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Ground-based telescopes have sniffed out evidence of life on Mars, although not necessarily biological life. Instead, the observations may show that Mars retains some geological life.

Ice coats layers of sand and rock near Mars' north pole in this image from Mars Reconnaissance Orbiter. As ice evaporates during summer, methane gas may erupt into the atmosphere.

The evidence consists of several outbursts of methane, which is the main ingredient of natural gas. Using two telescopes at Mauna Kea, Hawaii, a research team detected the chemical signature of the gas in the planet's atmosphere.

On Earth, most methane is related to biological activity, from the decay of dead plants to the digestive processes of humans and cattle. Methane is also a byproduct of some geological processes involving iron oxide. Since Mars gets its orange color from iron oxide, the compound is abundant at and below the surface.

Reactions with sunlight and other chemical compounds destroy methane in the Martian atmosphere, so any methane that is present must have been released recently.

The research team detected several outbursts of methane, which must have come from underground -- either from bacteria living in warm, wet chambers below the surface, or from volcanoes that retain some of their heat. Many volcanoes cover the surface of Mars, yet there is little evidence of recent eruptions.

Each outburst was recorded during summer in the planet's northern hemisphere. Team members speculate that the methane may erupt from chambers that had been sealed by thick layers of winter ice.

Another research team reported evidence of methane on Mars three years ago. Those observations found smaller amounts of methane, however. The more-recent study detected up to 20,000 tons of methane in a single outburst.

NASA's Mars Science Laboratory, which is scheduled to arrive at Mars in 2012, will carry instruments that can analyze the atmospheric methane in more detail, perhaps revealing whether the gas is produced by living organisms or geological processes on a not-quite-dead planet. -- DB