Ancient Mercury’s volcanic past

Volcanoes exploded on Mercury for a large proportion of the planet’s history according to detailed data from Messenger. The spacecraft’s first fly-bys of Mercury in 2008 pointed to its explosive history, but provided no clue of the eruptions’ timeframes. New research published in Journal of Geophysical Research; Planets suggests the planet’s volcanoes exploded over an extended period of time. These findings are surprising since Mercury wasn’t supposed to have explosive volcanism, and could have important implications for understanding how the crater-stricken planet formed. Researchers used data from Messenger’s cameras and spectrometers collected when the spacecraft entered orbit around the planet in 2011 to study 51 pyroclastic sites across Mercury’s surface. They discovered that some of the vents had eroded to a greater degree than others – suggesting explosions didn’t all happen at the same time. “If the explosions happened over a brief period and then stopped, you’d expect all the vents to be degraded by approximately the same amount,” said Tim Goudge from the Department of Geological Sciences at Brown University. “We don’t see that; we see different degradation state. So the eruptions appear to have been taking place over an appreciable period of Mercury’s history.” To see where this explosive period fits into Mercury’s geological history, the researchers took advantage of the fact that most of the sites are located within impact craters, the age of which provides an important constraint in the age of the pyroclastic deposit inside it. The deposit has to be younger than the crater; if it were the reverse the deposit would have been obliterated by the impact forming the crater. There is an established method for dating craters on Mercury; the extent of the degradation of the rims and walls of the crater points to its approximate age. Using this, Goudge and his colleagues showed that – geologically speaking – some of the pyroclastic deposits are in relatively young craters between 3.5 and 1 billion years old. This rules out the possibility that all pyroclastic activity occurred shortly after the planet formed 4.5 billion years ago. “These ages tell us that Mercury didn’t degas all of its volatiles very early,” said Goudge. “It kept some of its volatiles around to more recent geological times.” This finding could shed light on how the planet formed, eliminating hypothesis that the planet’s outer layers were fried by the Sun or blasted away by a huge impact. Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data