Curiosity adds to evidence that water once flowed on Mars

When a narrow stream, flowing downhill, meets a wide, significantly-flatter valley, you get an alluvial fan — a place where the flow of water spreads out, slows down, and leaves behind all the rocks and sediment it's no longer moving fast enough to carry. At least, that's how it works on Earth.

Once upon a time, it may have worked that way on Mars, too. Yesterday, NASA announced that the Curiosity rover had documented geology that looks very much like an alluvial fan and rocky deposits that also look very much like what would be left in an alluvial fan on Earth. You can see the comparison of some of those in the image above. In these Martian geological features — as in an Earth-bound stream bed — you find smooth, rounded pebbles and conglomerates, masses of pebbles cemented together over time. The rocks photographed by Curiosity are also too large to have been blown into this sort of arrangement by the wind.

All of this adds to the long string of evidence that Mars once had flowing water on its surface. In fact, reading up for this post, I was surprised to see how much evidence there actually is for this, some direct and some indirect, stretching all the way back to the Mariner 9 orbiter mission in the early 1970s. And, of course, there is water on Mars right now. It's just not flowing water. Previous probes have measured a small amount of water in the Martian atmosphere, and the planet's polar regions contain both frozen carbon dioxide and frozen water. Viking 2 took pictures of frost on the ground in the late 1970s, and in 2008, the Phoenix lander literally dropped out of the sky onto a patch of ice.

Frost on the surface of Mars, as photographed by Viking 2.

The bright spot near the lander's leg was later verified to be a patch of ice.

The new photos from Curiosity add to this growing body of evidence. Taken all together, it's pretty safe to assume that Mars was once a wetter place. Here's NPR's Bill Chappell:

"There is earlier evidence for the presence of water on Mars," the agency said in a press release, "but this evidence — images of rocks containing ancient streambed gravels — is the first of its kind."

Scientists have not yet estimated the age of the rocks, which may have been buried beneath the surface. Their age could be several billion years.

The next step will be to find a good spot to drill into the rock, NASA says. And they'll be looking for possible carbon deposits to determine whether the water on Mars once supported life.

Read the rest of the NPR story

Read astrobiologist Caleb Scharf's thoughts at the Life, Unbounded blog

Read Wikipedia's extensive entry on evidence for water on Mars

Check out this 2007 report prepared by the National Academies of Science, which discusses a strategy for looking for evidence of life on Mars. It includes a summary of evidence for water on the planet.


  1. Without the occasional ice age or the landscape-altering activities of plants and animals, how long would features like this persist on mars?   Even in a desert, a gully should silt in with dust, shouldn’t it?

    1. The surface of Mars is very similar to Death Valley. If you go off the path in Death Valley, you’re stepping on rocks that have been sitting in that exact position, exposed to the surface, for potentially hundreds of thousands of years. No exaggeration! They develop a patina on their surface due to such lengthy exposure, called desert varnish, which can be dated with reasonable accuracy. You can see this very clearly in most Mars photos; it’s the dark discoloration on the rocks (it’s very visible in the second photo here with the ice).

      Beyond that, these alluvial fan rocks already seem to be lithifying into a concrete-like rock (conglomerate or breccia). This can survive essentially forever – tens of millions of years, at least – especially without active tectonics. It can be buried, yes, of course – like your gully example – but just as in Death Valley there really isn’t enough dust around to do that on Mars. And I suppose it’s possible that it goes through periods where it’s covered with silt or sand or whatever for a while, and then uncovered. The alluvial conglomerate is hard enough to not be eroded beyond recognition from basic wind-powered erosion (best exemplified by e.g. Arches National Park, which formed the way it did because it’s all relatively loose/weak sandstone).

  2. Look, it’s really quite simple. Humans lived on Mars in the past. We fucked it up just like we’re fucking up this planet. Once it got too bad we left there and came here, the few that could. Lacking the industrial infrastructure to rebuild immediately our ancient ancestors regressed to hunter-gathering. Over time we have regained our technical prowess and are again fucking up our home planet. We’ll soon have to leave and go elsewhere again. But only a few will fit into the spacecraft. I wish them well on their journey. Hopefully the lessons we’ve learned will not be forgotten. There are only so many places we can survive.

  3. That looks like a chunk of old concrete to me… and I’ve seen enough construction sites to know that you need water to make concrete! Proof that Mars once had water, and possibly urban infrastructure.

    1. And Martian quality control guys arguing with the Martian foremen that they put too much damn water in that concrete, and it’s never going to reach 4000PSI!  My mind is blown.

      1. Mars has notoriously weak protections for worker unions. If you think things are bad in red states, just imagine a red planet.

  4. Note that that Viking 2 image is false-color: the sky on Mars is not so blue! This page has a presumably more accurately-colored image from the NASA website. The blue-skied image was created by a wikipedia editor from pictures taken through different color filters, but the wavelengths of the filters didn’t match the wavelengths that the rods in our eyes respond to, so he had to improvise his own color-correction scheme which he explains here. But I would trust the corrections made by NASA experts over a wiki editor (though there are conspiracy theories which say NASA is trying to hide the fact of Mars’ blue skies!)

  5. OK, so this is proof of liquid flow.
    How do we know this liquid was water, and not something else?

    I guess that the cold temperatures imply that is was likely not liquid chocolate flowing there, but maybe there are other possibilities?

    1. Water, with or without extra chemicals, is fairly common in the cosmos, plus is one of the few things we know that would be liquid under the likely temperature/pressure constraints of the Martian environment.  Occam’s Razor applies.

      e.t.a. Lava wouldn’t produce the same effects.

    2. Yes, any liquid could do it. But the set of naturally occurring liquids (which eliminates all but the simplest organics) that would *be* liquid at a temperature that would have been found on mars now or in the past (27C to -143C) is relatively small.

      Water, methanol, ethanol, mercury- not many others. And of those, water should be by far the most common.

    1. The term “alluvial” refers specifically to flowing water, and so alluvial fans are by definition formed from flowing water. There are separate technical terms for wind-formed features (which I assume is what you’re referring to by “drift”) but they’re normally just referred to as “dunes” and the implicit assumption among geologists with dunes, even though it is a catch-all term, is that they’re shaped by wind. True alluvial fans and dunes are quite distinct when seen from above, and the ones on mars look just like alluvial fans on earth (and are positioned next to a large mountain, just like alluvial fans on earth).

      That said, if I were not a geologist and thus not inclined to being so pedantic about geology, prior to this latest discovery one could have theoretically argued that the alluvial fans on mars are not alluvial but are wind-formed. Maggie covered it here, though – we now know that the pebbles forming the alluvial fans are too large to have been moved by wind.

      For parts of the martian surface that haven’t been seen up close and personal by rovers, it hasn’t historically always been easy to tell what we’re looking at in terms of geomorphology or basic geology. You can compare aerial photos of mars to aerial photos of earth and assume things that look the same are the same, but you can’t say for sure without getting up close to see if the details are actually the same.

      This discovery – just a handful of photographs – is huge for this reason. It means that we can say with much more certainty, now, that features we see on the martian surface are essentially the same as what we see on earth. Discoveries like this will be extrapolated to the rest of the surface and will result in reasonably accurate geologic maps, which up until this point were comprised of pretty much 100% speculation.

  6. So far as I know, no Martian lander has yet been equipped with a microscope and I haven’t seen any answer to why that might be. It seems to me a site such as this would be ideal to examine for microbial fossils. While I know that visual evidence of microbial fossils is open to debate as to whether they’re really biological in origin or the result of chemical processes, it would be interesting at least to see if anything like that exists.

    Update: Apparently Curiosity does have a microscope on its robotic arm. I don’t know how powerful its magnification is though.

  7. K.  There was water on Mars.  And lets take it as read that there was some microbial life.  Or not.    Now can a get a toaster that accurately toasts both sides the same.  And a jetpack.

    1. Right, we assumed water flowed on mars based on very limited evidence – basically just that it looks the same from the sky as similar features on earth. We can now say with certainty that it was water and not some other unknown geological process that formed those features – features which exist in great quantity across the martian surface.

      Nobody in their right mind would do any sort of geology research, or just basic geologic mapping, without visiting the area of study in person to check things out – there are always surprises in the field and it’s the field observations that are the very core of geology.

      This is basic science at, well, its most basic ;) It’s a hugely important finding, though – this is fundamental, groundbreaking stuff in the planetary geology field. This is like the age of enlightenment for planetary geology right now, basic low hanging fruit being discovered left and right.

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