MARS: ROCKS FROM ANOTHER WORLD

PIECES OF MARS THAT FELL TO EARTH AS METEORITES GAVE US OUR FIRST SAMPLES OF THE SURFACE OF ANOTHER PLANET.

THE MARTIAN LANDSCAPE LOOKS STRIKINGLY SIMILAR TO THE DESERTS OF EARTH. But billions of years ago, water flowed in abundance on Mars, carving huge canyons bigger than any on Earth, and immense volcanoes have gushed forth mountains of lava since that time.

Today, dust storms swirl over a barren surface scarred by impact craters-one or more of which may mark the site of impacts that dislodged the meteorites displayed here. But large amounts of water still lie frozen just beneath the Martian surface, raising hopes that some form of life might have arisen on Mars.

In 1996, one Martian meteorite caused a stir when some thought it contained evidence of ancient microbes. Most experts remain unconvinced, but all Martian meteorites do show the effects of a watery climate. The presence of water makes Mars a great place to start the search for life beyond Earth.

HOW DO WE KNOW?

TRACING METEORITES TO MARS

Most meteorites are pieces of asteroids, which solidified into cold, hard rock some 4.4 billion years ago. So experts took notice when volcanic meteorites were found that had crystallized within the past 1.3 billion years. Only a large planet could have remained volcanically active up to such recent times. The meteorites also showed evidence of an atmosphere, liquid water and chemical and geological processes that could only take place on a planet. Evidence of water pointed specifically to Mars.

Conclusive evidence that these meteorites are from Mars came in the form of gases trapped inside the rocks. These gases were a perfect match for the Martian atmosphere, as measured by NASA's Viking probes that landed on Mars in the 1970s. Some of the Martian atmosphere became trapped in the rock when it was shocked by an impact-probably the impact that launched the rocks on their way toward Earth.

The Martian atmosphere, though much thinner than Earth's atmosphere, is still thick enough to create weather that changes with the seasons and wind storms that kick up vast clouds of dust. The north and south poles of Mars are covered by ice caps that grow in the Martian winter and shrink in the summer.

THE JOURNEY TO EARTH

How did these rocks from Mars end up on Earth? Only one thing could send a piece of rock flying out fast enough to escape Mars's gravity: an asteroid or comet slamming into Mars. Mars's gravity is about one-third as strong as Earth's, and its atmosphere is about 125 times thinner, so it is easier to eject a rock from Mars than from Earth. After orbiting the Sun for more than a million years, some of the ejected pieces eventually struck Earth as meteorites.

Any impact powerful enough to launch a Martian rock into space would leave a crater several kilometers in diameter. These volcanoes in the Tharsis region show craters up to 25 kilometers (15 miles) wide—meaning they could be the source of the meteorites in this room.

A KILLER METEORITE

At 9:00 A.M. on June 28, 1911, a series of loud bangs was heard over a village in Egypt. About 40 kilograms (88 pounds) of rock from the Nakhla meteorite rained from the sky, in about 40 separate pieces. At the time, the most unusual aspect of this meteorite was that one piece struck and killed a dog. It wasn't until seven decades later that anyone realized that Nakhla came from Mars.

When more meteorites similar to Nakhla were found, they were called nakhlites. Two related groups, the shergottites and chassignites, were similar enough that scientists concluded they came from the same parent body as the nakhlites. Together they became known as the SNCs (pronounced "snicks"), a name that now refers to all meteorites from Mars.

A VOLCANIC PLANET

The outer layer of Mars is a thick crust of volcanic rock. In the heavily cratered southern highlands and northern basaltic lowlands, volcanic activity stopped over three billion years ago. But two areas near the Martian equator, the Tharsis and Elyssium regions, were active until much more recently. In Tharsis, huge volcanoes, bigger than any on Earth, spewed out lava that piled up into a layer of basaltic rock about 120 kilometers (75 miles) deep.

Most Martian meteorites probably come from this highly volcanic region of Mars. Shergotty is a piece of lava that flowed onto the Martian surface as recently as 180 million years ago. The Governador Valadares meteorite is lava that was trapped underground and slowly crystallized about 1.3 billion years ago. This lava never reached the Martian surface—until it was excavated by the impact of a comet or asteroid.