The Science of Sustainability

News From Mars: A River Ran Through It

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Mars streambed conglomerate compared to example on Earth. Credit: NASA/Mars Science Laboratory

Mars streambed conglomerate compared to example on Earth. Credit: NASA/Mars Science Laboratory

The news from Mars just keeps getting better. NASA's Curiosity rover, now exploring the alluvium at the base of Mount Sharp in Gale Crater for over two months, has struck pay dirt: the gravel and river stone conglomerate laid down by an ancient Martian stream!

Hearing that bit of news was tantalizing, and seeing the photographic evidence a true thrill for someone who loves scratching around in the rocks and sand of the desert, looking for signs and clues of the geologic history of the place. Death Valley is my favorite hunting ground for signs of water–and why, you might ask, would I go looking for water in a desert? To that I would answer, looking for signs of water in a lake or river is just too darned easy.

The pictorial evidence in question is a bed of gravel and rounded stones bound together by finer material that has solidified over time. The conglomerate rock is similar to examples of sediment found on Earth: solid layers a lot like very coarse concrete. The materials were washed down by river or stream action, transported from the locations where they originally eroded, and in the process of tumbling along with the flow the larger stones becoming rounded and smoothed.

On Mars, at the site of Curiosity's find near the bottom of a large alluvial fan (the pile of material deposited by stream action at the bottom end of a downhill flow), the layer of conglomerate material has been broken and upturned for the rover to see and examine—possibly by a meteorite impact in the past. Were it not for whatever had broken and exposed the layer, Curiosity's wheels may have rolled right over it, inches from the great find but unaware of it laying there under topsoil. From the size and appearance of the rocks in the conglomerate—from sand grain to golf ball sized—scientists estimate that the water which deposited the material probably moved along at a speed of three feet per second, and was anywhere from a few inches to two or three feet deep.

I recall some interesting formations out in the deserts of Northern Arizona, where I lived for a few years, which the Curiosity report from NASA reminded me of. In one particular spot, somewhere east of Flagstaff and maybe west of Winslow, are what I call "petrified rivers". Snaking around the flat desert just off the north shoulder of Interstate 40 is a winding network of raised "roadways" of rock, rising a foot or two off the ground and meandering about in the pattern of flowing streams. I learned that these features are sedimentary conglomerate rock laid down by streams that flowed there in the past and solidified over time. Then, long after the water stopped flowing as the region dried out, erosion by wind and rainfall gradually wore away the softer surrounding soils, leaving the harder streambed conglomerates intact, exposed for anyone passing on the Interstate to see and wonder about.

The water-laid conglomerate stone on Mars that Curiosity happened upon is a first. There has previously been plenty of evidence letting us speculate about past liquid water on Mars, from drainage channels viewed from orbit to various minerals found on the ground to ice buried under Mars' surface, but this new find is the first concrete evidence (so to speak) for actual water-transported material.

As always, each new discovery like this makes me ask, "what's next?" Really, I may be leaping too far ahead of conservative speculation here, but personally I'm hoping to see fossils….

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Category: Astronomy, Blog

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Ben Burress

About the Author ()

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • rex burress

    This is a fascinating conjecture well presented. The planets, and probably the entire universe seems to have a commonality of scattered stones.

    • bburress

      I thought you might like the rockhound aspect of this news….

  • http://www.facebook.com/craigrosa Craig Rosa

    Is there any way to tell how persistent the flow of water was? How long does it take for such conglomerates to form, in general? I'm just curious if these flows were around for thousands of years or more, or if they could have been formed as a result of a few intense, transient events. If they were persistent, it would seem to be a good place to look for evidence of life.

    • bburress

      NASA's report indicates that the rocks in the conglomerate, and those that have fallen out through erosion, are somewhat worn and rounded, and also that they were transported from above the crater rim, far away. As they mention, this means that the water flows that shaped and placed these rocks into the conglomerate layer was either continuous or repeated over a long period of time, "not just once or for a few years." So far, I haven't seen any numbers regarding how long the flow occurred, but it would seem that the action was part of a regular and long term water cycle.