Monday, April 8, 2019

How likely is it that there was life on Mars?

People have been thinking about life on Mars for a long time, ever since the writings on the illusive channels of Mars were mistranslated as being about canals if not even longer back.  The original Viking landers had some experiments to help detect life and people have been looking ever since.  If life had ever existed on Mars it's quite likely that the loss of hydrogen and breakdown of the magnetic field have ended it so I wouldn't bet on there being life now.  But for reasons I'll explain I think it's actually pretty likely that there was life at some time in the past.

A while ago I blogged about a timeline of life on Earth extending from its origin in Earth's path to its end in the future (if we don't do something about it).  Here it is again.

Of course that's not how things went or will go on Mars.  Well, Mars formed at the same time as Earth did and I can't find any reason that liquid water would have taken a very different amount of time to form.  But we don't know about life, we'd have to add some bits about liquid water going away to cold and low pressures, and it will take longer for the Sun's expansion to bake Mars 'cause it's further out.

How long did Mars stay wet?  We don't really know precisely but this article at least says that we know there was liquid water 3 billion years ago.  So a timeline like this.

As was pointed out when I wrote about that last timeline, we know that on Earth it took at most 120 million years for life to appear after liquid oceans did but we don't really have a minimum estimate.  The geology of Mars seems close enough to Earth's that it should have had the same sort of geothermal vents to provide free energy to pre-photosynthetic life as Earth had.

To nail down out how hard it is to evolve life really requires looking a bunch of planets that evolved life and a bunch that didn't.  Hopefully space exploration will get to there at some point.  But until then it sorts of seems that evolving life itself is pretty easy compared to photosynthesis or mitochondria or multiple cells.

Now, you can bring in anthropic reasoning and say that the fact that we're here to observe the result means that we're probably not looking at the typical case and that's fair enough.  But there's no reason to think that that luck should have been concentrated at the first stage as opposed to Earth lucking out in how quickly photosynthesis evolved.  Or having the many disasters that mostly ended life not having been a bit worse.

And Mars is a lot smaller than Earth is.  About 1/3 the surface area.  So you'd expect that it would take about 3 times as long for life or photosynthesis or whatever to arise on Mars as on Earth.  If we were to assume that everything would take proportionately as long to happen on Mars as on Earth that would mean life but no photosynthesis.

This is all highly speculative but it's the best guess I have.  That at some point Mars evolved simple chemosythetic life of the sort you'd find in deep sea vents or such on Earth but the planet dried out before they could even evolve photosynthesis.

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