June 1, 2012
Who discovered water on the Moon?
The Luna 24 lander descent stage on the Moon: Wishing Well? (NASA/ASU)
A recent article tells how Soviet scientists studying soil samples returned from the Moon in 1976 by the unmanned Luna 24 mission first discovered lunar water. This assertion is based on a paper published in the Russian journal Geokhimiia (vol. 285, p. 285-288, February 1978). The measurement used infrared absorption spectroscopy to look for the “water band” centered around 2.8 microns, the same technique used recently by several groups to map the water band on the lunar surface regionally from flyby (Cassini and EPOXI) and orbital (Chandrayaan-1) spacecraft. The Soviet paper claimed to detect water at a level of about 0.1 weight percent. This high concentration level of water raised my antennae.
The discovery of significant amounts of water would tell us about lunar processes and history as well as provide evidence that water might be manufactured on the Moon to support future exploration. The first lunar samples returned to Earth in 1969 by the Apollo 11 mission were intensely scrutinized for water content. Besides being exceedingly dry, the chemistry of the Apollo samples suggested they were created in a completely anhydrous, reducing environment. Samples from subsequent missions confirmed and extended this initial impression to the point where talk of water on the Moon was mostly dismissed.
A rock returned in 1972 by the Apollo 16 mission displayed visible brownish splotches which turned out to be “rust” in the form of the mineral akaganeite, an iron-hydroxyl phase, with minor amounts of chlorine. This mineral could have formed by the aqueous alteration of the iron-chlorine mineral lawrencite found in some meteorites. However a source of water is still needed to create the “rust,” so for several years the source of the water and the nature of the alteration were debated. Did water come from the inside of the Moon or from an impacting comet? Did the oxidation occur on the Moon or was it caused by the exposure of the highly reduced lunar sample to humid air (from inside the returning Apollo command module or the Houston summer humidity)? Different workers had a variety of opinions but with no resolution, interest faded.
But a few inquisitive types didn’t forget it. Jim Arnold, a chemist from UC-San Diego, resurrected an old idea about permanent cold and dark areas near the lunar poles. He concluded that over the course of history these areas were cold enough and old enough to have accumulated significant amounts of water from meteorites and comets. Groups studying the regolith (soil) from the Apollo missions measured variable amounts of hydrogen on dust grains; when heated, hydrogen in that dust reacted with metal oxides in the soil producing native metal (iron) and water vapor. Although done in the laboratory, it was shown that the process could occur naturally on the Moon during the impact of a micrometeorite, whose energy is mostly dissipated as heat. This heat and the hydrogen on dust grains could “reduce” the soil, creating measurable water release.
During the lunar “wilderness years” (i.e., 1976-1994, when no one was going to the Moon) all we could do was speculate and analyze existing samples. In 1982 a meteorite from the Moon was discovered in Antarctica. Lunar meteorites provided a new source of samples but even though all had significant exposure to the terrestrial hydrosphere, none of them showed evidence for water-bearing phases. Attempts were made to map the poles of the Moon from Earth using optical and radar telescopes but poor viewing geometry led to uncertain conclusions.
Two events re-ignited the water debate. The 1994 Clementine spacecraft probed the south pole of the Moon and found evidence for coherent backscatter near the dark areas. The team interpreted this as indicating the presence of water ice. Following Clementine, the Lunar Prospector neutron detector found elevated amounts of hydrogen near both poles of the Moon, resulting in new interest about the possibilities for water on the Moon. In recent years, a variety of robotic missions, carrying instruments designed to address the lunar water question one way or another, found large amounts of water in a variety of different forms, locations and concentrations. We are just beginning to decipher the origins, cycles, and eventual fate of this water.
So what can we say about the Soviet results published in 1978? No other scientist or group has repeated this measurement on the Luna 24 samples to confirm its validity. Under a reciprocal exchange agreement with the Soviet Union in the late 1970s, others studied the Luna 24 samples but none reported any traces of water in their samples. No one in Russia has studied the Luna 24 samples in years (at least to my knowledge), although they still exist and presumably are available for analysis. The spectral detection of water in the Luna 24 soil should be repeated and then followed up with analyses by other techniques to confirm the water’s presence and to cross-check the amounts claimed. The published value of 0.1 weight percent (1000 part per million) water seems very high for lunar soils from equatorial and mid-latitudes; typically, such soil contains 10-50 ppm hydrogen, almost two orders of magnitude less than the 1978 reported result. Finally, even if the old analysis is confirmed, questions about its source are still pertinent; we are still arguing about the origin of the water that made the rust in “Rusty Rock.”
If you’ve stayed with me this far, I hope that if nothing else, this brief history of a lunar controversy has shown that it is difficult (I would say impossible) to assign “credit” to any one paper or worker or group for the discovery of water on the Moon. In science we always proceed from the knowledge gained by previous work. Sir Isaac Newton put it well when he famously said that he saw more clearly because he stood on the shoulders of giants. A lunar scientist’s goal is to study, document and explain, thereby contributing to and advancing our knowledge and understanding of the Moon.
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“In science we always proceed from the knowledge gained by previous work.”
I’d say there’s not much more we can do to answer the water on the moon question with remote sensing from lunar orbit. It’s time to proceed and get to the lunar surface again. NASA has several small projects in the works (e.g., RESOLVE, Morpheus, ALHAT, etc.), that could be put together into a technology demonstration mission to one of the lunar poles. A mission like this could be ready to go in a few years. That is, if we can rid ourselves of the political shackles that have been put on NASA’s lunar plans. All of the other space-faring nations in the world see it as plain as day – the Moon is the ‘New World’ (the surface area of the Moon is about 90% of North and South America combined) and the only real destination for humans in the next couple of decades. It’s time to do the right thing.
Comment by JohnG — June 1, 2012 @ 4:07 pm
March 2010 was when my advocacy for going to the Moon caught fire in a big way. The reason was I had been puzzling for some time over the radiation problem detailed in Eugene Parker’s Scientific American Magazine article “Shielding space travelers”- and the water on the moon made Parkers guaranteed solution of a massive water shield practical.
A spaceship can “travel light” from Earth and then fill up with the several hundred tons of moon water required to shield a crew from deep space radiation hazards.
http://www.nasa.gov/home/hqnews/2010/mar/HQ_10-055_moon_ice.html
http://www.scientificamerican.com/article.cfm?id=shielding-space-travelers
This is an extremely important subject (I think anyway) that no one else seems to find very interesting.
Scientific controversy is interesting to some, but for others the stars are waiting.
Comment by GaryChurch — June 2, 2012 @ 4:04 pm
Paul:
Worst case, should further investigation determine that there is not a significant abundance of water at the lunar poles, or elsewhere, when we begin setting up human outposts, would it be possible to manufacture H2O from hydrogen compounds and oxygen compounds in the lunar regolith?
The alternative is to ship hydrogen to the Moon and combine it with the abundant oxygen…Or just bring along several tons of H2O and make sure that we recycle 100% of it.
Nelson
Comment by Nelson Bridwell — June 2, 2012 @ 5:17 pm
That’s a fascinating story. I’ve been following the progress of this line of work since Jim Arnold’s remarkable paper on the lifetime of ice at lunar polar temperatures. One has to wonder if there is anything about Mare Crisium, or the specific locale in which Luna 24 landed, that might speak to the detection of water from samples returned from that site. Does that site have a neutron spectrometer signature that is clearly more indicative of water than signatures at the Apollo sites? If not, and if you believe the Geokhimiia result, maybe one would have to presume that the Luna 24 rock was not representative of its surroundings.
Deriving an abundance by weight from spectroscopy isn’t an easy thing to do, but the near infrared band of water isn’t hard to look for. If this was reflectance spectroscopy, you don’t need a rock in a lab to look for the band. You can do it with a telescope if the signature is widespread. Has this been done? Though perhaps the samples examined from Luna 24 were not exterior surface ones?
Comment by Heinrich Monroe — June 3, 2012 @ 3:22 pm
Very interesting article.
I have wondered for some time how the existence of lunar water was missed in the Apollo samples in the 1960s/1970s.
This helps to clarify the situation.
Comment by Joe — June 3, 2012 @ 4:01 pm
Interesting article Dr. Spudis! Thanks for posting it.
Comment by Zach — June 4, 2012 @ 6:11 am
Nelson,
Worst case, should further investigation determine that there is not a significant abundance of water at the lunar poles, or elsewhere, when we begin setting up human outposts, would it be possible to manufacture H2O from hydrogen compounds and oxygen compounds in the lunar regolith?
Yes. Our best estimate is that there is at least 5-10 wt.% water throughout most of the polar areas, but my suspicion is that it is much higher than that (in the Spudis and Lavoie (2011) paper, we assumed only 10 wt.%). But even if such does not occur, there is enough solar wind implanted protons on the dust grains to extract water from any lunar soil. It will just take longer and cost more energy.
Comment by Paul D. Spudis — June 4, 2012 @ 11:23 am
Heinrich,
Does that site have a neutron spectrometer signature that is clearly more indicative of water than signatures at the Apollo sites? If not, and if you believe the Geokhimiia result, maybe one would have to presume that the Luna 24 rock was not representative of its surroundings.
Nothing in the remote sensing data suggest that there is anomalously high hydrogen or water at or near the Luna 24 site. I do not believe the result (for the moment) because it has not been confirmed by additional measurements, either by this group or anyone else.
The entire Moon was imaged at 3 microns by the M3 instrument on Chandrayaan-1 and it showed a strong correlation of water absorption with latitude (poleward of 65 degrees or so). There is no feature observed at the Luna 24 site.
Comment by Paul D. Spudis — June 4, 2012 @ 11:26 am
Joe,
I have wondered for some time how the existence of lunar water was missed in the Apollo samples in the 1960s/1970s
All of the Apollo sites were at latitudes of 25 degrees and lower (near-equatorial). They are all far-removed from the regions where we think water might reside.
Comment by Paul D. Spudis — June 4, 2012 @ 11:28 am
Recently there’ve been a flurry of articles on 1976 Russian discovery. Arlin Crotts seems to be the source.
Crotts seems to suspect that some of the PSR water isn’t exogenic, that outgassing from LTPs might contribute a significant fraction.
Besides the 1976 Russian sample return, Crotts mentions other missions finding evidence of lunar water. He talks a lot about Chandrayaan 1′s M3, but I’ve never seen Crotts mention the elevated CPR from mini-SAR.
Comment by Hop David — June 4, 2012 @ 11:34 am
“Worst case, should further investigation determine that there is not a significant abundance of water at the lunar poles, or elsewhere-”
Several times I have read outright skepticism (especially by private space advocates) or doubt between the lines about ice on the moon. Maybe I am missing something- is the Chandrayaan mini-SAR data indicating “several hundred million metric tons” of ice not to be believed?
http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html
I am reading that radar data indicates millions of tons; Please clarify Dr. Spudis.
Comment by GaryChurch — June 4, 2012 @ 1:17 pm
Comment by Paul D. Spudis — June 4, 2012 @ 11:28 am
“All of the Apollo sites were at latitudes of 25 degrees and lower (near-equatorial). They are all far-removed from the regions where we think water might reside.”
Check that was my main takeaway (along with an interesting history lesson) from the article.
I am an engineer, not a geologist, but it seems to me that the idea that the Moon was “bone dry” was based on the tacit assumption that the Moon (unlike the Earth) was extremely homogenous. It would be like someone coming to the Earth and saying that there was no oil (gold, silver, and fill in the blank) because none was found in the extremely limited number of sites they had the resources to survey.
Comment by Joe — June 4, 2012 @ 1:56 pm
Gary,
I am reading that radar data indicates millions of tons
Yes, that is what I believe that the Mini-SAR (Chandrayaan-1) and Mini-RF (LRO) radar data indicate, but it is not certain and needs to be confirmed by surface analyses in multiple places.
Comment by Paul D. Spudis — June 4, 2012 @ 2:48 pm
Joe,
it seems to me that the idea that the Moon was “bone dry” was based on the tacit assumption that the Moon (unlike the Earth) was extremely homogenous
Yes, that’s basically the story. The Moon was perceived to be a simple, uncomplicated object and samples from one location should tell us about its entire history, not only because of its alleged simplicity but also because impacts have thrown and scattered rocks all around the surface from great distances, so we can “sample” locations distant from the landing sites. The past few years have shown dramatically what many of us thought all along — the Moon is NOT simple and we have a lot yet to learn about it, its processes and its history.
Comment by Paul D. Spudis — June 4, 2012 @ 2:56 pm
“-but it is not certain and needs to be confirmed by surface analyses”
We spend huge sums on Mars rovers and we can’t send one or two to our backyard to verify a critical resource exists? Gee whiz, who did you Moon guys tick off? Why does that too far away rock with too deep a gravity well and not enough solar energy get all the attention? Why, why, WHY!?
Mars is a crummy place to go. Only a goofball with too much money and ego would want to retire there. I like the Moon and then Ceres- and then Callisto- and then Titan- and then…..
Ignoring the Moon seems completely backwards to my little mind.
Comment by GaryChurch — June 4, 2012 @ 4:16 pm
Paul,
No mention of LCROSS? I realize it was only one small area that was sampled, but my understanding is the entire mission was about obtaining ground truth to correlate the Clementine and Lunar Prospector data with optical and mass spectrometers. They reported very high water concentrations in this cold, dark, permanently shadowed crater.
Comment by Emory Stagmer — June 4, 2012 @ 6:03 pm
Emory,
My goal here was not to address all the facets of the lunar water topic, but to specifically discuss the issue of priority and how the alleged Luna 24 observation fits into the history of this controversy. LCROSS was not relevant to my topic.
Comment by Paul D. Spudis — June 5, 2012 @ 4:24 pm