NASA's MESSENGER spacecraft at Mercury has gathered new evidence indicating that the planet closes to our sun has decreased in size by about 7 kilometer in radius over the last 4 billion years.

This measurement is much more than earlier estimates, according to a Case Western Reserve University press release.

The new finding solves an enigma about Mercury's evolution.

Older images of Mercury's surface features showed that the planet had barely shrunk at all, despite cooling over its lifetime.

Paul K Byrne and Christian Klimczak from the Carnegie Institution of Washington used MESSENGER's images and topographic data to create a map of tectonic features. The map showed that Mercury shrunk as it cooled, as metal and rock that makes up its interior are "expected to," according to the press release.

"With MESSENGER, we have now obtained images of the entire planet at high resolution and, crucially, at different angles to the sun that show features Mariner 10 could not in the 1970s," said Steven A. Hauck, II, a professor of planetary sciences at Case Western Reserve University and the paper's co-author, according to the release.

Research was published in the journal Nature Geoscience on March 16.

Mercury's surface is different from Earth's, as its outer shell, known as the lithosphere, is made up of one tectonic plate instead of a number of plates, according to the release.

Researchers looked at tectonic features, commonly known as the lobate scarps and wrinkle ridges, which occur after interior cooling and surface compression, in order to gauge how the planet might have shrunk.

In total, scientists mapped approximately 5,934 of the tectonic features, according to the release.

The new data allowed researchers to see a greater number of these faults and estimate the shortening across broad sections of the surface.

The researchers believe the planet could have contracted around 3.6 to 7 kilometers in radius, according to the release.

"This is significantly greater than the 1 to maybe 2 kilometers reported earlier on the basis of Mariner 10 data," Hauck said.

Earth is the only planet to have tectonic plates instead of a single, outer shell, as far as scientists know, according to the release.

The discovery will help provide limits and a framework to understand how planets cool. It also helps researchers learn about planets in our solar system, and the increasing number of planets being discovered around other stars as well.

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