March 18, 2014
By looking for wrinkles at Mercury's surface, scientists now estimate the planet's radius shrunk 7 km since it formed. Contrary to previous estimates that put this number at 1 to 2 km, this fits the theoretical models.
The smallest planet of the solar system is getting smaller faster than we previously thought. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Ever since its formation 4.6 billion years ago, Mercury has been cooling down and thus shrinking. The cooling process is normal for planets that don't have a way to renew the heat internally. Mercury's iron core, which is unusually large for the planet's size – radius of the core is 2000 kilometers for the planet with the radius of 2400 kilometers – is cooling down slowly and the whole planet is losing heat. This leaves only about 420 kilometers for the mantle and crust, which only makes the cooling process faster.
"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," said planetary scientist Paul Byrne of the Carnegie Institution's Department of Terrestrial Magnetism, Washington, USA. He's the lead author of the paper published in the journal Nature Geoscience.
Until now, observations from Mariner 10 mission and theoretical models showed discrepancies in the extent to which the planet was shrinking. In 1970s, Mariner 10 took images of only 45% of Mercury's surface and based on those observations, scientists estimated that since its formation, planet lost 1 to 2 kilometer of its radius. For four decades, this was contrary to what models predicted, based on heat loss the planets must have suffered over time.
"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 added.
Now, Byrne and his team used data from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe and managed to come up with a better estimate of Mercury's shrinkage. MESSENGER mapped planet's entire surface which enabled them to look for ridges and scraps that form as the planet contracts. Since Mercury only has one tectonic plate, as the planet shrinks, the surface wrinkles up. MESSENGER identified over 5,900 such ridges and scraps and based on those observations, the new estimate of contraction of the radius is 7 kilometers.
"The discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved," says Sean Solomon, principle investigator for MESSENGER mission. "It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence."