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Meteoroid transfer of Earth life to Europa?
Guest_BruceMoomaw_*
post Mar 18 2006, 05:04 AM
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As promised, here are my conclusions after reading Mark Peplow's new report on Brett Gladman's paper to the LPSC meeting ( http://www.nature.com/news/2006/060313/full/060313-18.html ). I really don't see anything at all in it that wasn't in the earlier LPSC abstract of Gladman ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2165.pdf ) -- except that Peplow quotes Gladman as saying that (1) for every 600 million meteorites flung away from Earth by impacts, "about 100 objects would hit Europa", while Gladman's abstract actually says there would be "30-100"; and (2) outnof that many Earth meteorites, Peplow says "roughly 30" would hit Titan, while Gladman's actual abstract says "a few to 20" (and provides a graph).

This doesn't make any difference to the overall results -- based on numbers alone, it is (contrary to my previous fond wish) quite possible that Earth could have inoculated Europa with terrestrial life, so that the discovery of Europan life would NOT automatically prove that life had evolved in two separate world within the same solar system and that life must therefore be common in the Universe. However, both reports point out that those Earth meteorites would hit Europa at 20-30 km/sec, and that therefore (to quote Peplow) such crashes "would almost certainly sterilize the few rocks that made it that far." Peplow calls this "unfortunate" -- which is the exact opposite of my feeling about it; that impact speed really may allow us to consider Europa as a separate laboratory to look for the evolution of life, totally free of Earth biocontamination.

As for Titan, its atmosphere would slow any arriving Earth meteoroids down to such low speeds that they might very well biocontaminate that world -- IF the bacteria in the meteoroids could survive Titan's cryogenic near-surface temperatures. Note also that the problem of high impact speed onto Europa does NOT rule out the possibility that life might have originally evolved only on Europa, and then been transferred from there to Earth and/or Mars early in the Solar System's history!
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Guest_Richard Trigaux_*
post Mar 18 2006, 07:26 AM
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QUOTE (BruceMoomaw @ Mar 18 2006, 06:04 AM) *
As promised, here are my conclusions after reading Mark Peplow's new report on Brett Gladman's paper to the LPSC meeting ( http://www.nature.com/news/2006/060313/full/060313-18.html ). I really don't see anything at all in it that wasn't in the earlier LPSC abstract of Gladman ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2165.pdf ) -- except that Peplow quotes Gladman as saying that (1) for every 600 million meteorites flung away from Earth by impacts, "about 100 objects would hit Europa", while Gladman's abstract actually says there would be "30-100"; and (2) outnof that many Earth meteorites, Peplow says "roughly 30" would hit Titan, while Gladman's actual abstract says "a few to 20" (and provides a graph).

This doesn't make any difference to the overall results -- based on numbers alone, it is (contrary to my previous fond wish) quite possible that Earth could have inoculated Europa with terrestrial life, so that the discovery of Europan life would NOT automatically prove that life had evolved in two separate world within the same solar system and that life must therefore be common in the Universe. However, both reports point out that those Earth meteorites would hit Europa at 20-30 km/sec, and that therefore (to quote Peplow) such crashes "would almost certainly sterilize the few rocks that made it that far." Peplow calls this "unfortunate" -- which is the exact opposite of my feeling about it; that impact speed really may allow us to consider Europa as a separate laboratory to look for the evolution of life, totally free of Earth biocontamination.

As for Titan, its atmosphere would slow any arriving Earth meteoroids down to such low speeds that they might very well biocontaminate that world -- IF the bacteria in the meteoroids could survive Titan's cryogenic near-surface temperatures. Note also that the problem of high impact speed onto Europa does NOT rule out the possibility that life might have originally evolved only on Europa, and then been transferred from there to Earth and/or Mars early in the Solar System's history!



I don't believe too much that cross-seeding of life would be common, as the organisms would have to survive space, impact, etc. But I don't rule it out as impossible. I even put it on stage in one of my novels.


But I want to reassure you, Bruce, if we find life elsewhere in our system, there is a simple test to check if it comes frome earth: if it has the same DNA code or not. It is very likely that the DNA code is a very contingent result of an evolution (specialists envision an evolution from a two-codons code to the today three-codons). There could exist many possible different DNA codes, some eventually more efficient than ours (for instance allowing for a ten times faster evolution, or self-repairing bodies). On the other hand, once a code appears (more than three billion years ago on Earth) it becomes very stable. In fact we don't know, as we know only one example of life.

So if we find life with:
-the same DNA code than ours
-an evolution tree which starts from a given evolution stage, and not from zero
we can safely infer that they result from a cross-contamination from Earth (or from Venus, as there are some chances than life could have appeared here, and be transferred later on earth).

If we find life with:
-a different DNA code
-a whole evolution tree starting from scrap

so we can infer that life appeared here too, independently of Earth.


The only ambiguous case would be life with
-same or very similar DNA code from ours
-a whole evolution tree
so we may conclude that there is a few or only one solution for creating a DNA code.


In the case of my novel, they find algae with the same DNA system than theirs, which all look like the same family, with a very short evolution tree starting froma known algae of their own planet. So they can only conclude to a contamination (and consider this planer as theirs, and continue to terraform it, as the process was started naturally)
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Guest_BruceMoomaw_*
post Mar 18 2006, 08:03 AM
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That would not necessarily prove it -- because it's generally accepted that when earth life was initiated, it didn't use DNA at all. It used either RNA or some even more primitive nucleic acid (such as PNA) -- because, while these have much less ability than DNA does to store large amounts of genetic information stably, they do have the ability (which DNA totally lacks) to ALSO serve as enzymes to synthesize proteins. It was only considerably later that organisms evolved the use of DNA as a permanent genetic storage medium at all -- and they could only utilize DNA for that purpose because RNA or pre-RNA life had evolved first.

So the discovery of microbes on other planets with a radically different kind of DNA code would not at all prove that life had taken the huge stride of evolving from vastly simpler, definitely "nonliving" organic compounds separately on that world -- it might just mean that "RNA life" was what evolved on a single world in the Solar System and then got transferred to others via meteorite, after which it evolved into DNA life with a different particular kind of nucleotide coding sequence for different proteins. This would, God knows, be scientifically interesting enough in itself -- it would in itself provide us with vastly more information on the earliest stages in the evolution of life. But it would not prove that the really difficult initial step had been taken independently on more than one world in this Solar System, and must therefore be common in the Universe as a whole.

Actually, there is another indicator that would have a much better chance of doing that -- namely, the chirality of the organic compounds in alien microbes. Living things on Earth must have selected for one chirality or another extremely early in the game, much earlier than the appearance of DNA -- they must, in fact, have done so during their earliest transition from simpler organics to even RNA life or its equivalent. So if we find reverse-chirality life on Mars or Europa, in itself it will be a near-certain dead giveaway that the life on those worlds did evolve independently. The question is what we will do if we find life of the SAME chirality on other worlds. In that case, we may well never be able to prove that it appeared independently -- or even establish that such an independent origin was probable.
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deglr6328
post Mar 18 2006, 10:35 AM
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The radiation conditions at Europa are very extreme. It is hard to imagine one of those handful of meteoroids striking fortuitously in an area where it may be subducted to protective depth rapidly enough to preserve biologic materials.
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Bob Shaw
post Mar 18 2006, 02:04 PM
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I've always found it rather telling that, just about as soon as the Late Heavy Bombardmant is over, WHAM! it's goo-time on Earth. One argument goes that life arose as soon as it could, spontaneously, but the idea that there were impacts which themselves brought life here is quite telling. Add in the different conditions then extant in the outer solar system, and you really do raise the possibility of the clock of life having started somewhere else reasonably local *before* it infected the Earth. Note that these suggestions of debris transfer around the solar system are *not* classical panspermia, which envisaged a (practically untestable) drift of spores between the stars, but a much more quantifiable phenomenon.

Bob Shaw


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Remember: Time Flies like the wind - but Fruit Flies like bananas!
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centsworth_II
post Mar 18 2006, 03:35 PM
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QUOTE (Bob Shaw @ Mar 18 2006, 09:04 AM) *
I've always found it rather telling that, just about as soon as the Late Heavy Bombardmant is over, WHAM! it's goo-time on Earth.
Bob Shaw


With all this talk about how "easy" it is for life to originate and/or transfer in the solar system we'd BETTER find it somewhere else or someone's got some explaining to do.
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tty
post Mar 18 2006, 04:47 PM
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QUOTE (Bob Shaw @ Mar 18 2006, 03:04 PM) *
I've always found it rather telling that, just about as soon as the Late Heavy Bombardmant is over, WHAM! it's goo-time on Earth. One argument goes that life arose as soon as it could, spontaneously, but the idea that there were impacts which themselves brought life here is quite telling. Add in the different conditions then extant in the outer solar system, and you really do raise the possibility of the clock of life having started somewhere else reasonably local *before* it infected the Earth. Note that these suggestions of debris transfer around the solar system are *not* classical panspermia, which envisaged a (practically untestable) drift of spores between the stars, but a much more quantifiable phenomenon.

Bob Shaw


Things may even go back before the Late Heavy Bombardment. There are hyperthermophiles at the base of all three major realms (Archaea, Bacteria, Eucarya), but phylogenetic reconstruction of the last common ancestor suggests that it was not a thermophile. This suggest that at some time after the three realms branched something wiped out everything except thermophiles and the obvious candidate is the LHB.

tty
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Guest_Richard Trigaux_*
post Mar 18 2006, 06:28 PM
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QUOTE (BruceMoomaw @ Mar 18 2006, 09:03 AM) *
That would not necessarily prove it -- because it's generally accepted that when earth life was initiated, it didn't use DNA at all. It used either RNA or some even more primitive nucleic acid (such as PNA) -- because, while these have much less ability than DNA does to store large amounts of genetic information stably, they do have the ability (which DNA totally lacks) to ALSO serve as enzymes to synthesize proteins. It was only considerably later that organisms evolved the use of DNA as a permanent genetic storage medium at all -- and they could only utilize DNA for that purpose because RNA or pre-RNA life had evolved first.

So the discovery of microbes on other planets with a radically different kind of DNA code would not at all prove that life had taken the huge stride of evolving from vastly simpler, definitely "nonliving" organic compounds separately on that world -- it might just mean that "RNA life" was what evolved on a single world in the Solar System and then got transferred to others via meteorite, after which it evolved into DNA life with a different particular kind of nucleotide coding sequence for different proteins. This would, God knows, be scientifically interesting enough in itself -- it would in itself provide us with vastly more information on the earliest stages in the evolution of life. But it would not prove that the really difficult initial step had been taken independently on more than one world in this Solar System, and must therefore be common in the Universe as a whole.

Actually, there is another indicator that would have a much better chance of doing that -- namely, the chirality of the organic compounds in alien microbes. Living things on Earth must have selected for one chirality or another extremely early in the game, much earlier than the appearance of DNA -- they must, in fact, have done so during their earliest transition from simpler organics to even RNA life or its equivalent. So if we find reverse-chirality life on Mars or Europa, in itself it will be a near-certain dead giveaway that the life on those worlds did evolve independently. The question is what we will do if we find life of the SAME chirality on other worlds. In that case, we may well never be able to prove that it appeared independently -- or even establish that such an independent origin was probable.



With DNA code, I meaned the correspondence between amino acids and the DNA codons. This correspondence is the same with RNA or DNA. That DNA appeared much later don't changed this correspondence. It is this correspondence which is likely to be contingent, and thus be telltale of a different origin (if the correspondence is different). But it is likely that completelly alien forms of life may use the same nucleotides (guanin, cytosin, etc) and even the same DNA, as these molecules appear very soon if the proto-life chemistry. Seemingly they may be selected by clays, and thus be the similar everywhere.

Another tell-tale evidence would be into the coded proteins themselves. On earth we can trace the evolution of very primitive life forms because we find everywhere the same proteins, differing in just the way they evolved over ages and tree branches. A completelly different set of proteins, fitting with no known family on Earth, would be too an evidence of a completelly different origin.

So whatever we find, we have a battery of tests to assert if it has a common origin or not, and if yes, at what point of evolution it diverged. This point could even be dated and related to a cosmic event like the LHB.
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