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New research shows volcanism on Mercury extended to planet’s recent past

Mercury, long thought to be a planet without the ingredients necessary for volcanism, was volcanically active as recently as one billion years ago.

Scientists learned, as a result of a 2008 discovery by the MESSENGER spacecraft of pyroclastic ash deposits on the planet’s surface, that the solar system’s smallest planet has experienced volcanism.

The general assumption has been that Mercury’s volcanic activity must have occurred early in the planet’s history, about 4 billion years ago.

Newly-published research indicates that some of the pyroclastic ash deposits seen by MESSENGER were actually deposited between about 1 and 3.5 billion years ago.

That means volatile compounds, which drive the explosive eruption of volcanoes, are likely more prevalent on Mercury than has been thought.

“Mercury, contrary to predictions, is not deficient in volatiles, but instead has an abundance of them, “ Sean C. Solomon, the director of the Lamont-Doherty Earth Observatory at Columbia University and MESSENGER’s principal investigator, said.

Using data obtained by cameras and spectrometers on board MESSENGER, a team of scientists examined 51 sites at which the ash was deposited. They observed that the vents through which the ash was expelled onto Mercury’s surface exhibited varying degrees of erosion and determined the age of the craters in which the ash exists.

The difference in the erosion of the vents shows that ash was deposited on the surface of the planet at a variety of times during its history.

“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,” Timothy Goudge, a geology graduate student at Brown University and the lead author of the paper, said. “We don’t see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury’s history.”

The fact that some of the craters are older than the ash within them indicates that the ash must have been deposited in those craters after the impacts that produced the craters.

“These ages tell us that Mercury didn’t de-gas all of its volatiles very early,” Goudge said. “It kept some of its volatiles around to more recent geological times.”

Mercury’s large iron core has led to speculation that the planet may once have been significantly larger than it is now.

Solomon explained that scientists have tended to believe either that some of its mass was either burned away by the heat of the nearby Sun or blasted away during a collision with another celestial object. If either of those events had occurred, most, if not all, of Mercury’s volatile compounds would have been eliminated early in its history.

The new study’s confirmation that volatile compounds existed on Mercury until as recently as about 1 billion years ago casts doubt on those leading hypotheses about the planet’s formation.

“It really does tie the geochemistry and the geology together, pointing toward an origin we didn’t expect,” Solomon said.

Pyroclastic ash is produced by a volcano when it erupts. Volatile compounds in magma, such as carbon dioxide, hydrogen sulfide, sulfur dioxide, and water, change state from liquid to gas as the temperature of the magma rises and then expand, causing an increase in pressure that leads to the eruption.

The new paper appears in the March 28 edition of Journal of Geophysical Research: Planets.

MESSENGER was launched Aug. 3, 2004. Having completed its third year in orbit around Mercury last month, the probe is scheduled to crash into Mercury’s surface in March 2015.

The European Space Agency and the Japan Aerospace Exploration Agency plan a joint mission to Mercury to be launched in 2016.

Image

Image of Mercury obtained by MESSENGER on Jan. 14, 2008 courtesy NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington.

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