The potential amount of life that could have existed on Mars is tiny compared to the biomass early in Earth's history, say two experts from the University of Colorado at Boulder and Washington University in St. Louis.
Professor Bruce Jakosky of CU's Laboratory for Atmospheric and Space Physics and Professor Everett Shock of WU's Department of Earth and Planetary Sciences modeled geochemical reactions from rock weathering. They also estimated Martian volcanic activity over time and the associated activity of hydrothermal vents. They concluded a surprisingly small amount of life could have been produced through chemical reactions over billions of years.
"There has been a revolution in biology that has changed ideas about early life on Earth," Jakosky said. "These new ideas came after recent discoveries of life on Earth in extreme conditions where organisms use geochemistry rather than photosynthesis for energy."
Such organisms live on chemical energy obtained from near-boiling water created by mid-ocean rifts and continental hot springs like Yellowstone.
"We used this new knowledge as a springboard to estimate the amount of chemical energy available on Mars," said Jakosky, a science team member on NASA's Mars Global Surveyor spacecraft now orbiting the "Red Planet."
"This is the first modern estimate for the potential amount of life on Mars, past or present, and was calculated by using what we know about the planet's geological history," he said. The study will be published in the Aug. 25 issue of the Journal of Geophysics Research.
The researchers assumed life requires water, access to elements like carbon, hydrogen, oxygen and sulfur to build complex molecules, and a source of energy. The source can be either natural chemical reactions or photosynthesis, and provide energy that organisms can use for metabolism.
Earth can produce about 20 grams of organisms per square centimeter of land every 1,000 years because of the powerful forces of photosynthesis, according to Shock, an expert on chemical energy processes on Earth. But it would take Mars four billion years to produce that same 20 grams, assuming the organisms were using chemical energy.
This massive difference in the possible biomass produced on Earth relative to Mars is due almost entirely to the occurrence of photosynthesis on Earth, they said. "But there's no evidence of life on Mars yet, much less photosynthesis," said Jakosky, also a professor in geological sciences.
Jakosky and Shock estimated the amount of geochemical energy that has been available through time on Mars from evidence involving volcanism, the circulation of water on Mars that once flowed through its hydrothermal systems, and the weathering on the planet's surface and crust.
It appears the amount of volcanic rock that has erupted on Mars over its lifetime is several hundred times less than that on early Earth. Therefore, the global amount of energy accessible through hydrothermal vents would likely have been proportionately less on Mars as well, Jakosky said.
Within the next decade NASA plans to bring Mars samples back Earth to look for evidence of life. "However, the probable low abundance of life on Mars, if any, will make this difficult," Jakosky said. "The chance of picking up rocks containing fossils or even life during sample-return missions is small.
"Our best hope lies in targeting and exploring fossil or active hydrothermal systems, aqueous systems that could be exposed in walls of Mars' deep canyons, or active springs discharging at the surface," he said.
Jakosky and Shock also evaluated the potential biomass that could have been created on Europa, a moon of Jupiter. Europa gained attention in recent months when Galileo spacecraft scientists discovered what they believe to evidence for liquid water under thick sheets of ice.
Jakosky believes if there is even a slim chance for finding life there, it would not be in the water but in rocks underlying the water where internal heat sources may transfer energy for life. They estimate, however, that the energy available on Europa is even lower than on Mars.