Tuesday, September 29, 2015

How about that water on Mars?


NASA announced a big press conference over the weekend about the planet Mars, and I took time out of Algebra II yesterday to watch it with my students.  As most people expected, they announced evidence of liquid water on Mars.  So what's the big deal?

For some time now, scientists have observed seasonal streaking on slopes on Mars.  You can see some of the streaks in the image above (courtesy NASA/JPL/University of Arizona).  They've been called RSL, which stands for Recurring Slope Lineae, which means, "lines that appear repeatedly on slopes."  That certainly seems like the action of some kind of liquid running down the slope making the surface appear darker.

But wait, Mars is farther from the sun that we are, about 48 million miles farther than earth on average.  That makes Mars much colder than earth.  According to Space.com, the average temperature on Mars is -80 degrees F.  During the summer, some spots on the equator can get up to 70 degrees F, but at night the temperature can still drop to 100 below.  How could there be liquid water under those circumstances?  Wouldn't the water be frozen, except for those brief warm days on the equator?

Well, if you remember back to your general chemistry days (if you took general chemistry), you might recall a thing called "freezing point depression."  When you add salt to water, the freezing point goes down (and the boiling point goes up).  That means you can have liquid water at temperatures colder than the normal freezing point of water.  Is that what's happening on Mars?

NASA scientists and collaborators pointed an orbital spectrophotometer at the RSLs.  A spectrophotometer is an instrument that can detect the presence of all sorts of chemicals, and the one they used is on a satellite orbiting Mars.  When they measured the RSLs directly, they found evidence of salt water, but when the RSLs were gone (during the Martian winter), there was no evidence of salt water.  The salt they found was a chemical called perchlorate, which apparently can lower the freezing point of water quite a bit.

So there you have it, folks, liquid water on Mars.  What does it matter?  Well, you'll notice in the press release that they make a big deal about life on Mars, which has an interesting connection to origins.  Here's how the reasoning goes: If you think that there is a natural explanation of where life came from in the first place, then you will not be satisfied with explaining life by claiming that God created it.  Instead, you'll want to figure out how it could have happened, whether or not God was involved.  But you also notice that life is amazing and complex, and it seems unlikely that it would just happen.  This leads you to suspect that life "emerges" (whatever that entails) when the conditions are right.  So if you can find life on Mars, that makes the second option seem more credible.  Finding life on Mars would make life on earth seem less spectacular and unique and more of a natural consequence of having a particular sort of planetary system.

Liquid water factors into this because liquid water is required for life as we know it.  Finding water on Mars makes it at least plausible that life of some sort could exist there, although perchlorate brine is not something I would think of as favorable to life.  Let's be very careful about this hype though:  Finding liquid water on Mars does not change the probability that we will find Martian life.  Liquid water makes it more plausible or possible that life could exist there, but plausibility does not mean probability.  All kinds of crazy things are possible, but that doesn't make them likely.

As for me, I think water on Mars is neat, and if there were genuine Martian microbes, that would be neat too.  Since I already know that God created life, discovering life on Mars wouldn't really change anything I believe about origins.

Ojha et al. 2015. Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience doi:10.1038/ngeo2546.

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