1 December 2009
On August 6, 1996 NASA announced that scientists at the Johnson Space Center had found evidence for life on Mars, and everybody went crazy.
Yesterday, NASA announced two new papers by the same scientists at the Johnson Space Center claiming that they have found strong evidence of life on Mars. For the most part, there hasn’t been much of a reaction. No presidential press conferences, and only a few headlines. What gives?
Well, I suspect it is partially due to the fact that one of the papers is very long and esoteric, but even more this might be a case of “the boy who cried wolf”. This is the same research group saying, on the face of it, essentially the same thing more than a decade later. Still, these are respected scientists, and the fact that they are standing by their claim after all this time means they must be pretty confident. That’s why I decided to take a look at the two papers in question.
The first paper is a 47 page beast entitled “Origins of magnetite nanocrystals in Martian meteorite ALH84001“. And really, that’s what it is about. Biology only makes a few brief cameo appearances; for the most part this paper meticulously describes the study of the magnetite crystals in the famous ALH84001 meteorite. The significant thing about this paper is that they suggest that the magnetites could not have formed by alteration of the materials in the meteorite.
Back in 1996, magnetite crystals were proposed as evidence of martian microbes because some types of terrestrial bacteria form perfect, pure magnetite crystals in their guts. However, other scientists proposed hypotheses by which the magnetites could form when carbonate minerals in the meteorite were heated. This latest paper disagrees with that idea, pointing out that the magnetites in the meteorite have a completely pure composition, but the carbonate, from which they are supposed to have formed, has other elements in it that don’t appear in the crystals. The authors argue that since the crystals don’t show those contaminants, they must not have formed from the carbonates. Here is their conclusion:
We suggest that the majority of ALH84001 magnetites has an allochthonous origin and was added to the carbonate system from an outside source. This origin does not exclude the possibility that a fraction is consistent with formation by biogenic processes, as proposed in previous studies.
There you have it! It is possible that a fraction of the magnetite crystals are not inconsistent with life on Mars!
Ok, so that’s not exactly a resounding “yes”. I think the authors were being very conservative in this paper. In a companion paper entitled Life on Mars: New Evidence from Martian Meteorites, they show their feelings a little more. They give a concise and fairly readable summary of their original hypothesis and the subsequent alternatives that people have proposed and which in turn have been addressed. They also provide an easy-to-understand pdf slideshow summarizing their findings. At this point, people seem to agree that the carbonates in the martian meteorites formed on Mars, as did the few organic molecules detected in them. With the magnetite paper, the authors consider the various hypotheses about creating magnetite by heating to be disproved, leaving an organic origin as the best explanation.
They point out that on earth, pure magnetite crystals with the specific shape and properties of the ones found in ALH84001 would be considered definite biomarkers:
The unique properties of these magnetites (elongated along the c-axis, single domain grain size, extremely pure Fe oxide, tightly sorted grain size distribution) remains a suite of properties absolutely unique to magnetotactic magnetites on Earth.
The “Life on Mars” paper also shows lots of examples of electron microscope images of “biomorphs” in multiple Mars meteorites. Biomorphs are like mini-fossils. Some of them might be actual preserved remnants of single-celled organisms, while others are pits and textures formed by long-gone bacteria colonies. Picture them as the microscopic equivalent of finding fossil dinosaur tracks. They also make the case that the observed biomorphs show influence on the shapes of the minerals of the meteorite, suggesting that they formed long ago on Mars rather than more recently on Earth.
I’ll admit, a lot of their biomorphs look like bacteria, and they show examples of biomorphs from Earth that look quite similar. But many of their examples don’t look like much to me. Granted, that may be due to an untrained eye and overactive skepticism.
So, what does it all mean? Is this evidence for life on Mars? I give it a definitive “maybe”. It’s clear that the debate is far from over, and I expect to see some interesting rebuttal papers in the next few years. There’s a fine line to walk here. If this really is evidence of life on Mars, we should be shouting it from the rooftops! But it is so easy to trick ourselves into seeing what we want to see that we have to be cautious. Carl Sagan summed this up nicely: “Extraordinary claims require extraordinary evidence.” Right now I think the evidence is not extraordinary, but it may be getting there.
Thomas-Keprta, K., Clemett, S., McKay, D., Gibson, E., & Wentworth, S. (2009). Origins of magnetite nanocrystals in Martian meteorite ALH84001 Geochimica et Cosmochimica Acta, 73 (21), 6631-6677 DOI: 10.1016/j.gca.2009.05.064
David S. McKay, Kathie L. Thomas-Keprta, Simon J. Clemett, Everett K. Gibson, Jr., Lauren Spencer, & Susan J. Wentworth (2009). Life on Mars: New Evidence from Martian Meteorites Instruments and Methods for Astrobiology and Planetary Missions XII http://www.nasa.gov/centers/johnson/pdf/403089main_7441-1.pdf