Scientists: Mercury has been shrinking quicker than expected

An artist's concept shows the MESSENGER spacecraft in orbit around Mercury.

The innermost planet in our solar system was found to have contracted far more than the amount once estimated by scientists, according to findings published in the March 16 edition of the journal Nature Geoscience.

Analyzing global imaging and topographic data from NASA's MEcury Surface, Space Environment, GEochemistry and Ranging (MESSENGER) spacecraft, planetary geologist Paul Byrne and his colleagues had uncovered evidence that Mercury has shrunk by as much as 4.4 miles over the course of 3.8 billion years.

"When you look at the actual number, it's really pityingly small, compared to the size of a planet. But it doesn't need to change very much to have some effect," Byrne, a MESSENGER visiting investigator at the Carnegie Institution's Department of Terrestrial Magnetism, told Reuters.

The planet, which has one rigid, rocky layer, bears telltale cliffs and chasms caused by global contractions. Using images from MESSENGER, scientists studied more than 5,900 surface features, including cliff-like scarps and wrinkle ridges, and calculated how much Mercury has condensed.

The calculations matched the computer model predictions that scientists used to determine the planet's composition, chemistry and structure. These calculations directly contradicted the original data, gathered by earlier spacecrafts, which appeared to suggest that Mercury had only lost 1.2 to 3 miles in diameter.

"An awful lot of a planet's processes are driven by its heat loss -- that's a primary thing that drives a planet's evolution," Byrne told Reuters. "We didn't set out to prove the models right, but it turns out this number is exactly what the models have been predicting for 40 years."

Previous maps of Mercury's surface date back to the mid-1970s. The original data, pulled from NASA's Mariner 10 spacecraft's three flybys, was created with imaging that covered about 45 percent of the planet's surface. Now, with the addition of MESSENGER's imagery, about 98 percent of its surface can be seen in detail.

The spacecraft, since entering into orbit in 2011, has allowed for a deeper understanding of Mercury's distinctive features, which were formed after the planet's single crust was thrust upward. The most beautiful of all, according to Byrne, is the Enterprise Rubes in the southern hemisphere -- a single scarp system that is more than 600 miles long and nearly 2 miles high in places.

"Imagine standing in front of it. It's Mercury's version of a mountain belt. It would be a very dramatic landscape," Byrne told BBC News.

Scheduled to be launched in 2016, another spacecraft, BepiColombo -- a joint venture between Europe and Japan -- will follow up on the observations made by MESSENGER, reports the BBC.

In addition to learning more about the surface of Mercury and the planet's history, the discovery has implications for assessing the compositions of planets beyond the solar system, providing an example of what a planet does and how it behaves as it cools over time.