But when Mars Global Surveyor began mapping the Red Planet in sharp detail early in 1999, it disclosed startling evidence that water has shaped martian landforms within the past 10 million years.

The discovery challenges the prevailing view that Mars' surface has remained extremely cold and dry -- much as it is today -- for the past 3.9 billion years.

It confirms the idea that internal heat periodically triggers short-term warmer and wetter conditions -- conditions conducive to life -- in the global martian hydrological cycle, University of Arizona Regents' Professor Victor R. Baker says in a review article, "Water and the martian landscape," published by Nature July 12. Baker is head of the UA department of hydrology and water resources.

Mars is cold, dry and quiet for long periods of the hydrological cycle, periods spanning hundreds of millions of years, Baker says. It becomes actively warmer and wetter during brief episodes that last perhaps thousands to tens of thousands of years.

He summarizes evidence of permafrost and ground ice, surface water and glacial ice seen first in early 1970s images from Mariner 9 flybys and in later 1970s images from the Viking missions. New results from Mars Orbital Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) are not only consistent with the view that water shaped the geology of Mars -- they say it happened very recently.

New spacecraft images have revealed:

• Megafloods that have possibly triggered climate change within the past 10 million years.

• Extensive, uncratered (young) areas of the northern plains and southern highlands covered by contraction-cracked, polygonal terrain that closely mimics terrestrial permafrost areas.

• Lakes that held water for a thousand to ten thousand years, when climate must have been drastically different than it is now.

• Gullies that have drained water and debris on Mars' surface within the past several million years.

• A whole assembly of features that is evidence for very recent glaciers -- crevasse-like fractures, moraines drained by converging tributaries, debris aprons deposited within the past several million years.

"We don't know the answer to that yet -- that's very speculative, " Baker said. "But if it's true, it would have major implications for sending people to Mars, because it may mean that water is more available than otherwise thought."

UA planetary scientist William V. Boynton's Gamma Ray Spectrometer (GRS) heading to Mars on the Mars Odyssey spacecraft will begin its 917 Earth-day mapping mission in December 2001 or January 2002. The GRS will map the amounts of all the elements over the entire surface of Mars. Hydrogen would signal the presence of water or ice buried as deep as a meter in Mars' dust.

Other high-resolution spectrometers, cameras and robotic devices now in the planning stages also will search for martian water, Baker said. When scientists have learned as much as possible from these remote sensing instruments, one idea is to send a number of small, simple and inexpensive surface penetrators to be dropped at several places on the planet to test for the presence of water.

For now, however, new images from the Mars Orbital Camera and Mars Orbital Laser Altimeter further confirm that "brief episodes of water-related activity, including glaciation, punctuate the geological history of Mars." Baker wrote in Nature

Evidence for recent martian glaciers is among the most important and controversial, he said.

"If there are glaciers, then Mars is very different than many people have described it," he said. "Glaciers aren't ice cubes, sitting there to waste away. In order to have glaciers, you have to have precipitation. Water has to move through the atmosphere, and it precipitates and makes the ice grow."

And glaciers don't form in the coldest environments, he added. Glaciers form where there is cold and moisture. "But for there to be moisture, it can't be supercold," he noted. "The atmosphere has to be warm enough to evaporate water and move it through the air. The presence of glaciers means that Mars once was a lot warmer, and that there was much more water on the martian surface. The presence of glaciers suggests that there must have been standing bodies of water as well. "

How could it happen?

Confronted by Viking images of young fluvial and glacial features on Mars -- stream valleys that apparently were formed by precipitation and glacial features over large areas of the planet, Baker, Robert G. Strom and other UA scientists in 1991 theorized what has become known as the "MEGAOUTFLO" model.

Basically, the hypothesis says that over the long term, water and volatiles remain frozen as ground ice and ground water in the subsurface because Mars is so distant from the sun and extremely cold. The perennially frozen permafrost acts like a cap on a soda bottle. And just as gas and water in a capped soda bottle explode when heated, sporadic bursts of internal planetary heat trigger the dramatic release of gas and water locked under the permafrost.

They theorize that so much water is released in such episodes that a temporary ocean forms repeatedly over the northern hemisphere. Massive martian volcanism near the northern hemisphere's Thasis Bulge has -- and may again -- trigger an ocean.

Carbon dioxide released to the atmosphere promotes the warming greenhouse effect so that liquid water is stable near the martian surface. Mars lacks a soil layer like Earth's, so when it rains, water filters underground rather than collecting on the surface. Local valleys and other observed martian features form when near-surface water gushes up from below. But when it rains, water removes carbon dioxide from the atmosphere, and Mars chills to the point that permafrost reforms, plunging the planet into another dry, frigid long-lasting epoch.

The extensive hydrosphere implied by Mars' water-generated geology "may exist only as ground ice in the thick permafrost zone and as underlying groundwater," Baker wrote in Nature. "Yet, this is the type of environment in which the extremophile progenitors of Earth's biosphere probably evolved. Indeed, early Mars provided an arguably better habitat for the inception and incubation of early life than did early Earth."

And if that's the case, he added, there could be fossils.

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