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.
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.
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.
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.
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.
What can Mercury’s shrinking teach us about the planet and our solar system?
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.
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.
“New data from BepiColombo will open a door for understanding how Mercury has been shaped up to now,” says Nishiyama.
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