New radar observations and refined illumination maps reveal uneven water ice deposits twice the size of those found around the planet’s north pole, suggesting the source may be a recent comet impact.
The team also used standard and long-exposure camera images acquired by MESSENGER to estimate that 5.7% of Mercury’s south polar region is permanently shadowed, roughly 50% more than the same area encircling the north pole. These findings are consistent with the southern region’s older, more heavily cratered terrain, whose topography creates more permanently shaded areas with temperatures that are potentially low enough to harbor ice.
Although the researchers found that the radar-bright regions are consistently located in permanently shaded areas, they also discovered that nearly half of these nonilluminated regions don’t contain radar-bright deposits. This discrepancy, they argue, could be because the shaded areas lack water ice or because the ice is too deeply buried to be detected in those locations. Either way, this uneven distribution implies that the polar water ice was not delivered by a steady source—like planetary outgassing or solar wind generation—but rather by an episodic event, such as a large comet impact.
These results support the observation that Mercury’s south pole has a substantially higher volume of frozen water ice and other volatiles than Mercury’s north pole and provide strong new evidence for a recent impact event as the source. In addition to offering exciting new evidence regarding the source of Mercury’s ice deposits, these findings also provide additional resources to guide further exploration of the innermost planet’s ice deposits by the upcoming BepiColombo mission, which is scheduled to launch in October. (Journal of Geophysical Research: Planets, https://doi.org/10.1002/2017JE005500, 2018)
—Terri Cook, Freelance Writer