March 27, 2012
A Scientific Dispute
Uncollimated (left) and collimated (right) views of the Moon from the LEND instrument: What's being detected? (from Eke et al., 2012, LPSC 43, 2211)
Attendees at the recently concluded 43rd annual Lunar and Planetary Science Conference had front row seats to a heated debate on new data from the Moon. As opposed to how many envision scientific debate – coolly logical, white-frocked intellectuals, dispassionately discussing points of contention in a laboratory – what they witnessed was an impassioned and stormy exchange of differing opinions. There is good reason for passion. Subsequent decisions based on these data places the success or failure of future missions in the crosshairs.
Point in question: a team of scientists on NASA’s Lunar Reconnaissance Orbiter (LRO) mission claim that their new neutron mapping shows that locations of high hydrogen content are not well correlated with dark areas near the poles of the Moon. This relation seems to contradict (at least, it is not consistent with) one of the key concepts about water at the poles of the Moon – that it occurs in dark polar cold traps, where water is stable on the surface and cannot be ejected from the Moon (as appears to be the case for most water deposited there).
This new idea is current because LRO carries something called a collimated neutron spectrometer, named the Lunar Exploration Neutron Detector (LEND), an instrument provided to NASA by IKI, the Space Research Institute of the Russian Academy of Science. NASA flew a neutron spectrometer to the Moon over 10 years ago on a global mapping mission called Lunar Prospector (LP). That instrument had an omni-directional (4-pi) field-of-view (FOV), meaning that it simultaneously looked in all directions. As such, the resolution of features on the surface made by this instrument was fairly low, being effectively equal to the altitude of the spacecraft. The LP neutron mapping spectrometer obtained a best resolution of about 30 km, meaning that any smaller feature could not be resolved in the FOV of the detector. Unfortunately, most of the dark, cold areas near the poles are smaller than this. LP detected enhanced levels of hydrogen in both polar regions, but couldn’t detect whether these hydrogen reservoirs were confined to the permanently shadowed areas, thus increasing the likelihood that the hydrogen was in the form of water.
In order to identify zones of high hydrogen content and determine if they were truly associated with the cold, dark areas, as predicted by theory, scientists wanted higher resolution maps of the poles for the next mission to the Moon. The way to obtain higher resolution is to restrict the field of view of the neutron instrument to where it looks only at a small spot directly below the orbiter. This involves putting a shield on the detector (called a collimator) that restricts the FOV to the lunar surface only; this technique can resolve areas on the surface smaller than the orbital altitude during mapping. A drawback to using a collimator is that restricting the FOV means that the flux, or total number of neutrons that can be detected per unit time, is much lower, which greatly reduces precision of the measurements. However, the longer the counting is conducted, the more precise the data. LRO was to remain in lunar orbit for at least a two-year mission; it has now been orbiting the Moon and collecting data for almost three years.
Over the last year, the LEND team’s reports have appeared in the scientific literature. To the surprise of most lunar scientists, their team claimed that in all but two or three isolated cases, hydrogen detected by LEND does not correlate with the polar dark areas. This puzzling result would seem to indicate that perhaps we do not fully understand the nature of the polar hydrogen and the processes involved in their creation and retention.
Thus the debate commenced at last Monday’s scientific session, when several scientists (I will collectively call them the “skeptics”) who work with neutron data from LP and other missions, differed with the LEND team conclusions, who in turn vigorously defended their results as valid, citing as evidence the coincidence of laser altimetry and neutron data over one crater (Shoemaker) near the south pole of the Moon. Having studied the LEND data set themselves, the skeptics contended that the actual average count rate for neutrons is less than half of that quoted by the LEND team, meaning that the hydrogen content inferred from the LEND data are significantly less precise than claimed. Moreover, they estimate that the signal from the collimated (high resolution) detectors is only a few percent of the total signal, whereas the LEND team claims that it is roughly one-third of the total. The skeptics make the point that if the collimator is working as the LEND team claim, the map derived from the collimated detector should be a sharper, higher resolution version of the low-resolution map made in the uncollimated mode. In fact, the skeptics contend that the two maps look completely different (see figure at top of this post), suggesting that the collimated product is detecting something else; based on the observed pattern, it is probably related to the amount of iron in the lunar surface.
This is not some arcane, academic dispute. We will depend on the mapping results from LRO to identify potential landing sites for future missions, including the selection of the most hydrogen-rich areas for exploration and possible future utilization. Such decisions could involve the expenditure of hundreds of millions of dollars, so there is some pressure to make the correct ones.
So where does this impasse leave the lunar science community? Mostly befuddled. The vast majority of scientists simply do not have the time to read every scientific paper published, especially in fields peripheral to their own interests. However, in the course of their research, scientists often find that they must decide what to believe about uncertain or controversial ideas that may relate to their own studies. Is there a correct way to decide which interpretation to believe? After a quick and cursory review of the competing concepts, most scientists will adopt the majority, or “consensus” viewpoint. If they know someone with relevant expertise, they may ask for and rely on the considered judgment of that expert. Few scientists are able to read and make their own considered judgments about a field in which they have little understanding or no expertise. Thus, they tend to choose their position on the basis of non-scientific evaluations of the technical credibility of those arguing for or against a given viewpoint.
In this case, the detailed distribution of hydrogen at the poles of the Moon remains unclear. While both LP and LEND uncollimated (e.g., omni-directional) maps appear nearly identical, the collimated LEND polar hydrogen maps show widely varying concentrations, with little coherence over short distances. Repeatability of measurement is important in science. The fact that two completely different instruments on two different missions found nearly identical results suggests that the low resolution, uncollimated LP and LEND maps are currently the best reflection of reality we have. These uncollimated data most likely will remain the polar hydrogen maps of choice by working lunar scientists.
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From Wiki:
“In March 2010, NASA reported that the Mini-SAR radar aboard Chandrayaan-1 detected what appear to be more than 40 small craters hypothesized to contain up to 1.3 trillion pounds (600 million metric tons) of water ice.[8]”
What about the radar data?
Is this some kind of conspiracy to divert attention from the moon’s resources so funding can go to private space playboy clubs in Low Earth Orbit?
“However, in the course of their research, scientists often find that they must decide what to believe about uncertain or controversial ideas that may relate to their own studies.”
In other words…….
O.K., it sound’s crazy but sometimes I wonder what the heck is going on with our space program.
Comment by GaryChurch — March 27, 2012 @ 10:13 pm
Sorry I forgot to add,
” -the technical credibility of those arguing- ”
In other words, the guy or gal with the most paper on the wall get’s to decide where the money goes.
Very scientific.
Comment by GaryChurch — March 27, 2012 @ 10:17 pm
Isn’t there a way to get better resolution without using a collimator?
It seems if one simply flew nearer the surface you would get better resolution.
But also is there anyway by looking at result from a further distance comparing the “normal distance”.
Or do get different maps depending on the distance?
And if got difference could this tell you something?
Comment by gbaikie — March 28, 2012 @ 2:17 am
What about the radar data?
Apples and oranges. The neutron detector senses hydrogen only and is limited to sampling depths of less than a meter. Radar detects RF-transparent material and senses 2-3 meters below the surface.
Comment by Paul D. Spudis — March 28, 2012 @ 2:16 pm
Isn’t there a way to get better resolution without using a collimator?
Not really. Lunar Prospector flew in a 30 km high orbit for several months; you cannot really fly much lower than that without running the risk of collision with the Moon. A better collimator might help, but any shield will permit some neutron leakage, so it is not 100% efficient.
The best way to determine the actual amounts of hydrogen in the polar areas is to land there and measure them in place. Measurement form many different sites would be required, so doing this from a roving vehicle is probably the best solution.
Comment by Paul D. Spudis — March 28, 2012 @ 2:18 pm
I thought we already got “ground truth” data from the LCROSS impact plume analysis !
….the LEND polar data (see map) show very low count rates for both omni-directional AND collimated readings.
Wasn’t this part of the discussion ? …why not ?
Thanks.
Jaro
Comment by Jaro — March 28, 2012 @ 4:58 pm
[...] Data, Heated Debate Leave Lunar Science Community Befuddled At Recent 43rd LPI Conference; Data From LRO LEND Instrument Shows Locations Of High Hydrogen Do [...]
Pingback by Wednesday / 28 March 2012 | Lunar Enterprise Daily — March 28, 2012 @ 5:59 pm
Dr. Spudis, the technical jargon is kind of ambiguous to me; can you spell it out in plain terms?
Does this mean there very well may not be all those millions of tons of ice on the moon?
Comment by GaryChurch — March 29, 2012 @ 12:15 am
Not really. Lunar Prospector flew in a 30 km high orbit for several months; you cannot really fly much lower than that without running the risk of collision with the Moon.
I thought there was some improvement in instrument since Lunar Prospector.
But suppose you fly something like Lunar Prospector- very cheap mission and take more risk?
And wasn’t one aspect of more precisely measuring gravity of the Moon to allow more precision in orbits?
So instead lowering 30 km circular orbit, do a 5 by 30 km orbit [fly 500 to 1000 meters above highest mountain peak/elevation one could encounter]. LRO already did something like this [make lower apsis]. Here:
“The higher resolution of these images is possible because of adjustments made to LRO’s orbit, which is slightly oval-shaped or elliptical. “Without changing the average altitude, we made the orbit more elliptical, so the lowest part of the orbit is on the sunlit side of the moon,” said Goddard’s John Keller, deputy LRO project scientist.”
And:
“The maneuver lowered LRO from its usual altitude of approximately 31 miles (50 kilometers) to an altitude that dipped as low as nearly 13 miles (21 kilometers) as it passed over the moon’s surface. The spacecraft has remained in this orbit for 28 days, long enough for the moon to completely rotate”
http://www.nasa.gov/mission_pages/LRO/news/apollo-sites.html
But question is would it make much difference. Is there any significant difference between 30 and 50 or 100 km?
Comment by gbaikie — March 29, 2012 @ 1:16 am
Does this mean there very well may not be all those millions of tons of ice on the moon?
No, it does not mean that. It means that the high resolution neutron data set is a singular observation and cause has been shown to doubt it as a reflection of reality.
The various techniques detecting water on the Moon all measure different things. The spectral data measure monolayers of water and hydroxyl molecules on dust grains. The orbital mass spectrometer of the MIP on Chandrayaan-1 measures exospheric water vapor molecules in space above the Moon. The neutron detectors on LP and LRO measure total hydrogen, but only in the upper meter of soil. The radar detects ice in the upper 2-3 meters of the soil. And LCROSS measured soil water content (vapor and ice particles) in the upper 3 meters or so of its impact site. No one data set contradicts the findings of the others.
Comment by Paul D. Spudis — March 29, 2012 @ 11:32 am
Is there any significant difference between 30 and 50 or 100 km?
In terms of resolution, no. The resolution of an omni-directional instrument is fixed by the orbital altitude. Hence, the 30 km resolution of the LP neutron data is the current best data set. The theme of this post is that various other workers have found that the collimated neutron data from LRO, whose aim was to get higher resolution maps, is apparently not obtaining what was desired.
Comment by Paul D. Spudis — March 29, 2012 @ 11:35 am
Regarding the question “Isn’t there a way to get better resolution without using a collimator?” — I think the answer “No” is only true for passive neutron spectrometers like LEND.
If the spacecraft is equipped with an active neutron source, illuminating a small area on the surface, then the readings in the spectrometer will have high resolution, since the signal will swamp the one from background (cosmic) neutrons.
Comment by Jaro — March 29, 2012 @ 12:26 pm
If the spacecraft is equipped with an active neutron source, illuminating a small area on the surface, then the readings in the spectrometer will have high resolution
True enough, but you’d have to fly a nuclear reactor in lunar orbit to do that. As a matter of fact, there was a proposal for just such a mission back in the 1990′s. It was to use a Russian Topaz nuclear reactor and a cooled Ge detector gamma-ray spectrometer. The mission was called “Double Eagle” and was to have been flown by the Strategic Defense Initiative Organization (SDIO).
Comment by Paul D. Spudis — March 29, 2012 @ 2:17 pm
Re: “you’d have to fly a nuclear reactor” — there are other, much more compact neutron sources, some of which are commonly used in oil exploration. For short-duration missions, a nice intense source uses Beryllium-Polonium-210. For longer missions, Beryllium-Plutonium-238 is good. Another possibility is Californium-252.
Comment by Jaro — March 29, 2012 @ 3:28 pm
there are other, much more compact neutron sources, some of which are commonly used in oil exploration
Interesting. What kind of mass and power requirements are we talking about for these sources?
Comment by Paul D. Spudis — March 29, 2012 @ 4:17 pm
No power, other than possibly cooling (the radioactive decay can easily turn a small pellet red-hot, just like the RTG Pu238 pellets used on the Curiosity Mars rover & other missions, for power generation using thermoelectric converters, and for general heating of electronic and mechanical devices).
Depending on the neutron source strength required and the specific material chosen, the mass could be anywhere from, say, a 100 grams, to maybe a kilo.
I believe these types of neutron sources have been used on space missions before, for soil analysis at a distance, but I don’t remember which – possibly not even American.
Comment by Jaro — March 29, 2012 @ 4:29 pm
Astrobotic Technology is planning to fly a neutron spectrometer on its October 2015 mission to the lunar north pole. This mission is designed to determine the ISRU potential of that area. See http://affordablelunarbase.blogspot.com/2012/04/astrobotic-unveils-lunar-polar.html for more info.
Comment by Bob Carver — April 14, 2012 @ 6:46 pm
Here are other, much supplementary compact neutron sources, more or less of which are commonly used inside smear with oil exploration
Out of the ordinary. Could you repeat that? Kind of stack and power supplies are we discussion on the subject of in support of these sources?
Comment by salim — May 20, 2012 @ 7:19 am