Mars is the fourth planet from the Sun and the seventh largest, much smaller than the Earth. Despite this small size, it has a surface area that is about the same as the land area on Earth, so there is plenty to explore! Not only this, but Mars has perhaps the most varied and fascinating landforms of any of the rocky planets outside of the Earth. These include the largest volcano in the solar system (Olympus Mons), a massive canyon system called Valles Marineris that would stretch across the entire North American continent, and Hellas Planitia, a huge impact crater that would stretch halfway across North America! Mars is also famous for its startling dichotomy: most of the craters are located in the Southern hemisphere, while the plains and volcanoes are in the Northern hemisphere. Unlike the Earth, Mars appears to lack active plate tectonics, and there is no evidence of mountain building similar to that which we see on our own planet.

Mars follows a very elongated orbit around the Sun, and therefore suffers from a large temperature variation throughout its 687 day long year. A Martian explorer would have to be equipped to cope with temperatures ranging from as little as 133 K (-140 C, -220 F) to almost 293 K (20 C, 70 F), depending on where they are and what time of year it is. The average temperature is about 218 K (-55 C, -67 F). Mars does not have a breathable atmosphere, so any explorer would have to be equipped with breathing apparatus that would last for the full duration of the mission. The Martian atmosphere is composed mostly of carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%), oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is just 7 millibars (less than 1% of Earth's), but varies with altitude, and is thick enough to produce very strong winds and vast dust storms that have been know to smother the entire planet for months.

As with the Earth, ice caps are present at both Martian poles. On Mars, the ice caps are composed mostly of `dry ice' (frozen carbon dioxide), although layers of water ice are also known to be present in the North. It is not known if similar layers exist in the South. Many scientists think that ther may also be water ice below the surface at lower latitudes.

There is very clear evidence that, early in its history, Mars was much more like Earth. Many of the surfaces show signs of water erosion, including large floods and small river systems, so it is fairly obvious that there was water on the surface at some time in the past. This may even have taken the form of large lakes or oceans, but it does not appear to have lasted long, and occurred a very long time ago (probably about 4 billion years ago). This raises the possibility that at one time the conditions necessary for life to originate were present on Mars. The Viking landers performed experiments to try to determine the whether or not life existed on Mars, with somewhat arguable results.

The interior of Mars cannot be observed directly, and is difficult to estimate without carrying out extensive work on the ground. However, scientists predict that the most likely scenario is a dense core surrounded by a molten rocky mantle (probably a little denser than the Earth's) on top of which is a thin crust.

Mars also has two tiny moons, called Phobos and Deimos, which orbit very close to the surface. For further background reading on Mars, see The Nine Planets webpages. This site also has several links to other excellent pages on Mars.

Some questions that might be answered by sending a manned mission to Mars are:

• Is there still active volcanism on Mars?
• How much water is there?
• Is, or was there any life on Mars?
• Are we yet able to send people as far afield as Mars?
• Do we have the technology to set up a semi-permanent base on another world?

Try to think of some more yourself, and think of ways we may be able to answer them…