Mercury is shrinking more than we thought

New research indicates that Mercury is shrinking at a rate significantly higher than previously believed. Scientists now estimate that the planet has lost around 12 miles (19 kilometers) from its total diameter since its formation. This rapid shrinking has likely gone unnoticed in the past due to space debris creating craters that conceal the evidence.

The study's lead author, Gaku Nishiyama from the German Aerospace Center, explains that the greater the shrinkage, the more likely Mercury possesses a larger metal core, less silicon mixed into the core, or a hotter initial temperature.

Rocky planets like Mercury were formed 4.5 billion years ago during a chaotic period in the solar system's history, with rocks and asteroids colliding and merging together. As the planet's interior cooled and contracted, it resulted in geological features such as scarps and ridges across its rocky layers. However, asteroid impacts over time have obscured these signs of cooling by dispersing debris across Mercury's surface, making the wrinkles difficult to detect.

The research team used data from previous maps of Mercury's geology combined with new observations of the planet's surface roughness to estimate the extent of its shrinking. They found that areas with the roughest terrain generally had fewer wrinkles, suggesting that debris from asteroid impacts may be hiding the signs of shrinking.

By analyzing the ridges and scarps in regions less affected by debris, the scientists were able to determine an additional contraction of 10% to 30% over Mercury's 4.5 billion-year existence. This discovery could be an underestimate, as the research is based on data collected by NASA's MESSENGER probe, which ended in 2015. The upcoming BepiColombo mission in 2026 will provide higher-resolution scans of Mercury's surface, which may reveal even more evidence of the planet's shrinking.