June 19, 2010
A Wetter Moon Impacts Understanding of Lunar Origin
Microscopic view of a lunar sample, showing the various mineral phases. Over 1300 ppm hydroxyl (OH) was found in the black circle marked "4."
Is there water on the Moon?
We know now that the answer to that question is a resounding Yes! As information continues to emerge from a wide range of studies, it’s evident that we’ve just begun to understand the process of the creation, movement and history of water on the Moon and its prevalence.
A paper recently published in the Proceedings of the National Academy of Sciences describes lunar samples containing the calcium-phosphate mineral apatite. Using a sensitive technique, they detected water (in the form of its ion hydroxyl, -OH) within the crystal structure of this mineral. Moreover, these hydroxyl-bearing apatite grains are found in several different rocks from a variety of geological settings. This indicates that the presence of water in the lunar interior is not some fluke, but a general property of the Moon. So the story of water on the Moon advances.
Why did scientists believe for so long that the Moon was bone dry? Largely because the samples returned from the Apollo missions contained no obvious hydrous phases, such as mica or amphibole, common water-bearing minerals in terrestrial igneous rocks. In addition, the chemical composition of lunar samples indicated that they formed under very reducing conditions, indicating very low partial pressures of oxygen. Typically, water oxidizes metals in magma on Earth, creating minerals that contain ferric iron (Fe3+); the exclusive presence of ferrous (Fe2+) iron in lunar samples indicates that no water was present during their formation. Finally, there is the extremely reducing nature of the current surface environment of the Moon, in which solar wind protons (hydrogen ions) continuously impinge upon the surface and reduce metal oxides in the soil. This hydrogen reduction creates “free” metallic iron (0Fe) and hydroxyl ions, most of which are lost to space through a variety of mechanisms. But at least some of these ions migrate to cooler regions of the Moon, the poles.
We now have found traces of water in volcanic glasses, mare basalts and an alkali highland rock. All these samples are very different types of material, formed from different parent materials at different places within the Moon at different times and under different conditions. The rocks possess apatite grains that show evidence for the presence of water at the time of their formation. One reason that we are finding this water in lunar samples now is that the technology of laboratory instrumentation has vastly improved since lunar samples were first studied 40 years ago.
The ion microprobe used in the study by McCubbin and others resolved a spot size on a mineral grain about 8 microns (about 100th of a single millimeter) in diameter. Additionally, the composition of this spot was resolved with extremely high precision, measuring the presence of water at about 3 parts per million or better. We now have at our disposal a variety of brand new lunar samples in the form of meteorites – rocks that were blasted off the Moon during impact events. More than 130 individual lunar meteorites are now known; one of the samples in this new study is a piece of a lava flow (mare basalt), found as a meteorite from Northwestern Africa.
The results of the new discoveries indicate that water is (or at least was) present in the deep lunar interior. This water probably existed as gas as the pressures and temperatures within the Moon do not permit the existence of liquid water. The total amounts of water implied by this work are still very low; the bulk lunar water content is estimated to be between 0.064 to 21 parts per million, a low amount by almost any standard – except when compared to the previous estimate for the Moon, which was less than one part per billion of water. Thus, the new estimate suggests water contents inside the Moon that are several orders of magnitude higher than previously thought.
So what does all this mean? For models of lunar origin, some mechanism preserved primordial water inside the Moon immediately after it formed. It had been thought that if the Moon originated during a collision of a planet-sized object with the proto-Earth 4.5 billion years ago (the currently favored model), the high temperatures extant during such an event would “boil away” most, if not all, volatile substances. Indeed, the near absence of volatile substances in the Moon has long been cited as prima facie evidence for a high-energy environment of lunar formation, such as would be expected from a giant impact. It now appears that regardless of high temperatures prevalent during this time, some water was incorporated into the Moon. Does this make the giant impact model less likely? Perhaps. Clearly we do not fully understand the conditions created by such an event. Work continues on the problem of lunar origin with the handicap that a planetary-scale collision is something well beyond human experience or observation.
Some of this endogenous lunar water may have found its way into one of the permanently dark, extreme cold “traps” near the poles of the Moon. Thus, in addition to the water made on the Moon from solar wind reduction and deposited on its surface by the collision of water-bearing asteroids and comets, we must also consider the addition of water from the deep lunar interior. Considering that our estimate of the abundance of internal lunar water is still very low, it is likely that the vast bulk of the water found at the poles is of external origin. Therefore, the possible finding of indigenous “Moon water” in the polar areas makes detailed study and examination of the poles even more attractive.
The Moon continually surprises us as she reluctantly (but always provocatively) reveals her secrets. In recent months, a wholly new and totally unexpected picture of the processes and history of our nearest planetary neighbor has emerged. We are in the midst of a renaissance of lunar science.
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Pingback by Tweets that mention A Wetter Moon Impacts Understanding of Lunar Origin | The Once and Future Moon -- Topsy.com — June 19, 2010 @ 9:05 am
Do we have any information on the isotopic composition of lunar polar water? Could it be enriched in deuterium, as water apparently is on Mars and Venus?
Comment by Paul D. — June 19, 2010 @ 2:44 pm
Do we have any information on the isotopic composition of lunar polar water?
Not as yet. The LCROSS spectral data suggest multiple volatile species in the cold traps and at roughly “cometary” abundance. Thus, one might expect “solar” D/H ratios. But an actual measurement will have to await in situ analysis from mass spectroscopy on some future lander and/or rover mission to the poles.
Comment by Paul D. Spudis — June 19, 2010 @ 4:39 pm
Still more exciting reasons to return to the Moon!
Dr. Spudis, I hope you don’t mind me posting this link to another interesting blog on the Moon from Centauri Dreams called ‘Protecting the Lunar Farside’. You might want to make some comments yourself.
http://www.centauri-dreams.org/?p=13011
Comment by Marcel F. Williams — June 19, 2010 @ 8:32 pm
Dr. Spudis,
For many years the focus of Mars exploration has been to “follow the water.” Is a similar strategy emerging for lunar exploration? How would you like these findings to affect planning for future missions? What missions are planned that will build on this growing knowledge base?
Sincerely,
Jason
Comment by Jason — June 19, 2010 @ 9:58 pm
Jason,
You should keep in mind that this new work on lunar water is only marginally relevant to the issue of abundant polar water — most of the water at the poles is likely of external, non-lunar origin. However, I agree that in a sane world, we would be planning a strategy to systematically characterize and inventory the water, its various sources, and the processes involved in their disposal and fate. Sadly, we do not live in such a world.
Comment by Paul D. Spudis — June 20, 2010 @ 6:14 am
These results are interesting, nevertheless, even if they aren’t directly relevant for the polar ice fields. If we accept the result at face value, it would tend to favor the old Darwinian fission hypothesis for the origin of the Moon would it not? Presumably, there would be less energy released during a fission even as opposed to a massive collision, and so the Moon could condense faster thus preserving a higher concentration of volatiles.
Yes, the fission hypothesis would apparently require some sort of exotic mechanism to set it off (like georeactors); on the other hand, Velikovskian planets careening around the Solar System is rather exotic as well. The fission hypothesis better explains the oxygen isotope concentrations in that the concentrations are pretty much identical. The fossil angular momentum that we observe today is also difficult to square with the impact hypothesis, if I’m not mistaken.
One thing the hew data would seem to make more likely is outgassing as an explanation for at least some of the reliable observations of transient lunar phenomena, including the existence of active fumaroles on the Moon. Obviously, as a geologist involved in drilling for natural gas, I’m no believer in the abiotic origin of natural gas here on Earth–at least for commercial purposes. But there may be a grain of truth to the idea; if the lunar mantle has a higher concentration of volatiles than was formerly believed, perhaps its possible that it is still slowly migrating its way to the surface. Such fumaroles might eventually be good places to prospect for other volatiles besides water, such as methane.
Which leads to this that caught my eye: “This [primordial] water probably existed as gas as the pressures and temperatures within the Moon do not permit the existence of liquid water.” Are you sure about this Paul? A quick glance at the water phase diagram why there can’t be liquid water on the surface, but deeper within the Moon, surely there must be some combination of temperature and pressure where liquid water would be stable.
A simple BOTE calculation (assume an average surface temperature of 250 K, that temperature increases at 2.7 degrees per kilometer, and that pressure increases at the rate of about 4.9 million pascals per kilometer) suggests that liquid water should be stable from about a depth of about ~9 km down to a depth of roughly 130 km, at which point the temperature would exceed 600 K.
Comment by Warren Platts — June 28, 2010 @ 3:31 pm
Hi Warren,
the fission hypothesis would apparently require some sort of exotic mechanism to set it off (like georeactors); on the other hand, Velikovskian planets careening around the Solar System is rather exotic as well.
One difference in favor of giant impacts is that we have scars of such events preserved on the Moon (South Pole-Aitken basin) and on Mars (Utopia basin). So large impacts did occur early in the history of the Solar System. I am unaware of any independent evidence that planets can split into two pieces, ala the classic fission model. Doesn’t mean that it can’t happen — it just seems sort of ad hoc.
Are you sure about this Paul?
No. I haven’t done a detailed study — my statement was based on analogy to typical igneous systems on Earth, where water is almost always in vapor phase.
Comment by Paul D. Spudis — June 28, 2010 @ 4:10 pm
In order for the fission hypothesis to work, the body would have to be rotating so fast that the centrifugal force would almost match the force of gravity (on the order of 2 hours per rotation). Then some sort of catastrophe would have to happen to cause the split. I was thinking of the recent paper by de Meijer and van Westrenen “An alternative hypothesis for the origin of the Moon” (http://arxiv.org/abs/1001.4243), where a runaway georeactor (sort of like the Oklo georeactor, only much bigger) provides the missing energy source.
The fission hypothesis would better explain the similarity in composition, since most collision models require that most of the Moon’s mass come from the impactor. Also, the best simulations require that the proto-Earth be rotating in a retrograde fashion.
One thing’s for sure, we’re not going to settle the matter from our comfortable armchairs here on Earth. The authors do predict enhanced levels of certain helium and xenon isotopes deep within the Moon. On Earth, we usually recover these inert gases in natural (methane) wells. Which brings me back to my earlier post about the possibility of liquid water. Assuming a surface soil temperature of 250 K, and a 2.7 K increase per kilometer, then at 9 km total vertical depth, the temperature would be 274 K, a little above freezing. And since the gravity of the Moon is 1/6 of Earth’s, the pressure at 9 km would correspond to a depth of about a mile here on Earth. At these depths, we routinely drill for natural gas, and fractured basalt actually makes for good underground storage of natural gas, and even forms a reservoir rock in the right conditions. Meteor impacts might cause further, intense, local “frakking”. Geothermally heated water that was able to take advantage of the impact fractured system might significantly raise the zone of liquid water locally. At the logical extreme, there might even be gas seeps or even geysers as a result.
You see where I’m going with this. The fundamental premise of astrobiology is that where there’s liquid water, there is life. Yes, I know it’s a long shot, but it would be an irony of ironies, if, after galavanting all over the solar system in search of extraterrestrial life, we were to find it right under our noses on the Moon.
Comment by Warren Platts — July 4, 2010 @ 8:11 pm