29 million miles away from the Sun at its closest, Mercury is the nearest of the eight planets to the burning center of our Solar System. It's also the smallest.

And it's getting smaller.

Back in the 1970s, the Mariner 10 spacecraft swung by Mercury on three occasions, photographing about 45% of the planet's surface in the process. Examining those images, planetary scientists uncovered telltale signs of shrinkage: lobate scarps, geological structures where crustal rocks had been pushed up and over each other, sinking down in the process. They estimated that Mercury had lost one to two kilometers of its global 2,440-kilometer radius since forming and hardening approximately 4.6 billion years ago. (For comparison, Earth's radius is 6,371 kilometers.)

According to a new report, however, Mercury has shrunk more than we thought: as much as 7 kilometers! The new finding, published in the journal Nature Geoscience, comes courtesy of a team led by Paul Byrne, a planetary scientist at the Carnegie Institution for Science in Washington D.C. But Byrne and his colleagues couldn't have succeeded without a little help from a newfound mechanical friend.

In August 2004, the Messenger spacecraft blasted out of Earth's atmosphere. Armed with an array of cameras and fortified with special shielding to guard against the Sun's damaging radiation, it set out to become the first spacecraft to orbit Mercury, and help us Earthlings learn about our planetary neighbor. Three years ago, Messenger entered orbit around Mercury. Since then, it's buzzed around the planet 2,886 times. A dedicated and busy bee, Messenger has put those orbits to good use, imaging the entirety of the planet's surface.

Armed with this unprecedented view, Paul Byrne and his team found that the planet was littered with the aforementioned scarps. They also found lots of wrinkle ridges. Resembling veins on skin, they're clear signs of contraction. The moon has lots of aged wrinkle ridges.

Surveying 216 ridges and scarps, the researchers arrived at a new estimate of Mercury's shrinkage: 5 to 7 kilometers radially.

Referred to as a "Land of Confusion," Mercury is an enigmatic planet -- its day is actually longer than its year! While Mercury zips around the sun every 88 Earth days, it completes one rotation every 59 Earth days.* Thus, to a fictitious observer standing on Mercury's inhospitable surface, a solar day from sunrise to sunset would take the equivalent of 176 Earth days. Moreover, its crust and mantle appear to be joined together into a single tectonic plate, and its solid iron core comprises as much as 65% of the planet!

"That's twice the percentage of our own Earth," NASA's Charlie Plain wrote.

Mercury's oversized core is likely to blame for its dwindling radius. The iron is likely still cooling, compacting in the process. William B. McKinnon, a planetary scientist at Washington University in St. Louis, summed up the paper succinctly and poetically.

"As Mercury’s interior cools and its massive iron core freezes, its surface feels the squeeze."

Source: Paul K. Byrne, Christian Klimczak, A. M. Celâl Şengör, Sean C. Solomon, Thomas R. Watters and Steven A. Hauck. "Mercury’s global contraction much greater than earlier estimates. "Nature Geoscience. DOI: 10.1038/NGEO2097

(Images: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, NOAA)

*Correction 3/16: An earlier version of this article incorrectly stated Mercury's rotational period as six Earth months. This is incorrect.