September/October 1997

Roving Around Mars - Page 2

In some of Pathfinder's first pictures, the nearby hills resemble layer cakes. The layers of rock indicate that Ares Vallis may have undergone several massive floods, each of which deposited thick beds of rock and soil.

Pathfinder's early images also showed that rocks around the lander are not all alike. Some of them, like Yogi, are dark reddish-gray and gently rounded. Yogi and others look like one side has been sandblasted by Martian winds. Others, which are lighter colored with more angular faces, may have been blasted out of a large impact crater near Pathfinder's landing site. And a few wide, flat rocks may consist of hard-packed soil. "We put the rover on top of one of those and spun the wheels to try and abrade it, but we couldn't scratch it," says Peter Smith, a University of Arizona geologist and leader of Pathfinder's camera team.

The rover has done more than pop wheelies, of course. Despite early communication glitches and some snafus by its human operators on Earth, Sojourner became the mechanical star of the show.

Creeping along at a few inches per hour, the microwave-oven-size rover snapped pictures of both the rocks and the Pathfinder lander and tested the Martian soil (some is the consistency of fluffy, dry beach sand, while some is more like kitchen flour, Smith says). "It hasn't been a long distance, but it's done some interesting things," says Smith.

Most interesting of all are its measurements of the chemical composition of several rocks, beginning with Barnacle Bill, which sits near the base of the ramp that Sojourner descended to the surface.

Sojourner placed the Alpha Proton X-Ray Spectrometer against a smooth rock surface and bombarded it with radiation, then measured the energy reflected from the rock. Each element produces a unique radiation "signature," allowing the instrument to measure even small amounts of every element except hydrogen.

While the much larger "Yogi," was about what scientists expected, Sojourner found that Barnacle Bill contains a rich supply of silicon, which is a common ingredient in certain types of Earth rocks but was thought to be rare on Mars. The finding suggests that Barnacle Bill is similar to a type of Earth rocks called andesites, which contain lots of silica in the form of quartz.

But that creates a problem for planetary scientists. On Earth, andesite forms as part of the continual motions of Earth's crust. Where the plates that form the continents bump into each other, the edge of one plate plunges below the other. Rock in this boundary zone is melted and remelted, producing a unique chemistry and structure. Mars has no continental plates, though, so scientists have thought that Mars has long been geologically "dead," unable to melt its rocks and change their structure. If Barnacle Bill proves to be an andesite, scientists may need to revise their theories of Martian geology and evolution.

Pathfinder and Sojourner received plenty of sunlight to charge their batteries, and basked in daytime temperatures as high as 10 degrees during their early "sols" on Mars (a sol is one Martian day, or 24 hours, 37 minutes). But in September and October, summer in the northern hemisphere will give way to autumn. Temperatures will plunge, and giant dust storms stirred up as carbon dioxide frozen in the south polar ice cap vaporizes may dim the sunlight. With less power, Mars Pathfinder and Sojourner will curtail their workloads.

But scientists are hopeful that both will survive well into northern winter, if not beyond. "There are lots more rocks to see -- more chances to see how Mars evolved," says Barlow. "Who knows what else is out there?"

Damond Benningfield is executive editor of StarDate.

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