Although Mars is smaller and colder than Earth, it is still quite similar to our planet. It has a thin atmosphere and polar ice caps, and dry riverbeds crisscross its surface. And frozen or even liquid water may exist beneath the red Martian soil -- perhaps providing a home for living organisms. But it is not the planet described in science-fiction books and movies. There are no signs of civilizations on its surface -- past or present.

In August 1996, though, scientists announced the discovery of signs of possible ancient microscopic life in a meteorite from Mars. The meteorite was blasted into space when a huge boulder slammed into Mars. The meteorite eventually landed in Antarctica. Trace elements inside the meteorite prove that it came from Mars.

Evidence in the rock suggests that microscopic organisms a thousand times smaller than a human hair may have lived on Mars 3.6 billion years ago, when the planet was much warmer and wetter than it is today. The finding is disputed by many scientists, and additional tests are underway to try to confirm or refute the report.

Early in the next century, a spacecraft may scoop up samples of Martian rock and soil and return them to Earth. Scientists will select a Martian landing site that would have been likely to harbor life in the past.

Mars looks red because iron in its surface soil long ago reacted with the tiny amount of oxygen left on Mars, making rust. Its surface is marked by ancient volcanoes and a "grand canyon" that's as long as the United States is wide.

The largest volcano, Olympus Mons, may also be the largest volcano in the solar system. It towers 17 miles (27 km) above the surrounding desert. The base of Olympus Mons would cover the state of Missouri.

The heat from the Martian core, which powered Mars' volcanoes, is mostly gone now, and most of its early atmosphere has escaped into space or is frozen in the surface layer. A few thin clouds still punctuate the Martian sky, and scientists think that some pools of frozen or liquid water may be hidden underground. Although it is unlikely, the water could harbor simple forms of life similar to those found in Antarctic ice on Earth.

It is possible that Mars once had an atmosphere somewhat like Earth's, and almost certain that water once covered part of the Martian surface, in rivers, lakes, and perhaps small seas. Although no water flows over the Martian surface today, frost covers the north and south poles. These polar ice caps are made mostly of frozen carbon dioxide (dry ice), although the north polar cap contains much more water ice than the southern cap.

Mars is more like Earth than any other planet in our solar system, but it is still very different. You would have to wear a spacesuit to provide air and to protect you from the Sun's rays, because the planet's thin atmosphere does not block harmful solar radiation. Your spacesuit would also protect you from the bitter cold; temperatures on Mars rarely climb above freezing, and they can plummet to minus 220 degrees Fahrenheit (?161 C). You would need to bring water and oxygen with you, although if you brought the proper equipment you might be able to get these from the air or the ground.

Two small moons, Phobos and Deimos, orbit Mars. The larger moon, Phobos, passes across the Martian sky from west to east twice a day. It would look about half as big as the full Moon does on Earth. It's also so close to Mars that you couldn't see it from the Martian poles.

Deimos is farther away and moves slowly from east to west. Deimos would look like a small dot of light in the sky. Phobos is slowly moving closer to Mars. In another 50 to 100 million years, it will crash into Mars.

Phobos is small, dark, and airless. And it's one of the driest bodies in the solar system.

That doesn't mean that Phobos is dull, though. Quite the contrary. Phobos survived a powerful impact that may have fractured its interior. The impact gouged a large crater, and may be responsible for a series of long, deep grooves that appear to radiate away from the crater.

If the meteor that created the crater had been a bit larger, it might have destroyed Phobos. Instead, many scientists think the impact cracked the moon's interior. The cracks could contain water ice. If so, Phobos could serve as a refueling station for manned Mars-exploration missions. But observations by several spacecraft indicate that there's less water at the surface of Phobos than in almost any other body in the solar system.

Deimos is covered by a powdery dust that could be several hundred feet deep. The tips of giant boulders peek above the dust like icebergs floating in a dark sea. The dust formed as the result of billions of years of meteorite impacts, which pulverized the tiny moon's surface.

Many scientists believe that Phobos and Deimos are asteroids that wandered close to Mars billions of years ago. The Martian atmosphere may have been thick enough to act as a brake, slowing the small bodies enough to trap them in orbit.

But other scientists say that the moons formed at the same time as Mars, or that they're the remnants of a larger moon that was shattered by a collision with a large meteor.