1. INTRODUCTION TO MARS

Schiaparelli Hemisphere of Mars

Mars, the fourth planet from the Sun, is the closest planet whose surface we can see from Earth. It's only about half the size of the Earth and has one tenth as much mass, but its environment is the most Earthlike of any other planet in the solar system. Mars has many recognizable geological features -- volcanoes, rift valleys, landslide scars, desert dune fields, and polar ice caps -- but it has no oceans, lakes, or liquid surface water of any kind. It also has features that are unlike anything seen on Earth, or elsewhere in the solar system. Mars has nearly the same surface area as the dry land area of the Earth, so the exploration of the planet should keep us busy for generations.

Mars has very little atmosphere compared to the Earth. The pressure at the surface is less than 1% of that on Earth, and this results in major differences between the environments of the two planets. Even though it is much colder on Mars, at such a low pressure any liquid water on the surface would instantly boil away and evaporate into the air. But oddly enough, there's lots of water on Mars. It's frozen near the surface in the polar ice caps and in permafrost (frozen soil), and it's in liquid form as ground water deep below the surface, where the pressures and temperatures are higher.

The surface of Mars is a cold, dry desert. Without rain and rivers, the main agent of erosion is wind. The planet is locked in a global ice age. The place on Earth that most resembles Mars would be a dry valley in Antarctica.

We suspect, however, that long ago the climate on Mars was much more Earthlike. The Mariner 9 and Viking spacecraft in the 1970s discovered ancient networks of river valleys and flood channels on Mars. There may have been a time, probably a few billion years ago, when Mars had a much denser atmosphere, which permitted the existence of liquid surface water. A denser atmosphere, containing the greenhouse gases carbon dioxide and water vapor, would also have made the surface of Mars warmer.

Liquid water is an absolute requirement for life as we know it. Long ago when liquid water flowed on the Martian surface, life also may have flourished there. In August 1996 scientists announced the discovery of evidence in a meteorite from Mars suggesting the presence of past microbial life on that planet. The meteorite, called ALH 84001, was found in the Allan Hills of Antarctica in 1984. Most scientists agree that the rock came from Mars, but there's a major scientific controversy about whether or not it contains evidence of life.

If Mars was warmer and wetter in the distant past, perhaps with life on its surface, what happened to bring about the global ice age that now grips the planet? This may be the most important unsolved mystery of Mars. Why does it matter? Why should we explore Mars? For a thoughtful answer to these questions, see the article by Matthew Golombek, the Project Scientist for the Mars Pathfinder mission.