Mercury, the solar system’s smallest planet, has contracted considerably over the past 4.5 billion years—potentially 10% to 30% more than prior estimates, amounting to almost 12 miles of diameter loss since its formation. This finding, based on data from NASA’s MESSENGER spacecraft, offers insights into the planet’s origins and the evolution of rocky worlds like Earth, Mars, and the Moon. Scientists used the spacecraft’s laser altimeter and imaging systems to map “shortening structures”—wrinkles, ridges, and cliff-like scarps—on Mercury’s surface, indicating planetary contraction. The MESSENGER mission, which orbited Mercury from 2011 to 2015, gathered data using seven scientific instruments, including a Mercury Laser Altimeter capable of measuring surface features to within one meter.
Previous estimates of Mercury’s shrinkage ranged from 2.5 to 10 miles in diameter. The new study suggests that impact crater debris obscured evidence of greater contraction. Planetary scientist Gaku Nishiyama of the German Aerospace Center Institute of Space Research explained that the amount of contraction is a key indicator of Mercury’s cooling and evolution, providing clues to its internal composition—potentially a larger metal core with fewer light elements. The planet’s formation involved collisions of rock, gas, and asteroids, generating substantial heat that has dissipated over time, causing it to shrink.
While the extent of Mercury’s shrinkage is now becoming clearer, challenges remain in mapping these features due to the spacecraft’s varying distance from the planet during its elliptical orbits. The most detailed mapping was possible only when the spacecraft was over Mercury’s north polar region. The research highlights the importance of understanding Mercury’s history to better understand the formation and evolution of the solar system as a whole, and how rocky planets form in general. The question remains as to how Mercury’s unique composition and early conditions contributed to its greater-than-expected shrinkage.
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