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Recent exploration of Mars has revealed evidence of several defining moments in its history. For example, the remnants of a magnetic field suggest that something more than the mass of Mars once kept its interior molten; the ancient presence of water would have required an atmosphere thicker than that of today; and the Northern Basin records a massive and disruptive impact. Possible explanations include:
A massive satellite, perhaps a captured asteroid, caused enough tidal heating to melt the interior enough to generate a substantial magnetic field. The field protected the Martian atmosphere from the Solar wind, allowing liquid water to remain on the surface.
A massive impact removes the crust of one hemisphere and strips Mars of its atmosphere. This may have been the satellite, whose orbit could have decayed from tidal forces. The entire crust shifts 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 fades, and Solar wind striking the surface prevents the atmosphere from reforming.
The lack of stabilizing satellite allows significant wobble on the order of five million years. This periodically warms the polar regions enough for at least some liquid water to form, and leaves striations in the polar ice cap.
Orbit and rotation
Recent exploration of Mars has revealed evidence of several defining moments in its history. For example, the remnants of a magnetic field suggest that something more than the mass of Mars once kept its interior molten; the ancient presence of water would have required an atmosphere thicker than that of today; and the Northern Basin records a massive and disruptive impact. Possible explanations include:
A massive satellite, perhaps a captured asteroid, caused enough tidal heating to melt the interior enough to generate a substantial magnetic field. The field protected the Martian atmosphere from the Solar wind, allowing liquid water to remain on the surface.
A massive impact removes the crust of one hemisphere and strips Mars of its atmosphere. This may have been the satellite, whose orbit could have decayed from tidal forces. The entire crust shifts 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 fades, and Solar wind striking the surface prevents the atmosphere from reforming.
The lack of stabilizing satellite allows significant wobble on the order of five million years. This periodically warms the polar regions enough for at least some liquid water to form, and leaves 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.[71] The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth's cycle of 100,000 years.[72] 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.[73]
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.[71] The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth's cycle of 100,000 years.[72] 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.[73]
MOONS
Main article: Moons of Mars
Mars has two tiny natural moons, Phobos and Deimos, which orbit very close to the planet and are thought to be captured asteroids.[74] 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.[75]
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.[76]
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.[76]
It is not well understood how or when Mars came to capture its two moons. Both have circular orbits, very near the equator, which is very unusual in itself for captured objects. Phobos's unstable orbit would seem to point towards a relatively recent capture. There is no known mechanism for an airless Mars to capture a lone asteroid, so it is likely that a third body was involved — however, asteroids as large as Phobos and Deimos are rare, and binaries rarer still, outside the asteroid belt.[77]
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.[76]
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.[76]
It is not well understood how or when Mars came to capture its two moons. Both have circular orbits, very near the equator, which is very unusual in itself for captured objects. Phobos's unstable orbit would seem to point towards a relatively recent capture. There is no known mechanism for an airless Mars to capture a lone asteroid, so it is likely that a third body was involved — however, asteroids as large as Phobos and Deimos are rare, and binaries rarer still, outside the asteroid belt.[77]
DARK SLOP STREAKSThe photo below shows a dark streak. Such streaks are very common on Mars. They occur on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. Sometimes they start in a small area, then spread out and go for hundreds of metres. They have been seen to travel around obstacles, such as boulders. It is believed that they are dark underlying layers of soil revealed after avalanches of bright dust. However, several ideas have been advanced to explain them, some of which involve water or even the growth of organisms[78][79]. A notable aspect of these dark streaks is that new ones occur frequently.
LIFE
Main article: Life on Mars
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.[80] 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.[81]
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.[82]
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.[83] 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.[84] 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 organic compounds have been found in the meteorite ALH84001, which is supposed to have come from Mars. They concluded that these were deposited by primitive life forms 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.[85][86] It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.[58]
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.[82]
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.[83] 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.[84] 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 organic compounds have been found in the meteorite ALH84001, which is supposed to have come from Mars. They concluded that these were deposited by primitive life forms 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.[85][86] It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.[58]
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