The 13,000-year-old meteorite, named ALH84001, that was examined in 1996 and 2009. Credit: NASA

In 1996, NASA researchers reported that a meteorite contained evidence that life once existed on Mars. But others argued that the evidence was most likely caused by inorganic processes that could be recreated artificially. A second group of NASA researchers (containing some scientists from the first study) has reexamined the 1996 findings using a new analysis technique called ion beam milling, and they again claim that living organisms are most likely responsible for the materials found in the meteorite.

The new study not only reexamined the contents of the meteorite itself, named ALH84001, but tested the alternative, non-biological hypothesis. "In this study, we interpret our results to suggest that the in situ inorganic hypotheses are inconsistent with the data, and thus infer that the biogenic hypothesis is still a viable explanation," says Kathie Thomas-Keprta, a senior scientist for Barrios Technology at Johnson Space Center in Houston, in a press release. The researcher's study was published in the November issue of the journal Geochimica et Cosmochimica Acta. (The researchers have also conducted tests on the meteorite Nakhlite, images and data below.)

The life on Mars debate centers on magnetite, an iron-bearing, magnetic mineral in the meteorite. The 1996 researchers argued that some of the mineral crystals in the meteorite shared characteristics of bacteria found on Earth. Other scientists disagreed, saying that the magnetite was probably caused by a process called thermal decomposition.

NASA senior scientist, Everett Gibson, who was part of the 1996 research, says, "We believe that the biogenic hypothesis is stronger now than when we first proposed it 13 years ago."

But skeptics remain.

"It seems to me that they haven't really solved the whole thing," said Michael Fuller, who researches magnetism at the University of Hawaii's Institute of Geophysics and Planetology, in a Discovery News article. "Most of them [the grains discussed in the new research] appear too small. It doesn't look to me that they are very similar to magnetotactic bacteria."

Fuller says he is not convinced the magnetites in the Mars rock couldn't have been produced by shock when the meteorite blasted through Earth's atmosphere. A similar shock process produces small iron particles in the lunar soil, he notes.

However, the new study is not the only evidence that life once existed on Mars. Other clues includes evidence of past surface water and the recent release of methane into the Martian atmosphere, which might mean the presence of microbial life.

This is a scanning electron microscope (SEM) image shows a series of partly-filled pits on the surface of a mineral grain from Nakhla--another meteorite studied by the researchers. Similar pits are sometimes found on Earth minerals which have been exposed to microbial attack. Credit: NASA/David McKay

Complex biomorphs appear in another Nakhla image. Credit: NASA/David McKay

This is an image of a polished flat surface of chip cut from Nakhla and photographed with a SEM. At some point, the meteorite was fractured, then soaked in water on Mars. The Martian water precipitated a variety of materials in the cracks. Credit: NASA