Ancient Mars: Cosmogenic Dating Methods Allow Estimates of Erosion Rates on Mars

The NASA rover Curiosity has been making its way across the floor of Gale crater on Mars since 2012.  It has closely examined a wide range of rock types which reveal that this location on Mars has experienced a long history of variable climates.  From water-infused bedrock, water-deposited conglomerates, wind-formed fossilized dunes to active sand dunes, Mars has been shown to have had a very dynamic past.  The presence of large valleys and cliffs in Gale crater reveal that extensive wind erosion has taken place and measurement of wind on Mars today proves that erosion continues into the present.   The extent of observed erosion  is so great that NASA scientists believe that it has taken at least several hundred million years of erosion to expose the current rocky surface in this crater.

You might wonder, how can scientists sitting on Earth look at these rocks and have any idea how old they may be?  How can we know how long it takes to erode a rock on a different planet’s surface?   A few years ago I wrote a blog piece in which I interviewed a Martian rock  (see: My Interview with a Martian: A Story of Origins) in which I asked that rock how long it had been sitting on the surface of the crater floor and what its future may be.  At the time I was not given a very precise answer.  Well, that rock can be excused for being a bit confused about time given how boring it may be sitting on the surface so long with so little to do.   But the Curiosity rover we sent to Mars has some very sophisticated analytical instruments on board which have allowed us to answer some of our questions. One of them can take drilled rock dust, heat it up until it melts and the “smell” the molecules that emanate from the melting crystals.  The rover can also precisely measure the ratio of isotopes–different  forms of an atom–allowing it to calculate the radiometric ages of some rocks.  The precision of this instrument is not like we have in labs here on Earth but nonetheless it’s an impressive achievement and gives us our first direct test of age of martian rocks and even allow us to infer rates of erosion of rocks.

The first radiometric dates are reported in a paper entitled In Situ Radiometric and Exposure Age Dating of the Martian Surface published in Science provided a K-Ar radiometric age for the rocks that make up the original–basement–rocks of the Gale Crater area.  That measurement yielded an age of 4.2 billion years give or take a few hundred million years.  This is on par with the oldest rocks on Earth and was the age predicted for this region of Mars based on previous relative methods of dating such as crater density counts.

That date was expected but what was really interesting is that the form of dating used also allowed the scientists to estimate how long it has been since this particular set of rocks came to be exposed at the surface.   The whole area you see in the pictures above and below is thought to have been buried under dozens if not hundreds of feet-thick sedimentary rock. That rock has eroded, almost certainly by wind, over time to reveal the layers that we observe at the surface today. 

Emily Lakdawalla reviews the significance of the dates. In one of her blog posts “Curiosity results at AGU: Gale crater rocks are old, but have been exposed recently” Lakdawalla explains how the length of time a rock has been exposed at the surface can be calculated:

Mars lacks a significant magnetic field, so anything on its surface is exposed to bombardment by galactic cosmic rays. Galactic cosmic rays can produce specific isotopes of noble gases: argon-36, neon-21, and helium-3. These gases would not have been present in the rocks when they first formed, so if you know the rate at which the isotopes are produced on Mars, and if you measure their abundance in current rocks, you can estimate how long the rock has been exposed at, or near, the surface.

This type of analysis has been done for rocks on Earth as well.  It is called cosmogenic nuclide dating and it allows for rough estimates of the rates of erosion, or exposure times, in different locations (eg. how fast are rocks eroding in Utah vs deserts in other places in the world).    So what can we learn about erosion on the surface of Mars from this form of analysis? Lakdawalla goes on to explain:

You can imagine how the relatively weak mudstones of the Sheepbed Unit are getting sandblasted away by wind scouring, and that’s undercutting the more resistant layers like Gillespie Lake. In other words, the Sheepbed rocks are being exposed by a process of scarp retreat. Assuming that this is a continuous and ongoing process, if you follow the direction of the wind, then, until you get to the scarp, that’ll take you to Sheepbed rocks that should be freshly exposed. You can estimate the rate of scarp retreat, and it’s about 1 meter per million years.

I think the operators of the Curiosity rover have come up with a pretty nifty way of estimating erosion.  The 1 meter per million years is a very rough estimate so maybe it is only ½ meter or 2 meters per million years.  Regardless, this data reveals that erosion really is happening on Mars. You might think that 1 meter of rock erosion in 1 million years is a very very slow pace but remember the atmosphere is very thin on Mars and there is only wind and freeze/thaw cycles as the primary forces of change on the surface.

Back to our martian friend laying there on the surface (see picture to the right). In my interview (My Interview with a Martian: A Story of Origins) I asked this rock what would happen to it in the future? This was its reply:

Well, assuming you don’t poke me or shoot me with your little laser toy on your way by (referring to the Curiosity Rover obviously), I expect that I will continue to hang on for at least 10,000 more years but possibly several hundred thousand years or so after which I will probably have completely fallen apart into all those small stones that I was originally made of.  Now those parts are much tougher than I was and wind and thermal tension won’t be able to tear them down so quickly so someday you will probably find a little pile of stones here spread out a bit as they are pushed slightly by the wind and sand over time.  On the other hand, it is possible that another meteorite could hit near here and cover me with a blanket of rocks in which case those new friends would shield me from erosion and I might live for millions of years to come.

The prediction of 10,000 years or a bit longer seems about right given these new estimates of the rate of erosion. I had thought maybe it could last another million years but apparently erosion is occurring at a faster pace than I would have guessed.  Still, this little rock is but a very small piece of the geological terrain in the area that includes many small meteor craters and many hundreds of meters of rock that have been eroded in the area before our little friend even came to find its way to its present location.

The Curiosity rover science team has now collected enough evidence to convince them the rover is now driving around on rock that was deposited originally as sediments in a large lake.  But that lake has been dry for hundreds of millions of years if not billions of years.  Since then massive amounts of rock have eroded and small new craters have been created and eroded and then been covered with even more debris from other cratering events.  All the while wind has been slowing eating away at the rock surfaces and moving material from one place to another.  If we took a snapshot today and came back one million years from now it might look very similar but in many subtle ways the landscape is constantly changing.

Yes, Mars is a dynamic planet but that change is in slow motion.

A view from the lower levels of Gale Crater looking toward the mountains that exist in the center of the crater. The rocks here are thinly layered sedimentary rocks. Out of view are hills that contain thousands of thin layers of rock either laid down by water or wind. In the past these rocks would have covered the location that the rover was standing at the time it took its picture. Photo: NASA-CalTech/JPL 1516MR0077190000204658E01

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This is a revised and updated post from 2013.