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Long ago, Mars was alive with action. Space rocks pounded its surface. Water gushed down rivers. Volcanoes blasted ash and lava.

Today Mars seems quieter. Still, winds whip up giant sandstorms. Clouds float across the planet's pink sky. Boulders tumble down slopes. The polar ice caps melt and freeze with the changing seasons.

Not much else seems to happen on Mars. At least that's what astronomers, scientists who study space, used to think. Today they know that the face of Mars is changing constantly.

Mars is the fourth planet from the sun. Much of its surface is covered by reddish rocks and sand. As a result, people often call Mars the red planet. Windblown sand gives its sky a pink color.

Even though Mars is smaller than Earth, it has the largest known volcano and valley. Olympus Mons is about the size of Arizona and nearly three times taller than Mauna Kea, Earth's largest volcano.

Valles Marineris is the longest and deepest canyon. It is as large as the United States from coast to coast. It is so large that the Grand Canyon looks like a small ditch in comparison.

Then there are the craters. Thousands of impact craters scar the surface of Mars. About 43,000 of them are at least five kilometers (three miles) wide. Most of these craters were gouged out shortly after the planet formed.

Space rocks rained onto the planet, pounding its surface. Many of these rocks were asteroids. They are large rocks that orbit our sun between Mars and Jupiter. Others were meteoroids. They are smaller rocks flying through space. Still others were comets, chunks of ice and rock.

The smallest space rocks burned up in Mars’s atmosphere. They never made it to the ground. Larger ones plowed into the planet.

Speeding space rocks smashed into the surface, tossing stone and sand into the sky. Much of this material fell around the crater made by the collision.

The largest crater is the Hellas Impact Basin. It is about half the size of the United States. Had all the material tossed into the sky by the smashup fallen evenly across Mars, it would have made a layer much higher than the Washington Monument.

Astronomers call another crater the Happy Face Crater. It would take you a little under three hours to drive across this crater at a speed of 88 kilometers (55 miles) per hour. Features inside the crater make it look like it has two eyes and a smiling mouth.

Most of the impact craters on Mars were made long ago. Twenty craters, however, formed in the past seven years. They are very small. Between one and five school buses could stretch across each one.

They show that space rocks still pound Mars. The largest of these rocks was about the size of a car. It slammed into the planet at five kilometers (three miles) per second.

Gullies run down the sides of some craters and valleys. About 10,000 of these gullies have been found so far.

Two of the gullies look different from all the others. New material seems to line them. This material appears white.

Some astronomers say that underground water formed the gullies. The water flooded onto the surface, carving them. Small white particles fell out of the water, lining each gully.

Perhaps one of the biggest changes happening on the red planet is global warming. It is a worldwide rise in a planet’s temperature.

Astronomers say Mars's temperature has risen by 0.6 degree Celsius (1 degree Fahrenheit) in the past 30 years.

Wicked winds may be causing the global warm-up. Winds whip up sand on parts of the planet’s surface. That uncovers darker rocks that usually lie under the sand.

The darker rocks take in more heat from the sun than sand does. The warm rocks then heat air, which causes more winds. This, in turn, causes more sandstorms.

The worldwide warming is causing Mars's south pole to melt. Frozen carbon dioxide covers the south pole. It is a gas used in many fire extinguishers on Earth. No one knows whether the warming will cause all the ice to melt.

One thing is known: The face of Mars is changing. It changed in the past and is changing today. What will it look like in the future?

Article by Fran Downey. Top-of-page photo by Kees Veenenbos. "The Changing Face of Mars" appears on pages 18-23 of the October 2007 issue.