When the 13,000 year-old Alan Hills (ALH84001) meteorite was first analyzed back in 1996 it caused a sensation. LIFE had existed on Mars!!! The media played the story, of course. Unfortunately, the evidence for fossilized organisms in the meteorite was inconclusive, that is, it could be interpreted in different ways. Most scientists decided that what looked like a fossil worm was just an artifact of a chemical process. The sensation died down, way down. Guess what? It’s back. The meteorite has been re-examined with newer technology, and this time the evidence for life is clearer. So where is the media furor? Perhaps it’s coming, but in the meantime scientists will need to chew on the new research…

Here’s the existing (pre-revision) information about the meteorite from NASA:

A group of scientists led by David McKay of NASA’s Johnson Space Center published an article in the 16 August 1996 issue of Science magazine announcing the discovery of evidence for primitive bacterial life on Mars. An examination of a meteorite found in Antarctica and believed to be from Mars shows: 1) hydrocarbons which are the same as breakdown products of dead micro-organisms on Earth, 2) mineral phases consistent with by-products of bacterial activity, and 3) tiny carbonate globules which may be microfossils of the primitive bacteria, all within a few hundred-thousandths of an inch of each other. Based on age dating of the meteorite, the following scenario has been proposed:

1. The original igneous rock solidified within Mars about 4.5 billion years ago, about 100 million years after the formation of the planet. (Based on isotope ages of the igneous component of the meteorite)
2. Between 3.6 and 4 billion years ago the rock was fractured, presumably by meteorite impacts. Water then permeated the cracks, depositing carbonate minerals and allowing primitive bacteria to live in the fractures.
3. About 3.6 billion years ago, the bacteria and their by-products became fossilized in the fractures. (Based on isotope ages of the minerals in the fractures)
4. 16 million years ago, a large meteorite struck Mars, dislodging a large chunk of this rock and ejecting it into space. (Based on the cosmic ray exposure age of the meteorite)
5. 13,000 years ago, the meteorite landed in Antarctica.
6. The meteorite, ALH84001, was discovered in 1984 in the Allan Hills region of Antarctica.
[Source: NASA]

Note that at least so far, NASA has not commented on the new findings, which were first published online (Science Direct) in June, 2009. Here’s a relevant piece of the abstract (unfortunately the study is behind an expensive pay wall):

Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.

[Source: Science Direct (Elsevier Publishing)]

In less technical terms, the “allochthonous origin” means the carbonates where produced by the shells of living creatures and deposited in sediment, just as they are on Earth. This is the basis of what appears to be an attempt to put the findings once again before the public:

Although some have argued that the tiny magnetite crystals present in the surface layers of the meteorite are the result of a carbonate breaking down in the heat of Earth impact, the new analyses show that this is very unlikely to have resulted in the kinds of structures seen in the rock. Close examination has revealed that about 25 percent of the crystal structures are chemically consistent with bacterial formation.

“We feel vindicated,” said Dr. Mackay. “We’ve shown the alternate explanation is absolutely incorrect, leading us back to our original position that these structures are formed by bacteria on Mars.”

Other scientists agree. “Until now I was on the fence but this paper has finally thrown out the non-biological explanation,” said Dennis Bazylinski, an astrobiologist from the University of Nevada who peer-reviewed the findings.

[Source: Machines Like Us]

“Once burned…” It’s understandable that after the disappointment of the first ‘discovery of life on Mars,’ people in general and scientists in particular are not going to readily embrace this revised interpretation. It’s likely that additional pieces of unequivocal evidence are needed to shake disbelief. Also, the new interpretation needs to be carefully scrutinized. However, some skeptics are already turning. If this holds up, whether widely embraced or not, it means there was life on Mars. (It may still be there. Life may be many places in the universe. Gasp.) We shall see.