Moon’s Water Came From Comets, Study Says


A new study reveals that the water within the Apollo Moon rocks – and within the Moon itself — likely came from comets bombarding the nascent lunar surface, shortly after it formed following an impact event with a young Earth and Mars-sized protoplanet. The recent findings of abundant water at the lunar poles by the LCROSS impactor and across the Moon’s surface by various spacecraft have turned the long-standing notion of a dry Moon on its head, and the past year and a half, researchers have been trying to determine where this unexpected water came from.

“The water we are looking at is internal,” said Larry Taylor from the University of Tennessee, Knoxville, a member of an international team. “It was put into the moon during its initial formation, where it existed like a melting pot in space, where cometary materials were added in at small yet significant amounts.”

Using secondary ion mass spectrometry, the researchers measured the water signatures within rocks returned from the Apollo 11, 12, 14, and 17 missions that landed on the moon between 1969 and 1972. They found the chemical properties of the lunar water were very similar to signatures seen in three different comets: Hyakutake, Hale-Bopp and Halley.

The team found significant water in the lunar mineral apatite from both mare and highlands rocks, which indicates “a role for water during all phases of the Moon’s magmatic history,” the team wrote in their paper. “Variations of hydrogen isotope ratios in apatite suggest sources for water in lunar rocks could come from the lunar mantle, solar wind protons and comets. We conclude that a significant delivery of cometary water to the Earth–Moon system occurred shortly after the Moon-forming impact.”

Even though comet impacts may also have created the Earth’s oceans, Taylor said the water signatures from the mass spectrometer show that the water on the Earth and Moon are different, as apatite has a ratio of the deuterium and hydrogen that are distinctive from those in normal Earth water.

“The values of deuterium/hydrogen (D/H) that we measure in apatite in the Apollo rock samples is clearly distinguishable from water from the Earth, mitigating against this being some sort of contamination on Earth,” said James Greenwood of Wesleyan University, who led the research team.

Initially after the Apollo program, the Moon was believed to extremely dry. Many of the rocks returned by the astronauts and also the Soviet Luna program contained trace water or minor hydrous minerals, but those signatures were attributed to terrestrial contamination since most of the boxes of the Apollo program used to bring the Moon rocks to Earth leaked. This led the scientists to assume that the trace amounts of water they found came from Earth air that had entered the containers. The assumption remained that, outside of possible ice at the moon’s poles, there was no water on the moon.

Forty years later, a trio of spacecraft found evidence of water across the surface of the Moon: The Chandrayaan-1 spacecraft’s Moon Mineralogy Mapper (M Cubed) found that infrared light was being absorbed near the lunar poles at wavelengths consistent with hydroxyl- and water-bearing materials. A spectrometer on the re-purposed Deep Impact probe showed strong evidence that water is ubiquitous over the surface of the moon, and archival data from a Cassini Moon flyby also agreed with the finding that water appears to be widespread across the lunar surface.

“This discovery forces us to go back to square one on the whole formation of the Earth and moon,” said Taylor. “Before our research, we thought the Earth and moon had the same volatiles after the Giant Impact, just at greatly different quantities. Our work brings to light another component in the formation that we had not anticipated — comets.”

Taylor added that the existence of hydrogen and oxygen – water – on the moon can literally serve as a launch pad for further space exploration.

“This water could allow the moon to be a gas station in the sky,” said Taylor. “Spaceships use up to 85 percent of their fuel getting away from Earth’s gravity. This means the moon can act as a stepping stone to other planets. Missions can fuel up at the moon, with liquid hydrogen and liquid oxygen from the water, as they head into deeper space, to other places such as Mars.”

Their paper, “Extraterrestrial Hydrogen Isotope Composition of Water in Lunar Rocks” was published in the journal, Nature Geoscience.

Sources: Nature Geoscience, EurekAlert

10 Replies to “Moon’s Water Came From Comets, Study Says”

  1. “They found the chemical properties of the lunar water were very similar to signatures seen in three different comets: Hyakutake, Hale-Bopp and Halley.”

    I had no idea isotope ratios could be measured at a distance. Am I wrong?

    1. Why do you assume distance, when some of these have been visited and even “sniffed” by MS?

      Hyakatake’s tail was encountered by Ulysses (by sheer coincidence, apparently). Halley was beleaugere by a fleet of crafts, Giotto comes to mind.

      But deuterium is detected by spectroscopy, if the tail is near enough:

      “The close approach to the Earth of Comet C/1996 B2 (Hyakutake) in March 1996 allowed searches for minor volatile species outgassed from the nucleus. We report the detection of deuterated water (HDO) through its 101–000 rotational transition at 464.925 GHz using the Caltech Submillimeter Observatory. We also present negative results of a sensitive search for theJ(5–4) line of deuterated hydrogen cyanide (DCN) at 362.046 GHz.

      Simultaneous observations of two rotational lines of methanol together with HDO in the same spectrum allow us to determine the average gas temperature within the telescope beam to be 69 ± 10 K. We are thus able to constrain the excitation conditions in the inner coma and determine reliably the HDO production rate as (1.20 ± 0.28) × 1026s?1 on March 23–24, 1996. Available IR, UV, and radio measurements led to a water production rate of (2.1 ± 0.5) × 1029s?1at the time of our HDO observations. The resulting D/H ratio in cometary water is thus (29 ± 10) × 10?5, in good agreement with the values of (30.8+3.8?5.3) × 10?5 (H. Balsigeret al., 1995,J. Geophys. Res.100, 5827–5834). and (31.6 ± 3.4) × 10?5 (P. Eberhardet al., 1995,Astron. Astrophys.302, 301–316) determined in Comet P/Halley from in situ ion mass spectra. The inferred 3? upper limit for the D/H ratio in HCN is 1%.”

      [“Deuterated Water in Comet C/1996 B2 (Hyakutake) and Its Implications for the Origin of Comets*”, D. Bockelée-Morvan et al.]

      I seem to remember that D vs H spectra differs somewhat from my basic Quantum Phys courses. (D is twice as heavy as H and nuclear radius differs, both affecting the electron shells somewhat.) But I’m no spectroscopist.

      1. Also, since it was a rotational line of water, I now realize it is:

        a) several combinations of H and D, which broadens or separates band,

        but mostly b) it is the D mass factor that really bogs molecular rotational bands down! (And, I guess, stretch modes, if those show up in spectroscopy.)

      2. Thanks! I didn’t know about Hyakutake / Ulysses, and should have thought twice about Halley!

        After posting I did suspect H/D ratios should be accessible from spectra. Is that true for any other isotopes out there? O, C?

      3. Sorry, I can’t help, I suffer under the “I’m no spectroscopist” syndrome. You have to hunt the possible isotope distinguishing spectra down yourself, I’m afraid.

  2. Now that we now that the moon has abundant water – is the general theory still that the earth and the moon got water from the “comet” rain in the different quantities? Or do we now believe that the moon got the same amount of “rain” but it somehow never ended up making oceans? Is there as theory for that?

    1. My perception, which has been gathered at a recent rush through under an astrobiology course, is that there is still some to and fro on the subject.

      There are recent papers on different telltales that implies quite strongly that comets couldn’t have delivered the dominant part of Earth water. (Up to perhaps ~ 20 % at best, IIRC.) Coincidentally, there are recent papers that places asteroids on a rather continuous scale on porosity and water content towards comets (as I interpret it). So they should have been able to make up the difference, especially if Earth has a scarcity of water compared to those bodies in general.

      [Now to gather and point to the various references would be a major undertaking. But please don’t take my word for it but continue to ask and search for this; I’m just a layman here.]

      Personally I assumed the recent water finds on Moon [seems sort of silly to put “the Moon” around the term “Earth”] would test the theory that volatiles including water would have equilibrated during the Earth-Theia impact that seem to have equilibrated the remaining Earth-Moon rocks.

      But apparently not, perhaps enough dissociation and subsequent hydrogen escape allowed a drier Moon to capture enough comet water for it to be dominant. And that despite the capture cross section for comets would have been less on Moon than Earth. I’m sort of confounded here, and conclude “extra-ordinary results demand extra-ordinary evidence”. Unless we can scrape up more understanding elsewhere, personally I have to wait for a Moon sample return mission to understand if these results are robust.

      1. Oops, my bad! That comment is fuzzy on many points.

        Let me see:
        – When I say that asteroids could make up the Earth major water, I mean that asteroids are indicative remnants of the impactors that coalesced to form Earth. Comets would have formed farther out.

        – When I use the often seen here technical term “equilibrate”, it is my understanding that the large Moon-forming impact efficiently smashed up and mixed parts of both involved bodies (and the orbiting remains coalescing to form the Moon). If it didn’t, there would be no compositional “equilibrium”, no “equal parts” mix.

  3. The reason for deuterium enrichment is going to be preferential escape of the H while in vapour form. This could occur during the inital earth/moon condensation, but also again during the heavy bombardment phase. During the initial condensation the different escape velocities of the two bodies would explain the variation between Earth and Lunar highlands, with deposition of vapour during the later bombardment phase mainly on the plains, leading the comet like readings on the mares. This explains water-dry mare samples, IIt also doesn’t re

  4. Sorry, automatically posted mid comment. It is just that the amount of cometary flux required to deliver to necessary water to supply the moon with its water ice remains to be calculated.

Comments are closed.