Wednesday, December 13, 1995 Page A18
=A91995 San Francisco Chronicle

Charles Petit, Chronicle Science Writer

The more scientists learn about life on Earth, and how fast it appeared when the planet was young, the more they think there was life on Mars billions of years ago.

There is even reason to believe that Mars is still home to a few organisms protected from the intensely cold and harsh surface environment by living inside rocks or far underground= .

Yesterday at San Francisco's Moscone Center, scientists described plans to include a sophisticated search for life, or fossils of life, in a flurry of Mars explorations by the U.S., Russia, Japan, and the European Space Agency.

The renewed optimism over biology on Mars is in contrast to the gloom in the years after NASA's two Viking landers in 1976 discovered that Martian soil is sterile, blasted by ultraviolet radiation and without the organic chemicals believed necessary for life.

If Martian ``exobiology,'' or non-Earth biology, can be found, even in fossil form, it could provide important insight into the rise of life in general, on Earth and perhaps on planets near other stars.

At the least, many experts now believe, it makes excellent scientific sense to include ambitious attempts to find fossils on Mars.

``A lot of people think, when we say fossil, we're going to go look for dinosaur bones,'' said Jack D. =46armer of the NASA Ames Research Center, organizer of the session on Mars biology during the American Geophysical Union meeting. ``But we will be looking for microscopic or microbial remains, things like algae or even organic traces left by life.''

Even if the strong radiation shining through the thin Martian atmosphere and the powerful oxidizing chemistry of its soil have destroyed surface traces of organic molecules, there may be preserved telltale imprints of once-living cells inside Martian rocks near ancient, dried-up springs or on the shores of long-gone lakes, he said.

``After Viking, there was the feeling we had been there, done that, and so much for looking for life,'' =46armer said. ``But the tide of opinion has shifted a lot since then.''

More than 20 years ago, scientists studying space probe pictures of Mars saw clear evidence that it once had rivers and lakes, even though its atmosphere is now too thin for water on its surface.

And scientists now believe life got started on Earth extremely fast and under extraordinarily hostile conditions. The discoveries make Martian life easier to imagine.

When Viking was designed, biologists believed life took billions of years to evolve on a fairly pleasant, warm, wet and friendly young Earth. Now, it looks as though life here showed up in just a few million years while the planet was still forming, enduring titanic blasts from incoming meteors tens to hundreds of miles across.

And, only recently have scientists learned how profoundly rugged today's Earth life can be -- making survival on Mars more plausible. Bacteria and other single-celled life forms live in tiny pores in rocks hundreds or thousands of feet below the surface, in deep ocean hot springs far hotter than boiling water at the surface, and inside rocks and under permanently frozen lakes near the South Pole.

New evidence even suggests that primitive life could have moved from Mars to Earth, or Earth to Mars, hitchhiking on bits of rock blasted into space by giant meteor crashes. Such a hypothesis might be confirmed if Martian life were found and discovered to have qualities in common with Earth life, implying cross-fertilization long ago. A class of meteors found on Earth recently has been identified as being pieces of Mars. Although there is no sign of life in them, analysis suggests that some kinds of bacteria or viruses would have survived the trip if they had been encased inside the stones.

Norman Sleep, a geophysicist at Stanford University, believes that life may actually have gotten started several times on Earth, wiped out in its initial efforts by impacts so huge that their heat boiled away the primordial ocean.

In those days, he said, Mars would have been a more hospitable place for life to get started. He described calculations showing that Mars probably heated up faster, but also cooled down faster, during the period 4.4 billion to 3.8 billion years ago when the sun's planets endured their final bombardments by very large meteors. The short-lived heat would not have penetrated as deeply or as long under the surface on Mars as on Earth, where the deep oceans would hold the heat for thousands of years as they boiled off.

The United States plans to launch two small survey probes to Mars next year, a Global Surveyor to study its surface from orbit and a Pathfinder to put a small lander and solar-powered rover on its surface. Also next year, Russia hopes to launch an orbiter and landers to Ma= rs.

The missions come after deep disappointments. Two years ago, NASA's billion-dollar Mars Observer went dead just before arriving at the planet, and four years before that, a Russian probe failed just as it was to begin a close-up look at a Martian moon.

The next missions are smaller than those two recent failed projects, but the larger number of spacecraft being used makes failure less probable. Russia and the United States, and eventually Japan and the European Space Agency, plan to send automated missions to Mars every two years for the next two decades, including robots to send samples back to Earth. The missions could provide justification for sending humans to Mars some time after the year 2020.

=46armer said the missions are being redesigned to include more instruments to look for water, former hot springs, and mineral deposits that might preserve signs of ancient or even current life, and to equip landers and rovers with better instruments to detect organic molecules and run other biological tests.