lower in its
deep interior than Earth. The principal difference between the two planets is the lack of
plate tectonics on Venus, likely due to the dry surface and mantle. This results in reduced
heat loss from the planet, preventing it from cooling and providing a likely explanation
for its lack of an internally generated magnetic field.

Geography

About 80% of Venus’ surface consists of smooth volcanic plains. Two highland
‘continents’ make up the rest of its surface area, one lying in the planet’s northern
hemisphere and the other just south of the equator. The northern continent is called Ishtar
Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia.
Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak lies 11 km
above Venus’ average surface elevation; in contrast, Earth’s highest mountain, Mount
Everest, rises to just under 9 km above sea level. The southern continent is called
Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland
regions at roughly the size of South America. Much of this continent is covered by a
network of fractures and faults.

As well as the impact craters, mountains, and valleys commonly found on rocky planets,
Venus has a number of unique surface features. Among these are flat-topped volcanic
features called farra, which look somewhat like pancakes and range in size from 20–
50 km across, and 100–1000 m high; radial, star-like fracture systems called novae;
features with both radial and concentric fractures resembling spiders’ webs, known as
arachnoids; and coronae, circular rings of fractures sometimes surrounded by a
depression. All of these features are volcanic in origin.

Almost all Venusian surface features are named after historical and mythological women.
The only exceptions are Maxwell Montes, named after James Clerk Maxwell, and two
highland regions, Alpha Regio and Beta Regio. These three features were named before
the current system was adopted by the International Astronomical Union, the body that
oversees planetary nomenclature.

Surface geology

Map of Venus, showing the elevated ‘continents’ in yellow: Ishtar Terra at the top and
Aphrodite Terra just below the equator to the right
Much of Venus’ surface appears to have been shaped by volcanic activity. Overall,
Venus has several times as many volcanoes as Earth, and it possesses some 167 giant
volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is
the Big Island of Hawaii. However, this is not because Venus is more volcanically active
than Earth, but because its crust is older. Earth’s crust is continually recycled by
subduction at the boundaries of tectonic plates, and has an average age of about 100
million years, while Venus’ surface is estimated to be about 500 million years old.

Several lines of evidence point to ongoing volcanic activity on Venus. During the Russian
Venera program, the Venera 11 and Venera 12 probes detected a constant stream
of lightning, and Venera 12 recorded a powerful clap of thunder soon after it landed.
While rainfall drives thunderstorms on Earth, there is no rainfall on Venus. One
possibility is that ash from a volcanic eruption was generating the lightning. Another
intriguing piece of evidence comes from measurements of sulfur dioxide concentrations
in the atmosphere, which were found to drop by a factor of 10 between 1978 and 1986.
This may imply that the levels had earlier been boosted by a large volcanic eruption.

Impact craters on the surface of Venus

The Venusian surface is dominated by plains of basalt as it is shown here by a picture
from Venera 14.

There are almost 1,000 impact craters on Venus, more or less evenly distributed across its
surface. On other cratered bodies, such as Earth and the Moon, craters show a range of
states of erosion, indicating a continual process of degradation. On the Moon,
degradation is caused by subsequent impacts, while on Earth, it is caused by wind and rain
erosion. However, on Venus, about 85% of craters are in pristine condition. The number
of craters together with their well-preserved condition indicates that the planet underwent
a total resurfacing event about 500 million years ago. Earth’s crust is in continuous
motion, but it is thought that Venus cannot sustain such a process. Without plate
tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical
process in which mantle temperatures rise until they reach a critical level that weakens
the crust. Then, over a period of about 100 million years, subduction occurs on an
enormous scale, completely recycling the crust.

Venusian craters range from 3 km to 280 km in diameter. There are no craters smaller
than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects
with less than a certain kinetic energy are slowed down so much by the atmosphere that
they do not create an impact crater.

Atmosphere

Venus has an extremely thick atmosphere, which consists mainly of carbon dioxide and a
small amount of nitrogen. The pressure at the planet’s surface is about 90 times that at
Earth’s surface—a pressure equivalent to that at a depth of 1 kilometer under Earth’s
oceans. The enormously CO2-rich atmosphere generates a strong greenhouse effect that
raises the surface temperature to over 400 °C. This makes Venus’ surface hotter than

Mercury’s, even though Venus is nearly twice as distant from the Sun and receives only
25% of the solar irradiance.

Cloud structure in Venus’ atmosphere, revealed by ultraviolet observations
Studies have suggested that several billion years ago Venus’ atmosphere was much more
like Earth’s than it is now, and that there were probably substantial quantities of liquid
water on the surface, but a runaway greenhouse effect was caused by the evaporation of
that original water, which generated a critical level of greenhouse gases in its atmosphere.
Venus is thus an example of an extreme case of climate change, making it a useful tool in
climate change studies.

Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the
temperature of Venus’ surface does not vary significantly between the night and day
sides, despite the planet’s extremely slow rotation. Winds at the surface are slow, moving
at a few kilometers per hour, but because of the high density of the atmosphere at Venus’
surface, they exert a significant amount of force against obstructions, and transport dust
and small stones across the surface.
Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and
sulfuric acid droplets. These clouds reflect about 60% of the sunlight that falls on them
back into space, and prevent the direct observation of Venus’ surface in visible light. The
permanent cloud cover means that although Venus is closer than Earth to the Sun, the
Venusian surface is not as well heated or lit. In the absence of the greenhouse effect

caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus
would be quite similar to that on Earth. Strong 300 km/h winds at the cloud tops circle
the planet about every four to five earth days.

Magnetic field and core

In 1980, The Pioneer Venus Orbiter found that Venus’ magnetic field is both weaker and
smaller (i.e. closer to the planet) than Earth’s. What small magnetic field is present is
induced by an interaction between the ionosphere and the solar wind, rather than by an
internal dynamo in the core like the one inside the Earth. Venus’ magnetosphere is too
weak to protect the atmosphere from cosmic radiation.

This lack of an intrinsic magnetic field at Venus was surprising given that it is similar to
Earth in size, and was expected to also contain a dynamo in its core. A dynamo requires
three things: a conducting liquid, rotation, and convection. The core is thought to be
electrically conductive, however. Also, while its rotation is often thought to be too slow,
simulations show that it is quite adequate to produce a dynamo. This implies that the
dynamo is missing because of a lack of convection in Venus’ core. On Earth, convection
occurs in the liquid outer layer of the core because the bottom of the liquid layer is much
hotter than the top. Since Venus has no plate tectonics to let off heat, it is possible that it
has no solid inner core, or that its core is not currently cooling, so that the entire liquid
part of the core is at approximately the same temperature. Another possibility is that its
core has already completely solidified.

Orbit and rotation

Venus orbits the Sun at an average distance of about 106 million km, and completes an
orbit every 224.7 days. Although all planetary orbits are elliptical, Venus’ is the closest to
circular, with an eccentricity of less than 1%. When Venus lies between the Earth and the
Sun, a position known as ‘inferior conjunction’, it makes the closest approach to Earth of
any planet, lying at a distance of about 40 million km. The planet reaches inferior
conjunction every 584 days, on average.

Venus rotates once every 243 days – by far the slowest rotation period of any of the
major planets. A Venusian day thus lasts more than a Venusian year (243 versus 224.7
Earth days). At the equator, Venus’ surface rotates at 6.5 km/h; on Earth, the rotation
speed at the equator is about 1,600 km/h. To an observer on the surface of Venus, the Sun
would appear to rise in the west and set in the east every 116.75 days (which corresponds
to the period of continuous sunlight, on the Earth an average of 12 hours).

If viewed from above the Sun’s north pole, all of the planets are orbiting in an
anticlockwise direction; but while most planets also rotate anticlockwise, Venus rotates
clockwise in “retrograde” rotation. The question of how Venus came to have a slow,
retrograde rotation was a major puzzle for scientists when the planet’s rotation period
was first measured. When it formed from the solar nebula, Venus would have had a much

faster, prograde rotation, but calculations show that over billions of years, tidal effects on
its dense atmosphere could have slowed down its initial rotation to the value seen today.

A curious aspect of Venus’ orbit and rotation periods is that the 584-day average interval
between successive close approaches to the Earth is almost exactly equal to five
Venusian solar days. Whether this relationship arose by chance or is the result of some
kind of tidal locking with the Earth, is unknown.

Venus is currently moonless, though the asteroid 2002 VE68 currently maintains a quasi-
satellite orbital relationship with it.

According to Alex Alemi and David Stevenson of the California Institute of Technology,
their recent study of models of the early solar system shows that it is very likely that,
billions of years ago, Venus had at least one moon, created by a huge impact event .
About 10 million years later, according to Alemi and Stevenson, another impact reversed
the planet’s spin direction. The reversed spin direction caused the Venusian moon to
gradually spiral inward until it collided and merged with Venus. If later impacts created
moons, those moons also were absorbed the same way the first one was. The
Alemi/Stevenson study is recent, and it remains to be seen what sort of acceptance it will
achieve in the scientific community.

Observation

Venus as the Evening Star, next to a crescent moon

Venus is always brighter than the brightest stars, with its apparent magnitude ranging
from −3.8 to −4.6. This is bright enough to be seen even in the middle of the day, and the
planet can be easy to see when the Sun is low on the horizon. As an inferior planet, it
always lies within about 47° of the Sun.

Venus ‘overtakes’ the Earth every 584 days as it orbits the Sun. As it does so, it goes
from being the ‘Evening star’, visible after sunset, to being the ‘Morning star’, visible
before sunrise. While Mercury, the other inferior planet, reaches a maximum elongation
of only 28° and is often difficult to discern in twilight, Venus is almost impossible not to
identify when it is at its brightest. Its greater maximum elongation means it is visible in

dark skies long after sunset. As the brightest point-like object in the sky, Venus is a
commonly misreported ‘unidentified flying object’. In 1969, future U.S. President Jimmy
Carter reported having seen a UFO, which later analysis suggested was probably the
planet, and countless other people have mistaken Venus for something more exotic.

As it moves around its orbit, Venus displays phases like those of the Moon: it is new
when it passes between the Earth and the Sun, full when it is on the opposite side of the
Sun, and a crescent when it is at its maximum elongations from the Sun. Venus is
brightest when it is a thin crescent; it is much closer to Earth when a thin crescent than
when gibbous, or full.

Venus transits the face of the Sun on 2004-06-08. Here, the black drop effect is visible.

Venus’ orbit is slightly inclined relative to the Earth’s orbit; thus, when the planet passes
between the Earth and the Sun, it usually does not cross the face of the Sun. However,
transits of Venus do occur in pairs separated by eight years, at intervals of about 120
years, when the planet’s inferior conjunction coincides with its presence in the plane of
the Earth’s orbit. The most recent transit was in 2004; the next will be in 2012.
Historically, transits of Venus were important, because they allowed astronomers to
directly determine the size of the astronomical unit, and hence of the solar system.
Captain Cook’s exploration of the east coast of Australia came after he had sailed to
Tahiti in 1768 to observe a transit of Venus.

A long-standing mystery of Venus observations is the so-called ‘ashen light’—an
apparent weak illumination of the dark side of the planet, seen when the planet is in the
crescent phase. The first claimed observation of ashen light was made as long ago as
1643, but the existence of the illumination has never been reliably confirmed. Observers
have speculated that it may result from electrical activity in the Venusian atmosphere, but
it may be illusory, resulting from the physiological effect of observing a very bright
crescent-shaped object.

Studies of Venus

Early studies

Galileo’s discovery that Venus showed phases proved that it orbits the Sun and not the
Earth

Venus is known in the Hindu Jyotisha since early times as the planet Shukra. In the West,
before the advent of the telescope, Venus was known only as a ‘wandering star’. Several
cultures historically held its appearances as a morning and evening star to be those of two
separate bodies. Pythagoras is usually credited with recognizing in the sixth century BC
that the morning and evening stars were a single body, though he espoused the view that
Venus orbited the Earth. When Galileo first observed the planet in the early 17th century,
he found that it showed phases like the Moon’s, varying from crescent to gibbous to full
and vice versa. This could be possible only if Venus orbited the Sun, and this was among
the first observations to clearly contradict the Ptolemaic geocentric model that the solar
system was concentric and centered on the Earth.

Venus’ atmosphere was discovered as early as 1790 by Johann Schröter. Schröter found
that when the planet was a thin crescent, the cusps extended through more than 180°. He
correctly surmised that this was due to scattering of sunlight in a dense atmosphere.
Later, Chester Smith Lyman observed a complete ring around the dark side of the planet
when it was at inferior conjunction, providing further evidence for an atmosphere. The
atmosphere complicated efforts to determine a rotation period for the planet, and
observers such as Giovanni Cassini and Schröter incorrectly estimated periods of about
24 hours from the motions of apparent markings on the planet’s surface.

Ground-based research

Little more was discovered about Venus until the 20th century. Its almost featureless disc
gave no hint as to what its surface might be like, and it was only with the development of
spectroscopic, radar and ultraviolet observations that more of its secrets were revealed.
The first UV observations were carried out in the 1920s, when Frank E. Ross found that
UV photographs revealed considerable detail that was absent in visible and infrared

radiation. He suggested that this was due to a very dense yellow lower atmosphere with
high cirrus clouds above it.

Spectroscopic observations in the 1900s gave the first clues about Venus’ rotation. Vesto
Slipher tried to measure the Doppler shift of light from Venus, but found that he could
not detect any rotation. He surmised that the planet must have a much longer rotation
period than had previously been thought. Later work in the 1950s showed that the
rotation was retrograde. Radar observations of Venus were first carried out in the 1960s,
and provided the first measurements of the rotation period which were close to the
modern value.

Radar observations in the 1970s revealed details of Venus’ surface for the first time.
Pulses of radio waves were beamed at the planet using the 300 m radio telescope at
Arecibo Observatory, and the echoes revealed two highly reflective regions, designated
the Alpha and Beta regions. The observations also revealed a bright region attributed to
mountains, which was called Maxwell Montes. These three features are now the only
ones on Venus which do not have female names.

The best radar images obtainable from Earth revealed features no smaller than about
5 km across. More detailed exploration of the planet could only be carried out from
space.

Research with space probes

Early efforts

The first unmanned space mission to Venus, and the first to any planet, began on 12
February 1961 with the launch of the Venera 1 probe. The first craft of the highly
successful Soviet Venera program, Venera 1 was launched on a direct impact trajectory,
but contact was lost seven days into the mission, when the probe was about 2 million km
from Earth. It was estimated to have passed within 100,000 km from Venus in mid-May.

The United States’ exploration of Venus also started badly with the loss of the Mariner 1
probe on launch. The subsequent Mariner 2 mission enjoyed greater success, and after a
109-day transfer orbit on 14 December 1962 it became the world’s first successful
interplanetary mission, passing 34,833 km above the surface of Venus. Its microwave and
infrared radiometers revealed that while Venus’ cloud tops were cool, the surface was
extremely hot — at least 425°C, finally ending any hopes that the planet might harbor
ground-based life. Mariner 2 also obtained improved estimates of Venus’ mass and of the
astronomical unit, but was unable to detect either a magnetic field or radiation belts.[34]

Atmospheric entry

Venera 3, the first man-made object to strike the surface of another planet

The Venera 3 probe crash-landed on Venus on March 1, 1966. It was the first man-made
object to enter the atmosphere and strike the surface of another planet, though its
communication system failed before it was able to return any planetary data. Venus’ next
encounter with an unmanned probe came on October 18, 1967 when Venera 4
successfully entered the atmosphere and deployed a number of science experiments.
Venera 4 showed that the surface temperature was even hotter than Mariner 2 had
measured at almost 500°C, and that the atmosphere was about 90 to 95% carbon dioxide.
The Venusian atmosphere was considerably denser than Venera 4’s designers had
anticipated, and its slower than intended parachute descent meant that its batteries ran
down before the probe reached the surface. After returning descent data for 93 minutes,
Venera 4’s last pressure reading was 18 bar at an altitude of 24.96 km.

Another probe arrived at Venus one day later on October 19, 1967 when Mariner 5
conducted a flyby at a distance of less than 4,000 km above the cloud tops. Mariner 5 was
originally built as backup for the Mars-bound Mariner 4, but when that mission was
successful, the probe was refitted for a Venus mission. A suite of instruments more
sensitive than those on Mariner 2, in particular its radio occultation experiment, returned
data on the composition, pressure and density of Venus’ atmosphere.[35] The joint Venera
4–Mariner 5 data were analyzed by a combined Soviet-American science team in a series
of colloquia over the following year, in an early example of space cooperation.

Armed with the lessons and data learned from Venera 4, the Soviet Union launched the
twin probes Venera 5 and Venera 6 five days apart in January 1969; they encountered
Venus a day apart on May 16 and May 17 that year. The probes were strengthened to
improve their crush depth to 25 atmospheres and were equipped with smaller parachutes
to achieve a faster descent. Since the then current atmospheric models of Venus
suggested a surface pressure of between 75 and 100 atmospheres, neither were expected
to survive to the surface. After returning atmospheric data for a little over fifty minutes,
they both were crushed at altitudes of approximately 20 km before going on to strike the
surface on the night side of Venus.

Surface science

Venera 7 represented a concerted effort to return data from the planet’s surface, and was
constructed with a reinforced descent module capable of withstanding a pressure of
180 bar. The module was pre-cooled prior to entry and equipped with a specially reefed
parachute for a rapid 35-minute descent. Entering the atmosphere on 15 December 1970,
the parachute is believed to have partially torn during the descent, and the probe struck
the surface with a hard, yet not fatal, impact. Probably tilted onto its side, it returned a
weak signal supplying temperature data for 23 minutes, the first telemetry received from
the surface of another planet.

The Venera program continued with Venera 8 sending data from the surface for 50
minutes, and Venera 9 and Venera 10 sending the first images of the Venusian
landscape. The two landing sites presented very different visages in the immediate
vicinities of the landers: Venera 9 had landed on a 20 degree slope scattered with
boulders around 30-40 cm across; Venera 10 showed basalt-like rock slabs interspersed
with weathered material.

The Pioneer Venus orbiter

In the meantime, the United States had sent the Mariner 10 probe on a gravitational
slingshot trajectory past Venus on its way to Mercury. On February 5, 1974, Mariner 10
passed within 5790 km of Venus, returning over 4,000 photographs as it did so. The
images, the best then achieved, showed the planet to be almost featureless in visible light,
but ultraviolet light revealed details in the clouds that had never been seen in Earth-bound
observations.[36]

The American Pioneer Venus project consisted of two separate missions.[37] The Pioneer
Venus Orbiter was inserted into an elliptical orbit around Venus on December 4, 1978,
and remained there for over thirteen years studying the atmosphere and mapping the
surface with radar. The Pioneer Venus Multiprobe released a total of five probes which
entered the atmosphere on December 9, 1978, returning data on its composition, winds
and heat fluxes.

Venusian surface shot by Venera 13

Four more Venera lander missions took place over the next four years, with Venera 11
and Venera 12 detecting Venusian electrical storms; and Venera 13 and Venera 14,
landing four days apart on March 1 and March 5, 1982, returning the first color
photographs of the surface. All four missions deployed parachutes for braking in the
upper atmosphere, but released them at altitudes of 50 km, the dense lower atmosphere
providing enough friction to allow for an unaided soft landing. Both Venera 13 and 14
analyzed soil samples with an on-board X-ray fluorescence spectrometer, and attempted
to measure the compressibility of the soil with an impact probe. Venera 14, though, had
the misfortune to strike its own ejected camera lens cap and its probe failed to make
contact with the soil. The Venera program came to a close in October 1983 when Venera
15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with
synthetic aperture radar.

The Soviet Union had not finished with Venus, and in 1985 it took advantage of the
opportunity to combine missions to Venus and Comet Halley, which passed through the
inner solar system that year. En route to Halley, on June 11 and June 15, 1985 the two
spacecraft of the Vega program each dropped a Venera-style probe (of which Vega 1’s
partially failed) and released a balloon-supported aerobot into the upper atmosphere. The
balloons achieved an equilibrium altitude of around 53 km, where pressure and
temperature are comparable to those at Earth’s surface. They remained operational for
around 46 hours, and discovered that the Venusian atmosphere was more turbulent than
previously believed, and subject to high winds and powerful convection cells.[38]

Radar mapping

Magellan topographical map of Venus

The United States’ Magellan probe was launched on 4 May 1989 with a mission to map
the surface of Venus with radar. The high-resolution images it obtained during its 4½
years of operation far surpassed all prior maps and were comparable to visible-light
photographs of other planets. Magellan imaged over 98% of Venus’ surface by radar and
mapped 95% of its gravity field. In 1994, at the end of its mission, Magellan was
deliberately sent to its destruction into the atmosphere of Venus in an effort to quantify
its density. Venus was observed by the Galileo and Cassini spacecraft during flybys on
their respective missions to the outer planets, but Magellan would otherwise be the last
dedicated mission to Venus for over a decade.

Current and future missions

The Venus Express probe successfully assumed orbit around Venus on April 11, 2006. It
was designed and built by the European Space Agency and launched by the Russian
Federal Space Agency on November 9, 2005. On April 11 of the following year, its main
engine was successfully fired to place it in a polar orbit about the planet. The probe is
undertaking a detailed study of the Venusian atmosphere and clouds, and will also map
the planet’s plasma environment and surface characteristics, particularly temperatures. Its
mission is intended to last a nominal 500 Earth days, or around two Venusian years.[39]
One of the first results emerging from Venus Express is the discovery that a huge double
atmospheric vortex exists at the south pole of the planet.

Japan’s aerospace body JAXA (formerly ISAS) is planning to launch its Venus climate
orbiter, the PLANET-C, in 2010.

Future flybys en route to other destinations include the MESSENGER and BepiColombo
missions to Mercury.

Venus in human culture

Historic connections

As one of the brightest objects in the sky, Venus has been known since prehistoric times
and from the earliest days has had a significant impact on human culture. It is described
in Babylonian cuneiformic texts such as the Venus tablet of Ammisaduqa, which relates
observations that possibly date from 1600 BC. The Babylonians named the planet Ishtar,
the personification of womanhood, and goddess of love. The Ancient Egyptians believed
Venus to be two separate bodies and knew the morning star as Tioumoutiri and the
evening star as Ouaiti. Likewise believing Venus to be two bodies, the Ancient Greeks
called the morning star Φωσφόρος, Phosphorus, the “Bringer of Light” or Εωσφόρος,
Eosphorus, the “Bringer of Dawn”; the evening star they called Hesperos (Ἓσπερος, the
star of the dusk) — by Hellenistic times, it was realized they were the same planet.
Hesperos would be translated into Latin as Vesper and Phosphorus as Lucifer, a poetic
term later used to refer to the fallen angel cast out of heaven.[40] The Romans would later
name the planet in honor of their goddess of love, Venus, whereas the Greeks used the
name of its Greek counterpart, Aphrodite.

To the Hebrews it was known as Noga (“shining”), Ayeleth-ha-Shakhar (“deer of the
dawn”) and Kochav-ha-‘Erev (“star of the evening”). Venus was important to the Mayan
civilization, who developed a religious calendar based in part upon its motions, and held
the motions of Venus to determine the propitious time for events such as war. The Maasai
people named the planet Kileken, and have an oral tradition about it called The Orphan
Boy. In western astrology, derived from its historical connotation with goddesses of
femininity and love, Venus is held to influence those aspects of human life. In Vedic
astrology, where such an association was not made, Venus or Shukra affected wealth,
comfort, and attraction. Early Chinese astronomers called the body Tai-pe, or the
“beautiful white one”. Modern Chinese, Korean, Japanese and Vietnamese cultures refer
to the planet literally as the metal star (Chinese: ), based on the Five elements.

The astronomical symbol for Venus is the same as that used in biology for the female
sex, a stylized representation of the goddess Venus’ hand mirror: a circle with a small
cross underneath. The Venus symbol also represents femininity, and in ancient alchemy
stood for the metal copper. Alchemists constructed the symbol from a circle (representing
spirit) above a cross (representing matter).

In fiction

Lucky Starr and the Oceans of Venus

Venus’ impenetrable cloud cover gave science fiction writers free rein to speculate on
conditions at its surface; all the more so when early observations showed that it was very
similar in size to Earth and possessed a substantial atmosphere. The planet was frequently
depicted as warmer than Earth beneath the clouds, but still habitable by humans. The
genre reached its peak between the 1930s and 1950s, at a time when science had revealed
some aspects of Venus, but not yet the harsh reality of its surface conditions. Robert
Heinlein’s Future History series was set on a Venus inspired by the chemist Svante
Arrhenius’s prediction of a steamy carboniferous swamp upon which the rain dripped
incessantly. It probably inspired Henry Kuttner to the subsequent depiction given in his
novel Fury. Ray Bradbury’s short stories The Long Rain (found in the collection The
Illustrated Man) and All Summer in a Day (found in the collection A Medicine for
Melancholy) also depicted Venus as a habitable planet with incessant rain. Other works,
such as C. S. Lewis’s 1943 Perelandra or Isaac Asimov’s 1954 Lucky Starr and the
Oceans of Venus, drew from a vision of a Cambrian-like Venus covered by a near planet-
wide ocean filled with exotic aquatic life.

As scientific knowledge of Venus has advanced, the authors of science fiction have
endeavored to keep pace, particularly by conjecturing human attempts to terraform
Venus. In his 1997 novel 3001: The Final Odyssey, Arthur C. Clarke postulated humans
steering cometary fragments to impact Venus, the resulting addition of water to the
Venus environment intended to lower its temperature and absorb carbon dioxide. A
terraformed Venus is the setting for a number of diverse works of fiction that have
included Star Trek, Exosquad, Cowboy Bebop and the manga Venus Wars, and the theme
seems to be in little danger of dying out. A variation of this theme is Frederik Pohl’s The
Merchants of Venus (1972), which started his celebrated Heechee Series, where Venus
was colonised long ago by mysterious aliens whose abandoned dwellings and artifacts
make human colonization both materially easier and provide a strong economic incentive.

Earth

Earth

The Blue Marble, taken from Apollo 17.

Orbital characteristics (Epoch J2000)

Aphelion 152,097,701 km
(1.016 710 333 5 AU)

Perihelion 147,098,074 km
(0.983 289 891 2 AU)

Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)

Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)

Orbital circumference 924,375,700 km
( 6.179 069 900 7 AU)

Orbital eccentricity 0.016 710 219

Sidereal orbit period 365.256 366 d
(1.000 017 5 a)

Synodic period

Max. orbital speed

Average orbital speed

Min. orbital speed

Orbital inclination to
ecliptic

Longitude of the
ascending node

Argument of the
perihelion

Satellites

Physical characteristics

Aspect Ratio

Ellipticity

Equatorial radius

Polar radius

Mean radius

Equatorial
circumference

Meridional
circumference

Mean circumference

Surface area

Land area

Water area

Volume

Mass

Density

Equatorial surface
gravity

Escape velocity

Sidereal rotation
period

Rotational velocity at 465.11 m/s
equator

Axial tilt 23.439 281°

Right ascension of 0° (0 h 0 min 0 s)
North pole

Declination +90°

Albedo 0.367

Surface temperature 185 K (-88 °C) min
287 K (14 °C) mean
331 K (58 °C) max

Surface pressure 101.3 kPa (MSL)

Adjective Terrestrial, Terran, Telluric, Tellurian,
Earthly, Earthling (lifeforms)

Atmospheric constituents

Nitrogen 78.08 %
Oxygen 20.94 %
Argon 0.93 %
Carbon dioxide 0.038%

Water vapor Trace (varies with climate)

Earth (IPA: / ə (ɹ)θ/, often referred to as the Earth, Terra, or Planet Earth) is the
third planet in the solar system in terms of distance from the Sun, and the fifth largest. It
is also the largest of its planetary system’s terrestrial planets, making it the largest solid
body in the solar system, and it is the only place in the universe known to support life. It
is also the densest planet in the solar system. The widely accepted scientific theory states
that the Earth was formed around 4.57 billion years ago and its natural satellite, the
Moon, was orbiting it shortly thereafter, around 4.53 billion years ago.

Since it formed, the Earth has changed through geological and biological processes that
have hidden traces of the original conditions. The outer surface is divided into several
tectonic plates that gradually migrate across the surface over geologic time spans. The
interior of the planet remains active, with a thick layer of convecting yet solid Earth
mantle and an iron core that generates a magnetic field. Its atmospheric conditions have
been significantly altered by the presence of life forms, which create an ecological
balance that modifies the surface conditions. About 71% of the surface is covered in salt-
water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The
relatively large moon provides ocean tides and has gradually modified the length of the
planet’s rotation period. A cometary bombardment during the early history of the planet
is believed to have played a role in the formation of the oceans. Later, asteroid impacts are
understood to have caused significant changes to the surface environment. Changes in the
orbit of the planet may also be responsible for the ice ages that have covered significant
portions of the surface in glacial sheets.

The Earth’s only natural orbiting body is the Moon, although the asteroid Cruithne has
been erroneously described as such. Cruithne was discovered in 1986 and follows an
elliptical orbit around the Sun at about the same average orbital radius as the Earth.
However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit
around the Sun that avoids close proximity with the Earth.

Lexicography

In American English usage, the name can be capitalized or spelled in lowercase
interchangeably, either when used absolutely or prefixed with “the” (i.e. Earth, the Earth,
earth or the earth). Many deliberately spell the name of the planet with a capital, both as
“Earth” or “the Earth”. This is to distinguish it as a proper noun, distinct from the senses
of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the
electrical sense, etc.). Oxford Spelling recognizes the lowercase form as the most
common, with the capitalized form as a variant of it. Another convention that is very
common is to spell the name with a capital when occurring absolutely (e.g. Earth’s
atmosphere) and lowercase when preceded by “the” (e.g. the atmosphere of the earth).
The term almost exclusively exists in lowercase when appearing in common phrases,
even without “the” preceding it (e.g. it doesn’t cost the earth; what on earth are you
doing?).

Terms that refer to the Earth can use the Latin root terr-, as in terraform and terrestrial.
An alternative Latin root is tellur-, which is used in words such as tellurian and tellurium.
Such terms derive from Latin terra and tellus, which refer variously to the world, the
element earth, the earth goddess and so forth. Scientific terms such as geography,
geocentric and geothermal use the Greek prefix geo- (γαιο-, gaio-), from gē (again
meaning “earth”). In many science fiction books and video games, Earth is referred to as
Terra or Gaia. Astronauts refer to the Earth as “Terra Firma”.

The English word “earth” has cognates in many modern and ancient languages. Examples
in modern tongues include aarde in Dutch and Erde in German. The root has cognates in
extinct languages such as ertha in Old Saxon and ert (meaning “ground”) in Middle Irish,
derived from the Old English eorðe. All of these words derive from the Proto-Indo-
European base *er-.

Several Semitic languages have words for “earth” similar to those in Indo-European
languages. Arabic has ard; Akkadian, irtsitu; Aramaic, araa; Phoenician, erets (which
appears in the Mesha Stele); and Hebrew, ‫( ץרא‬arets, or erets when not preceded by a
definite article, or when followed by a noun modifier). The etymological connection
between the words in Indo-European and Semitic languages are uncertain, though, and
may simply be coincidence.

The standard name for people from Earth is Earthling, although Terran, Gaian, and
Earther are alternate names that have been used in Science fiction.

Words for Earth in other languages include: Terre (French), pr thvī

(Sanskrit), Maa (Finnish and Estonian), Pamînt (Romanian), Föld (Hungarian), Zemlja

(Russian and Serbian), Tierra (Spanish), Diqiu (Mandarin), Deiqao (Cantonese), Jigu

(Korean), Bumi (Malay), Chikyuu (Japanese), Jorden (Danish, Norwegian, Swedish), Gi,

Choma (Greek), Dunia (Swahili), Âlem, Dünya (Arabic), Dinê

(Kurdish), աշխարհ (Armenian), Jehun, Zamin (Persian), and Acun, Yeryüzü, Yerküre

(Turkish).

Shape

The Earth’s shape is very close to an oblate spheroid, although the precise shape (the
geoid) varies from this by up to 100 meters (327 ft). The average diameter of the reference
spheroid is approximately 12,742 km (more roughly, 40,000 km/π). The rotation
of the Earth causes the equator to bulge out slightly so that the equatorial diameter
is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky
surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana
Trench (10,924 m below local sea level). Hence compared to a perfect ellipsoid, the
Earth has a tolerance of about one part in about 584, or 0.17%. For comparison, this is
less than the 0.22% tolerance allowed in billiard balls. Because of the bulge, the feature
farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.

Composition

The mass of the Earth is approximately 5980 yottagrams (5.98 ×1024 kg). It is composed
mostly of iron (35.0%), oxygen (28.0%), silicon (17.0%), magnesium (15.7%), nickel
(1.5%), calcium (1.4%) and aluminium (1.4%).

Internal structure

Earth cutaway from core to exosphere. Partially to scale

The interior of the Earth, like that of the other terrestrial planets, is chemically divided
into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid
outer core that is much less viscous than the mantle, and a solid inner core.

The geologic component layers of the Earth are at the following depths below the
surface:

Depth Layer
Kilometers Miles

0–60 0–37 Lithosphere (locally varies between 5 and 200 km)

0–35 0–22 ... Crust (locally varies between 5 and 70 km)

35–60 22–37 ... Uppermost part of mantle

35–2890 22–1790 Mantle

100–700 62–435 ... Asthenosphere
2890–5100 1790–3160 Outer core
5100–6378 3160–3954 Inner core

Tectonic plates

A map pointing out the Earth’s major plates.

According to plate tectonics theory currently accepted by the vast majority of scientists
working in this area, the outermost part of the Earth’s interior is made up of two layers:
the lithosphere comprising the crust, and the solidified uppermost part of the mantle.
Below the lithosphere lies the asthenosphere, which comprises the inner, viscous part of
the mantle. The mantle behaves like a superheated and extremely viscous liquid.

The lithosphere essentially floats on the asthenosphere and is broken up into what are
called tectonic plates. These plates move in relation to one another at one of three types
of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity,
mountain-building, and oceanic trench formation occur along plate boundaries.
The main plates are

• African Plate, covering Africa - Continental plate
• Antarctic Plate, covering Antarctica - Continental plate
• Australian Plate, covering Australia (fused with Indian Plate between 50 and 55

million years ago) - Continental plate
• Eurasian Plate covering Asia and Europe - Continental plate
• North American Plate covering North America and north-east Siberia -

Continental plate
• South American Plate covering South America - Continental plate
• Pacific Plate, covering the Pacific Ocean - Oceanic plate
Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the
Nazca Plate and the Scotia Plate.

Surface

Surface of the Earth, colors reflect changes in elevation

The Earth’s terrain varies greatly from place to place. About 70% of the surface is
covered by water, with much of the continental shelf below sea level. If all of the land on
Earth were spread evenly, water would rise to an altitude of more than 2500 metres
(approximately 8000 ft.). The remaining 30% not covered by water consists of
mountains, deserts, plains, plateaus, etc.

Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting
permanent crops. Close to 40% of the Earth’s land surface is presently used for cropland
and pasture, or an estimated 3.3 × 109 acres of cropland and 8.4 × 109 acres of
pastureland.

Extremes

Elevation extremes: (measured relative to sea level)

• Lowest point on land: Dead Sea −417 m
• Lowest point overall: Challenger Deep of the Mariana Trench in the Pacific

Ocean −10,924 m
• Highest point: Mount Everest 8,844 m (2005 est.)

Hydrosphere

The abundance of water on Earth is a unique feature that distinguishes the “Blue Planet”
from others in the solar system. Approximately 70.8 percent of the Earth is covered by
water and only 29.2 percent is terra firma.

The Earth’s hydrosphere consists chiefly of the oceans, but technically includes all water
surfaces in the world, including inland seas, lakes, rivers, and underground waters. The
average depth of the oceans is 3,794 m (12,447 ft), more than five times the average
height of the continents. The mass of the oceans is approximately 1.35 × 10^18 tonnes, or
about 1/4400 of the total mass of the Earth.

Atmosphere

The Earth’s atmosphere has no definite boundary, slowly becoming thinner and fading
into outer space. Three-quarters of the atmosphere’s mass is contained within the first
11 km of the planet’s surface. This lowest layer is called the troposphere. Further up, the
atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.
Beyond these, the exosphere thins out into the magnetosphere (where the Earth’s
magnetic fields interact with the solar wind). An important part of the atmosphere for life
on Earth is the ozone layer.

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale
height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of other
gaseous molecules such as water vapor. The atmosphere protects the Earth’s life forms by
absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor,
and providing useful gases. The atmosphere is one of the principal components in
determining weather and climate.

Because hydrogen gas is light and based on Earth’s mean temperature, achieves escape
velocity, unfixed hydrogen leaves the Earth. For this reason, the Earth’s environment is

oxidizing, with consequences for the chemical nature of life which developed on the
planet.

Climate

A part of the earth as it looks in its round shape. This is not how it looks from space,
however it is what the earth’s shape is.

The most prominent features of the Earth’s climate are its two large polar regions, two
narrow temperate zones, and a wide equatorial tropical region. Precipitation patterns vary
widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents are important factors in determining climate, particularly the spectacular
thermohaline circulation which distributes heat energy from the equatorial oceans to the
polar regions.

Pedosphere

The pedosphere is the outermost layer of the Earth that is composed of soil and subject
to soil formation processes. It exists at the interface of the lithosphere, atmosphere,
hydrosphere and biosphere.

Biosphere

The planet’s lifeforms are sometimes said to form a “biosphere”. This biosphere is
generally believed to have begun evolving about 3.5 billion (3.5×109) years ago. Earth is
the only place in the universe officially recognized by the communities of Earth where
life is absolutely known to exist, and some scientists believe that biospheres might be
rare.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and
fauna. On land primarily latitude and height above the sea level separates biomes.
Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are
relatively barren of plant and animal life, while most of the more populous biomes lie
near the Equator.

Natural resources

• Earth’s crust contains large deposits of fossil fuels: (coal, petroleum, natural gas,
methane clathrate). These deposits are used by humans both for energy production
and as feedstock for chemical production.

• Mineral ore bodies have been formed in Earth’s crust by the action of erosion and
plate tectonics. These bodies form concentrated sources for many metals and
other useful elements.

• Earth’s biosphere produces many useful biological products for humans,
including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the
recycling of many organic wastes. The land-based ecosystem depends upon
topsoil and fresh water, and the oceanic ecosystem depends upon dissolved
nutrients washed down from the land.

Some of these resources, such as mineral fuels, are difficult to replenish on a short time
scale, called non-renewable resources. The exploitation of non-renewable resources near
the surface by human civilization has become a subject of significant controversy in
modern environmentalism movements.

Land use

Humans use the Earth’s land to support themselves through the production of food,
energy, and building material. They also live on the land by buliding shelters. Human use
of land is approximately:

• Arable land: 13.13%
• Permanent crops: 4.71%
• Permanent pastures: 26%
• Forests and woodland: 32%
• Urban areas: 1.5%
• Other: 30% (1993 est.)

Irrigated land: 2,481,250 km² (1993 est.)

Natural and environmental hazards

Large areas are subject to extreme weather such as (tropical cyclones), hurricanes, or
typhoons that dominate life in those areas. Many places are subject to earthquakes,
landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts,
and other calamities and disasters.

Many localize areas are subject to human-made pollution of the air and water, acid rain
and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss
of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction
of invasive species.

Long-term climate alteration from enhancement of the greenhouse effect caused by the
earth itself and human industrial carbon dioxide emissions is an increasing concern, the
focus of intense study and debate.

Human geography

Antarctica
Australia
Africa
Asia
Europe

North
America
South
America
Pacific
Ocean
Pacific
Ocean
Atlantic
Ocean
Indian
Ocean
Southern Ocean
Arctic Ocean
Middle East
Caribbean

Central
Asia

East Asia
North Asia
South
Asia

Southeast
Asia

SW.
Asia

China
Australasia
Melanesia
Micronesia
Polynesia
Central
America

Latin
America

Northern
America

Americas

C.
Africa

E.
Africa

N.
Africa

Southern
Africa

W.
Africa

C.
Europe

E.
Europe

N.
Europe

S.
Europe

W.
Europe

The Earth at night, a composite of satellite photographs showing human made
illumination on the Earth’s surface. Taken between October 1994 and March 1995.
Earth has approximately 6,500,000,000 human inhabitants (February 24, 2006 estimate).
Projections indicate that the world’s human population will reach seven billion in 2013
and 9.1 billion in 2050 (2005 UN estimates). Most of the growth is expected to take place
in developing nations. Human population density varies widely around the world.

It is estimated that only one eighth of the surface of the Earth is suitable for humans to
live on — three-quarters is covered by oceans, and half of the land area is desert, high
mountains or other unsuitable terrain.
The northernmost permanent settlement in the world is Alert, on Ellesmere Island in
Nunavut, Canada. The southernmost is the Amundsen-Scott South Pole Station, in
Antarctica, almost exactly at the South Pole.
There are 267 administrative divisions, including nations, dependent areas, other, and
miscellaneous entries. Earth does not have a sovereign government with planet-wide
authority. Independent sovereign nations claim all of the land surface except for some
segments of Antarctica. There is a worldwide general international organization, the
United Nations. The United Nations is primarily an international discussion forum with
only limited ability to pass and enforce laws.
In total, about 400 people have been outside the Earth’s atmosphere as of 2004, and of
these, twelve have walked on the Moon. Most of the time the only humans in space are
those on the International Space Station, currently three people who are usually replaced
every 6 months.