2: Life on Mars?
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7 August 1996
Today a group of scientists held a press conference at which they announced evidence that they interpret as indicating that a primitive form of life existed on Mars sometime in the past. The evidence is circumstantial, and the hypothesis is certainly not yet proven, but on reading the research paper and looking at the images I find it more plausible than it sounded from media news reports.
What seems certain:
The meteorite found in Antarctica (in 1984, I think) is from Mars -- ejected by some major impact on Mars around 16 million years ago. The rock of the meteorite was formed around 4.5 billion years ago. The meteorite fell to Earth around 13,000 years ago.
Found within fractures in the meteorite are globules of carbonate. These were formed around 3.6 billion years ago. Within these globules are fine grains of minerals such as magnetite and iron sulfides. Also in those regions are organic molecules: Polycyclic Aromatic Hydrocarbons. In addition there are small cylindrical structures, very reminiscent in appearance and size to bacteria.
What seems probable:
The hydrocarbons, the minerals, and these cylindrical structures occur in all the regions where the carbonate globules occur, and only in the regions where the carbonate globules occur. The carbonate globules were formed before the meteorite was knocked off Mars (the carbonate globules have shock faults, which could only have occurred while the rock was on Mars or in space).
What seems very plausible:
The hydrocarbons and minerals are not contamination acquired on Earth after the meteorite fell here. Reasons: These substances occur much less often close to the surface of the meteorite, the reverse of what one would expect if contamination had occurred. [That bit of reasoning is what I found least plausible in the news accounts, but seeing the case made in more detail in the Science paper was more convincing.] The specific types of hydrocarbon present do not match the types one would expect to contaminate it, if contamination happened. The minerals are also hard to explain as occurring through natural (er, non-biotic) processes, but are seen in fossils of Earth bacteria.
The life hypothesis:
That the small cylindrical structures are fossils of organisms that lived on Mars around 3.6 billion years ago, when the planet was warmer, and probably had a substantial atmosphere and surface water. (The carbonate shows evidence of forming in the presence of water, I think.) That the hydrocarbons are similarly remnants of these lifeforms. The carbonate itself could be biotic in origin.
What is not demonstrated (yet):
It has not been demonstrated that the cylindrical objects show any of the features that a fossil of a lifeform should have, other than being of a size and shape reminiscent of some terrestrial bacteria. e.g. Do they have a cell wall? It is not even known what the cylindrical features are made of. (I don't mean they're made of some unusual mystery substance never before seen by mankind -- I just mean it hasn't yet been determined what they're made of.) Are they the source of the hydrocarbons and the minerals? (All that's known is that all of these things occur where the carbonate globules occur.)
Links and stuff
For a huge range of links related to this development, check out the page devoted to it by the Federation of American Scientists (FAS).
Among the links they had were some to the American Association for the Advancement of Science (AAAS), which is the organization that produces the magazine/journal Science. You can look at a press release put on the web by Science. Alas, to see the research paper on-line you need to be signed up with Science magazine. (The print version appeared in the August 16 issue of Science.)
The David Bowie song about life on Mars has been playing through my head all day. Thankfully, the FAS helpfully provided a link to the lyrics of the song, allowing me to get the word less garbled.
And finally, on the topic of alien lifeforms, here's an explanation of the Drake Equation, as mentioned obliquely in The Fermi Paradox
Explained on a City Street Corner, by Graham P. Collins.
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