Geography edit

Animation of Mars' rotation from the vantage of an observer who moves south, then north, to hover over both poles, showing the planet's major topographic features.
Map of Mars made by Giovanni Schiaparelli in 1877 showed canals
Mars sketched as observed by Lowell sometime before 1914. All these canali were subsequently shown to be optical illusions.)

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars has with a thin atmosphere with surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two known moons, which are small and irregularly shaped. These may be captured asteroids.

Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface.In 2005, radar data revealed the presence of large quantities of water ice at the poles and mid-latitudes.

MOLA colorized shaded-relief maps showing the western (left)and eastern (right) hemispheres of Mars.
  • Western hemisphere (to the left)
  1. The Tharsis region in the large highlands (red and brown) that dominates the hemishpere.
  1. Tall volcanoes populate the highest elevations of this region.
  2. Valles Marineris (blue) is the long gash-like feature on the right side of the Tharsis region.
  • Eastern hemisphere (to the right)
  1. cratered highlands (yellow to red)
  2. Hellas basin is (deep blue/purple) at lower left is the largest confirmed impact crater on the planet.
  3. The Elysium is the smaller volcanic provence in the upper (northern) of the eastern hemisphere.
Volcanic plateaus (red) and impact basins (blue) dominate this topographic map of Mars
Top down view of Olympus Mons

The most conspicuous feature of Martian surface geology is a sharp contrast, known as the Martian dichotomy, between the rugged southern highlands and the relatively smooth northern basins. It has been speculated that, four billion years ago, the northern hemisphere of Mars might have been struck by an object one-tenth to two-thirds the size of Earth's Moon.

The shield volcano Olympus Mons (Mount Olympus) is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. Olympus Mons is roughly three times the height of Mount Everest, which in comparison stands at just over 8.8 km. (See also Volcanism on Mars).

The large canyon, Valles Marineris has a length equivalent to the length of Europe and extends across one-fifth the circumference of Mars. Valles Marineris was formed due to the swelling of the Tharsis area which caused the crust in the area of Valles Marineris to collapse.

Geology edit

Lava flow in Mare Tyrrhenum
Microscopic photo taken by Opportunity showing a gray hematite concretion, indicative of the past presence of liquid water
An electron microscope reveals bacteria-like structures in meteorite fragment ALH84001. To date, none of the lines of scientific evidence (that this represents an actual fossil of life) have been either discredited or positively ascribed to non-biological explanations.[1]

Like Earth, mars has undergone differentiation, resulting in a dense, metallic core region overlaid by less dense materials. Current models of the planet's interior imply a core region about 1794 km ± 65 km in radius, consisting primarily of iron and nickel with about 16–17% sulfur. This iron sulfide core is partially fluid, and has twice the concentration of the lighter elements that 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 dormant. 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.

Although Mars has no evidence of a current structured global magnetic field, observations show that parts of the planet's crust have been magnetized, and that alternating polarity reversals of its magnetic field have occurred in the past. One theory is that these bands demonstrate w:plate tectonics on Mars four billion years ago, before the planetary dynamo ceased to function.

During the Solar System's formation, Mars was created from the protoplanetary disk that orbited the Sun. 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 star's energetic solar wind. After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era. There is evidence of an enormous impact basin in the northern hemisphere of Mars. One 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.

Hydrology edit

The flow was from bottom left to right. Image is approx. 1,600 km (990 mi) across.
Maadim Vallis outflow channel is approximately 700 km long and much bigger than the Grand Canyon.

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, except at the lowest elevations for short periods. But solid water in the form of ice exists. The two polar ice caps appear to be made largely of water. Deep underground, a permafrost mantle is believed to stretch from the pole to latitudes of about 60°.

Landforms visible on Mars strongly suggest that liquid water has at least at times existed on the planet's surface. Martian outflow channels are extremely long, wide swathes of scoured ground, commonly containing the streamlined remnants of pre-existing topography and other linear erosive features indicating sculpting by fluids moving downslope. About 25 huge outflow channels are thought to record erosion which occurred during the catastrophic release of water from subsurface aquifers, though some of these structures have also been hypothesized to result from the action of glaciers or lava.

Gullies on Mars

The youngest of these channels are thought to have formed as recently as only a few million years ago. Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution very strongly imply that they were carved by runoff resulting from rain or snow fall in early Mars history.

Along crater and canyon walls, there are also thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process demands the involvement of liquid water, probably from melting ice, although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust. No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are very young features, possibly even active today.

Other geological features, such as deltas and alluvial fans preserved in craters, also argue very strongly for warmer, wetter conditions at some interval or intervals in earlier Mars history. Some authors have even gone so far as to argue that at times in the Martian past, much of the low northern plains of the planet were covered with a true ocean hundreds of meters deep, though this remains controversial.

Polar caps and atmosphere edit

North polar early summer ice cap (1999)
South polar midsummer ice cap (2000)
The tenuous atmosphere of Mars, visible on the horizon in this low-orbit photo

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice). The polar caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparatively 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.

Mars lost its magnetosphere 4 billion years ago, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer. Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars. Compared to Earth, the atmosphere of Mars is quite rarefied. The mean surface pressure is only 0.6% of that of the Earth (101.3 kPa).

The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.

References can be found in the permalink listed at the top of this article.
  1. Evidence for ancient Martian life. E. K. Gibson Jr., F. Westall, D. S. McKay, K. Thomas-Keprta, S. Wentworth, and C. S. Romanek, Mail Code SN2, NASA Johnson Space Center, Houston TX 77058, USA.