Mercury is nobody’s idea of a big cosmic deal. The solar system’s smallest planet, it measures just 3,023 miles across—or barely 3.6% of the 88,846 mi. diameter of Jupiter, the solar system’s largest. Saturn’s moon Titan and Jupiter’s Ganymede both also have greater diameters than Mercury. And according to a new study in Geophysical Research Letters, Mercury has actually put in the work to get as small as it is, spending part of the past 4.5 billion years shrinking—significantly more than planetologists once thought.
That matters. Figuring out the history of Mercury provides clues to how all rocky worlds like the moon and Mars formed. And what happens in our solar system surely happens in ones much farther afield in space.
The findings come courtesy of data gathered by the Mercury MESSENGER spacecraft—a ship whose name was derived from the decidedly clumsy acronym “MErcury Surface, Space ENvironment, GEochemistry, and Ranging.” MESSENGER launched in August 2004; reached Mercury in March 2011; and completed 4,105 orbits of the planet before running out of fuel and crashing into the surface on April 30, 2015. In those four-plus years on the job, MESSENGER did a lot of work, studying Mercury with seven different scientific instruments—including its Mercury Laser Altimeter, a ranging tool that could measure surface features to within one meter; and a dual imaging system that used two cameras to map the surface in wide- and narrow-angle and black and white and color.
Scientists have known for a long time that Mercury has gotten steadily smaller since its birth. The planet was formed when dust, gas, rocks, and asteroids left over from the formation of the sun collided and collected, coalescing into a discrete body and generating enormous amounts of heat in the process. Over the eons much of that heat dissipated, causing Mercury to contract by what astronomers estimated to be 2.5 miles to 10 miles in diameter.
To confirm that conclusion, MESSENGER scientists used the spacecraft’s laser altimeter and dual-imager to scan the planet looking for so-called shortening structures, which are distortions in the surface made up of wrinkles, ridges, and what are known as lobate scarps—cliff-like structures that form when the crust pushes rock upward along a fault. Shortening structures form when a planet contracts; the greater the number of those surface features, the greater the planetary shrinkage has been.
MESSENGER did its best to map shortening structures across the entirety of the planet, but two things made that job a challenge. For starters, the more powerful of its two imaging systems, the laser altimeter, could not operate effectively if the spacecraft was more than 930 miles away from the planet—a proximity it reached only when it was over Mercury’s north polar region during its sweeping elliptical orbits; at other points it was as much as 9,500 miles distant. The dual-imaging system could survey the surface during those more remote approaches, but it did not produce the detailed three-dimensional pictures the laser altimeter did.
Then too, there is the condition of Mercury’s surface itself. The shortening structures formed early in the planet’s history, and in the eons since, Mercury has been steadily pounded by incoming asteroids, gouging out craters and scattering debris, obscuring the telltale scars of planetary contraction. It’s possible to read through that rubble—tracing shortening structures until they are covered up by craters or rocks and estimating what their course and reach was—but that takes planet-wide three-dimensional maps, something that MESSENGER did not provide. There was, however, a way to create them.
With the cooperation of NASA, the custodian of the MESSENGER images, a team led by Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research and the lead author of the new paper, harvested two-dimensional pictures taken by the spacecraft’s dual-imaging system and combined them in such a way that every patch of surveyed land was captured twice—from slightly different angles each time.
“We used a digital terrain model generated through so-called stereophotogrammetric technique,” says Nishiyama. “Combining two images taken at different viewing angles [is similar to the way] human beings recognize stuff with both eyes. So two-dimensional images are combined to derive three-dimensional topography.”
With the help of that detailed terrain map, Nishiyama and his colleagues were able to study the entirety of Mercury with a resolution never before achieved. That helped the team infer the location and reach of shortening structures that were otherwise covered up over time, and that, in turn, led them to conclude that previous calculations of Mercury’s historical shrinkage were underestimates.
“We find a lack of shortening structures in rough regions,” the researchers wrote, “suggesting that roughness-related processes obscure pre-existing structures. This biases previous contraction estimates downward… implying that Mercury’s contraction is up to 30% larger than previously thought.”
That 30% factors out to about 4.5 miles more contraction than earlier calculations—small even on the scale of a world as modest as Mercury, but significant all the same. Increased shrinkage means planetologists have to rethink what they previously believed about the chemical composition and temperature of Mercury’s core, with lower sulfur or silicon content than previously believed, leading to faster cooling and more contraction. It could also suggest that Mercury’s core started out hotter than suspected, which resulted in a more dramatic volume change when it finally did cool down.
“More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,” said Nishiyama in a statement that accompanied the release of the study.
What Nishiyama and his colleagues learned about Mercury’s cooling and shrinking could have implications for the study of other rocky worlds like the moon and Mars. The moon has more craters and other rough features than Mercury, making it more important to try to interpret the shortening structures that did survive in order to estimate potential lunar shrinkage—learning if what happened on Mercury indeed happened on the moon and perhaps Mars. If so, says Nishiyama, all three worlds are likely to be contracting still, as heat continues to escape from their interiors into space.
Meantime, Mercury will get a closer look soon. In 2018, the European and Japanese space agencies launched the BepiColombo spacecraft on an eight-year journey to Mercury. The ship will arrive in December and begin science operations in April 2027. BepiColombo has a significantly more powerful laser altimeter than MESSENGER’s, able to resolve surface changes as small as 20 centimeters (8 in.). The spacecraft will also be in a less eccentric orbit than MESSENGER, putting more of Mercury’s surface within its crosshairs.
“New data from BepiColombo will open a door for understanding how Mercury has been shaped up to now,” says Nishiyama.
