Mercury is shrinking faster than thought

Overview

Updated Sep 20

Mercury is cooling. Cooling makes it shrink, and new research says it has shrunk more than scientists calculated: up to 23 kilometers of diameter lost since the planet formed 4.5 billion years ago, 10 to 30 percent more than prior estimates.

Earlier measurements missed the contraction because Mercury's cratered surface is rough enough to hide the ridges and scarps that record it. A German-Japanese team corrected for that bias, and the new figure matches physics-based predictions for how much the planet's interior should have cooled.

Why it matters

If the estimate holds, Mercury's core has fewer light elements or started hotter—and BepiColombo will test it in November.

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Key Indicators

23 km

Estimated total diameter shrinkage

Up from earlier estimates of 4 to 16 kilometers across the whole planet.

10-30%

Correction range for hidden contraction

Rough terrain obscured up to 30% of the shrinkage evidence in previous maps.

5 km

MESSENGER resolution limit

Features smaller than this could not be reliably measured, so the new figure may still be low.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Gaku Nishiyama

Planetary scientist, German Aerospace Center (DLR) Institute of Space Research

Lead author of the Mercury contraction study; member of the BepiColombo science team

Organizations Involved

German Aerospace Center (DLR)

Research organization

Home institute of lead author Gaku Nishiyama

Germany's national center for aerospace research, hosting the Institute of Space Research where Nishiyama works.

Hokkaido University

National university

Led the international collaboration behind the study

Japanese national research university that coordinated the study with DLR and the University of Tokyo.

NASA MESSENGER mission

Planetary probe mission

Source of the surface data used in the study; ended in 2015

NASA probe that orbited Mercury from 2011 to 2015, mapping the entire surface with cameras and a laser altimeter.

BepiColombo (ESA/JAXA joint mission)

Planetary probe mission

Will test the revised contraction estimate with high-resolution data starting November 2026

Joint European-Japanese mission to Mercury, only the third spacecraft to visit the planet, set to begin high-resolution mapping in late 2026.

Timeline

March 2011 September 2026

4 events Latest: September 17th, 2026 · 2 months ago

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September 2026
Nature publishes research highlight
Latest

Nature's editors spotlight the study, noting the revised figure matches physics-based cooling predictions.
September 17th, 2026
Study published in Geophysical Research Letters

Nishiyama team reports Mercury's rough terrain hid up to 30% of contraction evidence, revising diameter shrinkage to 23 kilometers.
September 10th, 2026
April 2015
MESSENGER mission ends

Probe runs out of fuel and crashes into Mercury after completing 4,105 orbits.
April 30th, 2015
March 2011
MESSENGER enters Mercury orbit
Mission

First spacecraft to orbit Mercury begins mapping the planet's surface with cameras and a laser altimeter.
March 18th, 2011

Scenarios

BepiColombo confirms the corrected contraction estimate

Likely Resolves by End of 2027

Discussed by: Gaku Nishiyama and the BepiColombo science team

BepiColombo begins high-resolution mapping in November 2026. Its laser altimeter (BELA) will measure surface roughness at finer scales than MESSENGER. If the data confirm the roughness correction, the 10 to 30 percent upward revision stands as the best available figure for Mercury's contraction.

BepiColombo finds even more shrinkage

Possible Resolves by Q2 2028

Discussed by: Gaku Nishiyama, who notes current figures may still underestimate contraction

MESSENGER data cannot resolve features smaller than about 5 kilometers. If BepiColombo's higher-resolution scans reveal additional small scarps and ridges, the contraction estimate rises above 23 kilometers of diameter loss. This would push the shrink rate beyond the current 10 to 30 percent correction.

BepiColombo shows the correction overstates shrinkage

Unlikely Resolves by Q2 2028

Discussed by: Planetary geologists expecting direct BepiColombo imaging to test the roughness model

If BepiColombo's high-resolution data reveal that some features counted as contraction wrinkles are actually impact-related, the corrected figure could come down toward earlier estimates. This would challenge the roughness-bias model that underpins the new calculations.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

2010

The Moon's discovered contraction (2010)

NASA's Lunar Reconnaissance Orbiter found lobate scarps, small cliffs, scattered across the Moon, showing the Moon had contracted by about 100 meters in radius. Apollo-era photos had missed most of them, leaving earlier contraction estimates far too low.

Then

LRO images proved the Moon is tectonically active despite its small size.

Now

Established that small rocky bodies shrink measurably as they cool, giving scientists a template for reading contraction features on other worlds.

Why this matters now

The same phenomenon, a cooling rocky body shrinking and wrinkling, and the same lesson: higher-resolution imagery revises earlier contraction estimates.

1990-1994

Venus revealed by Magellan radar (1990-1994)

NASA's Magellan spacecraft radar-mapped 98% of Venus's surface, revealing thousands of tectonic features, ridges, faults, and coronae, that earlier probes and Earth-based radar had missed.

Then

Magellan's imagery transformed understanding of Venus geology, showing a surface reshaped by both volcanism and tectonic deformation.

Now

Demonstrated that a new instrument with better resolution can expose geological structures previous measurements missed entirely.

Why this matters now

BepiColombo brings the same capability to Mercury that Magellan brought to Venus: a new instrument revealing structures older data could not resolve.