Over the course of its existence, Mercury may have contracted more than astronomers previously thought. This is indicated by a new study that analyzed folds on its surface. The planet’s diameter may have decreased by 19 km.
Shrinking Mercury
A new study by scientists from the Institute of Space Research at the German Aerospace Center shows that over the lifetime of the Solar System, the diameter of Mercury decreased by 19 km. This is significantly more than scientists had previously believed based on earlier studies.
In general, there is nothing surprising about a planet decreasing in diameter. Like all worlds orbiting the Sun, Mercury formed about 4.5 billion years ago as a result of collisions between primordial bodies. Since then, its interior differentiated, it first heated up, and then began to cool slowly and lose volatile substances.
The same processes occurred on all planets. However, on relatively small Mercury, where there is no wind or water erosion, this is especially noticeable. Its entire surface is covered with folds, and by estimating their size, scientists were able to determine how much the planet has “wrinkled” over its lifetime.
A New Estimate
The previous estimate of how much Mercury’s diameter had decreased ranged from 4 to 16 km. In the new study, however, scientists used a more detailed map of the planet and examined it very carefully. They concluded that impact craters, which densely cover the planet’s surface, may conceal some of the folds.
When the researchers revised their estimate of the number of these structures, they concluded that Mercury may actually have contracted by 10–30% more than previously estimated — by an average of 19 km and, at the upper end of the estimate, by as much as 23 km. Scientists suggest that this may be because Mercury was hotter in the past, or because its core contained a greater amount of volatile substances.
And this may still not be the limit. Scientists suggest that the planet may actually have contracted even more because very small folds are simply invisible to us. No existing map of Mercury shows details smaller than 5 km. This may change as early as November 2026, when the BepiColombo probe arrives at the planet. It will be only the third mission to visit Mercury, and scientists hope that we will be able to see its surface in much greater detail than before.
