Mercury鈥檚 surface may be covered in a very thin, baked crust created by the extreme heat from the planet鈥檚 proximity to the sun.
Like Earth鈥檚 moon, Mercury鈥檚 surface lacks a substantial atmosphere, meaning impacts should break down its surface into a thin powder over time. However, Mikhail Kreslavsky at the University of California, Santa Cruz, and his colleagues have found a key difference between the two bodies.
The team looked through thousands of images of Mercury鈥檚 surface taken by NASA鈥檚 Messenger spacecraft, which orbited the planet from 2011 to 2015. They identified features that resemble landslides on Earth, suggesting a hard surface on top of a softer interior.
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鈥淲e believe there is a formation of a thin crust in the top centimetres of the soil because of the very high temperature,鈥� says Kreslavsky.
Mercury is the closest planet to the sun, at an average of 58 million kilometres away. During a day on its surface, which lasts 176 Earth days, temperatures can reach upwards of 430掳C, and these extreme temperatures appear to be hardening the upper layer of Mercury.
This process is known as sintering, where a material is hardened by extreme heat, and is similar to how a 3D printer operates, says Kreslavsky, creating links within the material. 鈥淧hysically it means there are bridges between small dust and soil particles,鈥� he says. 鈥淭hey stick to each other.鈥�
This process doesn鈥檛 occur on the moon because temperatures there don鈥檛 get high enough for long enough. Instead, the moon鈥檚 surface remains powdery. 鈥淢oon astronauts walked and left footprints,鈥� says Kreslavsky. 鈥淥n Mercury, astronauts would walk on a more solid surface.鈥�
Landing on Mercury is so difficult that it has never been attempted 鈥� besides the high temperature, its proximity to the sun means it orbits rapidly and a lander would have to go very fast to match the planet鈥檚 speed. But new high-resolution images from Europe and Japan鈥檚聽BepiColombo orbiter mission, scheduled to arrive at Mercury in 2025, could tell us more about the planet鈥檚 unusual surface. 鈥淣ow we know what to look at,鈥� says Kreslavsky.
Icarus
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