MESSENGER reveals a more dynamic Mercury surface

This before-and-after animation of two images taken by the MESSENGER spacecraft shows the result (inside the red circle) of an impact event on Mercury that occurred sometime between 25 June 2012 and 11 June 2013. Credit: NASA

The surfaces of most other objects in the solar system seem mostly static compared to Earth. For a long time, planetary scientists have held that collisions from space debris are the main cause of change on these surfaces and that the rate of these impacts has been slowing down over time. This "cratering rate" determines the age of almost every surface outside Earth and the moon.

The rate of cratering varies with the solar distance. It is most loosely limited to Mercury, whose proximity to the sun has made it challenging to get high-resolution pictures. The latest photographs, with spatial resolutions as low as 5 meters per pixel in certain areas, were from NASA's MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) probe, which orbited Mercury from 2011 to 2015.

To increase the accuracy of the Mercury cratering rate estimate, Speyerer et al. searched 58,552 pairs of MESSENGER photos with overlapping surface coverage for freshly deposited surface features. They calculated the expected annual rate of change per square kilometer by comparing "before" and "after" photos to identify surface changes. Geophysical Research Letters is where the study was published.

In their data collection, the authors found 20 novel traits. 19 of them are quasi-circular formations, with sizes ranging from 400 meters to 1.9 kilometers, and one of them is encircled by the radial rays characteristic of Mercury impact craters.

Inferring a cratering rate for tiny impactors that is 1,000 times higher than the currently accepted figure, 19 new impact craters were discovered over the four years of the MESSENGER mission. Instead of accepting such a radical modification to the cratering rate, the researchers proposed the alternative theory that many of these characteristics are endogenic geologic processes.

The hollow, a circular depression without a strong, crater-like rim, is a symbolic small-scale Mercury surface structure. Prior research has shown that hollows mostly appear in areas of the planet's surface that reflect light the least, as well as those that massive impact craters have significantly altered. Twelve authors' 19 features are close to low-reflectance surface areas, according to comparisons with previously mapped geologic units. Six are close to craters where there are other recognized hollows.

These findings demonstrate that Mercury's surface is substantially evolving, regardless of the origin of the characteristics. 99% of the planet's surface might change in the next 25 million years if the modification rate suggested by these 20 characteristics is consistent with the long-term norm. That rate of change greatly outpaces what was previously thought possible for Mercury, indicating that the newly discovered characteristics will probably be the focus of the BepiColombo mission, which is presently travelling to the planet from the European Space Agency.

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