New Mars data may dispel theories

As the Mars Global Surveyor spacecraft slips closer and closer to its assigned orbit, scientists find their plates are already overflowing with exciting data, stuff they have barely been able to taste, much less digest.

Even before the big spacecraft settles into its circular path around the red planet by March, the new photos it is sending home are forcing researchers to rethink some older ideas. For example:

Although Mars was known to be a very windy place, the new evidence suggests it's windier yet, so the terrain is probably more rapidly remodeled than expected. The new photos show that a complex succession of events - erosion, followed by deposition and re-erosion - accounts for some of the surface sculpturing.

Layered terrain seen near the red planet's poles consists of many more layers, and far thinner, than expected. This indicates that episodes of climatic change occur more often than anticipated. Instead of episodes lasting millions of years, they occur at intervals at least 10 times shorter.

A few huge valleys seem to have smaller channels eroded into their floors. This is important because such features are similar to valleys seen on Earth. Thus similar processes may be involved in creating them.

For the first time, evidence shows that massive flows of fluid lava solidified into huge flat plates, which then cracked into smaller chunks and floated around on still-molten lava below.

The new, clearer photos of Mars' Elysium Basin show that the hardened lava stretches for hundreds of miles across the northern lowlands. What's visible now are large dark plates separated by intervening bright areas.

"Some scientists thought they could somehow be volcanic, while others thought they might be related to differences in the way the wind eroded a dried lakebed," said Alfred McEwen, a member of the science team from the University of Arizona.

"With these new images," he added, "it is now quite easy to understand the older, lower-resolution Viking images" that were taken in 1976 by two earlier American missions.

The scientists noted that other images show similar plate-like terrain in a nearby basin, Marte Vallis, which implies that some of the hot lava rolling over Elysium Basin spilled into the other valley and rolled on for thousands of miles to the northeast.

Because very few meteorite impact craters are visible on the lava plates, the scientists reason that the lava flows occurred late in the planet's life, after the major volcanoes such as Olympus Mons had finished erupting.

"The sparse occurrence of impact craters on these plate-like lava surfaces suggests that the eruptions happened relatively recently in Mars' history," McEwen said. "These eruptions could be much younger than the youngest of the large Martian volcanoes, but they would still have occurred many, many millions of years ago. So these images should not be treated as evidence that Mars is volcanically active today."

What is active today, according to the latest photos, are the wind-blown sand dunes, which are certainly migrating across the Martian surface.

Obvious changes in the dunes have occurred in the two decades between the Viking missions and the arrival of the new spacecraft, Global Surveyor.

Because of the new data, "it's becoming clear that Mars is a much more complex planet than seemed to be indicated by the Viking coverage," said planetary scientist Joseph Veverka, at Cornell University.

"One example is the layered deposits in the polar regions," he said. "Based on the earlier data, there were estimates for how thin these layers are. The number was tens of meters thick."

But Veverka said that according to Mike Malin, a specialist in wind erosion processes, "there is layering on a scale of meters, which indicates the cyclicity of deposition of the layers occurs on much finer scale. We thought it was millions of years, but now it's at least 10 times shorter," Veverka said.

That indicates that wind events that transport dust and sand change more often, are more cyclical, than the researchers had suspected.

12-03-98