astronomi: Mars

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.

A terrestrial planet with a thin atmosphere, Mars has 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 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.^[5] Observations by NASA's Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.^[6]

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^[1], 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.

Physical characteristics

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

Mars has 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.^[1] 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.^[7]

Geology

Main article: Geology of Mars

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 stones on Earth; however, these observations may also be explained by silica glass. Much of the surface is deeply covered by a fine iron(III) oxide dust that has the consistency of talcum powder.^[8]

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.^[9]

Current models of the planet's interior imply a core region approximately 1,480 kilometres in radius, consisting primarily of iron with about 15–17% 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.^[10] 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.

Hydrology

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 periods^[11]^[12] but water ice is in no short supply, with two polar ice caps made largely of ice.^[13] 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.^[14] Additionally, an ice permafrost mantle stretches down from the pole to latitudes of about 60°.^[13]

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.^[15] However, the morphology of this region is more consistent with the ponding of lava flows causing a superficial similarity to ice floes.^[16] These lava flows probably draped the the terrain established by earlier catastrophic floods of Athabasca Valles.^[17] 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. ^[17] Furthermore, the stoichiometric mass fraction of H₂O in this area to tens of centimeter depths is only ~4%,^[18] easily attributable to hydrated minerals^[19] 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 located in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30° latitude.^[20] 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.^[21] 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.^[22]^[23]

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.^[24]

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.^[25]

Geography

Main articles: Geography of Mars, List of mountains on Mars, and List of craters on Mars

See also: Category:Surface features of Mars

Opportunity at Victoria Crater

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."^[26]

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).^[27]

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.

Olympus Mons

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 approximately 0.6% of the sea level surface pressure on Earth.^[28]

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.^[29]

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.^[30] The largest of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.^[31] 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.^[32] 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.^[33]

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.^[34] The caves, named Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne after loved ones of their discoverers, are collectively known as the "seven sisters."^[35] 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.^[36] 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.

Atmosphere

Main article: Atmosphere of Mars

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.^[37]^[38] 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.^[1] 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.^[39]

Several researchers claim to have detected methane in the Martian atmosphere with a concentration of about 10 ppb by volume.^[40]^[41] Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting about 340 years in the Martian atmosphere,^[42] 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^[43] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.^[44]

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).^[45] 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.^[46]

Climate

Main article: Climate of Mars

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 approximately two Earth years in length. Martian surface temperatures vary from lows of approximately −140 °C during the polar winters to highs of up to 20 °C in summers.^[11] 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.^[47]

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 warmer than the equivalent summer temperatures in the north.^[48]

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.^[49]

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.^[50] The northern polar cap has a diameter of approximately 1,000 kilometres during the northern Mars summer,^[51] and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 kilometres thick.^[52] (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.^[53] 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.^[54] 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.^[55]^[56] Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons.

Orbit and rotation

Mars has a relatively pronounced orbital eccentricity of about 9%; 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 only 0.2%, much less than that of Venus or Neptune today.^[57] Although Mars takes twice as long as the Earth to orbit the Sun, its main cycle of eccentricity variation is slightly shorter than Earth's, with cycles taking 95,000 Earth years. However, there is a much longer cycle of eccentricity with a period of several million Earth years, and this overshadows the 95,000 year cycle in the eccentricity graph of the past three million years. Presently, Mars is approaching an eccentricity maximum, which will be reached in a thousand years.

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 aphelion in June 2006 and is now passing its perihelion since June 2007.

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.

Moons

Main article: Mars's natural satellites

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.^[58]

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.^[59]

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.^[60]

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.^[60]

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 of the asteroid belt.^[61]

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 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.

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.^[62]

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.^[63]

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.^[64]^[65] It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.^[44]

Exploration

Main article: Exploration of Mars

Viking Lander 1 site

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.

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.^[66]

Past missions

The first successful fly-by mission to Mars was NASA's Mariner 4, launched in 1964. 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 and remained operational for 6 and 3 years, for Viking 1 and Viking 2 respectively. The Viking landers relayed the first color pictures of Mars^[67] and also mapped the surface of Mars so well that the images are still sometimes used to this day. The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons, unfortunately Phobos 1 lost contact on the way to Mars, and 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. This mission was another big success, and received much publicity, partially due to the many spectacular images that were sent back to Earth.^[68]

Current missions

Spirit's lander on Mars

In 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of June 2007. 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.^[69]

In 2003, the 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.^[70] 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.^[71]

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.^[72]

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.

Future missions

Mars Polar Lander practices robotic arm control at a test site in Death Valley.

The next scheduled mission to Mars, not counting the brief flyby by the Dawn spacecraft to Ceres and Vesta, is the NASA Phoenix Mars lander, which launched August 4, 2007 and is scheduled to arrive 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 meter into the Martian soil. The lander will be in an area with an 80% chance of ice being less than 30 cm below the surface, and has a microscopic camera capable of resolving to one-thousandth the width of a human hair.^[73]

Phoenix will be followed by the Mars Science Laboratory in 2009, a bigger, faster (90 m/hour), and smarter version of the Mars Exploration Rovers. Experiments include a laser chemical sample that can deduce the make-up of rocks at a distance of 13 m.^[74]

The joint Russian and Chinese Phobos-Grunt sample-return mission, to return samples of Mars's moon Phobos, is scheduled for a 2009 launch. In 2012 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.^[75]^[76]

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 US President George W. Bush.^[77] 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.

The European Space Agency hopes to land humans on Mars between 2030 and 2035.^[78] This will be preceded by successively larger probes, starting with the launch of the ExoMars probe and a Mars Sample Return Mission.

On September 28, 2007, NASA administrator Michael Griffin stated that NASA aims to put a man on Mars by 2037: in 2057, "we should be celebrating 20 years of man on Mars." ^[79]

Astronomy on Mars

Main article: Astronomy on Mars

Photograph of a Martian sunset taken by Spirit at Gusev crater, May 19, 2005.

With the existence of various orbiters, landers, and rovers, it is now possible to study astronomy from the Martian skies. The Earth and the Moon are easily visible while Mars’ moon Phobos appears about one third the angular diameter of the full Moon as it appears from Earth. On the other hand Deimos appears more or less star-like, and appears only slightly brighter than Venus does from Earth.^[80]

There are also various phenomena well-known on Earth that have now been observed on Mars, such as meteors and auroras.^[71] 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

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. 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 favourably positioned, it can be lost in the Sun's glare for months at a time. At its most favourable times—which occur twice every 32 years, alternately at 15 and 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.^[81]

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.^[1] The next Mars opposition will occur on December 24, 2007.

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

The rotation of Mars as seen in a small telescope in 2003.

Mars oppositions from 2003-2018, viewed from above the ecliptic with the earth centered.

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 0.37284 AU, compared to 0.37271 AU on August 27, 2003, and the minimum distance on August 24, 2208 will be 0.37278 AU.^[82] The orbital changes of Earth and Mars are making the approaches nearer: the 2003 record will be bettered 22 times by the year 4000.

2007-2008

The next retrograde period will begin on November 16, 2007 and last through to January 31, 2008 with Mars appearing to move backwards through the sky from the constellation Gemini into Taurus.^[83]

This chart shows the path of Mars in the sky during the opposition of 2007. Each dot represents one day's motion. Mars shown on December 18 in Gemini, when it is closest to earth (0.59 AU distance, 15.9" visual diameter), 6 days before opposition on December 24.

Historical observations

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 approximately every 15–17 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 3, 1590, seen by M. Möstlin at Heidelberg.^[84]

In 1609, Mars was viewed by Galileo, who was first to see it via telescope.

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 was popularly mistranslated as canals.^[85]

Mars sketched as observed by Lowell sometime before 1914. (South top)

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 Perrotin and 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.^[86]

Map of Mars from Hubble Space Telescope as seen near the 1999 opposition. (North top)

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.^[87]

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.^[88]

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.^[89] 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 both Arabic and Persian, and Merih in Turkish. The etymology of al-Mirrikh is unknown. Ancient Persians named it Bahram, the Zoroastrian god of faith. 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.

Its 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.^[90] ♂ 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.^[91]

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever".^[92] 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.^[93]

”

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.^[94] 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."^[95]

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.^[96]

“

Early in December of 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. 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.^[96]

”

Pickering later proposed creating a set of mirrors in Texas with the intention of signaling Martians.

In fiction

Main article: Mars 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, 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.^[97]

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 prior to the mid-sixties.

Author Jonathan Swift made reference to the moons of Mars, approximately 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.^[98]

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.^[99]

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".

References and Notes

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^ ^a ^b Best fit ellipsoid
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External links

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Mars Portal

Solar System Portal

3D maps of Mars in NASA World Wind
Google Mars – Interactive image of Mars
Flight Into Mariner Valley – NASA/JPL/Arizona State University 3D flythrough of Valles Marineris
Marsgeo.com – Mars Rover photos, videos & surface geology
Guide to Mars – information about Mars and how to observe it.
Nine Planets Mars page
On Mars: Exploration of the Red Planet 1958–1978 from the NASA History Office.
Martian Law – a CATO white paper
Computer Simulation of a flyby through Mariner Valley
Mars Unearthed – Comparisons of terrains between Earth and Mars
Ralph Aeschliman's Online Atlas of Mars
Geody Mars – World's search engine that supports NASA World Wind, Celestia, and other applications.
Be on Mars – Anaglyphs from the Mars Rovers (3D)
NASA/JPL OnMars WMS Server for Mars Data – Work as Google Earth client overlays
Exploring Mars: Image Center
BBC News update on Mars Express' findings of polar water ice and water-eroded features on the surface
BBC News Mars pictures reveal frozen sea
04/02/07: ESA Prepares for a Human Mission to Mars
Mars' apparent relative size at opposition as seen by HST

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Retrieved from "http://en.wikipedia.org/wiki/Mars"

Mars Statistics
Mass (kg)	6.421e+23
Mass (Earth = 1)	1.0745e-01
Equatorial radius (km)	3,397.2
Equatorial radius (Earth = 1)	5.3264e-01
Mean density (gm/cm^3)	3.94
Mean distance from the Sun (km)	227,940,000
Mean distance from the Sun (Earth = 1)	1.5237
Rotational period (hours)	24.6229
Rotational period (days)	1.025957
Orbital period (days)	686.98
Mean orbital velocity (km/sec)	24.13
Orbital eccentricity	0.0934
Tilt of axis (degrees)	25.19
Orbital inclination (degrees)	1.850
Equatorial surface gravity (m/sec^2)	3.72
Equatorial escape velocity (km/sec)	5.02
Visual geometric albedo	0.15
Magnitude (Vo)	-2.01
Minimum surface temperature	-140°C
Mean surface temperature	-63°C
Maximum surface temperature	20°C
Atmospheric pressure (bars)	0.007
Atmospheric composition Carbon Dioxide (C02) Nitrogen (N2) Argon (Ar) Oxygen (O2) Carbon Monoxide (CO) Water (H2O) Neon (Ne) Krypton (Kr) Xenon (Xe) Ozone (O3)	95.32% 2.7% 1.6% 0.13% 0.07% 0.03% 0.00025% 0.00003% 0.000008% 0.000003%

Animations of Mars

Views of Mars

Mars With Cloud Cover
This image of mars came from a series of pictures taken by the Mars Global Surveyor wide angle cameras. A map was created and then wrapped around a sphere to generate this view of Mars. Here, bluish-white water ice clouds hang above the Tharsis volcanoes. (Copyright 2005 by Calvin J. Hamilton)

Large Mosaic of Mars
This image is a large mosaic of the Valles Marineris [VAL-less mar-uh-NAIR-iss] hemisphere of Mars. It is a view similar to that which one would see from a spacecraft. The lower center of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometers (1,860 miles) long and up to 8 kilometers (5 miles) deep, extending from Noctis Labyrinthus, the arcuate system of graben to the west, to the chaotic terrain to the east. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometers (16 miles) high, are visible to the west along with Olympus Mons the largest volcano on the planet. Very ancient terrain covered by many impact craters lies to the south of Valles Marineris. The polar cap can be seen to the north. (Copyright Calvin J. Hamilton)

The Interior of Mars
The current understanding of the interior of Mars suggests that it can be modeled with a thin crust, similar to Earth's, a mantle and a core. Using four parameters, the Martian core size and mass can be determined. However, only three out of the four are known and include the total mass, size of Mars, and the moment of inertia. Mass and size was determined accurately from early missions. The moment of inertia was determined from Viking lander and Pathfinder Doppler data, by measuring the precession rate of Mars. The fourth parameter, needed to complete the interior model, will be obtained from future spacecraft missions. With the three known parameters, the model is significantly constrained. If the Martian core is dense (composed of iron) similar to Earth's or SNC meteorites thought to originate from Mars, then the minimum core radius would be about 1300 kilometers. If the core is made out of less-dense material such as a mixture of sulfur and iron, the maximum radius would probably be less than 2000 kilometers. (Copyright 1998 by Calvin J. Hamilton)

Topography Map of Mars
This image is a newly released topographic map of Mars. The full range of topography on Mars is about 19 miles (30 kilometers), one and a half times the range of elevations found on Earth, The most curious aspect of the map is the striking difference between the planet's low, smooth Northern Hemisphere and the heavily cratered Southern Hemisphere," which sits, on average, about three miles (five kilometers) higher than the north. (Courtesy GSFC/NASA)

Schiaparelli Hemisphere
This image is a mosaic of the Schiaparelli hemisphere of Mars. The center of this image is near the impact crater Schiaparelli, 450 kilometers (280 miles) in diameter. The dark streaks with bright margins emanating from craters in the Oxie Palus region, upper left of image, are caused by erosion and/or deposition by the wind. Bright white areas to the south, including the Hellas impact basin at extreme lower right, are covered by carbon dioxide frost. (Courtesy USGS)

Central Candor Chasm - Oblique View
This image shows part of Candor Chasm in Valles Marineris. It is centered at Latitude -5.0, Longitude 70.0. The view is from the north looking into the chasm. Candor Chasm's geomorphology is complex, shaped by tectonics, mass wasting, wind, and perhaps by water and volcanism. (Courtesy USGS)

West Candor Chasm (Enhanced Color)
This picture (centered at latitude 4° S, longitude 76° W) shows areas of central Valles Marineris, including Candor Chasm (lower left), Ophir Chasm (lower right), and Hebes Chasm (upper right). Complex layered deposits in the canyons may have been deposited in lakes, and if so, are of great interest for future searches for fossil life on Mars. The pinkish deposits in Candor Chasm may be due to hydrothermal alterations and the production of crystalline ferric oxides. ((Geissler et al., 1993, Icarus 106,380). Viking Orbiter Picture Numbers 279B02 (violet), 279B10 (green), and 279B12 (red) at 240 meters/pixel resolution. Picture width is 231 kilometers. North is 47° clockwise from top.)

Ophir Chasma
Ophir Chasma is a large west-northwest-trending trough about 100 km wide. The Chasma is bordered by 4 km high walled cliffs, most likely faults, that show spur-and-gully morphology and smooth sections. The walls have been dissected by landslides forming reentrants; one area (upper left) on the north wall shows a young landslide about 100 km wide. The volume of the landslide debris is more than 1000 times greater than that from the May 18, 1980 debris avalanche from Mount St. Helens. The longitudinal grooves seen in the foreground are thought to be due to differential shear and lateral spreading at high velocities. The landslide passes between mounds of interior layered deposits on the floor of the chasma. (Courtesy USGS)

Landslide in Valles Marineris
Although Valles Marineris originated as a tectonic structure, it has been modified by other processes. This image shows a close-up view of a landslide on the south wall of Valles Marineris. This landslide partially removed the rim of the crater that is on the plateau adjacent to Valles Marineris. Note the texture of the landslide deposit where it flowed across the floor of Valles Marineris. Several distinct layers can be seen in the walls of the trough. These layers may be regions of distinct chemical composition or mechanical properties in the Martian crust. (Copyright Calvin J. Hamilton; Caption: LPI)

HST 3 Views of Mars at Opposition
These Hubble Space Telescope views provide the most detailed complete global coverage of the Red Planet ever seen from Earth. The pictures were taken on February 25, 1995, when Mars was at a distance of 103 million kilometers (65 million miles). To the surprise of researchers, Mars is cloudier than seen in previous years. This means the planet is cooler and drier, because water vapor in the atmosphere freezes out to form ice-crystal clouds. The three images show the Tharsis, Valles Marineris and Syrtis Major regions. (Credit: Philip James, University of Toledo; Steven Lee, University of Colorado; and NASA)

Outflow Source of Channel Ravi Vallis
This image of the head of Ravi Vallis shows a 300-kilometer (186-mile) long portion of a channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis orginates in a region of collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands. Structures in these channels indicate that they were carved by liquid water moving at high flow rates. The abrupt beginning of the channel, with no apparent tributaries, suggests that the water was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. Three such regions of chaotic collapsed material are seen in this image, connected by a channel whose floor was scoured by the flowing water. The flow in this channel was from west to east (left to right). This channel ultimately links up with a system of channels that flowed northward into Chryse Basin. (Copyright Calvin J. Hamilton; Caption: LPI)

Streamlined Islands
The water that carved the channels to the north and east of the Valles Marineris canyon system had tremendous erosive power. One consequence of this erosion was the formation of streamlined islands where the water encountered obstacles along its path. This image shows two streamlined islands that formed as the water was diverted by two 8-10 kilometer (5-6 mile) diameter craters lying near the mouth of Ares Vallis in Chryse Planitia. The water flowed from south to north (bottom to top of the image). The height of the scarp surrounding the upper island is about 400 meters (1,300 feet), while the scarp surrounding the southern island is about 600 meters (2,000 feet) high. (Copyright Calvin J. Hamilton; Caption: LPI)

Valley Network
Unlike the features shown in the above two images, many systems on Mars do not show evidence of catastrophic flooding. Instead, they show a resemblance to drainage systems on Earth, where water acts at slow rates over long periods of time. As on Earth, the channels shown here merge together to form larger channels.

However, these valley networks are less developed than typical terrestrial drainage systems, with the Martian examples lacking small-scale streams feeding into the larger valleys. Because of the absence of small-scale streams in the Martian valley networks, it is thought that the valleys were carved primarily by ground water flow rather than by runoff of rain. Although liquid water is currently unstable on the surface of Mars, theoretical studies indicate that flowing groundwater might be able to form valley networks if the water flowed beneath a protective cover of ice. Alternatively, because the valley networks are confined to relatively old regions of Mars, their presence may indicate that Mars once possessed a warmer and wetter climate in its early history. (Copyright Calvin J. Hamilton; Caption: LPI)

South Polar Cap
This image shows the south polar cap of Mars as it appears near its minimum size of about 400 kilometers (249 miles). It consists mainly of frozen carbon dioxide. This carbon dioxide cap never melts completely. The ice appears reddish due to dust that has been incorporated into the cap. (Courtesy NASA)

North Polar Cap
This image is an oblique view of the north polar cap of Mars. Unlike the south polar cap, the north polar cap probably consists of water-ice. (Copyright Calvin J. Hamilton)

Polar Laminated Terrain
One of the discoveries of the Mariner 9 spacecraft was that the south polar cap of Mars was made of thin layers or laminations of ice and sediment. Four years later, on October 10, 1976, the Viking 2 spacecraft took this picture of the Martian north polar cap. The visible layering occurred as a result of wind born dust settling upon the polar cap. As the caps experience climatic variations, they expand and contract. The layers of dust sediment tend to grow thicker near the poles where ice deposits remain for longer periods of time. The thickness of the deposits indicates they were formed during cyclical climatic variation rather than annual changes. As ice withdraws from a region, wind exposes the layers sculpting valleys and scarps. The formation of layered deposits is an active process today. (Copyright 1998 by Calvin J. Hamilton)

Dunefield
This image shows several dune types which are found in the north circumpolar dunefield. This thumnail image shows a section of transverse dunes. The full image has a field of traverse dunes on the left and barchan dunes on the right with a transition zone inbetween. Transverse dunes are oriented perpendicular to the prevailing wind direction. They are long and linear, and frequently join their neighbor in a low-angle "Y" junction. Barchan dunes are crescent-shaped mounds with downwind-pointing horns. These dunes are comparable in size to the largest dunes found on the Earth. (Copyright Calvin J. Hamilton)

Local Dust Storm
Local dust storms are relatively common on Mars. They tend to occur in areas of high topographic and/or high thermal gradients (usually near the polar caps), where surface winds would be strongest. This storm is several hundreds of kilometers in extent and is located near the edge of the south polar cap. Some local storms grow larger, others die out. (Copyright Calvin J. Hamilton; caption by LPI)

White Rock
This image shows a lesser known, but unusual feature on Mars. It is commonly called "White Rock". The white feature is eroded crater fill, but exactly how it was formed has not been satisfactorily explained. White Rock was not formed by polar processes because it lies near to the equator at latitude -8 degrees and longitude 355 degrees. It has been modified through aeolian erosion showing transverse and longitudinal erosional features. (Copyright 1998 by Calvin J. Hamilton)

Martian Atmosphere
This oblique image taken by the Viking orbiter spacecraft shows a thin band of the Martian atmosphere. This image looks northeast across the Argyre basin. The Argyre basin is about 600 kilometers across with a rugged rim of about 500 kilometers in width. (Copyright 1997 by Calvin J. Hamilton)