Washington, March 17 (IANS): The planet closest to the sun, Mercury, has seen its radius reduce by some 7 kilometers over the past four billion years, suggests new evidence.

This conclusion, based on data beamed back by NASA's Messenger spacecraft, would indicate that the planet has shrunk more than what was estimated earlier, according to findings published in the latest edition of the journal 'Nature Geoscience Sunday'.

The conclusion also solves an apparent enigma about Mercury's evolution, the findings suggest.

According to the journal, older images of Mercury's surface suggested that, despite cooling, the rocky planet had barely shrunk. But modelling of the planet's formation and ageing could not explain that finding.

Now, Paul K. Byrne and Christian Klimczak at the Carnegie Institution of Washington have led a team that used Messenger's images and topographic data to build a comprehensive map of tectonic features.

The map suggests Mercury shrunk substantially as it cooled, as rock and metal that comprise its interior were expected to.

"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," Steven A. Hauck of Case Western Reserve University and the paper's co-author was quoted as saying.

Mariner 10, the first spacecraft sent to explore Mercury, gathered images and data over just 45 percent of the surface during three flybys in 1974 and 1975.

Messenger, launched in 2004 and inserted into orbit in 2011, continues collecting scientific data, and is set to complete its 2,900th orbit of Mercury later this month.

Mercury's surface differs from Earth's in that its outer shell, called the lithosphere, is made up of one tectonic plate instead of multiple plates.

To help gauge how the planet may have shrunk, the researchers looked at tectonic features which result from interior cooling and surface compression. The features resemble long ribbons from above, ranging from 5 to more than 550 miles long.

Some of these are cliffs caused by thrust faults that have broken the surface and reach up to nearly 3.2 km high.

With the new data, the researchers were able to see a greater number of these faults and estimate the shortening across broad sections of the surface and thus estimate the decrease in the planet's radius.

They estimate the planet has contracted between 4.6 and 7 kilometres in radius.

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

One striking aspect of the form and distribution of surface tectonic features on Mercury is that they are largely consistent with some early explanations about the features of Earth's surface, before the theory of plate tectonics made them obsolete -- at least for Earth, Hauck said.

So far, Earth is the only planet known to have tectonic plates instead of a single, outer shell.

The findings, therefore, can provide a framework to understand how planets cool. So, by looking at Mercury, scientists learn not just about planets in our solar system, but about the increasing number of rocky planets being found around other stars.