Any Mars life would be hard to find.

Some researchers propose that life on Earth first evolved in warm environments such as hot springs and deep thermal vents. According to this theory, where there's heat and water there may be life.

Now, a study indicates that Mars' geophysical heat could have supported the development of a modest population of microorganisms over its 4-billion-year history. Those microbes may be so scarce, however, that a mission to sample Martian soil could easily miss them or their remains, report geochemists Bruce M. Jakosky of the University of Colorado at Boulder and Everett L. Shock of Washington University in St. Louis.

Mars could have produced, at most, a planetwide average of 20 grams of microorganisms per square centimeter during its entire history, Jakosky and Shock calculate. Jakosky says, however, that most of that biomass would have decayed away.

Their study, described in the Aug. 25 Journal of Geophysical Research, offers the first quantitative estimate of Martian biomass, says Kenneth H. Nealson of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "This puts in quantitative terms what biologists felt intuitively," Nealson says. "This is a very clever paper."

The study also bolsters previous arguments by biologists that expeditions should target thermally active areas of Mars to seek evidence of life, he says (SN: 11/1/97, p. 284). About half of the biomass would have derived its energy from hydrothermal vents, the report estimates. The authors suggest that oxidation of iron at the planet surface and underground could have provided sufficient energy to produce the remaining biomass.

Although scientists have not yet found conclusive proof of hydrothermal activity on Mars, Jakosky and Shock argue that it would have accompanied volcanism, which has occurred on the planet. Because there has been less volcanic activity on Mars than on Earth, they conclude that Mars must have generated proportionately lass hydrothermal energy.

Considering only biomass produced by geophysical processes, Jakosky and Shock suggest that Mars could have produced only one-fortieth the quantity of life estimated on Earth. Since Earth also supports life through photosynthesis, which may not occur on Mars, Earth's total biomass througthout its history could easily be more than 4 million times that of Mars, they say.

Jakosky concedes that the study relies on the conjecture that Mars' little-understood geology has similarities to Earth's. That represents "a big assumption," says Jonathan I. Lunine, a planetary scientist at the University of Arizona in Tucson. Mars may lack adequate amounts of many trace elements, such as phosphorus, necessary to sustain life on Earth, he suggests.

"It's not clear whether life can exist without phosphorus, but it's not clear that it's impossible, either," Jakosky responds. However, without geophysical energy, "you're stuck," he says.

The new study also speculates that any microbial life on Europa, a moon of Jupiter, would be even more rare than on Mars. Scientists suspect that an ocean that may contain life exists beneath that moon's frozen surface and have discussed sending a robotic probe to find out (SN: 7/6/96, p. 8).

The amount of life possible on Europa may be especially small because that moon may lack volcanic activity, the report says. If microbes must be sustained by oxidizing reactions alone, too few would be released into the ocean to be detectable by a probe, Jakosky speculates.