LIFE AS WE DO NOT KNOW IT

The NASA Search for (and Synthesis of) Alien Life

By Peter D. Ward

Viking. 292 pp. $25.95

Six years ago, Peter Ward had a run-in with alien hunters. He and a fellow scientist, Donald Brownlee, published a book called Rare Earth in which they argued that -- cosmically speaking -- warm, wet, stable planets like our own are rare exceptions. So, Ward and Brownlee asked, why are scientists spending millions of dollars pointing radio telescopes at the stars, hoping to hear someone say hello? Chances are there's no other planet within shouting distance where beings smart enough to build a radio could live.

In his new book, Life As We Do Not Know It, Ward has changed his tune. He's no more hopeful than before about the existence of alien electrical engineers, but on the subject of simple life forms on other worlds, he reveals himself to be a starry-eyed optimist. Life on Mars? Very likely, at least in the distant past. On Venus? Not on its scorching, acid surface, but perhaps floating in its cloud tops. On Saturn's giant moon, Titan? Its surface is a Superfund site of hydrocarbons and tarry sludge, but it could be "a veritable zoo of enormously alien life-forms."

These are speculations, of course. But in this fascinating and frustrating book, Ward, a University of Washington paleontologist who is also head of a NASA-funded astrobiology institute, grounds them in plenty of hard science. Discoveries over the past few years have revealed conditions on other planets and moons that look a lot more hospitable to life -- at least the simple, microbial kind -- than they used to, while findings on our home planet have shown that life is far more varied and adaptable than we thought.

On Mars, the robotic rovers that have been creeping across the ruddy desert for almost two years have shown beyond doubt that 3 billion years ago it was dotted with shallow lakes or even seas. Surely this was a place wet and warm enough for life. The Huygens probe that parachuted to the surface of Titan last January revealed an icy moonscape where water and organic compounds mingle, forming a giant biochemistry lab that could brew the complex molecules needed for life.

Even Earth boasts far more ways to be alive than biologists once thought. Volcanic hot springs on the ocean floor support organisms that draw energy not from the sun -- as surface life does -- but from chemicals spewing from the deep Earth. In the lab, scientists have tinkered with some of the basic molecules of life and found that altered versions work just as well, suggesting that there may be nothing unique about the DNA-based life we know. Combine an expanded definition of life with what we now know about other planets and moons, and prospects for at least simple life elsewhere look bright.

Ward creates an exhilarating sense of the possibilities, and he argues that we humans need to go see for ourselves what's out there. To him, the billions it will cost to send people back to the moon (as NASA plans to do in 2018) and ultimately on to Mars would be money well spent.The moon, he thinks, may be a planetary junk yard, littered with bits of rock blasted from other planets by giant meteorite impacts billions of years ago. And because the moon is geologically inert, these time capsules may hold fossils from ancient Mars or even the early Venus, which may have been wetter and more temperate than it is now.

Only astronauts could comb the moon for fossils, and it may take them to settle the question of life -- past or present -- on Mars. NASA's Mars rovers have been a stunning success, but Ward asks us to imagine whether any robot picking its way across a random patch of the Earth would find a fossil, abundant as they are on our planet. He even calls for a manned mission to Titan -- probably a one-way journey, he acknowledges, given the dangers and the distance (a billion miles or more). Titan could be the solar system's ultimate biodiversity preserve, home to perhaps three completely different kinds of life, one resembling some earthly microbes and the other two utterly alien, perhaps based on ammonia or silicon. Ward concludes: "We need to go there. We humans as well as robots."

If only he had written a better call to arms. His scientist's rigor seems to have deserted him when he wrote -- or dashed off -- this book. On page 218 he describes the landing of the Huygens probe on Titan; on 223 he declares that "no human-made object" has reached the surface. He says that ammonia was found in the atmosphere of Mars -- if real, a possible sign that vestiges of life remain on that desiccated planet -- but he never mentions that the finding was almost immediately retracted. He refers readers to chapter 3 to read about a concept that is nowhere to be found there. He repeats himself, doubles back, dances from speculation to speculation so quickly that the reader is left far behind.

But for an indulgent reader, there's plenty to enjoy: the play of ideas, the sense of wonder, the gossipy asides. Most of all, it is fun to be in the company, this time around, of a determined optimist. *

Tim Appenzeller is the science editor at National Geographic.