A ROCKY DEBATE - PART ONE - PART TWO - PART THREE - PART FOUR

My favourite rock
A Rocky Debate
by Bruce Moomaw
Cameron Park - April 28, 2000 - Meanwhile, the seemingly endless debate continued over whether Mars meteorite ALH84001 does or does not contain microfossil and chemical evidence of ancient Martian microbes.

The Johnson Space Center team who originally reported the evidence repeated that tiny magnetite crystals in the meteorite look very much like some crystals that seem to be made only by strains of Earthly bacteria (This seems to be their strongest piece of remaining evidence).

And Benjamin P. Weiss repeated his recent claim that microscopic magnetic patterns show that the meteorite's carbonate globules -- which contain the possible evidence -- were never above 40 deg C, contrary to the view of others that who hold that they were created by water heated far above boiling by a volcano or a meteor impact and so could never have contained live microbes.

But other scientists continue to doubt both opinions. And the JSC team, as they had done at the recent Lunar and Planetary Science Conference, also agreed with Allen Steele and J.K. Toporski that new studies show that this meteorite -- and apparently all other Martian meteorites -- have been contaminated to their very cores with identifiable Earth microbes.

Indeed, they now think it impossible to rule out the possibility that any organic compound found in any meteorite could actually be Earth contamination! Not only have they thus dropped one of their arguments for Martian life in ALH84001 (the "PAH" organic compounds found in it); they have concluded that using any meteorite for evidence of extraterrestrial life may be very difficult.

(Toporski added in his talk that analysis of carbon isotopes in a meteorite's organics might be able to settle whether they had an Earthly or extraterrestrial origin, but he was far from certain of this.)

However, there was considerable optimism over the more general question of whether rocks blasted off the surfaces of their planets by gigantic meteor impacts can carry live microbes between Earth and Mars.

In her talk reviewing the latest studies of the feasibility of such "panspermia", Gerda Horneck confirmed recent comments by other researchers that it is quite feasible -- at least between planets fairly near each other.

The most serious obstacle may be the high lethality of solar UV light in space to even the toughest spores -- it kills fully 98% of them within 10 seconds -- but they are adequately shielded from it by just a millimeter of rock.

Galactic cosmic rays are far more penetrating, but high-energy ones are also surprisingly rare; Horneck's calculations indicate that one spore out of 10,000 can avoid a lethal cosmic ray for 700,000 years even if totally unshielded, and for 1.1 million years if shielded by 70 cm of rock.

The best direct evidence is the spores carried by the LDEF satellite for six straight years before its recovery; they were completely unprotected from everything in the space environment except UV -- but 1/1000 of them survived in fine shape.

Norman Sleep's calculations indicate that nowadays 10 to 100 Martian meteorites strike Earth each year -- and that one out of 10,000 of those ejected by a giant Martian meteor impact spends less than 10,000 years in space before arriving here (although meteor impacts large enough to eject Martian rocks at escape velocity may happen a good deal more rarely than that).

The implications are obvious -- especially when you consider that the Mars-Earth meteorite transfer rate was 1000 times higher during the Solar System's early days.

Nor can the ejection of Earth rocks and their transfer to Mars be ruled out, although of course this is much harder given Earth's high gravity -- Sleep has calculated elsewhere that such transfers are at least 20 times rarer.

This still means that there is an excellent chance that the two planets have naturally cross-contaminated each other with at least a few species of whatever microbes they evolved.

Mars meteorite specialist H. Jay Melosh told me at the Conference that he thinks such transfers have in fact been common enough that (contrary to my opinion in a "SpaceDaily" piece last year) there is almost no point in worrying about any significant ecological dangers from the human transfer of Martian surface samples to Earth or the landing of contaminated Earth probes on Mars -- let alone human disease dangers from Mars germs.

(At least where the ecological dangers are concerned, though, his opinion is by no means unanimous among astrobiologists.)

In a poster, Kevin Zahnle repeated his own still more radical view (which is starting to catch on): the strong possibility that we may all be descendants of Martian microbes! (As he says, "This view of the solar system has gone from speculation to truism in less than five years.")

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