Study Reveals That Mercury Is the Not-So-Shrunken Planet

Findings from a recently published paper by Smithsonian senior scientist Thomas R. Watters reveal that Mercury has managed to retain much more of its interior heat than previously thought.
Images and data obtained by NASA’s Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft show that Mercury has experienced only a modest amount of global contraction. This is in contrast to other studies that cite large amounts of contraction, suggesting Mercury cooled more quickly. Mercury’s modest amount of global contraction suggests an evolutionary path for small rocky planets where interior heat is retained. “A case for limited global contraction of Mercury” was published in the journal Communications Earth & Environment–Nature.

“It flies in the face of conventional wisdom that a body the size of Mercury, the smallest rocky planet in our solar system, has not cooled and become geologically inactive long ago,” Watters said. “Somehow, Mercury has managed to insulate its interior and slowed the loss of heat that provides the driving force for geologic activity.”

Images and data obtained by MESSENGER have revolutionized the view of the innermost rocky planet in the solar system. Among the many discoveries was the detection of a global array of large, cliff-like fault scarps (steep slopes), indicating that Mercury has contracted and its crust shriveled, mostly from the loss of interior heat. The amount Mercury has shrunken, however, is debated with some estimates of the decrease in the planet’s diameter as much as 14 kilometers (8.7 miles) or more–a “super-contracted” Mercury. In a new study, the amount of contraction expressed by Mercury’s fault scarps has been reevaluated. The decrease in diameter is found to be no more than 2 to 4 kilometers (1.2 to 2.5 miles) at most, indicating that Mercury has managed to retain much more of its interior heat than previously thought.

The paper explains that these findings are based on best images and topographic data returned by the MESSENGER mission that ended April 30, 2015, when the spacecraft crashed on the planet’s surface. Large fault scarps, cliff-like landforms that look like a giant stair-step in the landscape that were first found in the flybys of Mariner 10 in the mid-1970s, were confirmed by MESSENGER to indicate the global contraction of Mercury. The large fault scarps were formed as Mercury’s interior cooled causing the planet to shrink and crustal materials to be pushed together, break and thrust upward along faults making cliff-like mountains, some more than a kilometer high and hundreds of kilometers long.

“Early in the orbital phase of the mission we discovered that images that had been acquired were not optimum for detecting the fault scarps on Mercury,” Watters said. “A campaign to obtain optimized images resulted in maps, that along with topographic data, greatly facilitated the identification of the fault scarps.”

A new global map of the population of fault scarps that was made using the best available images before the European Space Agency’s BepiColombo spacecraft returns to Mercury in 2025, shows that the amount the diameter of the planet has decreased is estimated to be at most 4 kilometers (2.5 miles). This is in stark contrast to the concept of a super-contracted Mercury. A relatively small change in diameter during the past 4 billion years indicates that Mercury has resisted shrinking by retaining its interior heat. This fits an emerging picture of Mercury as a planet that has maintained a hot interior, hot enough to preserve a liquid core, a long-lived magnetic field and very recent or current tectonic activity.

Watters is a senior scientist in the Center for Earth and Planetary Studies at the National Air and Space Museum and a member of the MESSENGER science team. MESSENGER was a NASA spacecraft launched Aug. 3, 2004, and it began orbiting Mercury March 18, 2011. The mission was managed by the Johns Hopkins University Applied Physics Laboratory. Although MESSENGER completed its primary science objectives by March 2012, the spacecraft’s mission was extended two times. The mission ended with a planned impact on the surface of Mercury April 30, 2015.

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