April 5, 2014

Prolonged Volcanic Activity Could Provide Insight Into Mercury’s Origins

redOrbit Staff & Wire Reports - Your Universe Online

Using new data obtained by the MESSENGER spacecraft, researchers from Brown University in Rhode Island have discovered that explosive volcanic activity has been occurring throughout most of Mercury’s history, meaning that the planet has had the volatile compounds necessary for those eruptions for longer than anticipated.

The findings, which are detailed in a recent edition of the Journal of Geophysical Research: Planets, are said to be surprising because explosive volcanism wasn’t initially believed to be occurring on Mercury at all. Furthermore, the authors report that the discovery could help experts better understand how the smallest planet in the solar system formed.

On Earth, large volcanic explosions occur because the planet’s interior is packed with volatile compounds with relatively low boiling points, such as water and carbon dioxide. As the lava rises towards the surface, compounds dissolving within it change from liquid to gas and expand, causing pressure that can cause the crust to burst.

On the other hand, experts had long believed that there were no volatiles on Mercury, and without those compounds, explosive volcanism cannot occur. However, in 2008, MESSENGER’s first flybys of the planet discovered surface deposits of pyroclastic ash.

Since pyroclastic ash is a sign that volcanic explosions are occurring, it alerted scientists that Mercury most likely had been home to volatile compounds at some point in its existence, the researchers said. The new study demonstrates that Mercury most likely held onto its volatiles over an extended period of time.

Tim Goudge, a graduate student in the Department of Geological Sciences, and his colleagues analyzed 51 pyroclastic sites throughout the planet’s surface. They used orbital data obtained by MESSENGER’s cameras and spectrometers collected by the spacecraft in 2011.

That information, combined with that collected during the initial flybys, gave Goudge’s team a more detailed look at the ash deposits, as well as the vents from which they originated. They discovered that some of those vents eroded to a far greater degree than others, which indicates that the explosions happened at different times.

“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,” Goudge said in a statement. “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.”

In order to figure out how that period of explosiveness fits on the planet’s geological timeline, the Brown University investigators seized on the fact that most of the sites were located inside of impact craters. The age of each deposit contained in those craters has to be younger than the crater itself, which means that the age of the crater provides an upper limit on how old the pyroclastic deposit contained within it could possibly be.

The researchers used an established method of Mercury crater dating which measures the erosion and degradation of the craters’ rims and walls in order to estimate how old they are. By using this technique, the authors figured out that some of the pyroclastic deposits are found in craters roughly 1.0 to 3.5 billion years old, which eliminates the possibility that all of this activity occurred shortly after the planet’s formation approximately 4.5 billion years ago.

“The extent to which Mercury’s volatiles stuck around could shed light on how the planet formed,” the university explained. “Despite being the smallest planet in the solar system… Mercury has an abnormally large iron core. That finding led to speculation the [sic] perhaps Mercury was once much larger, but had its outer layers removed – either fried away by the nearby Sun or perhaps blasted away be a huge impact early in the planet’s history.”

“Either of those events, however, would likely have heated the outer parts of Mercury enough to remove volatiles very early in its history,” it added. “In light of this study and other data collected by MESSENGER showing traces of the volatiles sulfur, potassium, and sodium on Mercury’s surface, both those scenarios seem increasingly unlikely.”

Image 2 (below): Two pyroclastic vents on the floor of Mercury’s Kipling crater, top, would likely not have survived the impact; they are more recent. The false color image of the same spot, bottom, marks pyroclastic material as brownish red.