Lessons about Mars taught by Pathfinder Scientists, geologists get new perspective from space mission


PASADENA, Calif. -- In the space of a week, the Pathfinder mission to Mars has prompted a profound shift in how the planet is perceived by geologists and scientists.

Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz., said Pathfinder's achievements have led to "a fundamental change in the way we think about Mars, and this will have a dramatic effect" on future Mars missions.

"Today Mars is a dry, cold, windy desert. In many areas dunes have buried Mars' geological history, masking to a large degree the real subsurface geology," Soderblom explained. But at Ares Vallis, where Pathfinder landed, "it serendipitously happens to be a zone where the dunes are very sparse" and where the surface is scoured by persistent winds.

In a sense, Soderblom said, the wind "is exposing the bones of Mars' complex geologic history. It's as if we were at an archaeological dig and the wind is our crew." It's a place, he said, "where the Martian sands of time are being drawn back, and Pathfinder is right at the edge of the curtain."

The lessons learned from Pathfinder include:

Mars is far more geologically interesting than previously imagined. Measurements of a rock that scientists have dubbed "Barnacle Bill" suggest that Mars has a longer, more intense history of heating than expected. Before the landing, geologists thought Mars' crust was relatively simple, made up mostly of ordinary basaltic lava rock. That is clearly not true.

At least one flood of massive proportions -- a surging tide hundreds of miles wide and hundreds of feet deep -- roared across the cold and barren Martian landscape, perhaps at 80 mph. Scientists suspected water once had eroded Mars' surface, but not in floods of such huge size.

The Martian atmosphere, especially at very high altitude, is thinner than researchers anticipated. So Pathfinder came in a little faster, and flew a little farther, than expected.

Martian soil seems to be the same everywhere. Two previous Viking landing sites, and now the Pathfinder site at Ares Vallis, all seem to have the same soil. This suggests there's a global mechanism that distributes soil over the planet. So perhaps the soil is just compacted dust.

"What Pathfinder has learned so far is amazing," said planetary scientist Maria Zuber, at the Massachusetts Institute of Technology. The relatively simple spacecraft "wasn't designed to do a great deal of science. The weight didn't allow for a lot of instruments," she said. "But it's making some of the most basic observations you can make."

Some scientists think the important early lesson came from Barnacle Bill. It's a football-size rock sitting very close to where Pathfinder landed and was the first one analyzed for its chemistry by Sojourner's little APX (alpha-proton-X-ray spectrometer) instrument. The readings suggest Barnacle Bill is a form of lava called andesite, typically loaded with quartz. So it seems more like an Earth rock than a Mars rock.

Scientists long have known that to produce quartz, ordinary basaltic lava must be melted and remelted over and over again, gradually refining the minerals to leave silicon dioxide, silica.

"A way to make a rock so rich in silica is melting it and decanting off the liquid that is rich in silica," said geologist Hap McSween, from the University of Tennessee. "Each time the liquid gets richer in silica -- sort of like skimming the cream from a bottle of milk."

If that scenario is true for Mars, "it implies a rather complex, protracted history of heating," McSween said. "And some people think this indicates Mars' interior may be wetter, because another way to make andesite is to have water present." And if that idea about water is correct, he added, "it would do all kinds of strange chemistry. And where that happened would be a good place to look for life" on Mars.

Zuber agreed that if Sojourner indeed has discovered andesite, it "means that the part of Mars where that rock originated was hot for a long time. So here's a solid piece of evidence that Mars stayed warm on the inside." That idea is exciting, she said, because "Twenty years ago we all thought Mars had started cold" and had warmed only enough to form volcanoes that pump out basalt.

Pub Date: 7/14/97

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