April 8, 2013
Though unremarkable in appearance compared to the roughly 4,000 craters on the Moon in its size range, the 20 km diameter crater Shackleton has been the source of relentless scientific controversy for the past 20 years. Shackleton is located at the south pole of the Moon; indeed, its near side rim is the precise location of the geographic pole itself. Its location makes observation by Earth-based telescopes difficult and it was not well photographed by the Lunar Orbiter series (our principal source of lunar images) of the 1960s. That all changed in 1994 with the flight of the joint DoD-NASA mission to the Moon, Clementine.
Clementine carried cameras that globally imaged the Moon in eleven visible and near-infrared wavelengths. In addition, it mapped the surface and lighting of the poles of the Moon at uniform resolution over the course of almost three lunar days (74 Earth days). When the Science Team first saw the south polar mosaic, the extent of darkness in the map was striking. Because the Moon’s spin axis is close to perpendicular to the ecliptic plane, the Sun is always at the horizon at the lunar poles. Instead of rising and setting, the Sun circles around the poles at or near the horizon. Because of this grazing incidence, an area in a topographic depression may be in permanent shadow. And so it appeared for Shackleton crater in the Clementine data, setting off bells in the heads of the Science Team.
A key controversy of the post-Apollo era was whether the lunar poles might contain water or not. Although the Apollo samples had been studied and found to be “bone-dry,” we had not been to the poles on any Apollo mission. We knew that any shadowed areas had to be extremely cold as well as permanently dark. As water-bearing debris in the form of asteroids and comets constantly strike the Moon, it was thought that some of that water might get into a polar “cold trap” and would be kept there (essentially) forever – billions of years of impacting cosmic “debris” can add up.
Clementine was not configured to measure the presence of water, but a cleverly improvised experiment used the spacecraft’s data transmitter to beam radio waves into the dark regions near the poles and listen to their reflected echoes on the enormous (70 m) dish antenna of NASA’s Deep Space Network. Interestingly, the reflections indicated an enhancement of “same sense” polarization within the (very large) resolution cell that contained Shackleton crater. A collect of data from a nearby sunlit area (taken as an experimental control) did not show this peak. The Clementine team interpreted the RF peak as evidence for the presence of a few percent water ice within the dark, cold interior of Shackleton crater. The media quickly spread the startling news about water on our “bone-dry” Moon.
Such a controversial conclusion did not go unchallenged. Some in the radar community argued that abundant wavelength-sized rocks on the surface were the source of the enhanced same sense reflection. Since the lunar surface is indeed rocky, this interpretation could not be ruled out. Then a few years later, the Lunar Prospector (LP) mission found an enhancement of hydrogen concentration at both poles of the Moon; as hydrogen is a major constituent of water, the idea ice exists in the dark areas gained credence and has lead to a decade-long scientific search (using a variety of techniques) for lunar polar ice. Though many areas near the poles were studied in detail, attention continued to be drawn back to Shackleton and the area near the south pole.
From studying Clementine images, we discovered that part of the rim crest of Shackleton is one of the most sunlit areas on the Moon. Now we had a double-attraction: constant sunlight with water ice nearby. At a press briefing in 1996, I called this area of water and sunlight “the most valuable piece of real estate in the Solar System.” Nothing found subsequently has changed my mind on that judgment.
So what have we learned about Shackleton lately? Many different, new sensors have flown to the Moon in the last few years, including radar, ultraviolet (UV) imaging, laser reflections, and low-light level imaging. And yet again, Shackleton crater continues to confound us with contradictory evidence, both for and against the presence of water ice in its interior.
In 2009, the question regarding the presence of water ice somewhere near the lunar south pole was answered when the LCROSS impactor threw up a cloud of water vapor and ice particles during its collision with the floor of the nearby crater Cabaeus. Spectral mapping instruments on three different spacecraft (Chandrayaan-1, Cassini, and EPOXI) documented the presence of adsorbed water on the lunar surface, increasing in concentration with latitude toward both poles. A small impact probe flown by India (MIP) passed through a water vapor zone in the exosphere just above the lunar south pole. And radar images from Mini-RF, our radar imaging experiment on both Chandrayaan-1 and Lunar Reconnaissance Orbiter (LRO), found evidence of high same sense reflections (just as Clementine had suggested in 1994) within the interior of Shackleton crater. These new lines of supporting evidence were countered by Japanese researchers, whose Kaguya spacecraft imaged the interior of the crater and found morphology similar to other lunar craters in the same size-class. But no one had ever claimed that the interior of Shackleton was a skating rink of pure ice – the lunar polar ice is partly covered by waterless dust and mixed with an unknown amount of dry regolith.
Interpretation of the new data continues to vex us. The LOLA (laser altimeter) team on LRO recently published a paper that documents the high reflectivity (at 1 micron wavelength) of the walls of Shackleton. Although the team’s favored interpretation is that this is caused by a constant exposure of fresh material on a steep slope, they also note that it is consistent with the presence of water ice on the walls of the crater. In addition, a team analyzing neutron spectrometer data from both LP and LRO found evidence in the fast neutron data (never before analyzed) that water in the interior of Shackleton is a possible explanation for its signal. Detailed analysis of the Mini-RF data for Shackleton corrected for its steep wall slopes and found that the presence of 5-10 wt.% water there provides the best model fit to the observed data. Newly obtained UV images from LRO show the existence of water frost in the interiors of the craters Haworth and Shackleton, and the neutron detector on LRO shows enhanced hydrogen within both Shoemaker and Shackleton craters. The Japanese team from Kaguya continue to insist that the no-ice interpretation is the correct one.
So we are left with a mystery. Some evidence is pro-ice and some is contra-ice. I find it interesting that for most of the investigators, new data does not necessarily change any minds, but tends to be interpreted in a way most favorable to their previously published ideas. This should not be terribly surprising; the people who have argued for some specific interpretation presumably did so for good reasons and desire hard and clear-cut evidence to the contrary before abandoning a previously held position, one no doubt reached after much thought and soul-searching.
The way to unravel the water-ice mystery is to go to the surface of the lunar south pole (or both poles) and measure the composition of the surfaces in question. Getting a definitive answer about the nature of lunar water would be game changing. Some say the bigger mystery is: Why hasn’t the United States sent a rover to the south pole of the Moon to take a closer look?
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