“The halos are really exceptional,” said MESSENGER science team member Clark Chapman of the Southwest Research Institute in Boulder, Colorado. “We’ve never seen anything like them on Mercury before and their formation is a mystery.”
Two of the craters are located in Mercury’s giant Caloris Basin, a thousand-mile-wide depression formed billions of years ago when Mercury was struck by a comet or asteroid. The larger of the two is about 40 miles wide. Both craters have dark rims or “halos,” and one is partially filled with an unknown shiny material.
Chapman offered two possible explanations for the halos:
1. The Layer Cake Theory: There could be a layer of dark material under the surface of Caloris Basin, resulting in chocolate-colored rims around craters that penetrate to just the right depth. If such a subterranean layer exists, however, it cannot be unique to the Basin. “We’ve found a number of dark halos outside of Caloris as well.”
2. The Impact Glass Model: Thermal energy from the impacts melted some of Mercury’s rocky surface. Perhaps molten rock splashed to the edge of the craters where it re-solidified as a dark, glassy substance. Similar “impact melts” are found around craters on Earth and the moon. If this hypothesis is correct, future astronauts on Mercury exploring the crater rims would find themselves crunching across fields of tiny glass shards.
Chapman noted that the moon also has some dark haloed craters. “Tycho is a well-known example,” he said. But lunar halos tend to be subtle and/or fragmentary. “The ones we see on Mercury are much more eye-catching and distinct.” – space