The presence of methane on Mars, first discovered a few years ago, has piqued the curiosity of researchers, who wonder if the gas results from geologic activity or, more intriguingly, from living organisms, as is largely the case on Earth. Though by no means settling the issue, new detections of methane at least point in the direction of further study.

Using ground-based telescopes, Michael J. Mumma of the NASA Goddard Space Flight Center and his colleagues monitored about 90 percent of the Red Planet's surface for three Martian years (equal to seven Earth years). They detected large methane belches during the summer of 2003 and located the areas of those emissions.

Mumma is careful not to overstate the significance of his study, published online January 15 by Science. Although the methane could have come from the activity of microbes living below the permafrost, an equally plausible explanation is that it came from reactions between minerals and water trapped in rocky layers underneath. The methane could also be a relic of past processes, somehow sequestered and then released. Still, by knowing that Mars's methane comes from discrete areas, scientists can look for new sources and target the regions for future lander missions.