NASA spacecraft photos have revealed that Mercury is gradually shrinking and has lost miles in diameter as the planet slowly cools over time.

The study published Sunday in Nature Geoscience shows how the solar system's smallest planet is contracting, National Geographic reported.

"We see the landscape literally crumpling up," said William McKinnon, a professor in the Department of Earth and Planetary Sciences at Washington University in St. Louis, as quoted by National Geographic. "Massive slabs of rock are sliding over one another."

Paul Byrne, who headed the study, described the downsizing as "Mercury's version of a mountain belt."

According to a Space.com report, the survey using NASA photos from the Messenger spacecraft revealed that the planet has shrunk by around 4.4 miles in global radius. The contraction of around 8.6 miles in diameter is substantially higher than earlier estimates.

"These new results resolved a decades-long paradox between thermal history models and estimates of Mercury's contractions," study lead author Paul Byrne of the Carnegie Institution for Science said in a statement, as quoted by Space.com.

Made of one continental plate that holds the whole planet together, Mercury has a huge iron core that scientists estimate is 2,500 miles wide. The core leaves a mere 260 miles for the mantle and crust, which compares with the Earth's mantle width of 1,800 miles.

The Messenger spacecraft, which launched in 2004 and is currently orbiting the planet, was used to capture 5,934 ridges and scarps that occur when Mercury contracts due to cooling.

Scientists had earlier been puzzled by the contradiction between the planet's heat loss and shrinking diameter, which didn't seem to be losing as much as it should.

"The discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved," Messenger principle investigator Sean Solomon said in a statement.

Because Messenger was able to map the entire surface of Mercury, the spacecraft picked up evidence missed earlier by Mariner 10, which captured 45 percent of the planet's surface.

The images from Mariner 10 had led researchers to estimate that Mercury had lost just 1.25 miles or less. The recent estimation of 4.4 miles in global radius fits better with the present theories of how the planet is cooling.

"It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence," Solomon said.

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