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A River Runs Through…Gale Crater

The views expressed are those of the author and are not necessarily those of Scientific American.


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The "Hottah" rock outcrop, an ancient stream or river bed (NASA/JPL-Caltech/MSSS)

It’s one thing to spot stuff from orbit above an alien world, quite another to get in close.

Earlier Mars Reconnaissance Orbiter imagery of Gale Crater, now home to NASA’s Curiosity rover, had shown signs of what appeared to be something akin to an ‘alluvial fan’ spreading downwards from the crater rim. It was extremely tempting to conclude that at some earlier time there had been a flow or flood of liquid water washing into and across a section of the crater floor.

Close up of the crusty stream bed, a larger 'clast' is circled (NASA/JPL-Caltech/MSSS)

Now Curiosity has come across the ground-truth, including a rocky outcrop that is made of gravelly pebbles (clasts) cemented together into a crusty conglomerate. This is a chunk of up-tilted, uplifted ancient stream bed. The pebbles probably originated from the crater rim a few hundred meters higher up and their range of sizes, somewhat rounded shapes and placement all point towards their having been washed and rolled in water that was somewhere between ankle and hip deep.

The view from orbit, color coded to show relative elevation. The alluvial fan from the Gale Crater rim is highlighted (NASA/JPL-Caltech/UofA)

It is a remarkable discovery. A great deal of speculation has been made about the nature of channel-like features across the surface of Mars. Water has always been a prime contender for carving and depositing these structures, and now it really does seem that it once flowed, albeit perhaps temporarily, on the planetary surface to leave this formation of gravel fixed into a mud-like cement.

Today Gale Crater may be drier than the driest desert on Earth, but a long time ago there was at least a brief respite as water gurgled and sparkled in the sunlight on Mars.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His latest book is 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos', and he is working on 'The Copernicus Complex' (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Glendon Mellow 12:21 am 09/28/2012

    Stuff like this makes me really happy to be alive right now.

    Link to this
  2. 2. curmudgeon 9:19 am 09/28/2012

    Yippee! There was once water on Mars! Just can’t wait to see all those practical offshoots of this important piece of knowledge transforming our …. oh!

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  3. 3. tharriss 9:59 am 09/28/2012

    Yes curmudgeon, gaining knowledge is useless unless it leads directly to a new type of cell phone or toaster strudel.

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  4. 4. Acoyauh2 11:20 am 09/28/2012

    Awesome discovery up there… lowsy estimating down here. “Somewhere between ankle and hip deep”? Really? Somewhere between a mouse and a truck sound good to you?
    The difference between a pee-creek and a full river is an estimation we could improve a bit, hmmm?
    Great job, buddy, keep on rovin’!

    Link to this
  5. 5. CherryBombSim 6:53 pm 09/28/2012

    Most likely, it was ankle deep at some times and hip deep at others. Like many streams on Earth. From the few pictures I’ve seen, it looks like the flow was probably variable.

    Link to this
  6. 6. Bibbs04569 11:00 am 10/4/2012

    This Blog gives a lot of good information about the structure on Mars. I learn new information about how Mars got its structure. The author gave a lot of new ideas to us readers.

    Link to this

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