10
$\begingroup$

The Ars Technica article NASA seeks industry help with lunar landings, potentially sample return discusses the potential value of ice on the moon as a source for fuel in future space missions.

Ice sublimates. Even if it is very cold hiding in permanent shadows in craters near the Moon's poles, I'd expect it to have a non-zero vapor pressure.

So what would keep ice from slowly sublimating over millions of years? Or is it being replenished somehow?

$\endgroup$
  • 1
    $\begingroup$ @LocalFluff ice sublimates in my freezer, does that mean that the light actually stays on when I close the door? (joke) I guess you mean the thermal sublimation rate is extremely low, and the existence of comets attests to this? $\endgroup$ – uhoh May 4 '17 at 13:55
  • 4
    $\begingroup$ Ice sublimates in your fridge, 200 degrees warmer than eternally shadowed Lunar craters. But the sublimated ice then sticks to the walls and shelves and freezes again. That's how I imagine how water ice could be accumulated in Lunar polar craters. Ice structures organizing themselves to hide from the evil Sun. Well, kitchen fantasies. $\endgroup$ – LocalFluff May 4 '17 at 14:51
  • 1
    $\begingroup$ "The ice in the ice cube tray disappears" I thought that I only imagined that this was happening to me. Now we can go on to explaining how the socks disappear in the washing machines. There can be good secret reasons for why they don't have a washing machine on the ISS. Nor do they need socks much in microgravity without stuff like floor and shoes. $\endgroup$ – LocalFluff May 4 '17 at 16:20
  • 4
    $\begingroup$ @developerwjk - lunar ice would be in the permanent shadow of polar craters, or underground, which is not anywhere we've explored. I can't tell if you're joking or trolling, but if you're legitimately a lunar landing denialist you will not find a warm welcome here. $\endgroup$ – Russell Borogove May 5 '17 at 21:45
  • 2
    $\begingroup$ @developerwjk -- That's a real paper published in 1961; you can go to your local university library and find the original paper journal. To add to Russell Borogrove's comment, you won't just find a non-warm welcome if you truly are a lunar landing denialist. The welcome will be ice cold. $\endgroup$ – David Hammen May 5 '17 at 22:07
10
$\begingroup$

Ice sublimates.

So does rock. Yet the planet Mercury is still there.

The reason Mercury still exists is because even though rock does indeed sublimate, the rate at which rock sublimates is extremely low, even at temperatures at the surface of Mercury. The same applies to water ice at the very low temperatures in those permanently shadowed craters on the Moon. Water ice those very low temperatures is essentially rock.

The extremely low sublimation rate of an exposed block of water ice at ~100 kelvins would result in that ice thinning by a millimeter per billion years. The temperatures in the permanently shadowed portions of the Shackleton Crater are ten kelvins lower than that. Even exposed water ice can easily survive for billions of years at those low temperatures. Water ice covered by material can survive even longer.

$\endgroup$
  • $\begingroup$ Thanks for the quantitative answer - that's a small number! Seems to drop by two or three orders of magnitude for every 10K below that as well. (Fig. 2) people.nwra.com/resumes/andreas/publications/Icarus_Moon.pdf So it seems that modeling the local thermal environment (conductive and radiative heating/cooling) is really important. $\endgroup$ – uhoh May 5 '17 at 4:53
6
$\begingroup$

You overestimate vapor pressure and underestimate dust power enter image description here source

0.05 Pa it equivalent to a layer 0.000018382 m of dust which would (if it being a sealant) to prevent the ice from further sublimations if we assume average density of 1700kg/m3 and gravity 1.6 m/s2

There are other factors like water being polar solvent and thus adhesion to the dust particles as an example(same way as of why there are gasses He,H, N in regolith), slowing or stopping their travel trough the dust and helping for better sealing on top of the ice.

The average velocity of ice molecule at 200K is about 16 m/s, which gives us that there is not enough energy(for most of them) to fly out of the 80m deep crater.

Essentially it is the same thing as with icy asteroids and comets.

But overall everything boils down to there was enough water and speed of escaping of the water was not fast enough, so we observe it presence.

$\endgroup$
  • 1
    $\begingroup$ I don't understand "dust as a sealant" against molecular diffusion. It may slow the diffusion, but it's not "air tight". And isn't the polar nature of water already part of the vapor pressure - can you explain how it would need to be taken into account again? What is the physics behind 16 m/s not getting out of an 80m deep crater? Thermal velocity distributions have tails. Can you add supporting links to your statements? $\endgroup$ – uhoh May 4 '17 at 15:54
  • 1
    $\begingroup$ @uhoh "already part of the vapor pressure" - it is, water-water, but not water-something. Which might include water complexes, but not only that, water complexes on surface of thing etc. Which lead to different sealing situations, which I can observe on my rusty water faucet, shich is not leaking anymore. So it is chemistry and chemistry on surface. 16 = sqrt(1.38×10⁻²³×6×10²³×3×200÷18); 80 = 1.6×(16÷1.6)²÷2 ; "Thermal ... tails." - that is true, there is a lot of factors to consider, it just not so simple as just ice vapor->no ice, and I definitely not the right person to exhaust them all. $\endgroup$ – MolbOrg May 4 '17 at 16:17
5
$\begingroup$

It is of course sublimating. And not only is it cold, but the sublimation cools the remaining ice even further.

Since it's in lunar dust, chances are additionally that a sublimated water molecule will hit a speck of dust and re-freeze. This effectively slows the speed at which water moves from deep down to the lunar atmosphere.

$\endgroup$
  • 2
    $\begingroup$ @uhoh: You're probably correct in assuming that the surrounding dust will also be cooled by the sublimation effect. That's why I'm pointing out the re-freezing effect. As for the "extremely low sublimation rate", well, that's the thing you were questioning in the first place! Note that this isn't really different from ice comets, except for the much higher gravity of the moon (and consequently much higher water vapor pressure) $\endgroup$ – MSalters May 4 '17 at 14:11
  • 2
    $\begingroup$ If there is a known, or even predicted non-zero water vapor pressure in craters on the moon, could you please add some kind of link or source? I'm not challenging the idea, I'd just really like to read more about it, and good stackexchange answers should support statements of fact, at least those that are not widely known. Thanks! $\endgroup$ – uhoh May 4 '17 at 14:20
  • 1
    $\begingroup$ @uhoh: NASA $\endgroup$ – MSalters May 4 '17 at 15:12
  • 1
    $\begingroup$ I don't see any mention of vapor pressure or a partial pressure of water in a crater's atmosphere. Maybe you can add a quote of the specific passage directly to your answer? Comments should be considered temporary, and not parts of an answer. Thanks! $\endgroup$ – uhoh May 4 '17 at 15:21
  • 1
    $\begingroup$ @uhoh And what would a covering of ... hum say a inch of Dust do to that patch of ICE sublimating? $\endgroup$ – Enigma Maitreya May 5 '17 at 3:50
2
$\begingroup$

I just ran across this reference again while writing this comment (same one as I mentioned here so I thought I would add it to the mix explicitly.

The paper is quite thorough and interesting, and I think deserves a careful read.

New estimates for the sublimation rate for ice on the Moon Edgar L. Andreas, Icarus 186 (2007) 24–30:

This is pretty amazing, the vapor pressure has been experimentally measured over a range of ten orders of magnitude!

New estimates for the sublimation rate for ice on the Moon, Edgar L. Andreas, Icarus 186 (2007) 24–30

Fig. 1. Measurements or reference data for the saturation vapor pressure over a planar surface of pure water ice from Hilsenrath et al. (1960), Jancso et al. (1970), Bryson et al. (1974), Buck (1981), and Marti and Mauersberger (1993). The functional expressions for esat,i are from Buck (1981), Wagner et al. (1994), and Murphy and Koop (2005) and are given in Eqs. (2)–(4). The Murphy and Koop curve is under the Buck and Wagner et al. curves in the region where they all overlap.


Here is the punch line, and it packs quite a punch! Note the annotation of 1 molecule per square centimeter per hour, and the fact that every major tick mark on the y axis represents ten orders of magnitude!

New estimates for the sublimation rate for ice on the Moon, Edgar L. Andreas, Icarus 186 (2007) 24–30

Fig. 2. The sublimation rate for a planar surface of pure ice calculated using the expressions from Buck (1981), Wagner et al. (1994), and Murphy and Koop (2005) for esat,i in Eq. (1). The left axis gives the sublimation rate as a mass flux; its units are µg cm−2 h−1. The right axis gives the sublimation rate as the number of molecules of water vapor leaving a square centimeter of the ice surface per hour. The arrow shows where the sublimation rate is only 1 molecule cm−2 h−1.

$\endgroup$
  • 2
    $\begingroup$ Poor solitary molecule: little chance of a meaningful relationship with fellow sublimaters, let alone settling down for some mutual hydrogen-bonding. If you find a similar curve for impure water, specifically sugar plus dairy fat, you can choose a suitable setting for the freezer temperature to stop the ice-cream developing crunchy crystals, though the anti-frost cycling may still cause enough sublimation that prompt consumption is still the best approach. $\endgroup$ – Tom Goodfellow Feb 13 at 7:54
  • 1
    $\begingroup$ @TomGoodfellow I always thought that was transparent, dendritic freezer-fungus. It's amazing what we can learn from scholarly discussions in Stack Exchange! $\endgroup$ – uhoh Feb 13 at 8:25
  • 1
    $\begingroup$ A sublimation rate of 10^-14 molecules per square centimeter and hour, that is one molecule per hour from a square area of 100 km by 100 km. Very far below what would be measurable in an experiment. $\endgroup$ – Uwe Feb 13 at 10:56
  • $\begingroup$ @Uwe I think that's a key point of the paper. They used measured values of vapor pressure over ten orders of magnitude, down to about 130 K, and then discuss physical models of sublimation rate that fit that data, and then use those models to predict sublimation rate down below ~50 K, where it impossible to measure as you mention. $\endgroup$ – uhoh Feb 13 at 11:00
  • $\begingroup$ @uhoh So the range from 10^10 down to 1 is measured and the range from 1 down to 10^-50 is modeled. But the fit of the different models is amazing. $\endgroup$ – Uwe Feb 13 at 11:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.