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Consider this scenario. One of the probes sent to Mars either very soon, or within the next century, finds conclusive micro-fossil evidence that life once existed on Mars.

One theory today that is quite common is that life might that have arisen on Mars was then ejected into space, floated to Earth and then seeded life on our planet.

I'm tracking with you and that could make sense, assuming we found the conclusive evidence for life on Mars. However, why would we assume that the early martian life originated on Mars, rather than Earth? Is it not likely that somewhere between 2.5-3 billion years ago, a major volcanic eruption or meteor impact could have hit an earth teeming with simple-life and sent it hurdling through space to Mars?

I understand that my theory has the gravity of the sun working against it, but I also see it not being entirely impossible. I mean, 100 years ago, one could assume that life originating on Mars and then being sent this was was ridiculous and impossible but now it's a rational, viable theory.

Considering such a scenario, how would scientist verify that these life-forms didn't originate from Earth, rather than Mars? It seems to me that this question is the first question that would need to be asked and answered after the fossil-records were found and verified.

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  • $\begingroup$ What is your astronomy or astrophysics question? Actually, it is generally thought that it would work the other way around, since Mars was "habitable" before Earth. We might all be Martians. $\endgroup$
    – ProfRob
    Oct 9, 2015 at 17:22
  • $\begingroup$ Question seems more appropriate for a biology forum or something. I'm kind of curious to hear the answer from someone knowledgeable in that field. $\endgroup$
    – Florin Andrei
    Oct 9, 2015 at 17:55
  • $\begingroup$ @FlorinAndrei I asked this hear, because this is where it seemed most appropriate. This is a question that would likely span astronomy, physics, geology and biology. $\endgroup$
    – RLH
    Oct 9, 2015 at 18:42

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Why would we assume that the early martian life originated on Mars, rather than Earth?

There's still a whole lot we don't know. As Wayfaring stranger points out in the comments, Origin is a whole different question. It's possible that life originated outside our solar system and came to either Mars and/or Earth from outside the solar-system. I don't think anyone who studies this idea is "Assuming" life originated on Mars, only that the idea has a chance of being true.

Is it not likely that somewhere between 2.5-3 billion years ago, a major volcanic eruption or meteor impact could have hit an earth teeming with simple-life and sent it hurdling through space to Mars?

As I understand it, volcanic eruptions are unlikely to send anything into space unless it's a smaller sized moon. The escape velocity (young earth, maybe 10 km/s, young mars, maybe 3-4), volcanic eruptions, as far as I know, don't shoot things out at 10,000 - 20,000 MPH. But meteor impacts of sufficient size can do that.

Mars is a better meteor debris making target than the Earth cause it's smaller, so the gravity is lower and presumably it's mostly had a thinner atmosphere too. We've found martial meteors on Earth. We might not find any Earth meteors on mars cause it takes a much bigger impact to knock bits of rock off Earth and because the Atmosphere slows objects down, both coming in and going out.

I understand that my theory has the gravity of the sun working against it, but I also see it not being entirely impossible.

The sun isn't as big a factor as you might think. Once something is knocked off a planet and it gets into orbit around the sun, gravitational assists can move it around further out or further inside the solar system. What presumably happens is that, with a big enough impact, many thousands if not millions of bits of debris get into solar-system orbit and from there, some of them land on other planets - probably much less than 1% of those hit Earth, but if it carries life that can survive the trip, all you need is one rock.

I mean, 100 years ago, one could assume that life originating on Mars and then being sent this was was ridiculous and impossible but now it's a rational, viable theory.

While that's true, the "People used to think this was impossible" isn't a scientific approach for what might be true. We should determine what is possible and/or thought to be likely, based on physical evidence, not what wasn't understood 100 years ago. Your example is a good point on why it's important to keep an open mind about the unknown. You can still make theories based on evidence, and keep an open mind on the unknown. There's really no conflict between the two.

Considering such a scenario, how would scientist verify that these life-forms didn't originate from Earth, rather than Mars? It seems to me that this question is the first question that would need to be asked and answered after the fossil-records were found and verified.

It's a good question.

The simple answer is that Mars cooled first and Mars (likely) had oceans first, so it's a better candidate to have developed life first though extremophiles can live in hot oceans, so . . . time will tell.

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If any amino acid chain on Mars is the same or similar to any amino acid chain on Earth, then we know with final certainty that they have the same origin. Simple combinatorics proves it within as many sigmas you wish. Do the math. A median protein on Earth is about 300 amino acids long. That combination has never occurred by chance anywhere in the visible universe ever. Evolution has no chance to converge them, because evolution can only test that which occurs by chance.

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    $\begingroup$ Of course, knowing that two things share a single origin does not tell us where that origin is. $\endgroup$
    – Wayfaring Stranger
    Oct 9, 2015 at 18:34
  • $\begingroup$ @WayfaringStranger Indeed, finding life on Mars might very well leave us none the wiser. $\endgroup$
    – LocalFluff
    Oct 9, 2015 at 18:55
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    $\begingroup$ This seems to make certain assumptions about the probabilities of particular amino acid chains arising, given that any arise at all. These sound unprovable to me. I get the impression that you are considering all possible amino acid chains--especially ones that could arise in whatever we might call "life"--as equally likely. Do you have a justification for this? $\endgroup$
    – zibadawa timmy
    Oct 9, 2015 at 23:40
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    $\begingroup$ @zibadawatimmy Just combinatorics. Evolution can only select what occurs by chance mutation. The average length protein is a sequence of 300 out of 22 amino acids. The shortest is longer than 100, the longest is over 30 000. And 22^100 is a number larger than what can occur twice by chance in the history of the visible universe. There cannot independently exist two similar proteins on Earth and Mars. For example, there are only about 10^31 virus particles in the biosphere. They could never twice by chance mutate the same protein if they didn't have the same origin. $\endgroup$
    – LocalFluff
    Oct 10, 2015 at 2:57
  • $\begingroup$ The really fundamental test would be if they used RNA the same way. It would be too much coincidence for the replicating molecules to end up the same on both planets. Probably RNA is even more fundamental than protein or DNA (look under 'RNA world'). $\endgroup$
    – AlaskaRon
    Oct 10, 2015 at 8:28
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We'd analyze the DNA and RNA sequences (assuming they had any) and the coding of amino acids, and compare them with the sequencing and coding on Earth. As far as we are evolutionary from Archaea and Bacteria, we're still a lot closer than an alien life form would be.

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In addition to already present good answers, I would say that if we found life on Mars on a century there is a noticeably probability of it being made from Earth microbes accidentally taken there by one of our probes or their descendants. Risk of contamination of other planets is addressed by carefully sterilization of probes but no sterilization is complete, so there is always a small risk.

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  • $\begingroup$ Could they not simply carbon date the fossil and show it was older than all the fossils on earth? $\endgroup$
    – Tim
    Sep 13, 2017 at 1:47

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