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For other uses, including the Roman god, see Mars (disambiguation).
Mars (pronounced /ˈmɑrz/) 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 as seen from Earth.
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. 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, it was 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 our Solar System other than Earth, Mars is the most likely to harbor liquid water, and perhaps life.
Mars is currently host to three functional orbiting spacecraft: Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet in the Solar System except Earth. The surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity). 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 in recent years.NASA Images Suggest Water Still Flows in Brief Spurts on Mars. NASA/JPL (December 6, 2006). Retrieved on 2007-01-04. Observations by NASA\'s Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.Webster, G.; Beasley, D. (September 20, 2005). Orbiter\'s Long Life Helps Scientists Track Changes on Mars. NASA. Retrieved on 2007-02-26.
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, though most of the time Jupiter will appear brighter to the naked eye than Mars.
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Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars.
Mars has approximately half the radius of Earth and only one-tenth the mass, being less dense, but its surface area is only slightly less than the total area of Earth\'s dry land.David R. Williams (September 1, 2004). Mars Fact Sheet. National Space Science Data Center. NASA. Retrieved on 2006-06-24. 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. The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.Peplow, Mark. How Mars got its rust. Retrieved on 2007-03-10.
Rock strewn surface imaged by Mars Pathfinder
Although Mars has no intrinsic 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.Goddard Space Flight Center. New Map Provides More Evidence Mars Once Like Earth. Retrieved on 2006-03-17.
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.Dave Jacqué. "APS X-rays reveal secrets of Mars\' core", Argonne National Laboratory, 2003-09-26. Retrieved on 2006-07-01. (English) 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:
Photo of microscopic rock forms indicating past signs of water, taken by Opportunity
Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periodsHeldmann et al., Jennifer L. (2005/05/07), "Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions", Journal of Geophysical Research 110: Eo5004, doi:10.1029/2004JE002261, <http://daleandersen.seti.org/Dale%20Andersen/Articles_files/Heldmann%20et%20al.2005.pdf>. Retrieved on 12 August 2007 \'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water\' ... \'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach 273 K for parts of the day [Haberle et al., 2001]\' but water ice is in no short supply, with two polar ice caps made largely of ice.Kostama, V.-P.; Kreslavsky, M. A. & Head, J. W. (June 3, 2006), "Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement", Geophysical Research Letters 33: L11201, doi:10.1029/2006GL025946, <http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml>. Retrieved on 12 August 2007 \'Martian high-latitude zones are covered with a smooth, layered ice-rich mantle\' 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.Mars\' South Pole Ice Deep and Wide. NASA (March 15, 2007). Retrieved on 2007-03-16. Additionally, an ice permafrost mantle stretches down from the pole to latitudes of about 60°.
Much larger quantities of water are thought to be trapped underneath Mars\'s thick cryosphere, only to be released when the crust is cracked through volcanic action. The largest such release of liquid water is thought to have occurred when the Valles Marineris formed early in Mars\'s history, enough water being released to form river valleys across the planet. A smaller but more recent event of the same kind 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.Murray et al., John B. (March 17, 2005), "Evidence for a frozen sea close to Mars\' equator", Nature 434: 352–355, doi:10.1038/nature03379, <http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36832>. Retrieved on 11 March 2007 However, the morphology of this region is more consistent with the ponding of lava flows causing a superficial similarity to ice floes.Kerr, Richard A. (March 04, 2005), "Ice or Lava Sea on Mars? A Transatlantic Debate Erupts", Science 307: 1390–1391, doi:10.1126/science.307.5714.1390a, <http://dx.doi.org/10.1126/science.307.5714.1390a>. Retrieved on 16 November 2007 These lava flows probably draped the terrain established by earlier catastrophic floods of Athabasca Valles.Jaeger et al., W. L. (September 21, 2007), "Athabasca Valles, Mars: A Lava-Draped Channel System", Science 317: 1709–1711, doi:10.1126/science.1143315, <http://dx.doi.org/10.1126/science.1143315>. Retrieved on 16 November 2007 Significantly rough surface texture at 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 H2O in this area to tens of centimeter depths is only ~4%,Boynton et al., W. V. (in press), "Concentration of H, Si, Cl, K, Fe, and Th in the low and mid latitude regions of Mars", Journal of Geophysical Research, Planets, doi:10.1029/2007JE002887, <http://dx.doi.org/10.1029/2007JE002887>. Retrieved on 16 November 2007 easily attributable to hydrated mineralsFeldman et al., W. C. (November 30, 2005), "Topgraphic control of hydrogen deposits at low latitudes to midlatitudes of Mars", Journal of Geophysical Research 110, doi:10.1029/2005JE002452, <http://dx.doi.org/10.1029/2005JE002452>. Retrieved on 16 November 2007 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.Malin, Michael C. (June 30, 2000). "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars". Science 288: 2330–2335. doi:10.1126/science.288.5475.2330. The researchers found no partially degraded (i.e. weathered) gullies and no superimposed impact craters, indicating that these are very young features.
Changing gully deposits on Mars
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.NASA Images Suggest Water Still Flows in Brief Spurts on Mars. NASA (December 6, 2006). Retrieved on 2006-12-06. 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.Water flowed recently on Mars. BBC (December 6, 2006). Retrieved on 2006-12-06."Water May Still Flow on Mars, NASA Photo Suggests", NASA, December 6, 2006. Retrieved on 2006-04-30.
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.NASA (March 3, 2004). "Mineral in Mars \'Berries\' Adds to Water Story". Press release. Retrieved on 2006-06-13.
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.A. S., McEwen et al (September 21, 2007). "A Closer Look at Water-Related Geologic Activity on Mars". Science 317: 1706-1709. doi:10.1126/science.1143987.
This approximate true-color image, taken by the Mars Exploration Rover Opportunity, shows the view of Victoria Crater from Cape Verde. It was captured over a three-week period, from October 16 - November 6, 2006.
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."Sheehan, William. Areographers. The Planet Mars: A History of Observation and Discovery. Retrieved on 2006-06-13.
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).Viking and the Resources of Mars (PDF). Humans to Mars: Fifty Years of Mission Planning, 1950–2000. Retrieved on 2007-03-10.
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.Topography. Think Quest. Retrieved on 2007-03-13.
The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is thus 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.Frommert, H.; Kronberg, C.. Christiaan Huygens. Retrieved on 2007-03-10.
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. It is over three times the height of Mt. Everest which in comparison stands at only 8.848 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.Wright, Shawn (April 4, 2003). Infrared Analyses of Small Impact Craters on Earth and Mars. University of Pittsburgh. Retrieved on 2007-02-26. The largest of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.Mars Global Geography. Windows to the Universe. Retrieved on 2006-06-13. 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.Wetherill, G. W. (1999). "Problems Associated with Estimating the Relative Impact Rates on Mars and the Moon". Earth, Moon, and Planets 9: 227. Retrieved on 2007-02-26. 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 4000 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 Tharis 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.Lucchitta, B. K.; Rosanova, C. E. (August 26, 2003). Valles Marineris; The Grand Canyon of Mars. USGS. Retrieved on 2007-03-11.
THEMIS image of cave entrances on Mars
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.G. E. Cushing, T. N. Titus, J. J. Wynne, P. R. Christensen. (2007). Themis Observes Possible Cave Skylights on Mars (PDF). Lunar and Planetary Science XXXVIII. Retrieved on 2007-08-02. The caves, named Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne after loved ones of their discoverers, are collectively known as the "seven sisters."\'Cave entrances\' spotted on Mars. NAU. Retrieved on 2007-05-28. 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.Researchers find possible caves on Mars. Paul Rincon of BBC News. Retrieved on 2007-05-28. Some researchers have suggested that this protection makes the caves good candidates for future efforts to find liquid water and signs of life.
Mars has two permanent polar ice caps: the northern one at Planum Boreum and the southern one at Planum Australe.
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.Philips, Tony (2001). The Solar Wind at Mars. Science@NASA. Retrieved on 2006-10-08. R. Lundin, S. Barabash, H. Andersson, M. Holmström, A. Grigoriev, M. Yamauchi, J.-A. Sauvaud, A. Fedorov, E. Budnik, J.-J. Thocaven,2 D. Winningham, R. Frahm, J. Scherrer, J. Sharber, K. Asamura, H. Hayakawa, A. Coates, D. R. Linder, C. Curtis, K. C. Hsieh, B. R. Sandel, M. Grande, M. Carter, D. H. Reading, H. Koskinen, E. Kallio, P. Riihela, W. Schmidt, T. Säles, J. Kozyra, N. Krupp, J. Woch, J. Luhmann, S. McKenna-Lawler, R. Cerulli-Irelli, S. Orsini, M. Maggi, A. Mura, A. Milillo, E. Roelof, D. Williams, S. Livi, P. Brandt, P. Wurz, P. Bochsler (2004). "Solar Wind-Induced Atmospheric Erosion at Mars: First Results from ASPERA-3 on Mars Express". Science 305: 1933–1936. doi:10.1126/science.1101860. Retrieved on 2007-02-26. 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 1155 Pa (1.155 kPa) in the depths of Hellas Planitia, with a mean surface level pressure of 600 Pa (0.6 kPa). This is less than 1% of the surface pressure on Earth (101.3 kPa). Mars\'s mean surface pressure equals the pressure found 35 km above the Earth\'s surface. The scale height of the atmosphere, about 11 km, is higher than Earth\'s (6 km) due to the lower gravity.
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. Lemmon et al. (2004). "Atmospheric Imaging Results from Mars Rovers". Science 306: 1753–1756. doi:10.1126/science.1104474.
Several researchers claim to have detected methane in the Martian atmosphere with a concentration of about 10 ppb by volume. V. Formisano, S. Atreya, T. Encrenaz, N. Ignatiev, M. Giuranna (2004). "Detection of Methane in the Atmosphere of Mars". Science 306: 1758–1761. doi:10.1126/science.1101732. "Mars Express confirms methane in the Martian atmosphere", ESA, March 30, 2004. Retrieved on 2006-03-17. Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting about 340 years in the Martian atmosphere,Martin Baucom (2006). "Life on Mars?". American Scientist 94 (2). Retrieved on 2007-02-26. its presence would indicate a current or recent 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 There are many serpentinization reactions. Olivine is a solid solution between forsterite and fayalite whose general formula is . The reaction producing methane from olivine can be written (in balanced form) as: Forsterite + Fayalite + Water + Carbonic acid → Serpentine + Magnetite + Methane , or: → involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars. C. Oze, M. Sharma (2005). "Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars". Geophys. Res. Lett. 32: L10203. doi:10.1029/2005GL022691. Retrieved on 2006-04-18.
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). J. T. Mellon, W. C. Feldman, T. H. Prettyman (2003). "The presence and stability of ground ice in the southern hemisphere of Mars". Icarus 169 (2): 324–340. doi:10.1016/j.icarus.2003.10.022. Retrieved on 2007-02-26. When the poles are again exposed to sunlight, the frozen CO2 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."Mars Rovers Spot Water-Clue Mineral, Frost, Clouds", NASA, Dec. 13, 2004. Retrieved on 2006-03-17.
Mars from Hubble Space Telescope October 28, 2005 with dust storm visible.
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.Haberle, R. M et al (2001). On the possibility of liquid water on present-day Mars. J. Geophys. Res.. Retrieved on 2006-10-06. 106(E10), 23,317–23,326. (abstract, full paper requires purchase or AGU subscription) 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.Mars\' desert surface.... MGCM Press release. NASA. Retrieved on 2007-02-25.
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 be up to 30 K (54 °F) warmer than the equivalent summer temperatures in the north.Goodman, Jason C (September 22, 1997). The Past, Present, and Possible Future of Martian Climate. MIT. Retrieved on 2007-02-26.
Mars\'s northern ice cap.
Mars also has the largest dust storms in the 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.Philips, Tony (July 16, 2001). Planet Gobbling Dust Storms. Science @ NASA. Retrieved on 2006-06-07.
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.Darling, David. Mars, polar caps, ENCYCLOPEDIA OF ASTROBIOLOGY, ASTRONOMY, AND SPACEFLIGHT. Retrieved on 2007-02-26. The northern polar cap has a diameter of about 1,000 kilometres during the northern Mars summer,
MIRA\'s Field Trips to the Stars Internet Education Program. Mira.org. Retrieved on 2007-02-26.
and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 kilometres thick.Carr, Michael H. (2003). "Oceans on Mars: An assessment of the observational evidence and possible fate". Journal of Geophysical Research 108 (5042): 24. doi:10.1029/2002JE001963. Retrieved on 2007-02-26. (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.Phillips, Dr. Tony. Mars is Melting, Science at NASA. Retrieved on 2007-02-26. 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.J. J. Plaut, G. Picardi, A. Safaeinili, A. B. Ivanov, S. M. Milkovich, A. Cicchetti, W. Kofman, J. Mouginot, W. M. Farrell, R. J. Phillips, S. M. Clifford, A. Frigeri, R. Orosei, C. Federico, I. P. Williams, D. A. Gurnett, E. Nielsen, T. Hagfors, E. Heggy, E. R. Stofan, D. Plettemeier, T. R. Watters, C. J. Leuschen, P. Edenhofer (2007). "Subsurface Radar Sounding of the South Polar Layered Deposits of Mars". Science 315. doi:10.1126/science.1139672. Retrieved on 2007-03-17. 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.Pelletier J. D. (2004). "How do spiral troughs form on Mars?". Geology 32: 365–367. Retrieved on 2007-02-27. MarsToday.Com. Mars Polar Cap Mysery Solved. Retrieved on 2007-01-23. Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons.
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 about twice as long given its longer year. Mars passed its perihelion in June 2007 and will come to aphelion in May 2008.
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.Mars\' Orbital eccentricity over time. Solex. Universita\' degli Studi di Napoli Federico II (2003). Retrieved on 2007-07-20. The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth\'s cycle of 100,000 years.Jean Meeus (March 2003). When Was Mars Last This Close?. International Planetarium Society. Retrieved on 2008-01-18. 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.Ron Baalke (22 Aug 2003). Mars Makes Closest Approach In Nearly 60,000 Years. meteorite-list. Retrieved on 2008-01-18.
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The image to the left shows a comparison between Mars and Ceres, a dwarf planet in the Asteroid Belt, as seen from the ecliptic pole, while the image to the right is as seen from the ascending node. The segments of orbits below the ecliptic are plotted in darker colors. The perihelia (q) and aphelia (Q) are labelled with the date of the nearest passage. |
Phobos (left) and Deimos (right)
Mars has two tiny natural moons, Phobos and Deimos, which orbit very close to the planet and are thought to be captured asteroids.Close Inspection for Phobos. ESA website. Retrieved on 2006-06-13.
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.ARES ATTENDANTS: DEIMOS & PHOBOS. Greek Mythology. Retrieved on 2006-06-13.
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.Arnett, Bill (November 20, 2004). Phobos. nineplanets. Retrieved on 2006-06-13.
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.
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.Ellis, Scott. Geological History: Moons of Mars. CalSpace. Retrieved on 2007-08-02.
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 requires that the orbit of a planet lie within a habitable zone, which for the Sun is currently occupied by Earth. Mars orbits half an astronomical unit beyond this zone and this, along with the planet\'s thin atmosphere, causes water to freeze on its surface. 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 life.Helen Briggs (BBC science reporter, Boston) (15 February 2008). Early Mars \'too salty\' for life. BBC News. Retrieved on 2008-02-16.
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. Hannsson, Anders (1997). Mars and the Development of Life.. Wiley. ISBN 0-471-96606-1.
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."New Analysis of Viking Mission Results Indicates Presence of Life on Mars", Physorg.com, January 7, 2007. Retrieved on 2007-03-02.
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 particles would quickly break down in the Martian atmosphere. Vladimir A. Krasnopolsky, Jean-Pierre Maillard, Tobias C. Owen (2004). "Detection of methane in the Martian atmosphere: evidence for life?". [[Icarus (journal)|]] 172: 537–547. doi:10.1016/j.icarus.2004.07.004. Retrieved on 2007-02-27. "Formaldehyde claim inflames Martian debate", Nature, February 25, 2005. Retrieved on 2006-03-19. It is possible that these compounds may be replenished by volcanic or geological means such as
