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

Why is there water?

  1. Feb 24, 2004 #1

    jimmy p

    User Avatar
    Gold Member

    Why is there water on earth? I mean when the earth was young and VERY hot, wouldnt any water have evaporated off, especially as there was little or no atmosphere?

    Which would lead to the next question. Obviously there is water on earth, but why is there so much?? (reference to the above question)
     
  2. jcsd
  3. Feb 24, 2004 #2
    Interesting question..

    Maybe it did evaporate and exist as vapor for many years while the earth was still being formed.

    I have always found it fascnating that our planet's temperature is mostly in the range where water is a liquid. Water in it's liquid form is so essential to life. The cycle from liquid to vapor to solid also proves beneficial to many ecosystems. Plus, water is is the only molecule (that I can think of anyway) that is less dense in its solid form. If it were not, who knows what may have happened with earth.

    Only the creator, god, really knows.......
     
  4. Feb 24, 2004 #3
    Science doesn't exactely know, but there are some pretty good scientifically sound guesses.

    First, the heat actually helped create much of the water in the first place, basically by baking the water out of rocks and pulling it into the atmosphere. They also believe earth was struck by a great many ice-containing comets in its earlier years, some carrier immense amounts of water.

    Even in its earlier days, earth had a atmosphere, it just wasn't like it is now, much more hostile. The massive build-up of steam or water vapor would have just created more atmosphere. It would not necessarily just have evaporated into space, it would need to reach escape velocity to do this and although surely much of it did, still alot did not and remained trapped on earth.
     
    Last edited: Feb 25, 2004
  5. Feb 24, 2004 #4
    At the risk of turning this into a religious debate, I find it interesting that the Bible says that prior to the great flood it did not rain.

    Genesis 2:5, 6 says "Now there was as yet no bush of the field found in the earth and no vegetation of the field was as yet sprouting, because Jehovah God had not made it rain upon the earth and there was no man to cultivate the ground. But a mist would go up from the earth and it watered the entire surface of the ground."

    I know that the Bible is not a science book, but I always found that description intriquing.
     
  6. Feb 24, 2004 #5
    Maybe the person that wrote this verse should have taken a biology class.

    Nautica
     
  7. Feb 24, 2004 #6

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Along with temperature gravity is a key factor in determining the composition of a planets atmosphere. Any gas with a mass; such that at the average temperature it is able to achieve the planets escape velocity, will be lost to space. This is the reason we do not find free He or H in the earths atmosphere. Water is a pretty massive molecule, so it would take a very high temperature to achieve earths escape velocity.

    The moon on the other hand has insufficient mass to hold any significant atmosphere.
     
  8. Feb 25, 2004 #7

    jimmy p

    User Avatar
    Gold Member

    But when the earth was goddamn hot and probably not quite as big, surely the water could have escaped then?
     
  9. Feb 25, 2004 #8
    As I said in my earlier post, of course some water escaped, some water even escapes today. Just not all of it(obviously)

    I am not aware of any data suggesting the earth's mass has changed in any significant amount since the theoretical impact that created the moon.

    So earth's gravity can, for our purposes be held constant through-out history and it would most likely be safe to assume earth always had about the same ability to hold water through gravity.

    As far as the biblical reference. I find it interesting that you would find that intriguing as it basically says nothing.
     
  10. Feb 25, 2004 #9

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    How much water does the Earth have? Well, there are the oceans, ice caps etc ... if the Earth were perfectly flat, and all this water distributed evenly, how deep would the global ocean be? And remind us again what the radius of the Earth is? Where's my calculator, ... hmm, seems there's actually very little water on Earth .

    How much water is there in a comet? Let's take Halley's, assume a sphere of radius 10km, assume H2O makes up x% of the comet, ... Now, how many comets' water is in the Earth's oceans? Assuming only y% of the water in a comet which collided with Earth actually remained on the Earth, how many comets would be needed to create the Earth's oceans? my goodness, so {many/few} .

    If the period of late bombardment (approx the time the Moon was formed to the earliest rocks we've found) lasted z years, how many cometary impacts per year would there be (on average) to produce the Earth's oceans (assuming the Earth was 'dry' after the Moon formed)?
     
  11. Feb 26, 2004 #10

    russ_watters

    User Avatar

    Staff: Mentor

    How hot was it? How hot would it need to be for the water vapor to reach escape velocity (remembering that it'll cool while trying to escape)? A partial answer lies in the composition of the atmosphere today: Water has a molar mass of 18. Nitrogen gas: 14. Hydorgen gas: 2. Hydrogen escapes pretty readily. The others do not. But hydrogen at the same velocity as water vapor would have to have 9x the kinetic energy: 9x the abosolute temperature, or roughly 2,600K (compared with the current average earth temperature).

    Artman, for your quote: thats not what it says. The issues comes from the fact that not all verses start at the beginning of a sentence. The phrase before the part you quoted is essential:
    The quote to which you are referring is about the first day of creation (and that is also why the part about no people to cultivate the earth didn't make sense in the context you posted).

    Sorry for the OT... the Bible isn't scientific anyway, but I get annoyed when anyone misquotes anything (on purpose or not).
     
    Last edited: Feb 27, 2004
  12. Feb 26, 2004 #11
    Its not terminal velocity but escape velocity that we should be looking at here. I'm sure thats what you meant but I thought I'd throw that out just in case anybody gets confused.
     
  13. Feb 26, 2004 #12
    Also this is slightly Ot, but on the idea of the earth "luckily" being in the temperature range for liquid water there are actually two major solutions for our current meterological models, one of which is what is happening the other is known as "white earth" where the planet is entirely glaciated. The massive amount of frozen water reflects 70% of sunlgiht back into space, leaving the earth cold and entirely inhospitable. Almost all long term weather simulations generate into this climate, so the fact that earth is the way it is si something of a miracle.
     
  14. Feb 26, 2004 #13

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is getting very close to topics discussed in a book called Gaia by James Lovelock. He proposes that the composition of the earths atmosphere and the salinity of the oceans are unstable equilibriums which are maintained by LIFE. Perhaps it is the biology of earth which maintains the temperature in a range around the triple point of water, thus ensuring the continuation of life. Before you poo-poo this idea you need to read the above book. He backs his words with data.
     
  15. Feb 26, 2004 #14

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It's also a point extensively discussed in Ward and Brownlee's "Rare Earth" and the sequel "The Life and Death of Planet Earth".
     
  16. Feb 26, 2004 #15
    There also have been studies looking into the carbon cycle that have suggested cold conditions encourage processes that release carbon into the atmosphere and hotter temperatures encourage processes that bind carbon thus acting as a mediating globle thermostat. I read this in a article in Scientific America.
     
  17. Feb 27, 2004 #16

    russ_watters

    User Avatar

    Staff: Mentor

    Typo, thanks: I had skydiving on the brain.
     
  18. Feb 27, 2004 #17
    wow what an interesting topic this is
    no one really knows the true answers.. all predictions

    science is about predictions is well
    for example no one has ever dug deep enough to see whats in the center of the earth.

    but the life form has been first started under the water.
    that was proved by scientists. using water, sunlight, and all sort of gases. i think the right gas they found was ammonia i forgot :/
     
  19. Feb 27, 2004 #18

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What is a 'true answer'? That pencil on your desk ... how do you know it's composed of molecules? atoms? protons, neutrons and electrons? up and down quarks? You've no doubt read that glaciers recently covered a large part of what is today the US and Canada, and northern Europe. Neither you nor I were there, and we've no written records from people who were - how do we know that the glaciers were indeed there?

    One of the most important parts of science is the testing of predictions; a good theory will make predictions which you or I can go and test, through an experiment or an observation. If the experimental results or observational data are inconsistent with the prediction, then the theory is wrong.

    Conversely, one indication of a bad theory is its inability to make any predictions that can be tested.

    (Of course, I'm oversimplying things quite a bit, but I think you get the idea).
     
  20. Feb 27, 2004 #19
    Which way would primordial lightning tend to drive the equation 2H2+O2<-->2H2O ?
     
  21. Feb 28, 2004 #20
    \

    It would most likely be H20 --> 2H2+O2.

    But this would be hardly significant compared to the other forces and processes at work at early earth.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why is there water?
  1. Why Water Blue? (Replies: 20)

  2. Why water is colourless (Replies: 19)

Loading...