Mars, the red planet
Mars is the fourth planet from the Sun. It is also the second nearest planet to the Earth and will probably be the first planet visited by humans. It has an orbital period equal to 687 Earth days and a rotational period equal to 24hrs 37mins and 22.6 seconds. Therefore there are 668 Martian days in a Martian Year. Mars has a very eccentric orbit that can vary from 249 million km to 207 million km. As a result, it also experiences seasons.
When it is nearest to Earth - 59 million km away - Mars can be seen in great detail even with small telescopes. The polar ice caps are visible; the southern ice cap can extend down to 50° latitude or become quite small depending on the season. There are many bright areas on the red planet, e.g. 'Hellas', that are in fact deep basins on the planet's surface. There are also dark areas on the planet's surface, e.g. 'Sirtis Major' (in the shape of a large 'V'), that were once thought to be seas, but when the atmospheric pressure was discovered to be too low for liquid water, the dark areas were then thought to be old sea beds filled with vegetation. However all this was disproved after the first fly-by mission by Mariner 4 in 1965. And two NASA rovers, Spirit and Opportunity, have since beamed back images of the Martian surface, which they explored.
The highest known surface point on Mars is a huge volcano known as 'Olympus Mons'. It stretches 24km high above the lava plains around it and it has a base measuring 600km. Mars has an average surface temperature of about -23°C. Its atmospheric content includes 95% carbon dioxide, 3% nitrogen and 1.6% argon. Mars is not as dense or as large as Earth and has an escape velocity of 5km/sec, only enough to sustain a thin, transparent atmosphere. However, some clouds can be seen and from time to time the occasional dust storm can completely cover the Martian surface. The storms occur when wind speed increases to 50-100 meters per second as dust from the surface is lifted up and bounces along the ground, colliding with other dust particles and forcing a cataclysmic reaction that could encompass the entire Martian globe. The technical name for this is 'saltation'.
Mars has two moons, Phobos and Deimos, both discovered in 1877 by Asaph Hall. These two moons are shaped irregularly and are probably asteroids that were caught by Mars' gravitational pull a long time ago. Neither is large enough to become spherical, and both have synchronous rotations enabling them to always keep the same face toward their parent planet.
Phobos orbits at a distance of less than 6000 km from the surface of Mars and, with a maximum diameter of 27 km, is larger than Deimos. Phobos has been falling very slowly toward the planet at a rate of 10 km every century, because of which it will collide with Mars in forty million years. Phobos' surface is covered with craters (the largest is the 10 km-long "Stickney," which was named after Asaph Hall's wife). Phobos has an orbital period of 7 hours and 39 minutes. Deimos is even smaller than Phobos. Its longest diameter is 15 km and it orbits 23400 km from the planet's centre and, unlike Phobos, has a stable orbit.
If you want to find out more about missions to Mars why not check out this article on ESA's project, Mars Express spacecraft.
Author: Astronomy Today Staff