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Mars is the fourth planet from the Sun and the seventh largest:

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

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 imaginary or some sort of mirage.

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.  The Mars Polar Lander that was supposed to land on Mars in December of 1999 has to date not been heard from and is supposed to have crashed.   NASA is still trying to locate the crash site on new photos from the projected landing area (taken by the Mars Global Surveyor).

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 dayside 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.   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);  It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.   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.

vlpan22.gif (11537 bytes)   yogi.gif (11205 bytes)  The rubble strewn Martian surface as shown here from lander photos indicates that geologic processes such as floods have shaped the planet surface in the not too distant past.  Photos taken from orbit also show channel systems resembling stream channels on Earth, and even evidence of recent groundwater seepage.

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. 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. At some time in the past there was clearly water on the surface There may have been large lakes or even oceans. 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.

Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was probably 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_crater_small.jpg (6568 bytes) Craters are one of the few landforms that Mars shares in common with the other planets and moons of our solar system. The Mars Global Surveyor gave scientists a close-up view of a 1.9 mile (3 km) wide impact crater on the floor of a larger crater in the Nepenthes Mensae region of Mars (photo from 1999). This smaller crater is three times wider than the famous Meteor Crater in northern Arizona (see detail image).   The high resolution image shows many small windblown drifts or dunes in the low areas both within the crater and outside on the surrounding terrain.  Some portions of the crater's walls have outcrops of bare, layered rock. Large boulders have been dislodged from the walls and have tumbled down the slopes to the crater floor. Many of these boulders are bigger than school buses
and automobiles.

Mars has permanent ice caps at both poles composed mostly of 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's not known if a similar layer of water ice exists below the southern cap (left) since its carbon dioxide layer never completely disappears. 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_polygon_small.jpg (14244 bytes) This photo, taken in October of 1999, shows a relatively smooth Martian "plain" covered with polygons like those commonly found at Earth's arctic and antarctic regions. Polygons form when the ground freezes and thaws repeatedly over the course of time. The polygons were photographed at Malea Planum in the far southern regions of Mars. Patches of frost from the retreating south polar ice cap are caught in the cracks making them more visible.  They are a key indicator of ice in the ground near the surface.  Scientists say finding polygons at the Malea Planum indicates that ice is not too deeply buried because only a thin veneer of material appears to have covered the crater at the top of the scene. (see detail image)
mars_layers_small.jpg (11097 bytes) In 1999, the Mars Global Surveyor's camera has sent back new pictures of layered deposits at the planet's
south pole. Both polar regions on the planet are blanketed by thick accumulations of layered material.  This pictures shows one of the clearest and highest-resolution images ever obtained of the area (see detail image). It  covers an area almost a mile (1.5 km) wide and two miles (4.6 km) long. The smallest objects that can be seen are about the sizes of automobiles. Small dark streaks in the upper right of the photo are formed by winds that have blown small patches of sediment across the surface of the layered material.  There are many more layers in these deposits than anyone thought previously. Based on data from the Mariner and Viking projects in the 1970s, the polar layered deposits have long been considered to hold accumulations of dust and ice. The layering may hold clues about climate change on Mars. An earthbound analogy would be tree rings -- periods of drought and rain affect the width of the rings.

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. 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 bee 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's interior and for the past history of its atmosphere and hence for the possibility of ancient life.

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