In 1996 the controversial discovery of what appeared to be Martian fossils in a meteorite from Antarctica ignited a furor in the scientific community.
The idea that a rock billions of years old was flung into space with traces of life aboard was intoxicating, fueling thoughts of panspermia — the idea that all life on Earth could have originated on Mars or some other alien planet.
Twelve years on, scientists still debate whether the tiny structures are Martian or not, or even fossils. But now a new study has shown it's possible for traces of life to survive a punishing interplanetary journey.
Frances Westall of the National Center for Scientific Research (CNRS) in France and a group of researchers attached a 2-centimeter-thick rock to the heat shield of a Foton M3 space capsule.
The desert-dwelling organisms were not so lucky, however.
Westall said the screws that held the rock to the heat shield loosened during re-entry, allowing temperatures between the shield and the rock to climb to between 570 and 930 degrees Fahrenheit (300 and 500 degrees Centigrade).
"[The bacteria] were carbonized," she said, "so it's a negative result for the idea of panspermia, but we'll have to run the experiment again."
"This certainly does not disprove the idea of panspermia," David Kring of the Lunar and Planetary Science Institute said.
For life on Mars to make it to Earth, it would have to survive a perilous interplanetary crossing. Scientists have calculated that the first stage — launching off the surface with debris created by a meteorite impact — is survivable.
Little is known about the long journey through the vacuum of space, but microbes living inside rock could be protected enough to make the trip.
Then comes re-entry. If life were to survive the heat of rocketing through Earth's atmosphere at 12-15 kilometers per second (33,000 mph), it would still have to live through the impact. At this point, no one knows if it can.
"This is a piece in the puzzle of the origin of life, and the distribution of life in the solar system," Kring said of the team's research. "If life did originate on Earth and was transferred elsewhere in the solar system, it would be interesting to everyone to know that, and vice versa — if life began on Mars and simply propagated better on Earth, that would be interesting, too."