Mars was our last hope for finding life on another planet within our solar system. In 1901, the Guzman prize was offered in Paris to the first person to contact beings from another world, but Mars was specifically excluded as being too easy. The reason Mars was such a great possibility was because it is the least hostile of all the planets, and it looks like there may once have been water there. However, the Viking landers performed experiments and didn't find any evidence of life on Mars. Some people point out that the results were not very exact and that only two samples were taken from locations that definitely could have been better. So, more experiments will have to be done by future missions to determine this for sure.
Mars is 6,787 km across-- a little bigger than half the size of Earth (right). Although it is so much smaller, the surface area is about the same as the amount of land on Earth. Mars is also only 1/10th the mass of earth, which causes it to have a very thin atmosphere. The inclination of the axis of Mars is 23°59' right now (left), but it can range between 35° and 14°, which definitely has its affect on the climate. Mars orbits the Sun in a very elliptical path. Because of this, the temperature varies about 30° C at the subsolar point between aphelion and perihelion. This greatly affects the climate of Mars. The average temperature on Mars is about -55° C, but temperatures can range from -133° C at the winter pole to 27° C on a summer day.
From what we've gathered so far, it looks like Mars doesn't have plate tectonics because we don't see any evidence of the surface moving horizontally, at least not recently. There is also no evidence of current volcanic activity. However, the Mars Global Surveyor just recently provided some evidence that suggests that Mars had tectonic activity when it was first forming. It seems that Mars used to be a lot like Earth. As already mentioned, there the evidence that suggests the possibility of ancient rivers and plate tectonics. Also like Earth, most of Mars' carbon dioxide was taken by carbonate rocks. However, without plate tectonics Mars can't recycle the carbon dioxide and without a global magnetic field it can't protect its atmosphere. So, Mars can't produce much of a greenhouse effect needed to keep its surface consistently warm.
We have no direct knowledge about the interior of Mars but based on what we know about its surface, it probably has a thin crust, a mantle of molten rock and a dense core of sulfur with some iron. Mars may have had a global magnetic field at one time but if so, it has degenerated into regional and weak magnetic fields.
Mars has been known since prehistoric times, probably because it is so conspicuous. The planet has always been a common place for science fiction writers to home aliens. Also, many things that can be seen from Earth have been interpreted to mean that there is life there. For example, in 1877, G. V. Schiaparelli (1835-1910) of Milan produced maps showing a network of lines that were thought to be channels of some type. Following in the same way of thinking, Percival Lowell (1855-1916), who later discovered Pluto, interpreted the lines as canals constructed by intelligent Martians for irrigation.
Much of current data about Mars comes not from Martians visiting us (as predicted in science fiction dramas) but our visiting Mars. In 1965, Mariner 4 became the first spacecraft to visit Mars. Following this came the Mars 2, which was the first to land on Mars, the two Viking landers (right) of 1976, and the Mars Pathfinder in 1997.
Winter on Mars is cold. So cold (-140° C; -220° F) that the carbon dioxide freezes out and is deposited on the southern polar cap. Still, the atmosphere of Mars is thick enough to support strong winds and dust storms. The atmospheric density does show seasonal variations but the ground density is always below ten millibars.
The Hubble Telescope has recently shown that the conditions during the Viking missions may not have been typical. So, Mars' atmosphere may be drier and even colder than we had thought when basing our calculations on the Viking landers.
These planitia or craters are numerous on Mars (left), particularly in the southern hemisphere. The northern hemisphere is mostly composed of wide plains that are geologically younger. The southern hemisphere has numerous impact craters that are older and rougher. Along the boundary of the two hemispheres, there is an abrupt elevation change of several kilometers. It's almost like a ball made of two different sizes, leaving a ridge where they are connected. There are many ridges, outcroppings and volcanoes, such as Pavonis Mons (at first thought to be a lake).
It looks like there used to be water on Mars. The erosion on the surface looks like it was caused by water running through valleys and river systems (right). There might even have been large lakes or oceans. However, it looks like this happened briefly and very long ago (say, 4 billion years ago, give or take a few hundred million).
There are permanent ice caps at both poles of Mars. But unlike having two Anartica, the ice caps of Mars was made of dry ice (solid carbon dioxide). As the surface oozes carbon dioxide, its added as another layer of dry ice at either pole. In summer (at the North cap), the carbon dioxide disappears, leaving behind a film of watery ice. In summer at the South cap (see picture at left), the same process may occur but this ice cap doesn't completely disappear so it's difficult to know for sure what happens there.
Mars may actually be a source of at least a few meteorites. In 1996, David McKay announced he'd found a Martian fragment on earth and that it contained organic compounds in it. Part of the great "Is There Life On Mars" debate centers around McKay's claims. There is no consensus about the presence of microorganisms on Mars, so there is sure to be more discussion on the matter.
Thus, the main crater of Phobos is called Stickney (the maiden name of Halls' wife)..
The outer moon, Deimos, was discovered on August 11, 1877), is elliptical in shape (about 9 miles long) and circles Mars about once a day. Phobos is longer (17 miles), covered with craters (the largest was called Stickney, the maiden name of Hall's wife), and circles the planet about 3 times a day. Phobos is slowing down and is going to go so slow that it falls out of its orbit and crashes back to Mars. And this is going to happen soon in terms of the universe (40 million years or so).