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What was the original claim for life on Mars? August 15, 2010

Posted by Simon in Uncategorized.

I thought it would be interesting to go back to the original 1996 paper that kicked off the whole controversy and see what it said. Here are some extracts:

“It is possible that all of the described features in ALH84001 can be explained by inorganic processes, but these explanations appear to require restricted conditions – for example, sulphate-reducing conditions in Antarctic ice sheets, which are not known to occur.”

“In examining the Martian meteorite ALH84001 we have found that the following evidence is compatible with the existence of past life on Mars:”

and the paper then proceeds to list 5 pieces of evidence that hint at possible life.

“None of these observations is in itself conclusive for the existence of past life. Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars.”

All in all, it’s not what you would call a strong argument, and one that’s fairly easy to attack.  Maybe this is the source of at least some of the controversy – people assume that the authors are saying “We found life on Mars” but what they really said was “We found some individual pieces of evidence that, when put together, are evidence for life on Mars”.

In my last post, I referred to David McKay speaking to a Stanford class in 2009 about ALH84001 –  I’ve found it on YouTube here. At the 48:00 mark, he reiterates that any individual piece of evidence might be discredited, but the conclusion follows from the whole collection of evidence. You’ve got to give him credit for sticking to his story through 14 years of attacks on his argument!



1. John Welch - September 10, 2010

how did anyone find a Martian meteorite in Antarctica? How do they know it is from Mars? Maybe the “Proudly made in Mars” sticker gave it away…

There must be one real interesting story as to how this martian rock turned up at all, let alone showing signs of life.

Simon - September 10, 2010


It turns out that Antarctica is the best place in the world to find meteorites – not because more of them land there, but because when they do land there, they don’t erode much, and they’re easy to spot against the white ice.

Finding them seems be as simple as walking or snowmobiling around, looking for dark rocks on (or in) the ice. Sounds like a fun job!

As far as how scientists know they’re from Mars – it was known for a long time that a small number of meteorites had different mineral composition and crystal structure from most other meteorites, but it wasn’t until the 1980s that it was accepted that they were from Mars. Chemical and mineral analysis was consistent with known Martian geology, and gases trapped inside the meteorites matched what the Viking landers found in the 1970s.

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