Why is the Moon so dry?

The Planetary Society puts out a magazine called The Planetary Report, and in the September issue, they pose an issue: why is there more water ice on Mercury than the Moon? This is an interesting question because it goes to the heart of data analysis and how to form theories. First we check our data, and while there is a degree of uncertainty, from neutron scattering as measured from orbiting satellites, Mercury has about 350 times the water than the Moon, not counting whatever is in the deep interior. Further, some of that on the Moon will not be water in the sense we think of water. The neutron scattering picks out hydrogen, and that can also come from materials such as hydroxyapatite, which is present in some lunar rocks. The ice sits in cold traps; parts of the body where the temperature is always below -175 ^oC. For the Moon, there are (according to the article) about 26,000 km² cold enough, while Mercury has only 7,000 km². So, why is the Moon so dry?

Before going any further, it might help some understand what follows if they understand what isotopes are, and what effects they have. The nature of an element is defined by the number of electrons, which also equals the number of protons. For any given number of protons, there may be a variation in the number of neutrons. Thus all chlorine atoms (found in common salt) have 17 protons, and either 18 or 20 neutrons. The two different types of atoms are called isotopes. Of particular interest, hydrogen has two such isotopes: ordinary hydrogen and deuterium with 0 and 1 neutron respectively. This has two effects. The first is the heavier one usually boils or sublimes at a slightly higher temperature and in a gravitational field, is more likely to be at the top, hence ice that spends a lot of time in space tends to be richer in deuterium. The second effect is the chemical bond is stronger for the heavier isotope.

So where do volatiles (water and gases) on the rocky planets come from? I raised some of the issues on where it did not come from earlier: https://wordpress.com/post/ianmillerblog.wordpress.com/564 To summarize, the first option is the accretion disk. That is where Jupiter’s came from. If the body is big enough before the gases are removed, they will remain as an atmosphere. We can reject that. The planets will have had such atmospheres, but soon after formation of the star, it starts spitting out extreme UV/Xray radiation, and intense solar winds, and these strip the volatiles. The evidence that this removed most of the atmosphere from Earth is that some very heavy inert gases, such as krypton and xenon, have heavy isotope enhancements suggesting they have been fractionally distilled, and some of the heavier isotopes were enhanced. These gases are extremely rare, but they also cannot be accreted by any mechanism other than gravity and adsorption, and unless they were so stripped, there would be huge amounts of neon here because physical mechanisms of accretion apply equally to all the so-called rare gases, and neon is very abundant in the accretion disk. However the krypton was accreted, very large amounts of neon would also be accreted. Neon is rare, so most gases that arrived with the krypton would have been similarly removed. As would be water. So after a few hundred million years, the rocky planets were essentially rocks, with only very thin atmospheres. That, of course, is assuming our star behaved the same way as other similar stars, and that the effects of the high energy radiation are correctly assessed.

So, where did our atmosphere and oceans come from? There are only two possibilities: from above and from below. Above means comets and asteroid-type bodies colliding with Earth. Below means the elements were accreted with the solids, and emitted through volcanoes. Which one? This is where the Mercury/Moon data becomes significant. However, as often is the case, there is a catch. Mathematical modeling suggests that the Moon might have changed its obliquity, and once upon a time it was almost lying on its side. If so, and if this occurred for long enough, there would have been no permanently cold points, and the Moon would have lost its ice. It did not, but the questions then are, did this actually happen, did that period last long enough, or did the water arrive on the surface after this tilting?

Back to Earth’s atmosphere. The idea that Earth was struck by comets has been just about falsified. The problem lies in the fact that the comets have enriched deuterium, and there is too much for Earth’s water to have come from there, other than in minor amounts. A similar argument holds for chondrites. It is not the water that is the problem, but rather the solid rock. The isotope ratios of the chondrite rocks do not match Terran rocks. The same applies for the Moon, because the isotope ratios of the surface of the moon are essentially the same as on Earth, and the Moon has not has resurfacing. That essentially requires the water on Earth to have come from below, volcanically. I gave an account of that at https://wordpress.com/post/ianmillerblog.wordpress.com/576

And now we see why the extreme dryness of the Moon is so important: it shows that very little water did land on the Moon from comets and chondrites. Yes, that was shown from isotopes, but it is very desirable that all information is consistent. When you have a set of different sorts of information that lead to the same place, we can have more confidence in that place being right.

The reason why the Moon is so dry is now simple if we accept the usual explanation for how it was formed. The Moon formed from silicates blasted out of the Earth when Theia collided with it. Exactly where Theia came from is another issue, but the net result was a huge amount of silicates were thrown into orbit, and these stuck together to form the Moon. We now come to a problem that annoys me. I saw a review where it said these silicates were at a temperature approaching 1100 ^oC At that temperature, zinc oxide will start top boil off in a vacuum, and the lunar rocks are depleted in zinc. According to the review, published in October, it could not have been hotter because the Moon is not significantly depleted in potassium. However, in the latest edition of Nature (at the time of writing this) an article argued there was a depletion of potassium. Who is right, and how does whoever it is know? Potasium is a particularly bad element to choose because it separates out and gets concentrated in certain rocks, and we do not have that many samples. However, for water it is clear: the silicates were very hot, and the water was largely boiled off.

So, we have a conclusion. I suppose we cannot know for sure that it is absolutely right because we cannot know there is not some other theory that might explain these facts, but at least this explanation is consistent with what we know.

To conclude, some personal stuff. There will be no post from me next week; I am having hip replacement surgery. Hopefully, back again in a fortnight. Second, for those interested in my economic thoughts, my newest novel, ‘Bot War, will be available from December 2, but it is available for preorder soon.

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