Volcanic Activity on Mercury Stopped About 3.5 Billion Years Ago

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Mercury

New research from North Carolina State University finds that major volcanic activity on the planet Mercury most likely ended about 3.5 billion years ago.

hese findings add insight into the geological evolution of Mercury in particular, and what happens when rocky planets cool and contract in general.

There are two types of volcanic activity: effusive and explosive. Explosive volcanism is often a violent event that results in large ash and debris eruptions, such as the Mount Saint Helens eruption in 1980. Effusive volcanism refers to widespread lava flows that slowly pour out over the landscape -- believed to be a key process by which planets form their crusts.

Determining the ages of effusive volcanic deposits can give researchers a handle on a planet's geological history. For example, effusive volcanism was active a few hundred million years ago on Venus, a few million years ago on Mars, and it still takes place on Earth today. Until now, the duration of effusive volcanic activity on Mercury, made of the same materials as these other planets, had not been known.

NC State assistant professor and planetary geologist Paul Byrne and colleagues determined when the bulk of Mercury's crust-forming volcanism ended by using photographs of the surface imaged by NASA's MESSENGER mission. Because there are no physical samples from the planet that could be used for radiometric dating, the researchers used crater size-frequency analysis, in which the number and size of craters on the planet's surface are placed into established mathematical models, to calculate absolute ages for effusive volcanic deposits on Mercury.

According to their results, major volcanism on Mercury stopped at around 3.5 billion years ago, in stark contrast to the volcanic ages found for Venus, Mars and Earth.

"There is a huge geological difference between Mercury and Earth, Mars or Venus," Byrne says. "Mercury has a much smaller mantle, where radioactive decay produces heat, than those other planets, and so it lost its heat much earlier. As a result, Mercury began to contract, and the crust essentially sealed off any conduits by which magma could reach the surface.

"These new results validate 40-year-old predictions about global cooling and contraction shutting off volcanism," Byrne continues. "Now that we can account for observations of the volcanic and tectonic properties of Mercury, we have a consistent story for its geological formation and evolution, as well as new insight into what happens when planetary bodies cool and contract."

The research appears in Geophysical Research Letters, with co-authors from the Carnegie Institution of Washington, Mount Holyoke College, the University of Georgia, Southwest Research Institute and Brown University. The MESSENGER mission provided substantial funding for this work.

Note to editors: An abstract of the work follows.

"Widespread effusive volcanism on Mercury likely ended by about 3.5Ga"

DOI: 10.1002/2016GL069412

Authors: Paul Byrne, NC State University; Lillian Ostrach, NASA Goddard Space Flight Center; Caleb Fassett, Mount Holyoke College; Clark Chapman, Southwest Research Institute; Brett Denevi, Johns Hopkins University; Alexander Evans, Southwest Research Institute and Columbia University; Christian Klimczak, Carnegie Institution and University of Georgia; Maria Banks, Smithsonian National Air and Space Museum and Planetary Science Institute; James Head, Brown University; Sean Solomon, Carnegie Institution and Columbia University

Published: Geophysical Research Letters

Abstract: Crater size-frequency analyses have shown that the largest volcanic plains deposits on Mercury were emplaced around 3.7?Ga, as determined with recent model production function chronologies for impact crater formation on that planet. To test the hypothesis that all major smooth plains on Mercury were emplaced by about that time, we determined crater size-frequency distributions for the nine next-largest deposits, which we interpret also as volcanic. Our crater density measurements are consistent with those of the largest areas of smooth plains on the planet. Model ages based on recent crater production rate estimates for Mercury imply that the main phase of plains volcanism on Mercury had ended by ~3.5?Ga, with only small-scale volcanism enduring beyond that time. Cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet.

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