There are no rocks on Mercury, and scientists don’t know why



A team of astronomers led by Mikhail Kreslavsky, from the University of California at Santa Cruz, has run into a new enigma that is difficult to explain: in Mercury there are practically no rocks over 5 meters, a certainly puzzling fact.

It is not easy to get to the first planet of the Solar system, so that the best images of Mercury that we have are those of the Messenger mission, from NASA, which was orbiting it for four years (between 2011 and 2015). Kreslavsky and his colleagues examined more than 3,000 of those images for rock formations, but only found them in 14 of them. The study already appears on the prepublications server

Surprised, the scientists decided to compare the amount of rocks found with those on the surface of the body that most resembles Mercury in the entire Solar System: our Luna. So they turned to images taken by the LRO mission (Lunar Reconnaissance Orbiter), from NASA, and compared them with those made by Messenger.

In order not to skew the results, Kreslavsky’s team selected only those LRO images that had been made under observation conditions similar to those of Messenger. That is, with the same angle of sunlight and with the same size of the photographed area. Then they used an algorithm to reduce the quality of the lunar images, so that it was the same as that of the Messenger photos. In this way, they got two sets of photos that were comparable.

What they found was that the rocks are roughly 30 times more abundant on the Moon than on Mercury, an unexpectedly large difference given the similarities of the surfaces that, in other respects, keep the two small worlds. In Kreslavsky’s words, “In many ways, Mercury is like the Moon, but in this it is very different. There are many more rocks on the Moon.

Three possible explanations

Without a convincing explanation, the researchers shuffled three possible causes for the absence of large rocks on Mercury. The first is that the planet could have a thick layer of dust that covers its bedrock. And since boulders are formed when impacts from space stones break up chunks of bedrock, that dense layer of dust would act as a protective layer that prevents it.

Besides, Mercury is much closer to the Sun than the Moon, so it experiences extreme changes in temperature, and it gets much hotter. Which could degrade any rock that forms, turning it into dust. Another possibility, the third, is that the continuous micrometeorite impacts that hit the planet’s surface would have the same effect, breaking up the largest rocks.

The true explanation, however, could be a combination of the previous three, say the researchers. Something that you can surely find out when the European ship BepiColombo, which is currently on its way to Mercury, reaches its destination in the year 2025.

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