MESSENGER reveals evidence of volcanism on Mercury
During its first two flybys of Mercury, MESSENGER (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) acquired images confirming that widespread volcanism occurred early in the planet’s history. Analysis of data from MESSENGER’s third and final flyby of Mercury in September 2009 has revealed evidence of younger volcanism than previously recognized. The results are reported in a paper published online on Science Express.
“After Mariner 10, it was thought that internal geologic activity on Mercury may have ended much earlier than on any other planet,” said MESSENGER imaging team member Brett Denevi, a faculty research associate in Arizona State University’s School of Earth and Space Exploration in the College of Liberal Arts and Sciences. “However, MESSENGER is showing that this may not be the case.”
MESSENGER’s third Mercury flyby revealed a 290‐kilometer‐diameter peak‐ring impact basin, among the youngest basins yet seen and recently named Rachmaninoff, having an inner floor filled with spectrally distinct smooth plains. These sparsely cratered Rachmaninoff plains postdate the formation of the basin and apparently formed from material that once flowed across the surface.
“We interpret these plains to be the youngest volcanic deposits we have yet found on Mercury,” said Louise Prockter of the Johns Hopkins University Applied Physics Laboratory (APL) and one of MESSENGER’s deputy project scientists, the paper’s lead author. “Moreover, an irregular depression surrounded by a diffuse halo of bright material northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on Mercury.”
MESSENGER results from the first two flybys showed evidence that Mercury had a very complex volcanic and tectonic history early on. Observations from the most recent flyby suggested that volcanism on the planet spanned a considerable duration much greater than previously thought, perhaps extending well into the second half of Solar System history.
“Mercury was a geologically active body well into its lifetime; its excitement wasn’t limited to just after its formation,” Denevi said.
A key missing element in scientists’ understanding of Mercury’s global thermal evolution is the temporal extent of volcanic activity and, in particular, the timing of most recent activity. But dating a surface without actually having any samples is complicated.
“Since we have no samples from Mercury, we use the relative number of superposed craters and compare that to the number of craters on different regions of the Moon,” said Denevi, who helped with the image analysis and interpretations of color. “We then factor in how we think the impact rate differs on the Moon and Mercury. It's all tied to the Moon because that's the only place we have samples from known locations with associated radiometric age dates.”
Previous analyses of the duration of geological activity led to the conclusion that volcanism ended prior to the beginning of Mercury’s Mansurian Period, ~3.5 to 1.0 billion years ago.
“Every time we’ve encountered Mercury, we’ve discovered new phenomena,” said MESSENGER principal investigator Sean Solomon, of the Carnegie Institution of Washington. “We’re learning that Mercury is an extremely dynamic planet, and it has been so throughout its history. Once MESSENGER has been safely inserted into orbit about Mercury next March, we’ll be in for a terrific show.”