Mercury May Have Shrunk by Seven Miles Since It Formed
Scientists Found Evidence of Ancient Cliff Formation Hidden Beneath Rough Terrain
WASHINGTON, D.C. — September 12, 2026
A team of planetary scientists has uncovered new evidence suggesting that Mercury, the smallest planet in our solar system, may have contracted by up to seven miles since its formation 4.5 billion years ago. This finding significantly revises previous estimates and opens new questions about how terrestrial planets cool and evolve.
Researchers published their findings this week in the Journal of Geophysical Research: Planets. The study combines data from NASA's MESSENGER mission with new analysis techniques that reveal previously hidden geological features on Mercury's surface.
Previous measurements indicated Mercury had shrunk by approximately two to three miles over its history. The new analysis doubles those estimates and suggests planetary contraction occurs more dramatically than previously understood.
Scientists examined data from NASA's MESSENGER mission which orbited Mercury from 2011 to 2015. The spacecraft returned over 280,000 images mapping more than 90 percent of the planet's surface. Researchers analyzed thousands of these images to identify geological features called lobate scarps — steep cliffs formed when a planet's crust contracts and breaks.
The study focused on Mercury's roughest terrain where researchers expected to find the most complete record of planetary contraction. Using advanced image processing, they identified approximately 3,000 previously overlooked fault features. Many were hidden beneath impact crater debris, leading to decades of undercounting.
"These features are scattered across the entire planet," said Dr. Sarah Martinez, planetary geologist at Johns Hopkins University Applied Physics Laboratory and lead author. "We found them in the northern hemisphere, in the southern hemisphere, even near the poles. They're everywhere, but they were hard to see because of the rough terrain."
The most compelling evidence comes from Mercury's south polar region where researchers identified a complete ring of thrust faults surrounding the Caloris Basin. This ring measures approximately 930 miles in diameter and indicates significant crustal shortening in the planet's southern hemisphere.
Mercury's unusually large iron core makes it particularly sensitive to thermal contraction. While Mercury's core accounts for approximately 85 percent of the planet's radius, Earth's core represents only about 50 percent. This composition means Mercury would contract more than planets with smaller cores as it cooled over billions of years.
The new findings have important implications for understanding other terrestrial planets. Scientists say similar processes likely occurred on Mars, Venus, and possibly Earth during their early histories. The research team is now planning follow-up studies on these worlds to compare contraction patterns across the inner solar system.
Researchers note their analysis also helps explain puzzling seismic data from Mercury's interior. NASA's InSight mission detected marsquakes that suggested the planet's interior remained warmer than expected. The new fault measurements align with thermal models predicting gradual cooling rather than rapid freezing.
The study team plans additional work to map fault orientations globally. Understanding how these features relate to each other across the entire planet will help refine models of Mercury's thermal history and internal structure.
Sources
- Journal of Geophysical Research: Planets, September 2026
- NASA MESSENGER mission data archive
- Johns Hopkins University Applied Physics Laboratory press release, September 12, 2026
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