Mercury’s Ongoing Mystery: Is the Smallest Planet Still Shrinking?

Ever since the 1970s, when NASA’s Mariner 10 reached Mercury, scientists have been intrigued by the phenomenon of the planet’s shrinkage due to its core cooling. Although the thermal contraction began roughly 3 billion years ago, the question that looms large today is whether this contraction is still ongoing.

Telescopic Evidence from the Past: As early as the 1970s, observations revealed that Mercury’s crust had shrunk, forming wrinkly ridges, or “scarps” due to its internal cooling. This led to the planet compressing an estimated total of about 8.8 miles in diameter over the last 3.8 billion years. This phenomenon can be visually compared to the wrinkles that form on an aging apple.

The New Findings: Grabens

A team from The Open University in the United Kingdom recently provided further insights into Mercury’s ongoing transformation.

  • Focus on Grabens: Unlike previous research that concentrated on scarps, the new research focused on “grabens.” Grabens, derived from the German word for “ditch,” appear as small depressions on the planet’s surface. They are often found on the backs of ridge-like scarps and are seen as evidence of the planet’s continued cooling and contracting. These structures are typically just over half a mile in length and less than 300 feet deep. Their formation, according to planetary scientist David Rothery, is somewhat akin to cracks that appear when bending a piece of toast.
  • Age Estimation: The majority of these grabens are believed to be less than roughly 300 million years old, a mere fraction when compared to the age of Mercury’s wrinkles. This discovery has led to the hypothesis that the planet might still be shrinking.

NASA’s MESSENGER Probe Insights

Between 2011 and 2015, NASA’s MESSENGER probe orbited Mercury and sent back intriguing images of the planet’s surface.

Images Reveal More: The probe captured photos of hundreds of possible grabens and 48 definite scarps that had grabens. Notably, some of these grabens might be as “young” as 50 million years old, indicating recent tectonic activity. The presence of these younger grabens amidst the much older scarps signals a potential continuity in Mercury’s contraction.

Upcoming Missions and Expectations

With mysteries still surrounding the smallest planet in the solar system, upcoming space missions may shed more light.

BepiColombo Mission: Launched in 2018 by the European and Japanese space agencies, the BepiColombo probe has already flown by Mercury twice. It’s expected to arrive back in 2026. Scientists anticipate that the spacecraft will provide higher-resolution photos than its predecessor, MESSENGER, and validate the observations made about grabens. The mission could ultimately reveal if Mercury remains seismically active and if its shrinkage is an ongoing process. More about BepiColombo Mission.

Future Implications

Understanding Mercury’s evolution is not merely a matter of academic interest; it has broader implications for our knowledge of the solar system and planetary science as a whole.

  • Blueprint for Understanding Planetary Evolution: Mercury’s unique position as the closest planet to the Sun provides a model for studying the extreme conditions and effects of solar proximity. As we gain more insights into Mercury’s shrinking, we might also be unveiling the mysteries of planetary formation, contraction, and tectonic activities, which could be applied to understand other celestial bodies both within and beyond our solar system.
  • Potential for Earth Analogy: Earth, too, is a tectonically active planet. Observing and understanding the tectonic activities of another planet might provide a different perspective on the mechanisms driving our own planet’s geological movements. While the two planets are vastly different, fundamental geological processes might hold commonalities worth exploring.


As scientists delve deeper into the mysteries of Mercury, the evidence seems to tilt towards the planet still being in a state of contraction. The recent findings surrounding the grabens have reignited the debate and increased anticipation for the results from the forthcoming BepiColombo mission. The question remains: is Mercury, against the vast backdrop of cosmic time, still shrinking? Only time and more research will tell.