Until recently, Mercury was not believed to contain the volatile elements required for making violent volcanic explosions. New data from the MESSENGER spacecraft has shown that this was not always the case. Not only was Mercury rife with volcanic activity early in the planet’s history, but it lasted much longer than anyone would have expected. The research comes from lead author Timothy Goudge from Brown University and was published in the Journal of Geophysical Research: Planets.
Earth is filled with chemicals like water and carbon dioxide which have relatively low boiling points and facilitate volcanic explosions. As they get cycled through with lava and get closer to the surface, they change from liquid to gas and expand, building up pressure. Eventually, the pressure just becomes too great, and the gasses and lava explode into an eruption.
Mercury, on the other hand, has not been thought to be as geologically exciting. When NASA’s MESSENGER probe began returning info about Mercury in 2008, scientists began to get the first clues of how dynamic the planet really is, or at least how it was. Some of the first images from the spacecraft revealed ash that is characteristic of volatile volcanic explosions.
While it was then clear that Mercury had volcanic activity at one point, there was not yet enough information about the duration of that activity. The ash could have accumulated over long periods of volcanic activity, or there could have been a short-yet-hectic time period in which the volatile elements caused these eruptions on Mercury’s surface.
Goudge’s team focused on 51 sites that had this characteristic pyroclastic ash by analyzing the images and data collected from spectrometers. The team noted that not all of the vents had experienced the same level of erosion, indicating that this activity did not occur in a short, chaotic period. In fact, after using the erosion to estimate the age of the vents, some were only 3.5-1 billion years old. Because Mercury itself is 4.5 billion years old, this means that Mercury spent much of its existence as a very volatile place with large amounts of the gases that cause those explosive eruptions.
Understanding the former volcanic activity could give clues to how the first planet formed. Mercury’s iron core is much larger than what would be expected for a planet of that size. This has led some to believe that Mercury used to be much larger than it is now. The loss of its size could have been due to a massive collision, or perhaps the intense radiation from the sun could have blown the outer layers off over time. If this turns out to be the case, it would explain why Mercury no longer has the volatiles that are necessary for explosive volcanic activity.