THE WOODLANDS, Tex.—Mercury is a world of extremes. Daytime temperature on the planet closest to the sun can soar as high as 400 degrees Celsius near the equator, hot enough to melt lead. When day turns to night, the planet’s surface temperature plunges to below –150 degrees C.
But some places on Mercury are slightly more stable. Inside polar craters on the diminutive planet are regions that never see the light of day, shaded as they are by their crater rims. The temperature there remains cold throughout the Mercury day—and during the Mercury year. Now new data from NASA’s MESSENGER spacecraft, presented at the annual Lunar and Planetary Science Conference, corroborate a long-held hypothesis that Mercury has squirreled away pockets of water ice in those shadowy craters, despite the sun’s proximity.
Beginning with a series of radar observations of Mercury two decades ago using some of the biggest radio dishes on Earth, planetary scientists have had good reason to suspect that the polar craters harbored ice deposits at or just below the surface. Radar images of the poles showed anomalously bright features—patches that reflected radio waves much better than the surrounding terrain, just as ice does. Many of those radar-bright features corresponded to the location of large impact craters as mapped by the Mariner 10 spacecraft in the 1970s. But Mariner saw less than half of Mercury, and researchers have long lacked a comprehensive atlas of the poles to compare with the radar imagery.
That all changed following MESSENGER’s 2011 arrival at Mercury. MESSENGER (a somewhat strained acronym for Mercury Surface, Space Environment, Geochemistry and Ranging) has orbited the innermost planet for just over a year and has charted Mercury’s surface in unprecedented detail. As Nancy Chabot of the Johns Hopkins University Applied Physics Laboratory demonstrated in a conference talk, the maps MESSENGER has made match up nicely with the radar imagery of the poles.
“There is an excellent correlation between the radar-bright features and the shadowed locations in the craters,” Chabot said. “All of the radar-bright regions are within a few pixels of a region that is shadowed on Mercury’s surface.” In other words, the putative ice deposits fall in the few perpetually cold locales on Mercury—the places where ice could plausibly remain stable over long periods of time. The available evidence, she noted, remains consistent with the hypothesized presence of water ice on Mercury.
The putative ice is essentially ubiquitous in the coldest northern craters, the large impact basins within 10 degrees of the north pole. “In this region, nearly every crater that’s greater than 10 kilometers hosts a radar-bright deposit, which I think is really striking,” Chabot said.
But the apparently icy craters cover more of the northern hemisphere than might be expected. “Craters hosting radar-bright features extend to latitudes as low as 67 degrees North,” Chabot said. “These lower latitudes are a thermally challenging environment.” The radar hot spots also line craters less than 10 kilometers across, where the heat radiating from the basin’s sunlit rim would make for ice-unfriendly temperatures across the crater floor. At lower latitudes or in smaller craters, any ice deposits would likely require a thin insulating blanket, perhaps a layer of fine-grained surface material, or regolith, to keep it from sublimating away.
In fact, MESSENGER’s altimeter, which has fired more than 10 million laser pulses at Mercury to make detailed maps of the planet’s topography, seems to confirm that some insulating material blankets whatever ice may line the craters. Whereas radar can penetrate a thin layer of regolith to bounce off the ice beneath, the laser pulses are sensitive to reflectivity at the surface. Ice is very reflective at the 1.06-micron laser wavelength of the Mercury Laser Altimeter, Gregory Neumann of the NASA Goddard Space Flight Center explained in a conference talk, so exposed ice would return laser pulses more readily than its surroundings. “Surprisingly, we’ve been reporting for some time that no, we don’t see this,” Neumann said. “In fact we see quite the opposite.”
In the permanently shadowed craters, where radar observations have pointed to the presence of ice, the altimeter recorded dark patches of diminished laser reflectance. “We never see in these regions the large increase of return energy that you would see if it were so cold that ice were exposed on the surface,” Neumann said. One possibility is that the radar-bright deposits, widely believed to be ice, could be overlaid by a dark material, such as a hydrocarbon, that can tolerate somewhat higher temperatures.
That hypothesis was supported by David Paige of the University of California, Los Angeles. He and his colleagues calculated surface and subsurface temperatures for the locations where radar-bright features tend to form and inferred a probable composition of ice pockets blanketed by regolith darkened by organic compounds. Peak temperatures in the shadowed craters, which can often be too warm for exposed water ice, mesh well with conditions at which dark organic molecules would be stable. But just below the surface, temperatures in radar-bright craters tend to be colder, hovering near –170 degrees C. That is exactly the temperature at which water ice would be expected to remain stable, Paige said. MESSENGER's new look at the features long ago spotted by Earth-based radars, he added, demonstrate “fairly conclusively now that they are predominantly composed of thermally stable water ice.”