Mars is the fourth
planet
from the
Sun in the
Solar System. The planet is named after
Mars, the
Roman god of
war. It is also referred to as the "
Red
Planet" because of its
reddish
appearance, due to
iron oxide
prevalent on its surface.
Mars is a
terrestrial planet with
a thin
atmosphere, having surface
features reminiscent both of the
impact
craters of the
Moon and the
volcanoes,
valleys,
deserts and
polar ice
caps of
Earth.
It is the site of
Olympus
Mons
, the highest known mountain
in the Solar System, and of Valles Marineris
, the largest canyon. Furthermore, in June
2008 three articles published in
Nature presented evidence of an
enormous impact crater in Mars's northern hemisphere,
10,600 km long by 8,500 km wide, or roughly four times
larger than the largest impact crater yet discovered, the Moon's
South Pole-Aitken basin. In
addition to its geographical features, Mars’
rotational period and
seasonal cycles are likewise similar to those of
Earth.
Until the first flyby of Mars by
Mariner 4
in 1965, many speculated that there might be liquid water on the
planet's surface. This was based on observations of periodic
variations in
light and
dark patches, particularly in the polar
latitudes, which looked like seas and continents,
while long, dark
striations were
interpreted by some observers as irrigation channels for liquid
water. These straight line features were later proven not to exist
and were instead explained as
optical
illusions. Still, of all the planets in the Solar System other
than Earth, Mars is the most likely to harbor liquid water, and
perhaps
life. Radar data from
Mars Express and the
Mars Reconnaissance Orbiter
have revealed the presence of large quantities of water ice both at
the poles (July 2005) and at mid-latitudes (November 2008). The
Phoenix Mars Lander directly
sampled water ice in shallow martian soil on July 31, 2008.
Mars is currently host to three functional orbiting
spacecraft:
Mars
Odyssey,
Mars
Express, and the
Mars Reconnaissance
Orbiter. With the exception of Earth, this is more than
any planet in the Solar System. The surface is also home to the two
Mars Exploration Rovers
(
Spirit and
Opportunity) and several inert
landers and rovers, both successful and unsuccessful. The
Phoenix lander recently completed its mission on
the surface. Geological evidence gathered by these and preceding
missions suggests that Mars previously had large-scale water
coverage, while observations also indicate that small
geyser-like water flows have occurred during the past
decade.
Observations by NASA
's
Mars Global Surveyor
show evidence that parts of the southern polar ice cap have been
receding.
Mars has two
moons,
Phobos and
Deimos, which are small and irregularly
shaped. These may be captured
asteroids,
similar to
5261 Eureka, a Martian
Trojan asteroid. Mars can be seen
from Earth with the naked eye. Its
apparent magnitude reaches −2.9, a
brightness surpassed only by
Venus, the Moon,
and the Sun, although most of the time
Jupiter will appear brighter to the naked eye than
Mars. Mars has an average
opposition distance of 78 million km
but can come as close as 55.7 million km during a close approach,
such as occurred in
2003.
Physical characteristics
Mars has approximately half the
radius of
Earth. It is less dense than Earth, having about 15% of Earth's
volume and 11% of the
mass. Its
surface area is only slightly less than the
total area of Earth's dry land. While Mars is larger and more
massive than
Mercury, Mercury has a
higher density. This results in a slightly stronger gravitational
force at Mercury's surface. Mars is also roughly intermediate in
size, mass, and
surface gravity
between Earth and
Earth's Moon (the
Moon is about half the diameter of Mars, whereas Earth is twice;
the Earth is about ten times more massive than Mars, and the Moon
ten times
less massive). The red-orange appearance of the
Martian surface is caused by
iron
oxide, more commonly known as hematite, or rust.
Geology
Based on orbital observations and the examination of the
Martian meteorite collection, the surface
of Mars appears to be composed primarily of
basalt. Some evidence suggests that a portion of the
Martian surface is more silica-rich than typical basalt, and may be
similar to
andesitic rocks on Earth;
however, these observations may also be explained by silica glass.
Much of the surface is deeply covered by finely grained
iron oxide dust.
Although Mars has no evidence of structured global
magnetic field, observations show that parts
of the planet's crust have been magnetized and that alternating
polarity reversals of its dipole field have occurred. This
paleomagnetism of magnetically susceptible
minerals has properties that are very similar to the
alternating
bands found on the ocean floors of Earth. One theory, published
in 1999 and re-examined in October 2005 (with the help of the
Mars Global Surveyor), is that
these bands demonstrate
plate
tectonics on Mars 4
billion
years ago, before the planetary
dynamo
ceased to function and caused the planet's magnetic field to fade
away.
Current models of the planet's interior imply a core region about
1,480 kilometres in radius, consisting primarily of
iron with about 14–17%
sulfur.
This
iron sulfide core is partially
fluid, and has twice the concentration of the lighter elements than
exist at Earth's core. The core is surrounded by a silicate
mantle that formed many of the
tectonic and volcanic features on the planet, but now appears to be
inactive. The average thickness of the planet's crust is about
50 km, with a maximum thickness of 125 km. Earth's crust,
averaging 40 km, is only a third as thick as Mars’ crust
relative to the sizes of the two planets.
The geological history of Mars can be split into many epochs, but
the following are the three main ones:
- Noachian epoch (named after Noachis Terra): Formation of the oldest extant
surfaces of Mars, 3.8 billion years ago to 3.5 billion years ago.
Noachian age surfaces are scarred by many large impact craters.
The
Tharsis
bulge volcanic upland is thought to have formed
during this period, with extensive flooding by liquid water late in
the epoch.
- Hesperian epoch (named after Hesperia Planum):
3.5 billion years ago to 1.8 billion years ago. The Hesperian epoch
is marked by the formation of extensive lava plains.
- Amazonian epoch (named after Amazonis Planitia): 1.8 billion years ago
to present. Amazonian regions have few meteorite impact craters but are otherwise
quite varied. Olympus Mons
formed during this period along with lava flows
elsewhere on Mars.
A major geological event occurred on Mars on February 19, 2008, and
was caught on camera by the
Mars Reconnaissance
Orbiter. Images capturing a spectacular avalanche of
materials thought to be fine grained ice, dust, and large blocks
are shown to have detached from a 700-metre high cliff. Evidence of
the avalanche is present in the dust clouds left above the cliff
afterwards.
Recent studies support a theory, first proposed in the 1980s, that
Mars was struck by a
Pluto-sized body about
four billion years ago. The event, thought to be the cause of the
Martian hemispheric dichotomy, created the smooth
Borealis basin that covers 40% of the
planet.
Soil
In June, 2008, the
Phoenix Lander
returned data showing Martian soil to be slightly alkaline and
containing vital nutrients such as
magnesium,
sodium,
potassium and
chloride,
all of which are necessary for living organisms to grow. Scientists
compared the soil near Mars's north pole to that of backyard
gardens on Earth, and concluded that it could be suitable for
growth of plants such as
asparagus.
However, in August, 2008, the Phoenix Lander conducted simple
chemistry experiments, mixing water from
Earth with Martian soil in an attempt to test its
pH, and discovered traces of the
salt perchlorate, while also confirming many
scientists' theories that the Martian surface is considerably
basic, measuring at 8.3. The presence of the perchlorate, if
confirmed, would make Martian soil more exotic than previously
believed. Further testing is necessary to eliminate the possibility
of the perchlorate readings being caused by terrestrial sources,
which may have migrated from the spacecraft either into samples or
the instrumentation.
Hydrology
Liquid water cannot exist on the surface of Mars with its present
low atmospheric pressure, except at the lowest elevations for short
periods but water ice is in no short supply, with two polar ice
caps made largely of ice. In March 2007, NASA announced that the
volume of water ice in the south polar ice cap, if melted, would be
sufficient to cover the entire planetary surface to a depth of 11
metres. Additionally, an ice
permafrost
mantle stretches down from the pole to latitudes of about
60°.
Large quantities of
water are thought to be trapped underneath Mars's thick
cryosphere. Radar data from
Mars Express and the
Mars Reconnaissance Orbiter
have revealed the presence of large quantities of water ice both at
the poles (July 2005) and at mid-latitudes (November 2008). The
Phoenix Mars Lander directly
sampled water ice in shallow martian soil on July 31, 2008.
A large
release of liquid water is thought to have occurred when the
Valles
Marineris
formed early
in Mars's history, forming massive outflow channels. A smaller but more
recent outflow may have occurred when the Cerberus Fossae chasm opened about 5
million years ago, leaving a supposed sea of
frozen ice still visible today on the Elysium Planitia
centered at Cerberus Palus. However, the
morphology of this region may correspond to the ponding of lava
flows, causing a superficial morphology similar to ice flows, which
probably draped the terrain established by earlier massive floods
of Athabasca Valles. Rough surface texture at decimetre (dm)
scales, thermal inertia comparable to that of the Gusev plains, and
hydrovolcanic cones are consistent with the lava flow hypothesis.
Furthermore, the stoichiometric mass fraction of water in this area
to tens of centimetre depths is only ~4%, easily attributable to
hydrated minerals and inconsistent with the presence of
near-surface ice.
More recently the high resolution Mars Orbiter Camera on the
Mars Global Surveyor has taken
pictures which give much more detail about the history of liquid
water on the surface of Mars. Despite the many giant flood channels
and associated tree-like network of tributaries found on Mars there
are no smaller scale structures that would indicate the origin of
the flood waters. It has been suggested that weathering processes
have denuded these, indicating the river valleys are old features.
Higher resolution observations from spacecraft like Mars Global
Surveyor also revealed at least a few hundred features along crater
and canyon walls that appear similar to terrestrial
seepage gullies. The gullies
tend to be in the highlands of the southern hemisphere and to face
the Equator; all are poleward of 30° latitude. The researchers
found no partially degraded (
i.e. weathered) gullies and
no superimposed impact craters, indicating that these are very
young features.
In a particularly striking example (see image) two photographs,
taken six years apart, show a gully on Mars with what appears to be
new deposits of sediment. Michael Meyer, the lead scientist for
NASA's Mars Exploration Program, argues that only the flow of
material with a high liquid water content could produce such a
debris pattern and colouring. Whether the water results from
precipitation, underground or another source remains an open
question.However, alternative scenarios have been suggested,
including the possibility of the deposits being caused by carbon
dioxide frost or by the movement of dust on the Martian
surface.
Further evidence that
liquid water once
existed on the surface of Mars comes from the detection of specific
minerals such as
hematite and
goethite, both of which sometimes form in the
presence of water.
Nevertheless, some of the evidence believed to indicate ancient
water basins and flows has been negated by higher resolution
studies taken at resolution about 30 cm by the Mars
Reconnaissance Orbiter.
Geysers on Mars
The seasonal frosting and defrosting of the southern ice cap
results in the formation of spider-like radial channels carved on 1
meter thick ice by sunlight. Then, sublimed CO
2 -and
probably water- increase pressure in their interior producing
geyser-like eruptions of cold fluids often mixed with dark basaltic
sand or mud. This process is rapid, observed happening in the space
of a few days, weeks or months, a growth rate rather unusual in
geology — especially for Mars.
Dark Slope Streaks
The inset photo of
Tharsis Tholus
shows an example of a dark streak. Such streaks are common across
Mars and new ones appear frequently on steep slopes of craters,
troughs, and valleys. The streaks are dark at first and get lighter
with age. Sometimes the streaks start in a tiny area which then
spreads out for hundreds of metres. They have also been seen to
travel around boulders and other obstacles in their path. The
mainstream theory is that they are dark underlying layers of soil
revealed after avalanches of bright dust, however several ideas
have been put forward to explain them, some of which involve
water or even the growth of organisms.
Geography
Although better remembered for mapping the Moon,
Johann Heinrich Mädler and
Wilhelm Beer were the first
"areographers". They began by establishing once and for all that
most of Mars’ surface features were permanent, and determining the
planet's rotation period. In 1840, Mädler combined ten years of
observations and drew the first map of Mars. Rather than giving
names to the various markings, Beer and Mädler simply designated
them with letters; Meridian Bay (Sinus Meridiani) was thus feature
"
a."
Today, features on Mars are named from a number of sources. Large
albedo features retain many of the older
names, but are often updated to reflect new knowledge of the nature
of the features. For example,
Nix Olympica (the snows of
Olympus) has become
Olympus Mons (Mount Olympus).
Mars’
equator is defined by its rotation, but the location of its
Prime Meridian was specified, as was
Earth's (at Greenwich
), by choice of an arbitrary point; Mädler and Beer
selected a line in 1830 for their first maps of Mars.
After the
spacecraft Mariner 9 provided extensive
imagery of Mars in 1972, a small crater (later called Airy-0
), located in
the Sinus Meridiani ("Middle Bay" or
"Meridian Bay"), was chosen for the definition of 0.0° longitude to
coincide with the original selection.
Since Mars has no oceans and hence no 'sea level', a zero-elevation
surface or
mean gravity surface
also had to be selected. Zero altitude is defined by the height at
which there is 610.5
Pa (6.105
mbar) of atmospheric pressure. This pressure corresponds to the
triple point of water, and is about
0.6% of the sea level surface pressure on Earth (.006 atm).
The dichotomy of Martian topography is striking: northern plains
flattened by lava flows contrast with the southern highlands,
pitted and cratered by ancient impacts. Research in 2008 has
presented evidence regarding a theory proposed in 1980 postulating
that, four billion years ago, the northern hemisphere of Mars was
struck by an object one-tenth to two-thirds the size of
the Moon. If validated, this would make Mars's
northern hemisphere the site of an impact crater 10,600 km
long by 8,500 km wide, or roughly the area of Europe, Asia,
and Australia combined, surpassing the
South Pole-Aitken basin as the
largest impact crater in the Solar System. The surface of Mars as
seen from Earth is divided into two kinds of areas, with differing
albedo.
The paler plains covered with dust and sand
rich in reddish iron oxides were once thought of as Martian
'continents' and given names like Arabia Terra
(land of Arabia) or Amazonis Planitia (Amazonian
plain). The dark features were thought to be seas,
hence their names
Mare Erythraeum,
Mare Sirenum and
Aurorae Sinus.
The
largest dark feature seen from Earth is Syrtis Major
.
The
shield volcano, Olympus Mons
(Mount Olympus), at 26 km is the
highest known mountain in the Solar System. It is an extinct
volcano in the vast upland region Tharsis
, which
contains several other large volcanoes. Olympus Mons is over
three times the height of Mount Everest
, which in comparison stands at just over
8.8 km.
Mars is also scarred by a number of
impact
craters: a total of 43,000 craters with a diameter of 5 km
or greater have been found.
The largest confirmed of these is the
Hellas
impact basin
, a light
albedo feature clearly visible from
Earth. Due to the smaller mass of Mars, the probability of
an object colliding with the planet is about half that of the
Earth. However, Mars is located closer to the asteroid belt, so it
has an increased chance of being struck by materials from that
source. Mars is also more likely to be struck by short-period
comets,
i.e., those that lie within
the orbit of Jupiter. In spite of this, there are far fewer craters
on Mars compared with the Moon because Mars's atmosphere provides
protection against small meteors. Some craters have a morphology
that suggests the ground was wet when the meteor impacted.
The large
canyon, Valles
Marineris
(Latin for
Mariner Valleys, also known
as Agathadaemon in the old canal maps), has a length of
4,000 km and a depth of up to 7 km. The length of
Valles Marineris is equivalent to the length of Europe and extends
across one-fifth the circumference of Mars.
By comparison, the
Grand
Canyon
on Earth is only 446 km long and nearly
2 km deep. Valles Marineris was formed due to the
swelling of the Tharsis area which caused the crust in the area of
Valles Marineris to collapse. Another large canyon is
Ma'adim Vallis (
Ma'adim is
Hebrew for Mars). It is 700 km long and again
much bigger than the Grand Canyon with a width of 20 km and a
depth of 2 km in some places. It is possible that Ma'adim
Vallis was flooded with liquid water in the past.
Images
from the Thermal
Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven
possible cave entrances on the flanks of the
Arsia
Mons
volcano. The caves, named after loved ones
of their discoverers, are collectively known as the "seven
sisters." Cave entrances measure from 100 m to 252 m wide
and they are believed to be at least 73 m to 96 m deep.
Because light does not reach the floor of most of the caves, it is
likely that they extend much deeper than these lower estimates and
widen below the surface. "Dena" is the only exception; its floor is
visible and was measured to be 130 m deep. The interiors of
these caverns may be protected from micrometeoroids, UV radiation,
solar flares and high energy particles
that bombard the planet's surface.
Mars has
two permanent polar ice caps: the northern one at Planum Boreum
and the southern one at Planum Australe
.
During a pole's winter, it lies in continuous darkness, chilling
the surface and causing 25–30% of the atmosphere to condense out
into thick slabs of
CO2
ice (
dry ice). When the poles are again
exposed to sunlight, the frozen CO
2 sublimes, creating enormous winds that
sweep off the poles as fast as 400 km/h. These seasonal
actions transport large amounts of dust and water vapor, giving
rise to Earth-like
frost and large
cirrus clouds. Clouds of water-ice were
photographed by the
Opportunity rover in 2004.
Atmosphere

Mars's thin atmosphere, visible on the
horizon in this low-orbit photo.
Mars lost its
magnetosphere 4 billion
years ago, so the
solar wind interacts
directly with the Martian
ionosphere,
keeping the atmosphere thinner than it would otherwise be by
stripping away atoms from the outer layer. Both
Mars Global Surveyor and Mars Express
have detected these ionised atmospheric particles trailing off into
space behind Mars.
The
atmosphere of Mars is
now relatively thin.
Atmospheric
pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Mons
to over 1,155 Pa (1.155 kPa) in the
depths of Hellas
Planitia
, with a mean
surface level pressure of 600 Pa (0.6 kPa). Mars's
mean surface pressure equals the pressure found 35 km above
the Earth's surface. This is less than 1% of the surface pressure
on Earth (101.3 kPa). The
scale
height of the atmosphere, about 11 km, is higher than
Earth's (6 km) due to the lower gravity. Mars' gravity is only
about 38% of the surface gravity on Earth.
The atmosphere on Mars consists of 95%
carbon dioxide, 3%
nitrogen, 1.6%
argon, and
contains traces of
oxygen and water. The
atmosphere is quite dusty, containing particulates about
1.5
µm in diameter which give the
Martian sky a
tawny color when seen from the
surface.
Methane
- See also: Atmosphere of Mars - Methane
Methane has been detected in the Martian
atmosphere with a concentration of about 30
ppb by volume; it occurs in extended
plumes, and the profiles imply that the methane was released from
discrete regions. In northern midsummer, the principal plume
contained 19,000 metric tons of methane, with an estimated source
strength of 0.6 kilogram per second. The profiles suggest that
there may be two local source regions, the first centered near 30°
N, 260° W and the second near 0°, 310° W. It is estimated that Mars
must produce 270 ton/year of methane.
The latest research suggests that the implied methane destruction
lifetime is as long as about 4 Earth years and as short as about
0.6 Earth years. This apparently rapid turnover would indicates a
current active source of the gas on the planet.
Volcanic activity,
cometary
impacts, and the presence of
methanogenic
microbial life forms are among
possible sources. It was recently pointed out that methane could
also be produced by a non-biological process called
serpentinization involving water,
carbon dioxide, and the
mineral olivine, which is known to be common on Mars.
Climate
Of all the planets, Mars's seasons are the most Earth-like, due to
the similar tilts of the two planets' rotational axes. However, the
lengths of the Martian seasons are about twice those of Earth's, as
Mars’ greater distance from the Sun leads to the Martian year being
about two Earth years in length. Martian surface temperatures vary
from lows of about −140
°C
(−220 °F) during the polar winters to highs of up to
20 °C (68 °F) in summers. The wide range in temperatures
is due to the thin atmosphere which cannot store much solar heat,
the low atmospheric pressure, and the low
thermal inertia of Martian soil.
The planet is also 1.52 times as far from the sun as Earth,
resulting in just 43 percent of the amount of sunlight.
If Mars had an Earth-like orbit, its seasons would be similar to
Earth's because its
axial tilt is similar
to Earth's. However, the comparatively large eccentricity of the
Martian orbit has a significant effect. Mars is near
perihelion when it is summer in the southern
hemisphere and winter in the north, and near
aphelion when it is winter in the southern hemisphere
and summer in the north. As a result, the seasons in the southern
hemisphere are more extreme and the seasons in the northern are
milder than would otherwise be the case. The summer temperatures in
the south can reach up to 30 °C (54 °F)warmer than the
equivalent summer temperatures in the north.

Mars's northern ice cap
Mars also has the largest
dust storms in
our Solar System. These can vary from a storm over a small area, to
gigantic storms that cover the entire planet. They tend to occur
when Mars is closest to the Sun, and have been shown to increase
the global temperature.
The polar caps at both poles consist primarily of water ice.
However, there is dry ice present on their surfaces. Frozen carbon
dioxide (dry ice) accumulates as a thin layer about one metre thick
on the north cap in the northern winter only, while the south cap
has a permanent dry ice cover about eight metres thick.The northern
polar cap has a diameter of about 1,000 kilometres during the
northern Mars summer,and contains about 1.6 million cubic
kilometres of ice, which if spread evenly on the cap would be 2
kilometres thick. (This compares to a volume of 2.85 million
cubic kilometres for the
Greenland
ice sheet.) The southern polar cap has a diameter of
350 km and a thickness of 3 km. The total volume of ice
in the south polar cap plus the adjacent layered deposits has also
been estimated at 1.6 million cubic kilometres. Both polar
caps show spiral troughs, which are believed to form as a result of
differential solar heating, coupled with the sublimation of ice and
condensation of water vapor. Both polar caps shrink and regrow
following the temperature fluctuation of the Martian seasons.
Evolution
Recent observational data and modeling techniques are providing
further insight into the history and evolution of Mars. For
example, the remnants of a magnetic field suggest that something
with a mass greater than that of Mars once kept the planet's
interior molten. The presence of bodies of water on Mars would have
required an atmosphere thicker than that of today. The Northern
Basin records a massive and disruptive impact. Possible
explanations include:
- A satellite could have caused enough tidal heating to melt the
interior enough to generate a substantial magnetic field. The field
would have protected the Martian atmosphere from Solar winds,
allowing liquid water to remain on the surface.
- An impact by large asteroid or comet could have removed the
crust of one hemisphere and striped Mars of its atmosphere. The
entire crust could have shifted to a more stable configuration with
the impact basin centered at the north pole and Mars' massive
volcanoes near the equator. Without tidal heating from the
satellite, the magnetic field could have faded, and Solar wind
striking the surface might have prevented the atmosphere from
reforming.
- The lack of a stabilizing satellite would have allowed
significant wobble on the order of five million years. These
irregularities in the motion of Mars would have periodically warmed
the polar regions enough for at least some liquid water to form,
leaving striations in the polar ice cap.
Orbit and rotation
Mars’ average distance from the Sun is roughly 230 million km
(1.5 AU) and its orbital period is 687 (Earth) days. The solar day
(or
sol) on Mars is only
slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244
seconds. A Martian year is equal to 1.8809 Earth years, or 1 year,
320 days, and 18.2 hours.
Mars's axial tilt is 25.19 degrees, which is similar to the axial
tilt of the Earth. As a result, Mars has seasons like the Earth,
though on Mars they are nearly twice as long given its longer year.
Mars passed its
perihelion in April 2009
and its aphelion in May 2008. It next reaches perihelion in May
2011 and aphelion in March 2010.
Mars has a relatively pronounced
orbital eccentricity of about 0.09; of
the seven other planets in the Solar System, only
Mercury shows greater eccentricity.
However, it is known that in the past Mars has had a much more
circular orbit than it does currently. At one point 1.35 million
Earth years ago, Mars had an eccentricity of roughly 0.002, much
less than that of Earth today. The Mars cycle of eccentricity is
96,000 Earth years compared to the Earth's cycle of 100,000 years.
However, Mars also has a much longer cycle of eccentricity with a
period of 2.2 million Earth years, and this overshadows the
96,000-year cycle in the eccentricity graphs. For the last 35,000
years Mars' orbit has been getting slightly more eccentric because
of the gravitational effects of the other planets. The closest
distance between the Earth and Mars will continue to mildly
decrease for the next 25,000 years.
Orbit of Mars (red) and Ceres (yellow)
 Orbit of Mars (red) and Ceres
(yellow)
The image to the left shows a
comparison between Mars and Ceres, a
dwarf planet in the Asteroid Belt, as seen from the north ecliptic pole, while the image to the right is as
seen from the ascending node. The segments of orbits south of the ecliptic
are plotted in darker colors. The perihelia (q)
and aphelia (Q) are labelled with the date
of the nearest passage. |
Moons
Mars has two tiny natural moons,
Phobos and
Deimos, which orbit very close to the planet.
Their known composition suggests the moons are captured asteroids
but their origin remains uncertain.
Both satellites were discovered in 1877 by
Asaph Hall, and are named after the characters
Phobos (panic/fear) and
Deimos (terror/dread) who, in
Greek mythology, accompanied their father
Ares, god of war, into battle. Ares was known
as Mars to the Romans.
From the surface of Mars, the motions of Phobos and Deimos appear
very different from that of our own moon. Phobos rises in the west,
sets in the east, and rises again in just 11 hours. Deimos, being
only just outside
synchronous
orbit—where the orbital period would match the planet's period
of rotation—rises as expected in the east but very slowly. Despite
the 30 hour orbit of Deimos, it takes 2.7 days to set in the west
as it slowly falls behind the rotation of Mars, then just as long
again to rise.
Because Phobos' orbit is below synchronous altitude, the
tidal forces from the planet Mars are gradually
lowering its orbit. In about 50 million years it will either crash
into Mars’ surface or break up into a ring structure around the
planet.
The origin of the two moons is not well understood. Their low
albedo and
carbonaceous
chondrite composition are similar to asteroids and capture
remains the favored theory. Phobos' unstable orbit would seem to
point towards a relatively recent capture. But both have
circular orbits, very near the equator, which
is very unusual for captured objects and the required capture
dynamics are complex. Accretion early in Mars' history is also
plausible but does not account for the moons' composition
resembling asteroids rather than Mars itself. A third possibility
is the involvement of a third body or some kind of impact
disruption.
Life
The current understanding of
planetary habitability—the ability of
a world to develop and sustain life—favors planets that have liquid
water on their surface. This most often requires that the orbit of
a planet lie within the
habitable
zone, which for the Sun currently extends from just beyond
Venus to about the
semi-major axis
of Mars. During perihelion Mars dips inside this region, but the
planet's thin (low-pressure) atmosphere prevents liquid water from
existing over large regions for extended periods. The past flow of
liquid water, however, demonstrates the planet's potential for
habitability. Recent evidence has suggested that any water on the
Martian surface would have been too salty and acidic to support
terran life.
The lack of a magnetosphere and extremely thin atmosphere of Mars
are a greater challenge: the planet has little
heat transfer across its surface, poor
insulation against bombardment and the
solar
wind, and insufficient atmospheric pressure to retain water in
a liquid form (water instead sublimates to a gaseous state). Mars
is also nearly, or perhaps totally, geologically dead; the end of
volcanic activity has stopped the recycling of chemicals and
minerals between the surface and interior of the planet.
Evidence suggests that the planet was once significantly more
habitable than it is today, but whether living
organisms ever existed there is still unclear.The
Viking probes of the mid-1970s
carried experiments designed to detect microorganisms in Martian
soil at their respective landing sites, and had some apparently
positive results, including a temporary increase of CO
2
production on exposure to water and nutrients. However this sign of
life was later disputed by many scientists, resulting in a
continuing debate, with NASA scientist
Gilbert Levin asserting that Viking may have
found life. A re-analysis of the now 30-year-old Viking data, in
light of modern knowledge of
extremophile forms of life, has suggested that
the Viking tests were also not sophisticated enough to detect these
forms of life. The tests may even have killed a (hypothetical) life
form. Tests conducted by the Phoenix Mars Lander have shown that
the soil has a very
alkaline pH and it contains magnesium, sodium, potassium and
chloride. The soil nutrients may be able to support life, but life
would still have to be shielded from the intense ultraviolet
light.
At the
Johnson
space center lab
, some curious shapes have been found in the Martian
meteorite ALH84001
. Some scientists propose that these
geometric shapes could be fossilized microbes extant on Mars before
the meteorite was blasted into space by a meteor strike and sent on
a 15 million-year voyage to Earth. Also, small quantities of
methane and
formaldehyde recently detected by Mars orbiters
are both claimed to be hints for life, as these
chemical compounds would quickly break
down in the Martian atmosphere. It is possible that these compounds
may be replenished by volcanic or geological means such as
serpentinization.
Exploration
Dozens of
spacecraft, including orbiters, landers, and rovers, have been sent to Mars by
the Soviet Union, the United States
, Europe
, and
Japan to study the planet's surface, climate,
and geology. The current price of transporting material from
the surface of
Earth to the surface of Mars is
approximately 309000
USD/
kg.
Roughly two-thirds of all spacecraft destined for Mars have failed
in one manner or another before completing or even beginning their
missions. While this high failure rate can be ascribed to technical
problems, enough have either failed or lost communications for
causes unknown for some to search for other explanations.
Examples
include an Earth-Mars "Bermuda Triangle
", a Mars
Curse, or even the long-standing NASA in-joke, the "Great Galactic Ghoul" that feeds on
Martian spacecraft.
Past missions
The first successful fly-by mission to Mars was NASA's
Mariner 4, launched in 1964. On November 14, 1971
Mariner 9 became the first space probe to
orbit another planet when it entered into orbit around Mars.
The first
successful objects to land on the surface were two Soviet
probes, Mars 2 and Mars 3 from the Mars
probe program, launched in 1971, but both lost contact within
seconds of landing. Then came the 1975 NASA launches of the
Viking program, which consisted of
two orbiters, each having a lander; both landers successfully
touched down in 1976.
Viking 1 remained
operational for six years,
Viking 2 for
three. The Viking landers relayed color panoramas of Mars and the
orbiters mapped the surface so well that the images remain in
use.
The Soviet probes
Phobos 1 and 2 were
sent to Mars in 1988 to study Mars and its two moons. Phobos 1 lost
contact on the way to Mars. Phobos 2, while successfully
photographing Mars and Phobos, failed just before it was set to
release two landers on Phobos's surface.
Following
the 1992 failure of the Mars Observer
orbiter, NASA launched the Mars Global Surveyor in 1996.
This mission was a complete success, having finished its primary
mapping mission in early 2001. Contact was lost with the probe in
November 2006 during its third extended program, spending exactly
10 operational years in space.
Only a month after the launch of the
Surveyor, NASA launched the Mars
Pathfinder, carrying a robotic exploration vehicle Sojourner, which landed in the Ares Vallis
on Mars in the summer of 1997. This mission
was also successful, and received much publicity, partially due to
the many images that were sent back to Earth.
The most recent mission to Mars was the NASA
Phoenix Mars lander, which launched
August 4, 2007 and arrived on the north polar region of Mars on May
25, 2008. The lander has a robotic arm with a 2.5 m reach and
capable of digging a metre into the Martian soil. The lander has a
microscopic camera capable of resolving to one-thousandth the width
of a human hair, and discovered a substance at its landing site on
June 15, 2008, which was confirmed to be water ice on June 20. The
mission was declared concluded on November 10, 2008, after
engineers were unable to contact the craft.
Current missions
2001 NASA launched the successful
Mars
Odyssey orbiter, which is still in orbit as of March 2009, and
the ending date has been extended to September 2010. Odyssey's
Gamma Ray Spectrometer
detected significant amounts of hydrogen in the upper metre or so
of Mars's
regolith. This hydrogen is
thought to be contained in large deposits of water ice.
In 2003,
the European
Space Agency
(ESA) launched the Mars Express craft, consisting
of the Mars Express Orbiter and
the lander Beagle 2. Beagle 2 failed
during descent and was declared lost in early February 2004. In
early 2004 the
Planetary
Fourier Spectrometer team announced it had detected methane in
the Martian atmosphere. ESA announced in June 2006 the discovery of
aurorae on Mars.
Also in 2003, NASA launched the twin
Mars Exploration Rovers named
Spirit (MER-A) and
Opportunity (MER-B). Both
missions landed successfully in January 2004 and have met or
exceeded all their targets. Among the most significant scientific
returns has been conclusive evidence that liquid water existed at
some time in the past at both landing sites.
Martian dust devils and
windstorms have occasionally cleaned both rovers' solar panels, and
thus increased their lifespan.
On August 12, 2005 the NASA
Mars Reconnaissance Orbiter
probe was launched toward the planet, arriving in orbit on March
10, 2006 to conduct a two-year science survey. The orbiter will map
the Martian terrain and weather to find suitable landing sites for
upcoming lander missions. It also contains an improved
telecommunications link to Earth, with more bandwidth than all
previous missions combined.
The Mars Reconnaissance Orbiter snapped the
first image of a series of active avalanches near the planet's north pole
, scientists said March 3, 2008.
The
Dawn spacecraft flew by Mars in
February 2009 for a gravity assist on its way to investigate
Vesta and then
Ceres.
Future missions
Phoenix will be followed by the
Mars Science Laboratory in 2011, a
bigger, faster (90
m/h), and
smarter version of the Mars Exploration Rovers. Experiments include
a laser chemical sampler that can deduce the make-up of rocks at a
distance of 13 m.
The joint Russian and Chinese
Phobos-Grunt mission to return samples of
Mars's moon Phobos (
grunt is the Russian word for soil)
was originally scheduled for October 2009, but the mission was
postponed till the next launch window in 2011. On September 15,
2008, NASA announced
MAVEN, a
robotic mission in 2013 to provide information about Mars'
atmosphere. In 2018 the ESA plans to launch its first Rover to
Mars; the
ExoMars rover will be capable of
drilling 2 m into the soil in search of organic
molecules.
The Finnish-Russian
MetNet mission will land
tens of small vehicles on the Martian surface to establish a
widespread surface observation network to investigate the planet's
atmospheric structure, physics and meteorology. A precursor mission
using one or a few landers is scheduled for launch in 2009 or 2011.
One possibility is a piggyback launch on the Russian Phobos-Grunt
mission. Other launches will take place in the launch windows
extending to 2019.
Manned Mars exploration by
the United States has been explicitly identified as a long-term
goal in the
Vision for
Space Exploration announced in 2004 by the then US President
George W. Bush. NASA and
Lockheed Martin have begun work on the
Orion spacecraft,
formerly the Crew Exploration Vehicle, which is currently scheduled
to send a human expedition to Earth's moon by 2020 as a stepping
stone to an expedition to Mars thereafter. On September 28, 2007,
NASA administrator
Michael D.
Griffin stated that NASA aims to
put a man on Mars by 2037.
ESA hopes to land humans on Mars between 2030 and 2035. This will
be preceded by successively larger probes, starting with the launch
of the ExoMars probe and a Mars Sample Return Mission.
Mars Direct, an extremely low-cost human
mission proposed by
Bob Zubrin, a founder
of the
Mars Society, uses heavy-lift
Saturn V class rockets, such as the
Space X Falcon 9, or, the
Ares V, to skip orbital construction, LEO rendezvous,
and lunar fuel depots. A modified proposal, called "
Mars to Stay", involves not returning the first
immigrant/explorers immediately, if ever.
Dean Unick has suggested the cost of sending a
four to six person team is one fifth to one tenth the cost of
returning that same four to six person team; twenty settlers could
be sent for the cost of returning four.
Astronomy on Mars
With the existence of various orbiters, landers, and rovers, it is
now possible to study
astronomy from the
Martian skies. While Mars’ moon Phobos appears about one third the
angular diameter of the full Moon
as it appears from Earth, Deimos appears more or less star-like,
and appears only slightly brighter than Venus does from
Earth.
There are also various phenomena well-known on Earth that have now
been observed on Mars, such as
meteors and
auroras. A
transit of the Earth as seen from
Mars will occur on November 10, 2084. There are also
transits of Mercury and
transits of Venus, and
the moon Deimos is of sufficiently small angular diameter that its
partial "eclipses" of the Sun are best considered transits (see
Transit of Deimos from
Mars).
Viewing

Apparent retrograde motion of Mars in
2003 as seen from Earth
To the naked eye, Mars usually appears a distinct yellow, orange,
or reddish color, and varies in brightness more than any other
planet as seen from Earth over the course of its orbit.
However
the actual color of Mars is closer to butterscotch, and the redness seen is actually
just dust in the planet's atmosphere; considering this NASA
's Spirit
rover has taken pictures of a greenish-brown, mud-colored landscape
with blue-grey rocks and patches of light red colored sand.
The
apparent magnitude of Mars
varies from +1.8 at conjunction to as high as −2.9 at
perihelic opposition. When farthest away from
the Earth, it is more than seven times as far from the latter as
when it is closest. When least favorably positioned, it can be lost
in the Sun's glare for months at a time. At its most favorable
times—at 15- or 17-year intervals, and always between late July and
late September—Mars shows a wealth of surface detail to a
telescope. Especially noticeable, even at low
magnification, are the
polar ice
caps.
The point of Mars’ closest approach to the Earth is known as
opposition. The length of time
between successive oppositions, or the
synodic period, is 780 days. Because of the
eccentricities of the orbits, the times of opposition and minimum
distance can differ by up to 8.5 days. The minimum distance varies
between about 55 and 100 million km due to the planets'
elliptical orbits. The next Mars opposition will
occur on January 29, 2010.
As Mars approaches opposition it begins a period of
retrograde
motion, which means it will appear to move backwards in a
looping motion with respect to the background stars.
2003 closest approach
On August 27, 2003, at 9:51:13 UT, Mars made its closest approach
to Earth in nearly 60,000 years: 55,758,006 km ( ). This
occurred when Mars was one day from opposition and about three days
from its
perihelion, making Mars
particularly easy to see from Earth. The last time it came so close
is estimated to have been on September 12,
57 617 BC, the next time being in
2287.However, this record approach was only very slightly closer
than other recent close approaches. For instance, the minimum
distance on August 22, 1924 was , and the minimum distance on
August 24, 2208 will be .
Historical observations

The rotation of Mars as seen in a
small telescope in 2003.
The history of observations of Mars is marked by the oppositions of
Mars, when the planet is closest to Earth and hence is most easily
visible, which occur every couple of years. Even more notable are
the perihelic oppositions of Mars which occur every 15 or 17 years,
and are distinguished because Mars is close to perihelion, making
it even closer to Earth.
Aristotle was
among the first known writers to describe observations of Mars,
noting that, as it passed behind the Moon, it was farther away than
was originally believed.
The only
occultation of Mars by Venus observed
was that of October 13, 1590, seen by Michael Maestlin at Heidelberg
. In 1609, Mars was viewed by Galileo, who
was first to see it via telescope.
Martian 'canals'

Map of Mars by Giovanni
Schiaparelli
By the 19th century, the resolution of telescopes reached a level
sufficient for surface features to be identified. In September
1877, a perihelic opposition of Mars occurred on September 5.
In that
year, Italian astronomer Giovanni
Schiaparelli, then in Milan
, used a
22 cm telescope to help produce the first detailed map of
Mars. These maps notably contained features he called
canali, which were later shown to be an
optical illusion. These
canali
were supposedly long straight lines on the surface of Mars to which
he gave names of famous rivers on Earth. His term, which means
'channels' or 'grooves', was popularly mistranslated in English as
canals.
Influenced by the observations, the
orientalist Percival Lowell founded
an observatory
which had a 300 and 450 mm telescope.
The observatory was used for the exploration of Mars during the
last good opportunity in 1894 and the following less favorable
oppositions. He published several books on Mars and life on the
planet, which had a great influence on the public.
The canali
were also found by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice
, using one
of the largest telescopes of that time.
The seasonal changes (consisting of the diminishing of the polar
caps and the dark areas formed during Martian summer) in
combination with the canals lead to speculation about life on Mars,
and it was a long held belief that Mars contained vast seas and
vegetation. The telescope never reached the resolution required to
give proof to any speculations. However, as bigger telescopes were
used, fewer long, straight
canali were observed. During an
observation in 1909 by
Flammarion
with a 840 mm telescope, irregular patterns were observed, but
no
canali were seen.
Even in the 1960s articles were published on Martian biology,
putting aside explanations other than life for the seasonal changes
on Mars. Detailed scenarios for the metabolism and chemical cycles
for a functional ecosystem have been published.
It was not until
spacecraft visited the
planet during NASA's
Mariner
missions in the 1960s that these myths were dispelled. The
results of the Viking life-detection experiments started an
intermission in which the hypothesis of a hostile, dead planet was
generally accepted.
Some maps of Mars were made using the data from these missions, but
it was not until the
Mars Global
Surveyor mission, launched in 1996 and operated until late
2006, that complete, extremely detailed maps were obtained. These
maps are now available online, for example, at
Google Mars.
In culture
Historical connections
Mars is named after the
Roman god of war. In
Babylonian astronomy, the planet was named after
Nergal, their
deity of fire, war, and destruction, most likely due
to the planet's reddish appearance. When the
Greeks equated Nergal with their god of war,
Ares, they named the planet Ἄρεως ἀστἡρ (
Areos aster), or
"star of Ares". Then, following the
identification of Ares and Mars, it was
translated into Latin as
stella Martis, or "star of Mars",
or simply
Mars. The Greeks also called the planet Πυρόεις
Pyroeis meaning "fiery". In
Hindu mythology, Mars is known as
Mangala (मंगल). The planet is also called
Angaraka in
Sanskrit, after the
celibate god of war, who possesses the
signs of
Aries and
Scorpio, and teaches the occult
sciences. The planet was known by the
Egyptians as "
Ḥr Dšr";;;; or
"
Horus the Red". The
Hebrews named it
Ma'adim (מאדים) —
"the one who blushes"; this is where one of the largest
canyons on Mars, the
Ma'adim Vallis, gets its name. It is known as
al-Mirrikh in Arabic, and
Merih in Turkish. In
Urdu and
Persian it is written as
مریخ and known as "Merikh". The etymology of
al-Mirrikh is unknown. Ancient Persians named it
Bahram, the Zoroastrian god of faith and it is written as
بهرام. Ancient Turks called it
Sakit.
The
Chinese, Japanese, Korean
and Vietnamese
cultures refer to the planet as 火星, or the fire
star, a name based on the ancient Chinese mythological cycle
of Five
elements.

symbol, derived from the
astrological symbol of Mars, is a circle
with a small arrow pointing out from behind. It is a stylized
representation of a shield and spear used by the Roman God Mars.
Mars in Roman mythology was the God of War and patron of warriors.
This symbol is also used in biology to describe the
male sex, and in
alchemy to
symbolise the element iron which
was considered to be dominated by Mars whose characteristic red
colour is coincidentally due to iron oxide. ♂ occupies
Unicode position U+2642.
Intelligent "Martians"

An 1893 soap ad playing on the popular
idea that Mars was populated.
The popular idea that Mars was populated by intelligent
Martians exploded in the late 19th century.
Schiaparelli's "canali" observations
combined with
Percival Lowell's
books on the subject put forward the standard notion of a planet
that was a drying, cooling, dying world with ancient civilizations
constructing irrigation works.
Many other observations and proclamations by notable personalities
added to what has been termed "Mars Fever". In 1899 while
investigating atmospheric radio noise using his receivers in his
Colorado Springs lab, inventor
Nikola
Tesla observed repetitive signals that he later surmised might
have been radio communications coming from another planet, possibly
Mars. In a 1901 interview Tesla said:
It was some time afterward when the thought flashed
upon my mind that the disturbances I had observed might be due to
an intelligent control.
Although I could not decipher their meaning, it was
impossible for me to think of them as having been entirely
accidental.
The feeling is constantly growing on me that I had been
the first to hear the greeting of one planet to
another.
Tesla's theories gained support from
Lord Kelvin who, while
visiting the United States in 1902, was reported to have said that
he thought Tesla had picked up Martian signals being sent to the
United States. However, Kelvin "emphatically" denied this report
shortly before departing America: "What I really said was that the
inhabitants of Mars, if there are any, were doubtless able to see
New York, particularly the glare of the electricity."
In a
New York Times article in
1901, Edward Charles
Pickering, director of the Harvard
College Observatory
, said that they had received a telegram from
Lowell
Observatory
in Arizona
that seemed to confirm that Mars was trying to
communicate with the Earth.
Early in December 1900, we received from Lowell
Observatory in Arizona a telegram that a shaft of light had been
seen to project from Mars (the Lowell observatory makes a specialty
of Mars) lasting seventy minutes.
I wired these facts to Europe and sent out neostyle
copies through this country.
The observer there is a careful, reliable man and there
is no reason to doubt that the light existed.
It was given as from a well-known geographical point on
Mars.
That was all.
Now the story has gone the world over.
In Europe it is stated that I have been in
communication with Mars, and all sorts of exaggerations have spring
up.
Whatever the light was, we have no means of
knowing.
Whether it had intelligence or not, no one can
say.
It is absolutely inexplicable.
Pickering
later proposed creating a set of mirrors in Texas
with the
intention of signaling Martians.
In recent decades, the high resolution mapping of the surface of
Mars, culminating in
Mars Global
Surveyor, revealed no artifacts of habitation by 'intelligent'
life, but pseudoscientific speculation about intelligent life on
Mars continues from commentators such as
Richard C. Hoagland. Reminiscent of the
canali controversy, some speculations are based on small
scale features perceived in the spacecraft images, such as
'pyramids' and the '
Face on
Mars'. Planetary astronomer
Carl
Sagan wrote:
Mars has become a kind of mythic arena onto which we
have projected our Earthly hopes and fears.
In fiction
The depiction of Mars in fiction has been stimulated by its
dramatic red color and by early scientific speculations that its
surface conditions not only might support life, but intelligent
life.

Alien tripod illustration from the
1906 French edition of H.G.
Wells' The War of the Worlds.
Thus originated a large number of
science fiction scenarios, the best known of
which is
H. G. Wells'
The War of the
Worlds, published in 1898, in which Martians seek to
escape their dying planet by invading Earth. A subsequent radio
version of
The War of
the Worlds on October 30, 1938 was presented as a live
news broadcast, and many listeners
mistook it for the
truth.
Also influential were Ray Bradbury's
The Martian Chronicles, in which
human explorers accidentally destroy a Martian civilization,
Edgar Rice Burroughs'
Barsoom series and a number of
Robert A. Heinlein stories before the
mid-sixties.
Author
Jonathan Swift made reference
to the moons of Mars, about 150 years before their actual discovery
by
Asaph Hall, detailing reasonably
accurate descriptions of their orbits, in the 19th chapter of his
novel
Gulliver's
Travels.
Another reference is found in
C.
S. Lewis'
Space Trilogy, and in particular
in the first book entitled
Out
of the Silent Planet (1938). Three men, Weston, Devine and
Ransom, set out for an interplanetary voyage from Earth to Mars
(called
Malacandra in the narrative). Ransom, who had been
brought along forcefully by Weston and Devine, in order to be
handed over to the Sorns, succeeds in escaping after they have
landed on the red planet, and thus comes to discover the geology,
flora, fauna, and cultures present on Malacandra. He also discovers
the relationship of planet Earth (called
Thulcandra, the
silent planet, in the narrative) with the other planets
and forms of life present in the Solar System.
A comic figure of an intelligent Martian,
Marvin the Martian, appeared on
television in 1948 as a character in the
Looney Tunes animated cartoons of
Warner Brothers, and has continued as part
of popular culture to the present.
After the
Mariner and
Viking spacecraft had returned pictures of
Mars as it really is, an apparently lifeless and canal-less world,
these ideas about Mars had to be abandoned and a vogue for
accurate, realist depictions of human colonies on Mars developed,
the best known of which may be
Kim
Stanley Robinson's
Mars
trilogy. However, pseudo-scientific speculations about the
Face on Mars and other enigmatic
landmarks spotted by
space probes have
meant that ancient civilizations continue to be a popular theme in
science fiction, especially in film.
Another popular theme, particularly among American writers, is the
Martian colony that fights for independence from Earth. This is a
major plot element in the novels of
Greg
Bear and
Kim Stanley
Robinson, as well as the movie
Total Recall (based on a short
story by
Philip K. Dick) and the television series
Babylon 5. Many video games also use this
element, including
Red Faction
and the
Zone of the
Enders series. Mars (and its moons) were also the setting
for the popular
Doom
video game franchise and the later
Martian Gothic.
In music
In
Gustav Holst's
The Planets, Mars is depicted as the
"Bringer of War".
The Flaming Lips's
Grammy-award-winning song "Approaching Pavonis Mons by Balloon"
from the album
Yoshimi Battles the Pink
Robots portrays travel across the Red Planet.
See also
Notes
- Best fit ellipsoid
- There are many serpentinization reactions.
- Olivine is a solid solution between forsterite and fayalite
whose general formula is (Fe,Mg)_2SiO_4.
- The reaction producing methane from olivine can be written as:
Forsterite + Fayalite + Water + Carbonic acid → Serpentine +
Magnetite + Methane , or (in balanced form): 18 Mg_2SiO_4 + 6
Fe_2SiO_4 + 26 H_2O + CO_2 → 12 Mg_3Si_2O_5(OH)_4 + 4 Fe_3O_4 +
CH_4
References
External links
Cartographic resources