1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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 the Moon

  1. Nov 14, 2009 #1
    Being not very tech minded I wonder how water (ice) could exist for billions of years in a vacuum without turning to vapor? Ice evaporates at atmospheric pressure on earth and I guess it is called subliming? Maybe it's because of the super low temps on the moon? Comets are supposed to have lots of water in them too and I have never understood this.
  2. jcsd
  3. Nov 15, 2009 #2
    Two things come to mind. I hypothesize that:
    There is no wind on the moon so the evaporated water would not blow away.
    There is microgravity on the moon - so the water would fall back down and condense back into the water. ... ??
  4. Nov 15, 2009 #3

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    where is it going to go?
    i do not know what the surface temperature of the moon is but suspect it is below freezing since the vacuum of deep space is this cold.
    when water evaporates it goes to a gas state in the atmosphere. if you do not have an atmosphere , where is it going to go?
    out of my domain of knowledge here so just opinion
  5. Nov 15, 2009 #4
    There is a "wind" on the moon and on comets. It is "solar wind". This is the wind that causes comet's tails.
  6. Nov 15, 2009 #5

    There is a "wind" on the moon, on comets and on any object in the solar system. It is "solar wind". This is the wind that causes comet's tails. With gravity so low on comets, for example, any vapor or gas like sublimed water vapor produced on the surface is blown away by the solar wind. I suspect this is why the moon has no atmosphere. It's gravity is not strong enough to hold gases and as they are produced are blown into space by the solar wind. So my question still stands I think.
  7. Nov 16, 2009 #6

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    Moon dust, or regolith, is "like a finely ground powder. When you look at it under a microscope, it almost looks like volcanic ash. So when you step on it, it can compress very easily into the shape of a boot." And those shapes could stay pristine for a long while thanks to the airless vacuum on the moon.

    per wikepedia -the Earth's Moon has no atmosphere or intrinsic magnetic field, and consequently its surface is bombarded with the full solar wind. The Project Apollo missions deployed passive aluminum collectors in an attempt to sample the solar wind, and lunar soil returned for study confirmed that the lunar regolith is enriched in atomic nuclei deposited from the solar wind. There has been speculation that these elements may prove to be useful resources for future lunar colonies.

    Earth and planets with magnetic fields prevent the solar wind from effecting it. moon does not have magnetic field or not big enuff to prevent this.
    i learned something today, thanks

    Attached Files:

    Last edited: Nov 16, 2009
  8. Nov 16, 2009 #7
    Can someone tell us a bit more about the characteristics of solar wind on the moon. If you witnessed a puff of smoke or vapour - would the solar wind actually grab that and accelarate it in a direction away from the sun - in an appreciable manner? like an observable movement?

  9. Nov 16, 2009 #8

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    I got the following from NASA google - it ia about solar wind and earth...indicates how it effects Moon

    Because auroras are caused by the interaction of solar winds with the Earth's magnetic field, you can see them most often near the poles, both north and south. In the north, they're called aurora borealis, or Northern Lights. Aurora is the name of the Roman goddess of the dawn, and "boreal" means "north" in Latin. In the southern hemisphere, auroras are called aurora australis (Latin for "south").

    Auroras follow solar cycles and tend to be more frequent in the late autumn and early spring (October, February and March are the best months for seeing them). Around the Arctic Circle in northern Norway and Alaska, you can see them almost nightly. As you travel south, their frequency diminishes. Around southern Alaska, southern Norway, Scotland and the United Kingdom, they might show up about one to 10 times per month. Near the United States/Canada border, you may see them two to four times a year. Once or twice a century, they might pop up in the southern United States, Mexico and the equatorial regions.
    Let's take a closer look at auroras and what causes them.

    Auroras are indicators of the connection between the Earth and the sun. The frequency of auroras correlates to the frequency of solar activity and the sun's 11-year cycle of activity.
    Illustration of how solar winds impact the Earth's magnetosphere

    As the process of fusion occurs inside the sun, it spews high-energy particles (ions, electrons, protons, neutrinos) and radiation in the solar wind. When the sun's activity is high, you'll also see large eruptions called solar flares and coronal mass ejections. These high-energy particles and radiations get released into space and travel throughout the solar system. When they hit the Earth, they encounter its magnetic field.

    The poles of the Earth's magnetic field lie near, but not exactly on, its geographic poles (where the planet spins on its axis). Scientists believe that the Earth's liquid iron outer core spins and makes the magnetic field. The field is distorted by the solar wind, getting compressed on the side facing the sun (bow shock) and drawn out on the opposite side (magnetotail). The solar winds create an opening in the magnetic field at the polar cusps. Polar cusps are found on the solar side of the magnetosphere (the area around the Earth that's influenced by the magnetic field). Let's look at how this leads to an aurora.

    1. As the charged particles of solar winds and flares hit the Earth's magnetic field, they travel along the field lines.
    2. Some particles get deflected around the Earth, while others interact with the magnetic field lines, causing currents of charged particles within the magnetic fields to travel toward both poles -- this is why there are simultaneous auroras in both hemispheres. (These currents are called Birkeland currents after Kristian Birkeland, the Norwegian physicist who discovered them -- see sidebar.)
    3. When an electric charge cuts across a magnetic field it generates an electric current (see How Electricity Works). As these currents descend into the atmosphere along the field lines, they pick up more energy.
    4. When they hit the ionosphere region of the Earth's upper atmosphere, they collide with ions of oxygen and nitrogen.
    5. The particles impact the oxygen and nitrogen ions and transfer their energy to these ions.
    6. The absorption of energy by oxygen and nitrogen ions causes electrons within them to become "excited" and move from low-energy to high-energy orbitals (see How Atoms Work).
    7. When the excited ions relax, the electrons in the oxygen and nitrogen atoms return to their original orbitals. In the process, they re-radiate the energy in the form of light. This light makes up the aurora, and the different colors come from light radiated from different ions.

    Note: The particles that interact with the oxygen and nitrogen ions in the atmosphere don't come from the sun, but rather were already trapped by the Earth's magnetic field. The solar winds and flares perturb the magnetic field and set these particles within the magnetosphere in motion.

    hope this helps a little

    Attached Files:

  10. Nov 16, 2009 #9

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    from what I have just learned i think solar wind does hit the monn
    the frequency is not predictable
    the area of impact is not predictable
    so the comment about foot prints being o n the moon for a long time depends on if and when the solar winds sweep that landing zone
    thing 2 - the water detected on the moon was in sub surface strata propelled into space caused by the man made impact of a projectile into the moons surface. earlier analysis of the moon showed no water , i think..may be wrong..
    what did i miss?
  11. Nov 16, 2009 #10

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Think of it this way: How can rocky asteroids and metallic satellites exist in the vacuum of space without subliming away? Sublimation is not something unique to water. Any and every solid undergoes sublimation. The answer to the conundrum lies in the rate at which sublimation occurs. Rock and metal do sublime at room temperature, but the rate is so slow that in a billion those rocky asteroids and metallic satellites will have lost very little to sublimation. The same happens to water ice at extremely low temperatures

    At about 70 Kelvin, the sublimation rate of water ice in vacuum is about one molecule per square centimeter of surface area per hour. For a circular crater 19 km in diameter (e.g., Shackleton crater), that corresponds to a sublimation rate of less than 1 kilogram per billion years -- and that is a temperature of 70 Kelvin. The temperatures in those polar craters have been measured to be about 33 Kelvin. Between 70 and 33 Kelvin, the sublimation rate drops by a factor of 1030 or more. For all practical purposes, water ice simply does not undergo sublimation at the extremely low temperatures observed at the bottoms of those polar craters.

    Reference for sublimation rates:
    Edgar L Andreas, "New estimates for the sublimation rate for ice on the Moon," Icarus, 186:1, pp 24-30 (2007)
    Last edited: Nov 16, 2009
  12. Nov 16, 2009 #11
    Thank you DH. The demonstrable very low sublimation rate of water ice at extremely low temperatures answers my question.
  13. Nov 16, 2009 #12
    Here is a question.

    The previous poster showed that the ice in the craters does not sublimate fast enough to have significant losses. But the sublimation rates might be fast enough outside of the craters, where the mean surface temperature is between 220k - 130k (according to wiki). Would we expect to have no water in the soil outside of the craters? Could there be water deeper in the soil?

    Curious reader who doesnt know too much about thermodynamics.
  14. Nov 16, 2009 #13
    Can we also say that there is some ice being deposited on the moon from meteor impacts as well? So the net amount may in fact be rising.

    Also is it not possible that in some crates, particularly at the poles, sunlight and solar wind is never incident?
  15. Nov 17, 2009 #14
    Water can get bounded up in minerals, these are called hydrous minerals. In fact there is a lot of water in the Earth's mantle, and considering that the moon is basically a huge wedge of the Earth, composed primarily of mantle material, there is more than likely going to water bound up in those minerals that comprise the moon.

    Water may also exist in the cracks and pore spaces between the grains of rock. Geological layers of rock that contain "economic" proportions of rock are called aquifers. On Earth, rocks near the surface that contain water are commonplace, and their water is commonly mined for drinking and agricultural purposes -- these rocks are predominantly sedimentary rocks which usually have lots of pore space. On the moon the rocks are igneous - and these rocks do not normally have much pore space - in fact igneous rocks are formed of interlocking crystals that have solidified out of a melt and this usually precludes the forming of pore spaces. An exception is when there is trapped gas and the rock is cooling quickly - trapped bubbles of gas in basalts on Hawaii for instance can form vesicles when the rock cools and if these vesicles are linked they may form an effective conduit for water.

    I'm no expert on the geology of the moon but I believe there has been volcanism on the Moon in the past, so perhaps it is possible that the conditions were right for rocks to form on the near surface with pore spaces that could contain water...?

    Yes, why not? It's possible at least. Not only do meteorites contain significant quantities of water, but they would also shock the rock forming cracks in which the water could seep.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook