The closest planet to our sun is shrinking. Here’s what that means

Mercury’s Surface May Hide Evidence of a Greater Planetary Contraction

Earthguardiansonline.com – Mercury’s battered, fractured landscape may be concealing a larger story about the planet’s long-term cooling. New research suggests the solar system’s smallest planet could have contracted more than earlier estimates indicated, with impact debris masking some of the scars left behind as its interior lost heat.

The planet closest to the sun travels around its star at an average distance of roughly 36 million miles, or 58 million kilometers, and finishes a trip around the sun every 88 days. It formed about 4.5 billion years ago from gas and dust in the young solar system, a period when collisions between orbiting objects were common and immensely energetic.

Like other newly formed rocky worlds, Mercury began hot. Planet-building impacts added still more heat to its interior. Over billions of years, that heat gradually escaped into space. As the inside cooled, the planet contracted, causing its outer rocky shell to buckle and break.

Cliffs and ridges record a cooling world

Mercury’s crust is marked by ridges, scarps and cliffs that planetary scientists describe as shortening structures. Some rise as high as 1 mile, or 1.6 kilometers, while stretching hundreds of miles across the surface. They are among the clearest visible signs that the planet’s volume changed after it formed.

But Mercury has also endured repeated strikes from asteroids and comets. Those impacts left countless craters and scattered debris across terrain that already had a rugged, folded appearance. The material ejected during collisions may have buried or softened the appearance of many contraction-related features.

A study published Thursday in Geophysical Research Letters examined whether this debris has led scientists to underestimate Mercury’s total shrinkage. Understanding the planet’s contraction is important because it can offer clues to its internal structure, tectonic development, volcanic past and magnetic field.

“There are lessons about the formation and evolution of large, rocky bodies like our Earth, that can be learned on Mercury better than anywhere else in our Solar System.”

Dr. Hannes Bernhardt, an assistant research scientist in the department of geological, environmental and planetary sciences at the University of Maryland, College Park, made that observation in an email. Bernhardt did not participate in the new research but has studied Mercury’s contraction.

A global look at Mercury’s roughness

Lead author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research in Berlin, and colleagues used observations from NASA’s MESSENGER spacecraft to create a worldwide map of surface roughness.

Their analysis found that the roughest regions tended to show fewer visible wrinkles than smoother areas. That pattern raised the possibility that impact debris and heavily disturbed terrain were obscuring structures created as Mercury compressed.

“It made us think that there’s a process obscuring shortening structures.”

Nishiyama said debris may be covering evidence of contraction that would otherwise be easier to identify. The team calculated how many such features could exist if they were not hidden. Its results suggest Mercury may have shrunk 10% to 30% more than past work proposed.

That difference corresponds to a possible reduction in the planet’s radius of about 7.2 miles, or 11.6 kilometers. Earlier estimates have ranged from 0.6 to 1.2 miles, or 1 to 2 kilometers, with some studies placing the change as high as 4.3 miles, or 7 kilometers.

Dr. Paul Byrne, an associate professor of Earth, environmental and planetary sciences at Washington University in St. Louis, said it is plausible that prior examinations, including one of his own, did not identify every geological fracture. Some features may be difficult to detect in rough terrain, while others may not have developed where Mercury’s unusually rugged crust resisted deformation. Byrne was not involved in the latest study.

“Yet if we can accurately measure how much Mercury has contracted, we can make better estimates of its interior layering, the size and make-up of its core, its tectonic and volcanic histories, how its magnetic field is generated, and a whole lot more.”

Why Mercury remains difficult to understand

Despite its proximity to the sun, Mercury has remained one of the least explored rocky planets. Extreme solar heat and the challenging conditions near the inner solar system have made missions there difficult. Only two spacecraft missions had previously entered this environment: NASA’s Mariner 10, which made flybys in 1974, and MESSENGER, which began orbiting Mercury in 2011.

Mariner 10 first revealed many of the planet’s remarkable folds and cliffs. MESSENGER later supplied a far more complete view of Mercury’s global geology, helping researchers map its varied terrain and examine the consequences of its ancient cooling.

The next opportunity for a deeper look is close. In November, an unprecedented mission is expected to place two orbiters around Mercury. Their observations could help resolve longstanding questions about the planet’s surface, interior and evolution.

For scientists, Mercury is more than a small, scorched world at the edge of the sun’s domain. Its preserved faults, cliffs and cratered plains provide a natural record of how rocky planets respond as their interiors cool. If a significant share of that record lies under impact debris, the planet may still have much more to reveal about its own past — and about the forces that shaped Earth and other rocky worlds.