NASA's "follow the water" approach to exploring Mars has led to tantalizing clues that microbial life may have existed at some point on the Red Planet. But any organisms would have to have been extremely hardy to survive, according to new calculations. At the places visited so far, at least, it may have been too salty for any Earth-like life at all.
Here on Earth, life seems to have permeated every nook and cranny, from temperate oceans to million-year-old permafrost. But not every environment is hospitable. Curiously enough, it is the food industry that has explored these most extreme conditions. Cram the maximum amount of salt or sugar into a water solution--as in salting meat or making strawberry preserves--and microbes are hard-pressed to survive, much less grow. That's because the ions of dissolved salt hold on to so many water molecules that few are left to support microbial life.
So was early Mars a warm bath or salt pork? Geochemist Nicholas Tosca of Harvard University and his colleagues calculated the salinity of long-gone waters from the composition of the salts left behind both at Meridiani Planum, where the Opportunity rover found the remains of salty groundwater, and at Gusev crater, where Spirit found volcano-related hydrothermal deposits. They also looked at subsurface rocks blasted off Mars that became meteorites collected on Earth.
Even the most salt-loving organisms of Earth couldn't handle the most concentrated martian brines of 4 billion years ago, Tosca and his colleagues report tomorrow in Science. By then, martian waters were 10 to 100 times more saline than Earth's seawater, they found. What's more, the high acidity and chemically oxidizing conditions of the time--previously inferred from mineral compositions--made these waters even more inhospitable. "Our paper compresses the window of opportunity [for life] to a very short span very early in Mars's history," says Tosca, when water may have been far fresher.
"Tosca et al. are making some very good points," writes planetary geochemist Jeffrey Kargel of the University of Arizona, Tucson, in an e-mail, but "they carry it too far." Perhaps early exploration has been drawn to the most saline and therefore most obvious sites, he writes, missing more hospitable places. Microbiologist Kenneth Nealson of the University of Southern California in Los Angeles also holds out hope for life. Faced with greater challenges, martian life may have evolved even better ways to cope with salty water than Earth's microbes have devised. "Keep on following the water" is the message, say these optimists--and the Phoenix lander is doing just that (ScienceNOW, 27 May). Within weeks, it will be analyzing far younger and presumably far fresher water in the martian arctic.