Ice on Mercury and the Moon: Why So Different?

 

 A comparison of the poles of Mercury and the Moon illustrates similarities and differences that PVL PhD Candidate Jake Kloos explores in this blog post. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory.

By Jake Kloos

The research I am conducting for my PhD pertains to the polar regions of the Moon, which have been active regions of study within the planetary science community for over half a century. For a variety of reasons, interest in the lunar polar regions is centered around the presence of volatile compounds, principally water ice. Ice deposits have been detected within permanently shadowed regions (PSRs), which are regions within impact craters near the poles that are permanently shielded from the Sun. Due to the lack of direct sunlight, temperatures within PSRs are extremely low, enabling them to trap, and potentially preserve, molecules such as water that are wandering about the surface. While ice has been detected within lunar PSRs, the concentrations that have been inferred from remote sensing observations appear to be unexpectedly low, at only a few percent by weight.

The low concentrations of ice found on the Moon is surprising given what we know about ice concentrations on the planet Mercury. Mercury and the Moon share certain key similarities that led many to predict that the two bodies would posses similar amounts of ice: both are considered “airless” bodies and host PSRs near the poles that exist within similar temperature regimes (although Mercury’s PSRs are slightly warmer). Despite this, ice appears to be abundant at the polar regions of Mercury, with inferred concentrations in the range of 50 to 100 % by weight. Moreover, radar data unambiguously show enhancements in nearly all of Mercury’s PSRs, whereas many PSR craters on the Moon lack similar radar enhancements. In fact, some of the lunar PSRs that do show radar enhancements are subject to debate, as some researchers feel that ice may not be the best explanation as to the cause of the enhanced signal. The large discrepancy in ice concentrations on the Moon and Mercury does raise the question: why?

David Lawrence, a researcher at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland recently published a review article in the Journal of Geophysical Research: Planets that details the history of scientific theories and measurements pertaining to the polar regions of the Moon and Mercury. As noted in the paper, discussion of PSR ice dates back to 1952, when Harold Urey first speculated that “condensed volatile substances” might be present within craters near the Moon’s pole which are shielded from direct sunlight. To date, there have been a multitude of attempts to detect water ice using techniques such as Radar, Neutron Spectroscopy, passive and active remote sensing, as well as the more unconventional approach of crashing a Centaur upper stage shell into the Moon and analyzing the resulting plume. The most interesting aspect of this paper, perhaps, was a discussion of the possible explanations for the dissimilar ice concentrations at the Moon and Mercury. The author explores four possible differences to explain the dichotomy, which are summarized below.

1. One way in which ice is delivered to planetary bodies is through cometary impacts. Comets are rich in water ice and throughout the history of the solar system have frequently collided with the Moon and Mercury, wherein they transfer their volatiles in the process. A relatively straightforward explanation is that Mercury has been hit with a comet more recently than the Moon, however as the author points out, this explanation seems too simple and isn’t entirely satisfying. Despite its simplicity, there does seem to be multiple lines of evidence that support this idea. One such example is that the ice deposits on Mercury appear to be relatively pure, which is to say that have had little modification over time from processes such as space weathering. On the Moon, there is evidence of ancient deposits of hydrogen which have been modified over long time periods, suggesting that the Moon has not had a recent delivery of ice.
An explanation of this sort presupposes a mechanism of ice destruction, burial or other loss processes that are operating proportionally on each body. There are a few such processes that are known, such as burial of ice through impact gardening, destruction via sputtering, or migration through diffusion.

2. A second explanation involves the types of impactors that have hit these two bodies. It could be the case the Mercury has been hit mostly with water-rich comets whereas the moon has been hit with water-poor asteroids. Detailed analysis of the ice present on both bodies could give a clue as to its origin, however such detailed measurements have not been made due to the difficult nature in obtaining useful measurements of PSRs.

3. A third explanation is that there are different processes operating within the PSRs of the Moon and Mercury as a result of the different environmental conditions. The PSR temperatures at Mercury are slightly warmer than those on the Moon meaning processes such as diffusion would have different effects on each body. Additionally, Mercury, unlike the Moon, has a magnetic field that shields the surface from energetic particles from the Sun, thereby helping to protect the ice deposits from modification and/or destruction.

4. Finally, Lawrence postulates that differences in planetary composition could be responsible. Based on data from the MESSENGER spacecraft, it was (unexpectedly) found that Mercury is relatively rich in volatiles elements such as K, S, Na, Cl, and C. If these volatiles are released from the surface, they would eventually reach the PSRs through a series of ballistic hops and become trapped. This process could potentially contribute to the disparity in volatile content between the Moon and Mercury given that the Moon is a volatile-poor body. The author points out, however, that “the idea that endogenous volatiles might supply a substantial portion of water to Mercury’s PSRs is currently only speculation.”

A fifth explanation, only tangentially covered by Lawrence, pertains to differences in orbital stability between the two bodies. For Mercury, its orbit and obliquity (the angle between the spin axis of the planet and its orbital plane) has likely been stable for the the last 4 billion years, resulting in a stable thermal environment at the poles, enabling ice to accumulate. In contrast, the Moon’s obliquity has changed considerably over the past few billion years, with past obliquities calculated to be as high as 77 degrees (its current value is 1.5). Research led by Matt Siegler, a researcher at the Jet Propulsion Laboratory, shows that for past lunar epochs, temperatures at the lunar poles positively correlate with obliquity. In other words, as we rewind the clock, we find higher lunar obliquities, leading to significantly higher temperatures at the poles and fewer PSRs. If we rewind the clock back 2.8-4.2 billion years ago to when the lunar obliquity was ~16 degrees, there are no longer regions at the surface where the temperature is low enough for ice to be thermodynamically stable.

Lawrence concludes his review article by noting that a combination of the above explanations could in fact explain the difference in observed volatile content at the Moon and Mercury, and that there is simply not enough information available to resolve this planetary puzzle. This unsolved problem in part speaks to the difficult nature of obtaining useful measurements of a surface region devoid of light, as many of the available data in this field of study (particularly from the Moon) are ambiguous and open to speculation. These initial measurements however, made in the first half-century of PSR exploration, have served to illuminate an interesting problem in planetary science  - a fortunate happenstance for those of us interested in studying such problems.