Mercury, which is about one-third the size of Earth, may have shrunk more in its evolution than scientists originally thought.
A new study suggests that the tiny planet may have contracted 10% to 30% more than previous models suggested — meaning the planet’s total diameter shrunk almost 12 miles (19 km) since Mercury was formed. (
Mercury and the rest of the solar system planets formed due to the gradual collisions and aggregations of rock and gas upon asteroids and comets. Some of those impacts that created the planets also released a lot of heat. Mercury has been slowly cooling from those ancient impacts, and its interior has been shrinking along the way.
While this kind of cooling affects the entire planet’s landscape, impact craters can mask the effects because each crash into the surface generates its own surface changes, including debris and new divots. The craters build up over time, making it difficult to spot more ancient geology underneath.
Nishiyama’s team compared two sets of maps from Mercury, using data from NASA’s MESSENGER mission, which orbited Mercury between 2011 and 2015. One set of maps charted geological features showing contraction, and the other set estimated the roughness of the planet’s surface. Their work covered the entire surface and demonstrated that the most rugged spots on the planet have fewer wrinkles from shrinking. In other words, according to Nishiyama, the debris from more recent impacts is covering up the evidence of shrinking.
Nishiyama said more information will help to confirm the estimate. Such data could come from the European Space Agency’s BepiColombo spacecraft, which began its arrival sequence at Mercury earlier this month after nearly eight years of space travel. Once the mission settles in, the spacecraft will take higher-resolution imagery of the surface than MESSENGER did. Nishiyama’s team expects BepiColombo to pick up smaller impact craters, as well as scarps and ridges, that could further refine the measurements.
“Stay tuned for future topography measurements by BepiColombo,” Nishiyama said. “Future data from laser altimetry on BepiColombo will collect more information on planetary contraction by measuring topography more precisely.”
