March 27, 2012
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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