ARE WE ALONE?

As far back as there is human memory, people have wondered if there is life elsewhere in the universe. Are the inhabitants of earth alone, or do living creatures inhabit planets orbiting distant stars? The stuff of science fiction novels, of Twilight Zone TV programs by Ray Bradbury, of movies like Contact, these ideas have burst onto the scientific scene this last year with the report that rocks from Mars bear evidence of ancient life. Suddenly extraterrestrial life is a legitimate scientific question, with evidence to analyze and conclusions to debate. And hot debate has indeed resulted. Every aspect of the _Mars rock_ is the subject of careful scrutiny and extended discussion. All of this creates a great hunger among scientists for more information and more concrete data to help resolve the issue. Plans to bring rocks back to earth from Mars are afoot, and orbiters are photographing Europa, a more promising candidate than Mars, scanning its icy surface for any signs of life.

ANCIENT BACTERIA ON MARS?
A dull grey chunk of rock collected in 1984 in Antarctica ignited this uproar with the possibility that it contains evidence of life on Mars. Analysis of gases trapped within small pockets of the rock indicate it is a meteorite from Mars. It is, in fact, the oldest rock known to science _fully 4.5 billion years old. Back then, when this rock formed on Mars, that cold, arid planet was much warmer, flowed with water, and had a carbon dioxide atmosphere_conditions not too different from those that spawned life on earth.

Intrigued by the parallel, a group of NASA scientists set out to investigate the meteorite for evidence of biological activity. The search did not yield conclusive results, but several features of the meteorite seem to point in the direction of bacterial life! Three aspects of the Mars meteorite suggested the presence of ancient bacteria:

1. Biotic combinations of minerals. In acid-pitted portions of the meteorite, the iron oxide called magnetite and the iron sulfide called pyrrhotite are found together, associated with microfossil-like elements. Magnetite is, as its name suggests, magnetic. Many earthly bacteria secrete particles of magnetite within themselves, probably to help them navigate by orienting to the earth_s magnetic field. Iron sulfide is also a common product of bacteria that do not need oxygen to respire. These two minerals do not precipitate together under acid conditions, but many times they do when associated with bacterial growth. However, independent work published on the same meteorite a year earlier concluded that the mineral deposits formed within the meteorite at temperatures much too hot for life.

2. Organic residues. Polycyclic aromatic hydrocarbons (PAHs) are found in the interior of the meteorite, where they could not be the result of contamination. These large complex organic molecules have been found in other meteorites as residues of nonbiological reactions. However, the PAHs in the Martian meteorite are unusual. They resemble neither common terrestrial nor meteoritic ones. Instead, they look surprisingly like the sort of PAHs which bacteria dissolve into when subjected to rock-forming processes.

3. Bacteria-like structures. When examined with powerful electron microscopes, carbonate patches within the meteorite exhibit what look like microfossils, some 20-100 nanometers in length. One hundred times smaller than any known bacteria, it is not clear they actually are fossils, but the resemblance to bacteria is striking. While carbonate spheres tend to grow on rocks brought out of the Antarctic cold, the spheres within this meteorite must have formed before the meteorite arrived on earth, because some of them were cracked by whatever shock blasted the rock from the Martian surface. Also, the iron oxide and iron sulfide deposits lie within the spheres. Closer examinations will reveal whether the possible microfossils have cell walls, as all bacteria do, and whether they contain amino acids.

Because other meteorites that supposedly harbored evidence of life have proven to be contaminated, the report of possible bacterial fossils on the Martian meteorite has met with considerable skepticism. But contamination does not seem to be a problem in this case; the PAHs, present in the interior of the meteorite, are absent from the outer rind, where the meteorite melted during atmospheric entry. Viewed as a whole, the evidence of bacterial life associated with the Mars meteorite is at best indirect and suggestive, rather than compelling. Clearly, more painstaking research remains to be done before the discovery can claim a scientific consensus. However, while there is no conclusive evidence of bacterial life associated with meteorites, it seems very possible that life has evolved on other worlds in addition to our own.

BACTERIA IN ALIEN EARTH ENVIRONMENTS
The possibility that life on earth actually originated in the vicinity of deep-sea hydrothermal vents is gaining popularity. At the bottom of the ocean, where these vents spewed out a rich froth of molecules, the geological turbulence and radioactive energy battering the land was absent, and things were comparatively calm. The thermophilic archaebacteria found near these vents today are the most ancient group of organisms living on earth. Perhaps the gentler environment of the ocean depths was the actual cradle of life.

Has life evolved on other worlds within our solar system? There are planets other than ancient Mars with conditions not unlike those on earth. Europa, a large ice-covered moon of Jupiter, is a promising candidate. Photos being taken in close orbit in the winter of 1998 are anxiously anticipated.

LIFE IN OTHER SOLAR SYSTEMS
There are undoubtedly many other worlds in the universe with physical characteristics that resemble those of our planet. The universe contains some 1020 (100,000,000,000,000,000,000) stars similar to our sun. We don_t know how many of these stars have planets, but it seems increasingly likely that many do. In 1996 astronomers succeeded in actually detecting several planets orbiting distant stars. At least 10% of stars are thought to have planetary systems. If only 1 in 10,000 of these planets is the right size and at the right distance from its star to duplicate the conditions in which life originated on earth, the _life experiment_ will have been repeated 1015 times (that is, a million billion times). Consequently, it does not seem likely that we are alone.

Nor should we overlook the possibility that life processes might have evolved in different ways on other planets. A functional genetic system, capable of the accumulating and replicating changes and thus of adaptation and evolution, could theoretically evolve from molecules other than carbon, hydrogen, nitrogen, and oxygen in a different environment. Silicon, like carbon, needs four electrons to fill its outer energy level, and ammonia is even more polar than water. Perhaps under radically different temperatures and pressures, these elements might form molecules as diverse and flexible as those carbon has formed on earth.

The final question about life, of course, is whether it has an end. Does evolution tend toward any fixed form or state? Some have suggested that if life might have emerged on any of 1015 worlds, we should have heard from some other life forms by now_unless the life experiment does not work, with life always tending to destroy itself. We can only hope that this is not so and that peace and progress are perpetual possibilities.