Size: 6794 km in diameter
Distance from the sun: 228 million km
Atmosphere: 95% carbon dioxide, 3% nitrogen, 1.6% argon
Rotation time: 24.6 hours
Revolution time: 687 days
Temperature range: -185 C to -35 C
Natural satellites: 2
Since ancient times, Mars has been an object of great interest to astronomers. Unlike Venus, Mars generally has no obscuring layer of clouds. It passes relatively close to Earth in its orbit, so it is and ideal subject for telescopic observation. Over the centuries, observers have noted various unusual phenomena on the planet's surface , including a seasonal growing and shrinking of the polar caps and a wave of darkening that seems to sweep from pole to equator during each hemisphere's spring.
Mars is about half the size of Earth. Its atmosphere is composed mostly of carbon dioxide and is very thin, exerting about 1% of the surface pressure that Earth's atmosphere exerts. The temperature at the planet's surface varies widely during the course of the Martian day. These temperatures range from about 190 k just before dawn to 240 k in the afternoon.
Mars, like Earth, is tilted on its rotational axis. Consequently, it is subject to seasonal variations in climate. Liquid water cannot exist on Mars' surface because of low temperature and pressure. Water exists only as ice deposited in the poles and maybe trapped below the surface. It may also exist as vapour in the atmosphere. However, there is evidence that, in the past, water in the form of a liquid may have existed on the planet's suface.
Mars experienced a period of volcanic activity that peaked a few billion years ago. The planet has the largest volcano in the Solar System, Olympus Mons, which has a height of 17 miles. This volcano is three times larger than mount everest, and covers an area the size of the state of Arizona. Other regions on Mars include smooth plains, densely cratered areas, and rolling hills formed by various combinations of fracturing, volcanism and atmospheric-related erosion and disposition.
Once each Martian year, at the beginning of the southern hemisphere's spring, Mars is engulfed by global dust storms. Local temperature differences generate strong winds that lift the dust from the surface to form thick clouds. The clouds block the sunlight, gradually causing surface temperatures to even out, and the winds to subside. Some of the atmospheric dust is deposited in a snowfall of dust in the winter hemisphere. The snow forms a winter cap of carbon dioxide ice, water ice and dust. During the spring, most of the cap evaporates, but some remains as a permanent deposit. As a result, a geologic record of these storms and their variations over the planet's lifetime must be preserved in the permanent layers of dust and ice at the Martian poles.
Mars has two small satellites, Phobos and Deimos, that may be captured asteroids. Both are so small that they do not have enough interal gravity to draw them into spheres, so they are pretty much shaped like potatoes. Phobos is about 17 miles long and Deimos is about 9.5 miles long. Both have rotational periods equal to their orbital periods, so that the same side is always facing Mars, much like our moon.
Phobos is very close to Mars, and it's orbit is gradually decaying, so that it is drawing closer to Mars with each orbit. Astronomers estimate that Phobos may fall to the Martian surface sometime in the next 100 million years. Deimos is in a more distant orbit and is gradually moving away from Mars.
