Back in 2004 when the MESSENGER craft was launched to begin its trip to study the planet Mercury planetary scientists had some basic ideas of what the planet would be like but now that images are finally coming back those same scientists are having to re-evaluate much of what they thought about Mercury.
Scientists were pretty sure that they would see some sort of volcanic deposits on the surface of the planet but what they weren’t expect is that there are vast expanses of volcanic plains that cover almost 6% of the planet surface.
The volcanic deposits are thick. “Analysis of the size of buried ‘ghost’ craters in these deposits shows that the lavas are locally as thick as 2 kilometers” (or 1.2 miles), explains James Head of Brown University, the lead author of one of theScience reports. “If you imagine standing at the base of the Washington Monument, the top of the lavas would be something like 12 Washington Monuments above you.”
Additionally they have discovered vents with some measuring up to 25 kilometers in length and appear to be the source of the huge volumes of lava that created the lava plains.
One of the unusual things found during the flybys of Mercury was the visibility of what scientists referred to a bright crater deposits that had a blue color to them. Now additional images from MESSENGER has helped them figure out some answers.
“To the surprise of the science team, it turns out that the bright areas are composed of small, shallow, irregularly shaped depressions that are often found in clusters,” says David Blewett, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of the Science reports. “The science team adopted the term ‘hollows’ for these features to distinguish them from other types of pits seen on Mercury.”
Hollows have been found over a wide range of latitudes and longitudes, suggesting that they are fairly common across Mercury. Many of the depressions have bright interiors and halos, and Blewett says the ones detected so far have a fresh appearance and have not accumulated small impact craters, indicating that they are relatively young.
Additionally thanks to MESSENGER’s on board Gamma-Ray Spectrometer scientists have also been able to learn more about Mercury’s surface; which is apparently different from the Moon and other terrestrial planets.
“Measurements of the ratio of potassium to thorium, another radioactive element, along with the abundance of sulfur detected by XRS, indicate that Mercury has a volatile inventory similar to Venus, Earth, and Mars, and much larger than that of the Moon,” says APL Staff Scientist Patrick Peplowski, lead author of one of the Science papers.
These new data rule out most existing models for Mercury’s formation that had been developed to explain the unusually high density of the innermost planet, which has a much higher mass fraction of iron metal than Venus, Earth, or Mars, Peplowski pointed out. Overall, Mercury’s surface composition is similar to that expected if the planet’s bulk composition is broadly similar to that of highly reduced or metal-rich chondritic meteorites (material that is left over from the formation of the solar system).