Mercury’s radius has shrunk by over 10 kilometers, Japanese team finds

Since its birth around 4.6 billion years ago, Mercury’s radius has shrunk by over 10 kilometers as the inside of the planet cooled down, a team from Hokkaido University has found.

The team’s findings were published in the U.S. science journal Geophysical Research Letters on Thursday.

Mercury’s radius currently stands at some 2,440 km. Observations by the U.S. space probe Messenger and other equipment have confirmed that there are many wrinkle ridges on the surface of the planet that formed as the ground contracted.

While attempts had been made to estimate the planet’s shrinkage based on the length and other characteristics of such wrinkles, there was a possibility that the shrinkage was underestimated because the geological features were unevenly distributed.

Using a new analysis method, Gaku Nishiyama, a postdoctoral researcher at the university, focused on the possibility that the wrinkles may have been obscured by craters formed by meteor impacts and lava eruptions.

The team measured the degree of surface roughness created through such events and compared this data to the distribution of the contractional features.

Through this method, the team discovered that contractional features were generally scarce in areas with high terrain roughness, such as near craters.

Taking into account such a trend, the team found that the planet’s radius had decreased by nearly 12 km.

A spacecraft from the Japan-Europe joint exploration mission BepiColombo is set to arrive on Mercury in November.

As the mission will include high-precision terrain observations of the planet, Nishiyama said, “We believe that the estimated shrinkage of the radius will increase as the mission provides more detailed observations of the planet’s topography.”