Men are from Mars and women are from Venus. That's one theory ... another is that all life on Earth descended from organisms that originated on the Red Planet before hitching an interplanetary journey aboard meteorites to Earth. In an effort to provide a definitive answer, researchers at MIT and Harvard are developing an instrument to compare the genetic makeup of Martian microbes with that of terrestrial life. If they find correlations between the two it could prove that we are all descended from Martians, which would make us invaders from Mars.

The instrument concept was devised by MIT research scientist Christopher Carr and postdoctoral associate Clarissa Lui, working with Maria Zuber, head of MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS), and Gary Ruvkun, a molecular biologist at the Massachusetts General Hospital and Harvard University. Their strategy is to search for particular sequences of DNA or RNA in Martian microbes that are nearly universal in all forms of terrestrial life.

The researchers' device, which they have called the Search for Extra-Terrestrial Genomes (SETG), would take samples of Martian soil from below the surface and isolate any living microbes that might be present or any microbial remnants that can be preserved for around a million years and still contain viable DNA. Using the same techniques used for forensic DNA testing on Earth, the device would amplify the microbe's DNA or RNA and then use standard biochemical techniques to analyze their genetic sequences.

The concept relies on several facts that are now well established. Firstly, the climates of Earth and Mars used to be much more similar, so life that originated on one planet could have survived on the other. Secondly, an estimated one billion tons of rock have made the trip from Mars to Earth after being blasted loose by asteroid impacts. And thirdly, there is evidence to suggest that not only are microbes capable of surviving such an initial asteroid impact, but that they could also survive a trip through space that could take thousands of years. Additionally, orbital dynamics shows that it's around 100 times easier for potentially life-harboring rocks to travel from Mars to Earth than the other way around.

While the researchers believe their instrument could answer some fundamental questions about the origins of life on Earth, Christopher McKay, an astrobiologist at NASA-Ames Research Center in California who specializes in research related to the possibility of life on Mars, points out another reason for the research. If we are in fact descended from Martian microbes, then any organisms presently on Mars that are closely related to us could pose a higher risk of infection to any humans heading to the red planet than a totally alien microbe.

While potential evidence of past life on Mars has been found in asteroids on Earth and the level of methane on the planet points to life on the planet, there is still no concrete proof that there is or ever was life on the planet. The Viking probes of the mid-1970's carried several experiments designed to detect organic materials and organisms, but they produced ambiguous, albeit tantalizing, results. As part of NASA's Mars Exploration Program, the Mars Science Laboratory (MSL) rover is scheduled to launch late this year to assess whether Mars ever was, or is still today, an environment able to support microbial life by investigating the chemistry relevant to life.

Unfortunately the SETG won't be ready for that mission, as the MIT and Harvard researchers estimate it will take two more years to finalize the design and testing of a prototype SETG device. And although such a device hasn't been selected for any upcoming Mars mission, they say their SETG instrument could be carried on a future mission with a lander or rover equipped with a drill.

"It's a long shot," Carr concedes, "but if we go to Mars and find life that's related to us, we could have originated on Mars. Or if it started here, it could have been transferred to Mars." Either way, "we could be related to life on Mars. So we should at least be looking for life on Mars that's related to us."