Two separate studies published Thursday both provided new evidence that life could exist on the planet Mars while raising questions about the validity of some of the evidence presented for Martian life.

One paper, published in the latest issue of the journal Nature, discussed the discovery of a microbe living deep within the Earth in conditions that resemble those below the surface of Mars.

The microbe, found 200 meters below the surface in the mountains of Idaho, are completely cut off from the Earth's ecosystem, thriving in conditions like those on the early Earth, and perhaps within Mars or other worlds.

"The microbial community we found in Idaho is unlike any previously described on Earth," said Derek Lovley, head of the microbiology department at the University of Massachusetts, Amherst, and one of the authors of the Nature paper. "This is as close as we have come to finding life on Earth under geological conditions most like those expected below the surface of Mars."

Unlike other subterranean life, which survives on organic material carried underground by groundwater seepage, the microbes discovered in Idaho instead exist by combining hydrogen and carbon dioxide gasses dissolved in water to produce methane, releasing energy needed to sustain life. The microbes don't require organic carbon or sunlight to survive.

Such "Archaean" microbes were likely commonplace on the Earth early in its history, when hydrogen was commonplace but other organic compounds were not. The existence of the microbes also raises the possibility that such life forms could exist on other worlds, in similar conditions.

"On Mars and other planets or moons in our solar system on which life might exist, liquid water is only available below the surface where there is no sunlight," said Lovley. "So, if there is life, it must sustain itself with alternative energy sources. This study demonstrates, for the first time, that certain microorganisms can thrive in the absence of sunlight by using hydrogen gas released from deep in the Earth's surface as their energy source."

Mars is the most likely world in the solar system, other than the Earth, where life could have taken root and might still exist today. One controversial piece of evidence for past life on Mars were carbonates found in Martian meteorite ALH84001, first reported over five years ago. However, a separate paper published in Nature suggests that those carbonates may not have an organic origin.

Studies by European astronomers, using ESA's Infrared Space Observatory, have turned up evidence of carbonates in two nebulae formed by dying stars. Those carbonates, astronomers say, could not be formed by water, as scientists previously believed was the only way to create them.

"The amount of carbonates we find is equivalent to at least 30 Earth masses, far too large to be the relic of a hypothetical planetary system present before the star became a planetary nebula," said Ciska Kemper of the University of Amsterdam. "On the other hand, the age of the dust shell in the nebula is about ten thousand years, which is too short for a new planetary system to form."

This implies that the carbonates were formed by another method, although scientists haven't determined what that could be. It does mean that the existence of carbonates is alone not evidence of water, so the discovery of carbonates in a meteorite does not mean that it has been exposed to water, as was assumed for ALH84001. It may also mean that earlier beliefs that liquid water was available within 20 million years of the solar system's formation, based on the discovery of carbonates in other meteorites, may have to be reexamined.