Why is Mercury gradually becoming smaller?

Astronomers have long known that Mercury has become smaller in size over billions of years. Despite being the closest planet to the Sun, Mercury’s surface cools as its internal heat leaks out. Because of this, the rocks and metals that make up the planet shrunk slightly in volume. But no one could say whether this process was still ongoing – at least until recently.

A scientific work published in the journal Nature Geoscience lifted the veil of secrecy a little. Due to the fact that Mercury’s core is compressed, its surface gradually begins to cover a smaller and smaller area. The result of this phenomenon is the appearance of special faults: some tracts of the landscape are pushed onto others. They look like wrinkles, like on a dried apple. The only difference is that the apple shrinks from rotting, and Mercury shrinks from thermal contraction.

The first evidence that Mercury is shrinking came in 1974: the Mariner 10 mission sent back to Earth images of miles-long mounds snaking across hundreds of kilometers of the planet. The MESSENGER robot, which was on the planet Mercury in 2011-2015, showed even more mounds.

From these data we can conclude that the appearance of the mounds was the result of geological faults under the surface of the planet. In fact, they are a kind of reaction to the compression of Mercury – its radius has become smaller by about 7 kilometers.

But when exactly this happened is a completely different question. Typically, the age of Mercury’s surface is calculated by the density of craters: the older the surface, the more craters. But this technique cannot be called one hundred percent correct, because previously the percentage of collisions that left craters was much higher.

Scientists have now reached a consensus that the age of most of the mounds is somewhere around 3 billion years. But are they all really that old? Do they keep moving? The first question has not yet been answered, but the second has been answered – yes, they continue. Small channels can be seen on the embankments: they appear due to the fact that a fragment of the surface bends under the pressure of compressed soil layers. Presumably, most of these channels appeared about 300 million years ago – this means that the compression of Mercury continued until relatively “recent” times.