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Study Finds Mercury Shrank Up to 14.5 Miles More Than Previously Measured

A reanalysis shows impact debris has hidden many contraction features, implying a larger iron core and prompting model revisions.

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In a Mercury flyby in 2024, the European Space Agency's BepiColombo spacecraft captures the solar system's innermost planet.

Overview

  • The peer-reviewed study published Saturday in Geophysical Research Letters reanalyzed NASA MESSENGER data and estimates Mercury’s cumulative radial contraction is about 10–30% larger than earlier counts, equal to roughly 12 to 14.5 miles (19–23 km) of lost diameter.
  • Researchers compared global maps of shortening structures with surface-roughness maps and found rugged, impact-blanketed regions often lack visible shrinkage marks because ejecta and debris conceal older tectonic features.
  • If the larger contraction is confirmed, the measurement points to a proportionally bigger iron-rich core with fewer light elements, a result that strengthens formation scenarios such as crust-stripping collisions early in Mercury’s history.
  • The revision depends on MESSENGER’s kilometer-scale imaging and remains provisional until higher-resolution topography and laser altimetry from ESA/JAXA’s BepiColombo can confirm or refine the numbers.
  • Beyond Mercury, the method could help close a long-standing gap between observed surface shortening and physics-based cooling models and may reveal similar undercounts of contraction on other rocky bodies like the Moon or Mars.