Mercury has Shrunk Significantly, Revelation Solves Old Enigma

Mercury is cooling and contracting at a rate much quicker than previously believed, according to planetary scientists, who report the discovery solves a long-standing conundrum for scientists studying our solar system's innermost planet.

According to data from NASA's Messenger spacecraft, which studies Mercury, the tiny world has shrunk up to 7 kilometers in radius in the past 4 billion years. The find runs contrary to previous observations, which used photographic evidence to suggest that the rocky planet had barely shrunk over its lifetime.

However, that photographic evidence presented its own conundrum: Although the planet seemed to be the same size, thermal data suggest it should have been smaller because the planet has cooled over its lifetime.

Now, writing in the journal Nature Geoscience, a team led by Paul K. Byrne and Christian Klimczak at the Carnegie Institution of Washington report they have an explanation for Mercury's cooling with evidence of planetary shrinkage. The evidence comes from a more complete picture of Mercury, as documented by the Messenger mission. Prior to the Messenger mission only about 45 percent of Mercury's surface had been mapped. This non-global data led scientists to predict that had contracted no more than 3 kilometers.

Using the more complete Messenger data, the scientists recalculated Mercurial shrinkage, finding that Mercury contracted radially by as much as 7 kilometers - far more than previous estimates. The findings, which are based on geographic data, are in agreement with thermal data used to calculate the planet's contraction.

To make their calculations, scientists studied nearly 6,000 geological landforms on Mercury's surface including wrinkled ridges and curved cliffs known as lobate scarps that are a result of the planet's contraction as it cooled.

Mercury's modern radius is 2,440 kilometers (1,516 miles), the scientists report.

"These new results resolved a decades-old paradox between thermal history models and estimates of Mercury's contraction," said Byrne, a planetary geologist and Messenger investigator. "Now the history of heat production and loss and global contraction are consistent."

"Interestingly, our findings are also reminiscent of now-obsolete models for how large-scale geological deformation occurred on Earth when the scientific community thought that Earth only had one tectonic plate," Byme said in a statement. "Those models were developed to explain mountain building and tectonic activity in the nineteenth century, before plate tectonics theory."

Sean Solomon, principal investigator of the Messenger mission, commended the discovery. "This discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved. It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence."