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.
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.
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