Mercury has barely any boulders. Rocks of at least 5 metres across are far less abundant on the planet鈥檚 surface than expected, and figuring out why could help us understand conditions on the closest planet to the sun.
Because Mercury is extremely difficult to reach with a spacecraft, we have few high-resolution images of its surface, most of them from , which orbited the planet from 2011 to 2015. at the University of California, Santa Cruz, and his colleagues examined about 3000 of the clearest of these images, searching for boulders.
They spotted only 14 images with visible boulders. To determine whether this number was unusual, they compared the Messenger images with pictures of the moon taken by NASA鈥檚 Lunar Reconnaissance Orbiter (LRO). They selected LRO images with similar observation conditions 鈥� such as the angle of the sunlight and the size of the imaged area 鈥� and used an algorithm to lower the quality of the pictures to match the ones from Messenger so the two data sets were comparable.
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The researchers found that boulders are about 30 times less abundant on Mercury than on the moon, an unexpectedly large difference given how similar the surfaces of the two small worlds are in other respects. 鈥淚n many senses, Mercury is like the moon, but in this sense it is different,鈥� says Kreslavsky. 鈥淲e see plenty of boulders on the moon.鈥�
There are three potential causes for this dearth of large rocks. The first is that Mercury may have a thicker layer of dust covering its bedrock. Boulders are formed when impacts break off bits of bedrock, so thicker dust would act as a protective layer that makes this more difficult.
Also, Mercury is much closer to the sun than the moon is, so it experiences extreme temperature variations and gets far hotter. This could degrade any boulders that do form, as could the larger amount of micrometeorites that hit Mercury.
The true explanation is probably a combination of the three, says Kreslavsky, but the BepiColombo spacecraft 鈥� which is on its way to Mercury and is due to arrive in 2025 鈥� should help us figure it out.
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