Are Martian gullies formed by water or not?

By Phil Plait | May 5, 2010 7:00 am

The idea of liquid water on Mars is an enticing one. We know life on Earth needs liquid water, and if we find it on Mars… We know there’s plenty of frozen water on Mars; we see it there in abundance. But Mars is cold, and the air is thin, making liquid water on the surface difficult to achieve, let alone sustain.

But there’s been tantalizing evidence. Ever since Mars Global Surveyor got to the Red Planet in 1997, we’ve seen gullies sprinkled here and there. These gullies form on slopes near the tops of the hills, and are clearly the result of something moving downslope and digging furrows. But is that something water, or just sand and dust? The conclusions flip-flop back and forth; I’ve seen papers come out saying water-not-sand and others saying sand-not-water several times.


A new paper has just come out saying it’s sand and dust, and not water, that’s doing the trick. The authors did a clever experiment. They assumed that it was dry ice — frozen carbon dioxide — that was behind the gullies, and not water. As the dry ice turns into a gas in warmer weather, they supposed, it blows out of the ground and gets in between the sand particles, causing them to run downslope like a fluid. That’s a fair assumption, given how common dry ice is on Mars. They then set up a tub filled with Mars-like sand, piled it into a slope, and used an air pump to force air under the sloping pile. Sure enough, the sand flowed down, making gullies that seem very much like what’s seen on Mars!


Here’s one of their experimental results (left) compared to Mars (right). You can see a lot of the gross features are very similar. Their idea also naturally explains why these gullies form mostly at mid-latitudes: closer to the equator it doesn’t get cold enough to make much frozen CO2, and closer to the poles it never gets warm enough to turn to gas and drive the sand flows. And their experiments do reproduce a lot of the features of the actual Martian gullies…

A lot, but not all. As I wrote a few weeks ago, new observations of some gullies show peculiar features. One of the most provocative is that the trenches carved by the moving material don’t fan out as you might expect at their downslope terminals — look at the image at the top of this post to see what I mean (and click it to embiggen it most cromulently; it’s beautiful). As I said in that post, "They just kinda stop." Flowing sand tends to spread out, or form a raised feature at a trench terminus. But these new gullies simply don’t do that. That does seem to point to some other material driving the motion. Water? Maybe.

There are still problems with the idea of water; if it erupts out to form some of these gullies, how does it get resupplied? In the paper, they note that CO2 will freeze out of the air on Mars, so the supply is re-established. Water, not so much. It hasn’t rained on Mars in a billion years. However, an underground aquifer of some sort isn’t a totally crazy idea.

So clearly there are issues with both ideas, just as both ideas have their strong points as well. But I wonder… are we seeing more than one process here? After all, there’s no rule that says all gullies must form the same way, and the different morphologies (structures and shapes) hint that perhaps different mechanisms are at work on Mars.

I know a way we can find out: send more probes. Lots of probes. Because while I love a mystery as much as the next scientist, another thing I love is answers. Mars is Ellery Queen, Agatha Christie, and Scooby Doo all rolled into one big red ball. The clues are tantalizing, the evidence is scattered, but dagnappit, the answers are lying right there on the surface.

All we have to do is go there and flip to the last page.

Tip o’ the sandworm to Emily at The Planetary Society Blog for the image, which is from HiRISE. I found out about the new research from Slashdot.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science
MORE ABOUT: gullies, HiRISE, Mars

Comments (53)

  1. Great post. Luckily, there’s no political or monetary payoff for either answer, so this debate can stay about as purely in the scientific realm as any…

  2. Steve Norley

    Dude, those are sooo obviously the tracks left by giant sand worms.

  3. Sarafan

    Yeah, I suppose a Newtonian Fluidist would say that…

  4. DennyMo

    Tom, good luck with that. It’ll cost money to solve the mystery, so debate will wander from the scientific about as quickly as it does for any other expenditure…

    Love the Ellery Queen reference, haven’t thought about those books in forever. But I think the answer is simple: those in the top picture are worm trails. Giant worm trails. Or not.

  5. elgarak
  6. Douglas Troy

    #4 elgarak – my thoughts exactly … there’s Spice in them dar hills!


  7. Torbjörn Larsson, OM

    It is always nice to see areas passing from everything but the kitchen sink stage, but to actually see them ending up in the tub is even better. Hooray for actual tests!

    papers come out saying water-not-sand and others saying sand-not-water

    I doubt they can conclusively predict and test that. As the article says it can be plural processes and it is very difficult to make a “no go” on theories without testing. Especially by using another theory altogether. That is more likely a turf (here perhaps better “surf”) war.

  8. Dennis

    It’s obviously martian gophers – well, possibly mars moles.

  9. Jason

    I admit liquid water on Mars must be a challenge, but are there any known chemicals on Mars that could be mixed with the water that would allow the solution to last longer in the Martian environment? Has there been any attempted spectrographic analysis of the recent gullies to see if anything shows up?
    I don’t know if it is possible, but on some of the recent gullies that may be water generated we could check for ice. If the concentration of Ice is higher than the surrounding area that could indicate that water was a formative force. I would think that IF liquid water was the cause then not all of it would boil away but some would also freeze and leave a higher concentration of ice in the area. The downside being that we know the Ice will sublimate away so any measurements would have to be done fairly quickly and probably rely on getting lucky to catch one forming or just formed.

  10. ND

    elgarak beat me to it!

    Sandworms of Dune, er, Mars.

  11. “I know a way we can find out: send more probes. Lots of probes. ”

    Spoken like a true science lover. :) Now all we have to do is convince someone with money.

  12. Adrian Lopez


    “But I think the answer is simple: those in the top picture are worm trails. Giant worm trails.”

    That was my thought as well. Martian sandworms.

    I can’t wait for evidence of the ones that build pyramids of increasing size.

  13. They need to scale up that experiment. Go from a tub to a pool, see if the artifacts scale up, too.

    This sort of thing has science fair written all over it. I hope some kids get inspired to join in the fun.

  14. Sandworms? Nah, it’s the guy who owns the old movie theater on Olympus Mons. And he’d have gotten away with it too, it it weren’t for them darn rovers.

  15. Messier Tidy Upper

    So clearly there are issues with both ideas, just as both ideas have their strong points as well. But I wonder… are we seeing more than one process here? After all, there’s no rule that says all gullies must form the same way, and the different morphologies (structures and shapes) hint that perhaps different mechanisms are at work on Mars.

    Yes I agree that’d be my thinking here too. No reason it has to be either /or when it could very well be both /and! 😉

    @12. Adrian Lopez :

    That was my thought as well. Martian sandworms.

    Nice but .. wrong planet I’d say. The sandworm one would have to be Dune or Arrakis! 😉

    Which, B.T.W. is the actual proper name for the star Mu Draconis – also known as Arrakis or Alrakis – a nice double star 88 light years away.

    Source :

    Plus Page 138, Collins Guide to Stars & Planets, Ian Ridpath & Wil Tirion, Collins , 1984 (old version) & Page 144, Collins Guide to Stars & Planets, Ian Ridpath & Wil Tirion, Collins, 2007. (revised version.)

    Old version of Collins Guide has ‘Arrakis’ as the name but new version lists ‘Alrakis’ instead.

  16. Dave

    Sorry to be priggish – I really enjoyed the article. But, to be priggish, I hope the answers are laying right there, not lying right there. :)

  17. Pi-needles

    I know a way we can find out: send more probes. Lots of probes.

    True enough but I know an even better way – send people! 😉

    This sounds like a good excuse for a Human mission to Mars to me. :-)

  18. Dave– laying is an active verb – “I lay the rock down” – while lying is passive – “The rock lies there.”

  19. oldebabe

    And what about Mars’ atmospheric pressures, gravity, etc. which would affect flow…?

  20. Terryl

    Look at the sides of trails, they appear to be raised. It must be worms because erosion would collapse the sides. Something is pushing it way thou the soil.

  21. I don’t know – looking at the first image under higher res and additional magnification, I suspect something else entirely is at work here.

    Look at the shadows of the central large furrow, and most especially to the right of it. Unless I’m misinterpreting them, that furrow is actually running a ridge line, and while it turns downslope slightly (lower right) it then turns away. That somehow doesn’t seem right.

    The other thing that seems counterintuitive is that the furrows, nearly all of them, show pushed-up edges on both sides. This might simply be an artifact of crosswind sand deposits, but it doesn’t seem likely that this would occur on both sides. Fluids digging furrows don’t typically push up deposits on both sides unless they’re moving pretty fast, and then they display turns with greater washouts and “elbows.” They also seem unlikely to peter out so abruptly without either fanning or “pointing.”

    The lack of points also seems to rule out some kind of erosion “slipping,” the crust cracking and splitting towards the downslope side – the ends shouldn’t look like that.

    Then there’s the idea that when the forks come together, the channels don’t widen, seeming to indicate that the channels were carved singly at different times.

    Now, here’s what’s occurring to me right now: is there such a thing as a CO2 glacier? Because this seems to fit the furrows closer than anything else I’ve thought of so far, and the cohesiveness of a glacial body might prevent sideslipping a bit. Not a perfect idea, I admit, but the best I’ve come up with so far…

  22. Steeev

    I’m with Just Al — those sure look like crevasses to me. Or old pudding skin. Same principle.

  23. Regarding that first picture: Have Frank Herbert, Bootsy Collins, and Fatboy Slim taught us NOTHING? “Walk without rhythm, and you won’t attract the worm.” Future Mars-o-nauts had best keep that in mind.

  24. Cody

    Haha Dave, Dave, Dave… as those in the gaming community would say.. get owned.

  25. Albert J. Hoch

    I’d think that water would be salty. Maybe very salty. The salt would remain behind and should be visible. Is anyone looking for salts? The air pressure sand experiment may not be realistic. If the falling sand is a mixture of dry ice and sand it should be out gassing as it falls. Further, vacuum conditions should prevail. The angle of repose increases as air pressure diminishes. This latter might account for the abrupt stop without fanning out.

  26. DanO

    Fractures, eroded by winds is my bet.

  27. Allen

    I dunno. All of these people are saying the sandworms of Dune, but I grew up with Beetlejuice, so I’m thinking the sandworms from that. Either way, the consensus here is some kind of sandworm, so that must be the answer!

  28. Mr Anonymous

    Wish we would send some there soon. There are alternative engines out right now that can get us to Mars in a month. We already know how to get “stuff” into orbit. What’s the hold up here? Send up the ship pieces. Build it in orbit perhaps by the space station. Go. Come Back. 10 week trip. No problem.

  29. mike burkhart

    Rember for years it was thought there were canals on Mars and many thought they were dug by Martians to get water from the poles. If there are sandworms there like on Dune maybe theres melage I would love some . One more thing on Dune in the 4 Dune novel : God empeor of Dune Dune became a green planet and in the 5 Hertics of Dune while becomeing a desert again Dune was destoryed

  30. @ Phil @ Dave:

    Maybe the furrows are laying there. Horny bastards.

  31. Who cares if there’s water on Mars right now? All we need to do is slam a couple comets into that rock, and then there’ll be water there all right!

  32. Gary Ansorge

    I think it’s just MArtian Otters going sand sliding/surfing.

    I wonder how the relative gravity difference would affect these experiments? Shouldn’t THAT result then be dependent upon the resistance to sliding vs the lubricant quality of the sublimating CO2?

    Gary 7

  33. Rivenburg

    I’m with Just Al on this,
    I suspect a combination of CO2 & water glaciation pushing up the edges of the channels.
    the arrow-head shaped piece of ice then sublimates leaivng the pointed furrow with raised edges all the way to the point. A combination of the two posited theories more or less.

    And too all who mentioned it:gravity & vacuum would both affect the fluidity and repose angle of the sand, as would it’s chemistry which determains the crystal structure of the sand, causing it’s grain size, roughness and any electric properties. If for instance the sand is composed of highly piazo electric crystals (common on earth), the wind blowing over a super dry dune could cause all kinds of unguessable effects. Leavitating sand for one. Ok I guessed.

  34. Rivenburg

    OK it just occurred to me that in those conditions the sand might be highly electrically charged, between piazo-electric effects & unfiltered sun. This MIGHT create intense surface tension effects creating the raised sides of the channels, actually PULLING the sand up as the water runs OVER the highly charged surface of the dune instead of cutting into it and cementing it in place as the “water” (probably a super-saturated mix of water & soluble stone/salts) sublimates leaving the salts /stone. Gypsum has been found in abundance, this is a very water soluble stone. OK, manned mission. Not enough data for meaningful answer.


    RE: “Lying” or “Laying”?

    This is according to

    Usage note

    LAY and LIE are often confused. LAY is most commonly a transitive verb and takes an object. Its forms are regular. If “place” or “put” can be substituted in a sentence, a form of LAY is called for: Lay the folders on the desk; The mason is laying brick; She laid the baby in the crib. LAY also has many intransitive senses, among them “to lay eggs” (The hens have stopped laying), and it forms many phrasal verbs, such as LAY OFF “to dismiss (from employment)” or “to stop annoying or teasing” and LAY OVER “to make a stop”.

    LIE, with the overall senses “to be in a horizontal position, recline” and “to rest, remain, be situated, etc.”, is intransitive and takes no object. Its forms are irregular; its past tense form is identical with the present tense or infinitive form of LAY: Lie down, children; Abandoned cars were lying along the road; The dog lay in the shade and watched the kittens play; The folders have lain on the desk since yesterday.

    In all but the most careful, formal speech, forms of LAY are commonly heard in senses normally associated with LIE. In edited written English such uses of LAY are rare and are usually considered nonstandard: Lay down, children; The dog laid in the shade; Abandoned cars were laying along the road; The folders have laid on the desk since yesterday.

    So, according to the above Usage note, it is rare and considered nonstandard to use “laying” instead of “lying” — i.e., it is over formal.

  36. Vern Wall

    It was electrical attraction and repulsion. Earthlings of a few thousand years ago recorded vast amounts of electrical activity between the planets. Modern scientists ignore electrical effects, even while watching lightning bolts in dust devils, because they never learned much about the subject.

  37. Brian Too

    I admit this is unlikely, but borderline possible. What if the entire slope were slumping off to the right (or left)? Surface cracks often develop when slopes slump. Surface slopes slump slightly!

    The sand composition does make this possibility problematic. However even sand can develop a crust.

  38. Don Gisselbeck

    Compare the result of an 25cm rain event on scree north of Great Northern Mountain near Glacier Park. The storm was in Nov 2006 (I think) and there are still many such gullies around and in the park.,-113.787632&spn=0.005476,0.02105&t=h&z=16

  39. mike burkhart

    This will be off topic but when Dune was mentioned I just had to give an oppion on the Dune novels .I started reading Dune after seeing the 1984 movie on tv .And I found Dune was unlike any scifi I’ve ever read before .I found Dunes antitecnlogcal,metipysical future was differet form other scifi that normaly shows a hitec future .Also the return of the fuedal order on an intersteler scale,seem to show the death of demorcy .Also no aliens,humans are the only intelgent life forms in the universe .For some one who grew up on Star Trek and Star Wars Dune was certenly diferent.

  40. Autumn

    @ Vern Wall,
    Which particular Earthlings “a few thousand years ago” recorded “vast amounts” of a force which was unknown to them?
    And how could they have possibly measured it “between planets?”

    To all others, I do apologise for feeding the troll, but I’m really curious.

  41. Could someone explain to me why liquid water is called “liquid water”. As I understand it, water _is_ liquid. In its solid form it is called “ice” and in its gaseous state its called “steam”. If someone said to me “hey I have found water” I wouldn’t need to be told it is liquid.

    Is there a scientific reasoning here that I am not aware of??

  42. James

    Because “ice” is also used to refer to non-water ices (ammonia, methane, etc), and “water” can be used as a shorthand for dihydrogen monoxide in both liquid and solid form.


    Vern Wall (#37):

    It was electrical attraction and repulsion. Earthlings of a few thousand years ago recorded vast amounts of electrical activity between the planets. Modern scientists ignore electrical effects, even while watching lightning bolts in dust devils, because they never learned much about the subject.

    From Dictionary.comLIE1:

    1. a false statement made with deliberate intent to deceive; an intentional untruth; a falsehood.

    2. something intended or serving to convey a false impression; imposture.

    3. an inaccurate or false statement.

    4. the charge or accusation of lying.

    —verb (used without object)
    5. to speak falsely or utter untruth knowingly, as with intent to deceive.

    6. to express what is false; convey a false impression.


    Enough said! 😎


    @ Dwight (#42),

    To answer your question, this is from Wikipedia — Properties of water:

    Forms of water
    Like many substances, water can take numerous forms that are broadly categorized by phase of matter. The liquid phase is the most common among water’s phases and is the form that’s generally denoted by the word “water”. The solid phase of water is known as ice and commonly takes the structure of hard, amalgamated crystals, such as ice cubes, or loosely accumulated granular crystals, like snow. […] The gaseous phase of water is known as water vapor (or steam), and is characterized by water assuming the configuration of a transparent cloud. The fourth state of water, that of a supercritical fluid, is much less common than the other three and only rarely occurs in nature. […] Since water only becomes supercritical under extreme temperatures or pressures, it almost never occurs naturally. One example of naturally occurring supercritical water is in the hottest parts of deep water hydrothermal vents, in which water is heated to the critical temperature by scalding volcanic plumes and achieves the critical pressure because of the crushing weight of the ocean at the extreme depths at which the vents are located.


  45. Thank you Ivan!

  46. Gary Ansorge

    37. Vern Wall

    Since we observe electrical discharges as a catastrophic breakdown of the dielectric resistance of a material, there can be none such across a vacuum, since there’s nothing to break down.

    Plus, it’s kinda hard to SEE electrical discharges between planets when that light emission comes from the ionization of a material that doesn’t exist.

    GAry 7

  47. Dave

    Clearly I’m an idiot! No more priggishness for me….


    @ Gary Ansorge,

    It appears that Mr. V. Wall has buggered off!

  49. Jefferson

    Just hook a dowsing stick up to a rover! lol

  50. Electric Universe Dude

    Good job IVAN3MAN AT LARGE and GAry 7! You ran off the only guy on this post that was saying something intelligent. “Space” isn’t a vacuum. Even NASA admits that much. Read up on your plasma physics before you start beating your chest and high-fiving.


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