Mars, in astronomy, 4th planet from the sun, and the seventh largest. With an orbit next in order beyond that of the earth.

 	orbit:227,940,000 km (1.52 AU) from Sun
        diameter: 6,794 km
        mass: 6.4219e23 kg

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth's mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide, with some nitrogen and argon. Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80°F (27°C) at noon to about −100°F (−73°C) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.

It appears that there may be liquid water on Mars after all! This is exciting news. But we shall see if it holds up to the scrutiny of the scientific community over the next weeks and months. I'm sure that the authors of the paper have been exceedingly careful. More evidence is needed.

   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. 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 (left). Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4 (right).

   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.

   Except for Earth, Mars has 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 (right).
       - 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.

   The southern hemisphere of Mars is predominantly ancient cratered highlands (left) 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. But there is new evidence from Mars Global Surveyor that Mars may have had tectonic activity in its early history, making comparisons to Earth all the more interesting!

   There is very clear evidence of erosion in many places on Mars including large floods and small river systems (right). 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. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (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.

     Mars has permanent ice caps at both poles 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. It seems likely that a similar layer of water ice exists below the southern cap (left) 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).

   Recent observations with the Hubble Space Telescope (right) 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 the first identification of organic compounds in a Martian meteorite. The authors further suggest that these compounds, in conjunction with a number of other mineralogical features observed in the rock, may be evidence of ancient Martian microorganisms. (left)
    Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact of extraterrestrial life. There have also been several contradictory studies published since the McKay paper. Remember, "extraordinary claims require extraordinary evidence." Much work remains to be done before we can be confident of this most extraordinary claim.

   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. Its apparent 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 such as Starry Night.

Mars' Satellites

          Distance Radius  Mass
Satellite (000 km)  (km)   (kg)   Discoverer Date
--------- -------- ------ ------- ---------- ----
Phobos        9      11   1.08e16   Hall     1877
Deimos       23       6   1.80e15   Hall     1877

More about Mars, Deimos, and Phobos

• more Mars images
• Mars Global Surveyor high resolution mosaic and map
• first images from Mars Global Surveyor
• selected images from Pathfinder
• Life on Mars?
  • info and pictures from NASA GFSC (mirrored at JSC)
  • prerelease of "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001" by David S. McKay et al
  • from LANL
  • Life on Mars?, an index of breaking news
  • more on the meteorites page
  • Life on Mars!, another opinion on the Viking results by Dr. Gilbert V. Levin
  • On the Question of the Mars Meteorite from LPI
  • bacterial remains in the Nakhla meteorite?
• SNC Meteorites
• Mars Meteorites from Ron Baalke at JPL (lots of images!)
• from LANL
• from JPL
• from RPIF
• from StarDate
• from RGO
• from NASA Spacelink
• Welcome to Mars from the American Museum of Natural History
• The Planet Mars: A History of Observation and Discovery, by William Sheehan (a complete online book!)
• The Geological History of Mars
• The Surface of Mars from LANL
• MGS images of layered terrain 
• Martian Volcanoes from LANL
• Martian Clouds from LANL
• Comparative Terrestrial Planet Thermospheres
• from NSSDC
• a Martian dust storm seen by HST; another from MGS; 2001 global storm
• Mars Atlas and Viking Orbiter image-finder (access high-res images of the entire surface of Mars!)
• Mars Explorer, allows you to get an image map of any area on Mars
• The Daily Martian Weather Report from Mars Global Surveyor Radio Science Team at Stanford
• The original "MARS" by Percival Lowell, 1895.
• Mars exploration:
  • A Crewed Mission to Mars, a case study by NASA
  • The Mars Society
  • Mars Pathfinder, new images from the surface!
  • Planetary Missions and Materials Exploration Server
  • Center for Mars Exploration
  • The Martian Chronicle, The Electronic Newsletter for Mars Exploration at JPL
  • The Rational for Exploring Mars by Dr. Michael Duke of NASA
  • An Exobiological Strategy for Mars Exploration
  • Mars Mission Launch Sequence, a catalog of all Mars missions
  • Mars Watch, Linking Amateur and Professional Mars Observing Communities for Observational Support of the Mars Pathfinder Mission
  • Exploring Mars, many resources from LPI
  • Rocky 7, a prototype Mars rover
  
• "The Face" on Mars:
  • April 2001 images from MGS
  • fact sheet from NASA
  • detailed analysis from MSSS; (another copy of it at LANL)
  • images of the "Happy Face" from Viking and a higher res image from MGS
  • raw MGS image, which to no one's surprise, doesn't look like a face at all
• Mars in the Arts:
  • Mars in the Mind of Earth; links to Mars related fiction
  • Mars Bibliography, science fiction about Mars
  • Edgar Rice Burroughs' A Princess of Mars
  • Edgar Rice Burroughs' The Gods of Mars
  • Edgar Rice Burroughs' The Warlord of Mars
  • Edgar Rice Burroughs' Thuvia, Maid of Mars
  • H.G. Wells' The War of the Worlds
  • Study guide to Ray Bradbury's The Martian Chronicles
  • C. K. Anderson's A Step Beyond
  • more references
  • Mars in Popular Culture
• Mars FAQ for amateur astronomers
•  

Open Issues

• Why are the northern and southern hemispheres of Mars so different? Why are the northern and southern polar caps different?
• Is there still active volcanism on Mars?
• What exactly caused the erosion patterns that look so much like stream beds on Earth?
• How much subterranean ("sub-martian"?) water is there? (see links at the top of this page)
• Mars remains at the top of the list of possible life-bearing planets. The Viking probes found little evidence of life on Mars. But they sampled only two isolated locations. Is there life elsewhere or was there life at some time in the past on Mars? The recent meteoric evidence needs to be confirmed. Ultimately, a sample return mission will be necessary.
• The future of Mars exploration is more hopeful than for the other planets. NASA's Mars Global Surveyor an orbiter which includes most of the science instruments from the ill-fated Mars Observer is now in orbit around Mars. Mars Pathfinder, which includes a lander and mini-rover landed successfully on Mars on 4 July 1997. Several more robotic missions are planned by NASA and others. But no one seems willing to put any money toward a manned expedition.