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Diameter: 6,779 km
Distance from sun: 227,900,000 km
Surface area: 144,798,500 km²
Length of day: 1d 0h 40m
Gravity: 3.711 m/s²
Circumference: 21,297 km
Mass: 639E21 kg (0.107 Earth mass)
Length of Year: 686.98 Earth Days
Surface temp: -87 to -5 C

Mars is the fourth planet from the Sun and the seventh largest.Mars (Greek:   Ares) is the god of War.  The planet probably got this name due to its red color; Mars is sometimes  referred to as the Red Planet. (An interesting side note: the Roman god Mars was  a god of agriculture before becoming associated with the Greek Ares; those in  favor of colonizing and terraforming Mars may prefer this symbolism.) The name  of the   month March derives from Mars.Mars has been known since prehistoric times.  Of course, it has been  extensively studied with ground-based  observatories. But even very large telescopes find Mars a difficult target, it’s just too  small. It is still a favorite of   science fiction writers as  the most favorable place in the Solar System (other than Earth!) for human  habitation. But the famous “canals” “seen” by   Lowell and others were, unfortunately, just as  imaginary as   Barsoomian princesses.

The first spacecraft to visit Mars was   Mariner 4 in 1965. Several others followed  including   Mars  2, the first spacecraft to land on Mars and the two   Viking landers in 1976. Ending a long 20 year hiatus,   Mars  Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the   Mars Expedition Rovers “Spirit” and “Opportunity” landed on Mars sending back geologic data and many  pictures; they are still operating after more than three years on Mars.  In  2008, Phoenix landed in the northern plains to search for water. Three Mars orbiters (Mars  Reconnaissance Orbiter, Mars  Odyssey, and Mars  Express) are also currently in operation.

Mars’ orbit is significantly elliptical. One result of this is a  temperature variation of about 30 C at the subsolar point between aphelion and  perihelion. This has a major influence on   Mars’  climate. While the average temperature on Mars is about 218 K (-55 C, -67  F), Martian surface temperatures range widely from as little as 140 K (-133 C,  -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during  summer.

Though Mars is much smaller than Earth, its surface area is about the same  as the land surface area of Earth.

Olympus Mons

Olympus Mons, Mars

Mars has some of  the most highly varied and interesting terrain of any of  the   terrestrial planets, some of it quite  spectacular:

  • Olympus Mons: the largest mountain in the Solar  System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more  than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.
  • Tharsis: a huge bulge on the Martian surface that is about 4000 km  across and 10 km high.
  • Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km  deep (top of page);
  • Hellas Planitia: an impact crater in the southern hemisphere over 6  km deep and 2000 km in diameter.

Much of the Martian surface is  very old and cratered, but there are also much  younger rift valleys, ridges, hills and plains.  (None of this is visible in any  detail with a telescope, even the Hubble Space Telescope; all this information  comes from the spacecraft that we’ve sent to Mars.)

Southern Highlands, Mars

Southern Highlands

The southern hemisphere of Mars is predominantly ancient cratered highlands  somewhat similar to the   Moon. In contrast, most of the  northern hemisphere consists of plains which are much younger, lower in  elevation and have a much more complex history. An abrupt elevation change of  several kilometers seems to occur at the boundary. The reasons for this global  dichotomy and abrupt boundary are unknown (some speculate that they are due to a  very large impact shortly after Mars’ accretion).   Mars  Global Surveyor has produced a nice   3D  map of Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface  and the bulk statistics of the planet. The most likely scenario is a dense core  about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth’s  and a thin crust. Data from Mars Global Surveyor indicates that Mars’ crust is  about 80 km thick in the southern hemisphere but only about 35 km thick in the  north. Mars’ relatively low density compared to the other terrestrial planets  indicates that its core probably contains a relatively large fraction of sulfur  in addition to iron (iron and iron sulfide).

Like   Mercury and the Moon, Mars appears to  lack active   plate tectonics at present; there  is no evidence of recent horizontal motion of the surface such as the folded  mountains so common on   Earth. With no lateral plate  motion, hot-spots under the crust stay in a fixed position relative to the  surface. This, along with the lower surface gravity, may account for the Tharis  bulge and its enormous volcanoes. There is no evidence of current volcanic  activity.  However, data from Mars Global Surveyor indicates that Mars very  likely did have  tectonic activity sometime in the past.

There is very clear evidence of    erosion in many places on Mars  including large floods and small river systems. At some time in the past there  was clearly some sort of fluid on the surface. Liquid water is the obvious fluid  but    other  possibilities exist. There may have been large lakes or even oceans; the  evidence for which was strenghtened by some very nice    images  of layered terrain taken by Mars Global Surveyor and the mineralology  results from MER Opportunity. Most of these point to wet episodes that  occurred only briefly and very long ago; the age of the erosion channels is  estimated at about nearly 4 billion years. However, images from Mars Express  released in early 2005 show what appears to be a frozen  sea that was liquid very recently (maybe 5 million years ago).  Confirmation of this interpretation would be a very big deal indeed! (Valles Marineris was  NOT created by running water. It was formed by the stretching and cracking of  the crust associated with the creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost  all of its carbon dioxide was used up to form carbonate rocks. But lacking the  Earth’s    plate tectonics, Mars is unable to  recycle any of this carbon dioxide back into its atmosphere and so cannot  sustain a significant    greenhouse effect.  The surface of Mars is therefore much colder than the Earth would be at that  distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of  remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces  of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars  is only about 7  millibars (less than 1% of  Earth’s), but it varies greatly with altitude from almost 9 millibars in the  deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick  enough to support very strong winds and    vast  dust storms that on occasion engulf the entire planet for months. Mars’ thin  atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much  less than what we see on Venus and Earth.

South Polar Cap, Mars

South Polar Cap

Early telescopic observations revealed that Mars has permanent ice caps at  both poles; they’re visible even with a small telescope.  We now know that  they’re composed of water ice and solid carbon dioxide (“dry ice”). The ice caps  exhibit a layered structure with alternating layers of ice with varying  concentrations of dark dust. In the northern summer the carbon dioxide  completely    sublimes, leaving a residual layer  of water ice.  ESA’s Mars  Express has shown that a similar layer of water ice exists below the  southern cap as well. The mechanism responsible for the layering is unknown but  may be due to climatic changes related to long-term changes in the inclination  of Mars’ equator to the plane of its orbit. There may also be water ice hidden  below the surface at lower latitudes. The seasonal changes in the extent of the  polar caps changes the global atmospheric pressure by about 25% (as measured at  the Viking lander sites).

Mars shot by HST

Mars by HST

Recent observations with the Hubble  Space Telescope  have revealed that the conditions during the Viking  missions may not have been typical. Mars’ atmosphere now seems to be both colder  and dryer than measured by the Viking landers  (more  details from STScI).

The Viking landers  performed experiments to determine the existence of life on Mars. The results  were somewhat ambiguous but most scientists now believe that they show no  evidence for life on Mars (there is still some controversy, however). Optimists  point out that only two tiny samples were measured and not from the most  favorable locations. More experiments will be done by future missions to Mars.

A small number of meteorites (the SNC  meteorites) are believed to have originated on Mars.

On 1996 Aug 6, David McKay et al announced what  they thought might be  evidence of ancient Martian microorganisms in the meteorite ALH84001.   Though  there is still some controversy, the majority of the scientific community has  not accepted this conclusion.  If there is or was life on Mars, we still haven’t  found it.

Large, but not global, weak magnetic fields exist in various regions of  Mars. This unexpected finding was made by    Mars  Global Surveyor just days after it entered Mars orbit. They are probably  remnants of an earlier global field that has since disappeared. This may have  important implications for the structure of Mars’ interior and for the past  history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily  visible with the unaided eye.  Mars is a difficult but  rewarding target for an amateur telescope though only for the three or four months each Martian year when it is  closest to Earth. Its apparent size and brightness varies greatly according to  its relative position to the Earth. There are several    Web  sites that show the current position of Mars (and the other planets) in the  sky. More detailed and customized charts can be created with a    planetarium  program.

Mars’ Satellites

Mars has two tiny satellites which orbit very close  to the Martian surface, Deimos and Phobos




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