Water on the Moon? The battle continues

LEND data of lunar south pole

A little over a month ago I reported here on Behind the Black some recent results from the LEND instrument on Lunar Reconnaissance Orbiter (LRO) that had found significantly less water in the permanently shadowed craters at the lunar poles than previously thought. To quote again from that paper’s abstract, which I will henceforth refer to as Sanin, et al:

This means that all [permanently shadowed regions], except those in Shoemaker, Cabeus and Rozhdestvensky U craters, do not contain any significant amount of hydrogen in comparison with sunlit areas around them at the same latitude.

And from the paper’s conclusion:

[E]ven now the data is enough for definite conclusion that [permanently shadowed regions] at both poles are not reservoirs of large deposits of water ice.

Paul Spudis of the Lunar and Planetary Institute in Houston, Texas and one of the world’s top lunar scientists then commented as follows:

You neglect to mention yet another possibility — that this paper and its conclusions are seriously flawed in almost every respect. The veracity of the LRO collimated neutron data [produced by the LEND instrument] have been questioned on serious scientific grounds. Other data sets (spectral, radar) suggest significant amounts of water at both poles, billions of metric tons in total.

Spudis also discussed this scientific dispute at length on his own blog.

When I read Dr. Spudis’s comment I immediately emailed William Boynton of the Lunar and Planetary Laboratory at the University of Arizona, one of the authors of the Sanin et al paper, to get his reaction. Today he sent me the following detailed explanation, describing the basis of the controversy and why he believes the LEND data is valid.

Paul Spudis questions the validity of the LEND data citing a recent manuscript that claims that all but a few percent of the signal in the LEND collimators is due to hydrogen. This manuscript is wrong in its quantitative assessment of the amount of signal due to hydrogen, and Spudis is wrong to suggest that our “conclusions are seriously flawed in almost every respect.”

LEND achieves its high spatial resolution by virtue of surrounding four of its neutron detectors with tubular shields, called collimators. The collimator tubes are open at the ends pointing toward the lunar surface allowing neutrons from directly below the spacecraft (within ± 5.6 degrees, which is about a 10-km diameter circle at the 50 km altitude of LRO) to be detected by the sensors located at the opposite ends of the tubes. The collimator walls cannot be made completely opaque to neutrons, so some small fraction of the neutrons outside the field of view of the opening will also be recorded by the sensors. The concern in the quality of the LEND data revolves around a disagreement concerning the amount of signal that is due to neutrons coming through the shield vs. those coming from the field of view.

LEND data over Shoemaker Crater

Without getting into all of the “inside baseball” arguments on the proper way to estimate the contribution of the background signal coming through the walls of the collimator, it is instructive to simply look at the results. In the figure on the right I show a trace of the count rate measured in the four collimated sensors relative to the counts recorded in a presumably low-hydrogen area at low latitudes. Also shown is the surface elevation of the Shoemaker crater as determined by the Lunar Orbiter Laser Altimeter (LOLA) on LRO. An increase in the amount of hydrogen is accompanied by a decrease in the count rate of neutrons as seen here at the location of the crater. The data are taken along a line of constant longitude (45° and 225°) passing through the center of the crater. It can be seen that a very significant decrease in the flux of neutrons is seen that coincides with the topography of the crater.

The statistical uncertainty of the data is also shown. This uncertainty is something like the margin of error that one often hears associated with surveys of people on a particular subject. When scientists make measurements, there is always some error associated with the measurement. For example one could measure the diameter of a rod with a ruler and get it accurate to about 1/32 of an inch. If one could measure it with a good pair of calipers, one could get an accuracy of about 1/1000 of an inch, but there is still some uncertainty. The uncertainty shown implies that the true count rate of lunar neutrons at the location of the crater is likely to be between those limits of the value actually plotted. In this case the measured value is 0.18 counts per second (cps) less than that of the low-hydrogen region, but with the uncertainties, the true value is likely to be anywhere between 0.165 and 0.195 counts per second below that of the low-hydrogen region. In this case, even with the statistical uncertainty, we can be absolutely certain that the depletion of neutrons seen at the crater is real, and it indicates an enhanced amount of hydrogen in the crater compared to its surroundings.

The way we know that this suppression is due entirely to signal in LEND’s field of view is because the background comes from a very large region that extends all the way from horizon to horizon. The area over which the background is generated is so large, that moving the spacecraft a small distance, say 50 km away from Shoemaker, will have a negligible difference on the amount of background that must be subtracted. I have calculated the difference in background and find that it changes by only 0.002 counts per second over a 50 km distance. The disagreement between the two groups on the amount of signal coming through the walls of the collimator ranges between factors of 1.2 to 2.2. Even if I multiply the difference in background by the factor of 2.2, the change in background over 50 km is still just 0.005 counts per second, which is still completely insignificant compared to the observed suppression of 0.18 counts per second.

It would be better if the two groups could agree on the extent of the background leakage through the walls of the collimator, but the bottom line is that the data can speak for themselves. LEND has shown here that it can detect neutron suppressions with very high spatial resolution.

So, is the Moon a desert or not? I suspect we really won’t know until we get there. However, if I had to guess, my skeptical nature would favor a drier environment. It is remarkable enough that there is any water ice on the Moon. To believe that — as some data has suggested — there is a lot there is very counter-intuitive. And now that at least one instrument on LRO has found a lack of water at most locations is further confirmation of this intuitive conclusion.

Moreover, if I was a mission planner aiming to build a base on the Moon, I would take the LEND data very seriously and aim for those few craters (Shoemaker, Cabeus and Rozhdestvensky U) that LEND says do have water. Other locations may have water ice, as suggested by other instruments, but if they don’t, as LEND’s data indicates, any lunar base at these locations would have very significant problems. Better to be safe and aim for the most likely places to find water.

One interesting side note: The LEND instrument was built in Russia, and was chosen over another instrument designed by American scientists, some of whom are apparently leading the criticism of the LEND instrument. This makes me wonder if a political turf war battle is contributing to this conflict, partly fueled by leftover Cold War resentments. Just a thought.

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18 comments

  • Coastal Ron

    Certainly the lack of water doesn’t support the narrative Spudis promotes for lunar water ISRU, and it makes the likelihood of success of the Spudis/Lavoie plan questionable too.

    But until we get there and find out, we won’t know. The least risky way of doing that is with robotic explorers, maybe even cousins of the MSL currently on it’s way to Mars.

  • The Shoemaker crater correlation shows that they are measuring *something* which is reasonably collimated and correlated with the crater. But is it hydrogen? I’m not entirely certain that I even agree it is “neutron suppressions” they are measuring. It would be much more convincing if they weren’t cherry-picking a single crater to justify their data — and if the measured collimated LEND data correlated with the uncollimated LEND data.

  • Joe

    “To believe that — as some data has suggested — there is a lot there is very counter-intuitive. And now that at least one instrument on LRO has found a lack of water at most locations is further confirmation of this intuitive conclusion.”

    Basing a “scientific” judgment on” intuition” and a “skeptical” nature is in large part how people for centuries remained convinced that the earth was flat.

  • Coastal Ron

    Joe said:

    Basing a “scientific” judgment on” intuition” and a “skeptical” nature is in large part how people for centuries remained convinced that the earth was flat.

    The converse is true too Joe.

    And because there is so little hard evidence, it makes no sense to fund a $87B program (the Spudis/Lavoie plan) around mining lunar water until 1) we know there is enough water to mine (i.e. hard evidence, not conjecture), and 2) we know there is a reason to mine and process large volumes of water (i.e. demonstrated demand, not wishful thinking).

    • Joe

      “The converse is true too Joe.”

      Yes, that would mean there is no bottom line only points of contention to be endlessly debated. I am sure that works for you as that is what you like to do. Have fun.

  • Morris Jones

    The best way to settle this question is to physically land something in this region and perform in situ measurements. Dig we must.

  • Joe

    Agreed.

    Below is a link to what Coastal Ron refers to as the “Spudis/Lavoie” Plan.
    http://www.spudislunarresources.com/Bibliography/p/102.pdf

    On page seven you will find a section titled: A. Phase I: Resource Prospecting

    The first paragraph reads:

    “To begin our return to the Moon, we first launch a series of robotic spacecraft to: 1) emplace critical
    communications and navigational assets; 2) prospect the polar regions to identify suitable sites for resource mining and processing; and 3) demo the steps necessary to find, extract, process and store water and its derivative products (Fig. 2).”

    This fulfills everything Coastal Ron says he wants. Yet Coastal Ron continues to try to create an argument on the subject. That should tell you what you need to know about how seriously to take Coastal Ron’s comments.

  • pathfinder_01

    To begin our return to the Moon, we first launch a series of robotic spacecraft to: 1) emplace critical
    communications and navigational assets; 2) prospect the polar regions to identify suitable sites for resource mining and processing; and 3) demo the steps necessary to find, extract, process and store water and its derivative products (Fig. 2).”

    Ah step one should have been verify that there is ICE there and that the quantity is sufficent to mine. That does not need a fleet of communications and navigational assets. (i.e. The probe could communicate directly to earth or use a lunar orbiting probe to relay the information).

    Step 2 depends alot on information gathered from step three. How do you know what regions are suitable when you don’t know what steps you are going to take to mine it? On earth there are mulitple methods of mining things.

    • Joe

      “Ah step one should have been verify that there is ICE there and that the quantity is sufficent to mine.”

      Ah step two is to do exactly what you say. If you want misunderstand short declarative sentences that is of course your choice.

      “That does not need a fleet of communications and navigational assets. …”

      If you really believe you know more about what is required to do practical lunar geology by tele-roboitcs than Spudis and Lavoie (who headed up the Constellation Systems Robotic Precursor program) maybe you should educate them.

      • Joe,

        Let me quote from each and all of your recent comments in connection with the water-on-the-Moon story:

        “Ah step two is to do exactly what you say. If you want misunderstand short declarative sentences that is of course your choice.”

        “If you really believe you know more about what is required to do practical lunar geology by tele-roboitcs than Spudis and Lavoie (who headed up the Constellation Systems Robotic Precursor program) maybe you should educate them.”

        “Yet Coastal Ron continues to try to create an argument on the subject. That should tell you what you need to know about how seriously to take Coastal Ron’s comments.”

        “Yes, that would mean there is no bottom line only points of contention to be endlessly debated. I am sure that works for you as that is what you like to do. Have fun.”

        “Basing a ‘scientific’ judgment on ‘intuition’ and a ‘skeptical’ nature is in large part how people for centuries remained convinced that the earth was flat.”

        Do you see a pattern? I do. Rather than debate these points rationally, based on your knowledge and opinion, you seem to relish putting everyone down, insulting them in a sarcastic and sometimes childish manner. This is not helpful and does your arguments no good at all. If anything, it discredits you. And I do not mean this as an insult. I think you are better than this.

        I really appreciate your contributions to Behind the Black. You know your stuff and provide a good voice for opposing points of view. I would appreciate, however, if you would cut back on the sarcasm and insults and focus instead on simply proving your points based on actual data. I want the discussion here to rise up above the typical troll-like flamewars we see on too many other websites.

        Thank you.

        • Joe

          Robert,

          If any sarcasm I may have used irritates you I am truly sorry.

          But the fact remains that (in this case for example) the “Spudis/Lavoie Plan” definitely calls for robotic precursor missions to analyze prospective sites. When individuals ignore that fact and state that they want robotic precursors done first it also irritates me. I will admit that kind of thing brings out the “worst” in me; however the original provocations might also bear a little scrutiny.

          I know you disagree that there is significant water on the moon, but that should not extend privileges to others to repeatedly misstate someone’s (in this case Spudis) opinions and positions.

          You might look at the post by pathfinder_01 below. First he accuses me of trying to dodge the subject (which was whether or not the proposed precursor missions would characterize the amount and nature of lunar ice at the selected locations) and then starts talking about diamond mining.

      • pathfinder_01

        Ah nice way to dodge, but it states: “prospect the polar regions to identify suitable sites for resource mining and processing”

        Suitability is more than is there ice present or not. There are resources on earth that are present but not easily mined like say diamonds in earth’s mantle. You can’t tell what is or is not a suitable place to mine without some detail (or idea) of how you plan to mine it. I mean for instance if you are expecting a rover to drive in and out of a crater, then the crater walls probably should not be too steep. If you are planning to lift material out the crater using a crane then the crater’s walls are less a problem, but the crater itself should not be too deep. Can the ground support the weight of the equipment or not? How much power do I need and how will it be supplied are all factors in just how suitable an area is.

        “If you really believe you know more about what is required to do practical lunar geology by tele-roboitcs than Spudis and Lavoie (who headed up the Constellation Systems Robotic Precursor program) maybe you should educate them.’

        I know enough to know that putting communications and navigations assets in lunar orbit ahead of determining if you can mine the stuff or not is a waste. It would be like building a mining town in Kentucky before determining if there is coal present.

  • Patrick

    I have one question or so.

    How long will unprotected ice last on the moon?

    How long will ice protected by a meter of rigolith(sp?) survive one the moon?

    A simple test should be able to be sent to the moon real cheap. We already have a place to do it and something could designed for a simple experiment.
    Two sizable blocks of ice sent to ISS. One insulated and one not. Installed on a cheap platform outside the station. A camera on each to watch what happens.

    Obviously the unprotected one will not last as long as the protected one but just how long will the protected one last. If it can’t last basically forever in the vacuum of space next to the station then it has little chance of lasting on the moon for a thousand years.

    In my opinion the only ice available on the moon could only be pretty deep. Deeper than a meter at least.

    • Joe

      Hi Patrick,

      “How long will unprotected ice last on the moon?”

      The point is that the ice in the lunar Polar Regions is not “unprotected”. It is protected by virtue of being in permanently shaded areas that are referred to as Cold Traps. In these areas in the moons vacuum environment the temperatures are very cold and thus allow any ice there to be protected.

      Hope this helps.

      Joe

      • Interestingly the team of scientists behind the LEND instrument published a new paper this weekend. I will be writing about sometime in the next few weeks. They agree completely that there is evidence of water ice on the Moon, but their data does not place it in as many places or even in the same places as other instruments have suggested. They even detect evidence of water in non-permanently shadowed areas, and outline a scenario whereby water molecules might survive in such circumstances. And, as I have already noted, they generally find less evidence of water than past studies.

        It is all very intriguing. I really don’t know if their data is definitive or trustworthy, but if I had to commit a large amount of funds for any missions to the Moon, I would be remiss if I simply dismissed it. Since they do find evidence of water in some of the same places as other instruments (Shoemaker and Cabeus craters for example), it seems logical to make those locations the high priority targets.

      • Patrick

        Whats the temperature of the moon twenty feet down. In the sunlit areas?

  • James Fincannon

    In addition to the questions Dr. Boynton attempts to answer, it would be nice for a clarification as to why they did not include Shackleton Crater in their paper. It is conspicuous by its absence.

    The paper specifically offered the criteria for being reviewed by the paper’s analysts, namely they “tested” permanently shadowed regions (PSRs)…“The set selected for analysis in this study all have areas >100 km^2. The list of tested PSRs is presented in Table 1.“

    So, if you examine this Table 1 list, you find 29 South Pole PSRs and 17 North Pole PSRs, which are ALL the PSRs they “tested” for that study (at least offered in the paper’s results). If you look at the list, there is no Shackleton Crater (which has an area of 350 km^2, well over the criteria limit of 100 km^2).

    Also, if you look carefully at the locations, they get no closer than -88.6 deg latitude to the South Pole. They “test” craters AROUND Shackleton namely Shoemaker, Faustini, Sverdrup and de Gerlache, but not Shackleton. Curious.

    They do offer polar maps of epithermal neutrons, but it is hard to correlate their “tested” results to the map based on the colors and PSR locations.

  • Paul D.

    We know from the data on material kicked up by the Centaur stage that at least one lunar polar crater contains a very weird mix of elements, including silver. Now, cadmium is similar to silver; could the polar craters also be enriched in this element, or in other elements that have high thermal neutron capture cross sections? If so, might this suppress the neutron signal from the craters?

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