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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 is currently host to three functional
orbiting spacecraft: Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. On the surface are the two Mars Exploration
Rovers (Spirit and Opportunity) and several inert landers and rovers, both successful and unsuccessful. The Phoenix lander
completed its mission on the surface in 2008. Observations by NASA's now-defunct Mars Global Surveyor show evidence that parts
of the southern polar ice cap have been receding. Mars can easily be seen from Earth with the naked eye. Its apparent magnitude
reaches -2.91, a brightness surpassed only by Jupiter, Venus, the Moon, and the Sun. Mars has an average opposition distance
of 78 million km but can come as close as 55.7 million km during a particularly close approach, such as the opposition of
2003. Rock strewn surface imaged by Mars PathfinderBased 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(III) oxide dust.
Although
Mars has no evidence of a current structured global magnetic field,[24] observations show that parts of the planet's crust
have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past. 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 four 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 1480 km
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 one third as
thick as Mars’ crust, relative to the sizes of the two planets. During the Solar system formation, Mars was created
out of the protoplanetary disk that orbited the Sun as the result of a stochastic process of run-away accretion. Mars has
many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points
such as chlorine, phosphorus and sulphur are much more common on Mars than Earth; these elements were probably removed from
areas closer to the Sun by the young Sun's powerful solar wind. This same effect is thought to have originally provided Mars
with more oxygen than the Earth had; reactions between iron and the excess oxygen may be the reason Mars has much more iron
in its crust and mantle than does the Earth.
After the formation of the planets, all were subjected to the "Late
Heavy Bombardment". About 60% of the surface of Mars shows an impact record from that era. Much of the rest of the surface
of Mars is probably underlain by immense impact basins that date from this time—there is evidence of an enormous impact
basin in the northern hemisphere of Mars, spanning 10600 km by 8500 km, or roughly four times larger than the Moon's South
Pole-Aitken basin, the largest impact basin yet discovered. This theory suggests 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.
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 CO2 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. However, an exclusively inorganic origin for the shapes has also been proposed.
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 instead
be replenished by volcanic or geological means such as serpentinization.
The geological history of Mars can be split into many
epochs, but the following are the three primary epochs:
Noachian epoch (named after Noachis Terra): Formation of the
oldest extant surfaces of Mars, 4.5 billion years ago to 3.5 billion years ago. Noachian age surfaces are scarred by many
large impact craters. The Tharsis bulge, a 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.
Some geological activity is
still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows up to about 200 Mya. Water flows in the
grabens called the Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions. On February 19,
2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700 m high cliff.
The
Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium,
potassium and chloride. These nutrients are found in gardens on Earth, and are necessary for growth of plants such as asparagus.
Experiments performed by the Lander showed that the Martian soil has a basic acidity of 8.3, and may contain traces of the
salt perchlorate.
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 terrestrial
life.
Size comparison of Earth and Mars.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.[5] While Mars is larger and more massive
than Mercury, Mercury has a higher density. This results in the two planets having a nearly identical gravitational pull at
the surface—that of Mars is stronger by less than 1%. 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 nine
times more massive than Mars, and the Moon one-ninth as massive). The red-orange appearance of the Martian surface is caused
by iron(III) oxide, more commonly known as hematite, or rust. Mars:
Size: 7th largest planet - 6,794 kilometers - 0.533 Earths
Orbit: 227,920,000
kilometers - 1.524 Earth orbits
Axial tilt: 25.19 degrees (only 1.74 degrees more than Earth's)
Number of moons:
2
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