MARS

Mars is the fourth planet from the Sun and the seventh largest in all. Its orbit is 227,940,000 km. from Sun,diameter 6,794 km. and total mass is 6.4219e23 kg.

Mars is sometimes referred to as the Red Planet. It has been known since prehistoric times, but the first visit was from the Mariner spacecraf in 1965. Others followed including Mars 2, the first spacecraft to land on Mars and the two landings by Viking in 1976. It was another 20 years before the spacecraft Mars Pathfinder landed successfully on Mars on July 4, 1997.

One result of Mars' elliptical orbit 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), 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.

Mars' surface is very similar to that of Earth's. Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets. Areas like:

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 Mars' 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 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. Some speculate that the reason for this is due to a very large impact shortly after Mars' accretion.

The most likely scenario, since little is known about Mars' interior, 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).

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. There is; however, new evidence that Mars may have had tectonic activity in its early history.

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. 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 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 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).

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.

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. 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.

On Aug. 6, 1996 David McKay and team announced the first identification of organic compounds in a Martian meteorites. The authors also 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. 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 may have important for the structure of Mars' interior and for the past history of its atmosphere and the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible without aid. Its apparent brightness varies greatly according to its relative position to the Earth.