Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Water on Mars

  1. Dec 6, 2004 #1
    We had a substitute teacher today, and he was rather bland and tended to go off on tangents. He started talking about water (which was supposed to be the topic), only water on Mars. He said that there's evidence that water once flowed on Mars, but it's all gone now. I asked why, and he said "The water molecule has such a low molecular weight, that it just drifted away."

    Now, to me, that sounds like a really, really generalized answer. I talked to him after class, and I told him that the water wouldn't just drift away just because it's lightweight. Even the lightest molecules are still subject to gravity. I told him they still needed some other force to push or pull them away from the planet. He didn't agree, but then later proved my point by saying something along the lines of "No, they can just drift away if they are small enough...it's caused by solar winds and other space weather."

    I kinda cut it off there (it was lunchtime), but I still have some problems with that theory:

    1. Why would that cause the 'light' molecules of water to drift any more than the other solids on mars? The molecule is light, but unless the water water was vapor the molecules would be in the form of compounds. And, if anything, the density or weight of the compounds would cause a bigger effect than the molecular weight.

    2. Even if the water was vapor, why would the water disappear and other gases (such as methane) still remain?

    3. If the water was vapor, why would there be evidence that it was once there? (From what I know, he made the point that there was once water because of canals and river beds and such. I think there now we have found ice, but I don't think he knew about that).

    What does everyone think? Are my concerns valid? Is his theory valid?
  2. jcsd
  3. Dec 8, 2004 #2
    Has anybody thought about it and come up with something worth sharing?
  4. Dec 8, 2004 #3
    A general rule about substitute teachers: Unless they are retired teachers(and even then in some cases), anything they say should be subject to extreme suspicion. I've seen a lot of quacks.

    To say that the water would just drift away makes no sense. Temperatures on Mars are never high enough to have water gas, and are generally too low to have water liquid (path finder did measure soil temperatures of 81 F, but viking measurements were from -178 F to + 1 F). Liquid d water could not jsut 'drift away'.

    1)Water is always a compound.

    2)They wouldn't, it would not work that way. By this guy's logic, venus should not have an atmosphere either, its much closer to the sun.

    3)There wouldn't be evidence if it was only a vapor, and as i said temperatures would not allow that.

    This guy is making stuff up.
  5. Dec 8, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Google finds this link

    but it doesn't say much about the actual rate at which water is escaping Mars, just that the phenomenon is currently being studied.
  6. Dec 8, 2004 #5

    HMM a very convoluted subject....there are many shcools of thought on this and they are all, just theories ..but a few things to consider are: a large cataclysm befell the planet long ago,such as a heat source so strong it blew the atmosphere off (planets collide?) with the cooling surface molten and runny (canals?) and or near unceasing meteor hits with the same effect as a planetary collission ...another theory is a thing that is at work right here on earth ..it is believed that a large percent of our weather is driven by a natural internal rotating machine (heavy metal liquid core) that acts as a generator of sorts..that if stopped would be a disaster of biblical proportions,and indeed if would leave the atmophere open to any number of degragating levels till gone...your teacher has just the last piece of the puzzle i think, in that erosion by solar wind ect just finished the job ...try this thought : a spinning ball creates a venturi effect of sorts holding any gas nearby its surface (an atmosphere) if the spinning were to slow down sufficiantly the hold would be gone ,and said gas would be open to these proccesses....but again ....thats only a theory .. :eek: c-ya
  7. Dec 8, 2004 #6
    DUDE ...Venus is not a water vapor atmosphere ...its HEAVY boiling acid gasses are way too heavy to be effected by the proccess we speak of... you wanna see that solar wind blow-off look at mercury ....nuff said
  8. Dec 8, 2004 #7
    boil off speed/velocity/realative time? ..of "water" is largely determined by atmospheric "Pressure" as well as tempeture ..not so heavier elements as on the outer planets..which like our own compressed gases boil off at relative low "Pressures/Tempeture" (out of the container/can) ...c-ya
  9. Dec 8, 2004 #8


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    In a thin atmosphere and low gravity environment, such as mars, it is not that hard for water molecules to achieve escape velocity. It appears likely mars had a much denser atmosphere in the past, but slowly bled it off due low gravity. The water molecules followed the lighter molecules.
  10. Dec 8, 2004 #9
    Low gravity doesn't cause things to 'bleed off'. I'm no Astro/Cosmo guru of the year, but even I know for a fact that thing's don't just start moving without some force to move them. Are you saying it is because of solar winds?
  11. Dec 8, 2004 #10


    User Avatar

    Staff: Mentor

    The motion is due to the temperature. The "bleeding off" is due to the low gravity: the lower the gravity, the lower the escape velocity and the lower the particle speed necessary for the atmosphere to "bleed off."
  12. Dec 8, 2004 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    water gets to the top of the atmosphere, where it is dissociated into H and O by the Sun's UV; some H atoms have speeds - due to their temperature - that are greater than the escape velocity, so they are lost to Mars. Oxygen in Mars' atmosphere is removed by oxidation reactions with minerals on the surface. As there is no on-going source of water (unlike on Earth - volcanism), over time most surface water either freezes (polar cap), migrates into the crust (and freezes, to form permafrost), or is lost from the top of the atmosphere (as above).

    There are some interesting and relatively simple calculations you can do to estimate

    There are other mechanisms by which H may be lost from the top of the atmosphere. I saw a very nice photo, in the Lyman alpha wavelength, of the Earth (from a spacecraft), showing H streaming away from Earth (yes, we are losing a lot of H, every day), but when I searched for it I couldn't find it :cry:

    Edit: If you read the material in the link which pervect gave, you'll find some discussion of the mechanisms and estimates of the rates through time (double edit: I see that pervect has said the same thing, only more clearly).
    Last edited: Dec 9, 2004
  13. Dec 8, 2004 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    If you read the link I posted, you'll see that there are a lot of effects that can lead to the loss of atmosphere. One of the effects is simply that there will be a Maxwell-Boltman velocity distribution of velocities for a gas, with light gases (like the disassociated hydrogen from water) having the highest velocity.

    If the velocity of a particular atom is above escape velocity, it will escape. This is well out on the "tail" of the statistical distribution of velocities, so it doesnt' happen a lot. Given the details of the atmosphere, one can calculate the loss rate due to this particular mechanism.

    Escape velocity is lower for a smaller planet, making escape via this mechanism more likely. In fact, the lower escape velocity probably increases the loss due to other mechanisms as well.
  14. Dec 10, 2004 #13
    There is also evidence on the face of Mars that suggests that the planet was struck by an extremely large object. There is a theory that suggests that this blast caused extreme volcanic activity and the following dust storms (nuclear winter) sped up the freezing process. Just another possibility.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Water on Mars
  1. Life on mars? (Replies: 22)

  2. Conveyor belt to Mars (Replies: 4)

  3. Carbon dating in Mars (Replies: 3)