Whenever we send a probe to Mars we spend a lot of effort scrubbing it to make sure that there aren't any terrestrial microbes hitching a ride to the red planet. There's a lot of sense in doing that. Scientists are curious as to whether Mars has any native life. If we spread earthly bugs to Mars then it might be hard to tell if whatever we find had its origins on Mars or Earth. But I wonder if the cat isn't already thoroughly out of the bag.
Way back in '84 some scientists found a meteorite that they realized had initially come from Mars. Some larger rock had hit Mars 17 million years ago and this future meteorite had been blasted out of the crater so hard it actually left Mars altogether. For a very long time it orbited the sun until eventually, 11,000 years ago, it managed to land on Earth.
All well and good but you'd think that any rock being blasted off a planet would be pretty thoroughly sterilized in the process. Some years after the meteorite was discovered, in 1984, some scientists gave it a hard look and made a pretty amazing discovery. There were structures inside the rock that looked a lot like fossils. That isn't conclusive evidence that there was once life on Mars since people have found non-biological ways that those structures could have formed though it is suggestive. What it does prove, however, is that neither the shocks to the rock nor the heating it endured were enough to destroy them. In fact, small things tend to deal with acceleration very well and bacteria are very small. And no matter how much the outside of a meteor might warm up when it enters the atmosphere the inside tends to remain pretty cool as this one did.
Could some spore remain viable for the 17 million years it took for this meteor to get from Mars to Earth? I have no idea but there are probably rocks that didn't take that long. Earth has been around for 4.5 billion years and life got going pretty fast, maybe 4 billion years ago. So that's an awful lot of chances for some ejecta to leave Earth and make the trip in a lot less than 17 million years. The speed with which life evolved after Earth got oceans is sort of evidence that Mars should have developed life too when it had oceans but that's far from certain.
Of course Earth's gravity well is a lot steeper than Mars's is at 10 km/s to 5 km/s. So an impact that would hurl a terestrial rock into space would be much rougher than one that would do the same on Mars. Maybe life can make the trip from Mars to Earth but not vice versa? That seems improbable to me but I can't rule it out.
And on the other hand again it seems very odd to me that Earth microbes could be transported to Mars and out compete any native Martian microbes. In a warm sea, sure, the evolution of Eukaryotes was improbable and I could easily see them beating up Martian bacteria. But the sort of extremophyle bacteria that could live under Martian conditions are much simpler and haven't had the opportunity to gradually evolve along with Martian conditions. It might be that Mars is actually totally dead but life could live there if seeding by ejecta really is impossible.
This whole post is totally speculative of course and I don't really know that much about the current rationale behind NASA's policies. Maybe they were put in place in the 60s when we didn't know so much about the solar system and have persisted through inertia. Maybe they're carefully and continuously evaluated. I've tried to find places to ask these questions but without much luck. Maybe, if I'm lucky, someone will read this and enlighten me as to what I might be missing.
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