Over the past four billion years Mercury has shrunk by as much as 14 kilometers. The fact that this planet closet to the sun is shrinking isn’t the surprise; scientists have known about it since the 1970s. But new images show that the shrinkage is several kilometers more than they previously observed.
Mercury is a small, iron planet -- about 5,000 kilometers across -- with a thin rock crust and a large iron core that’s been cooling over billions of years. When an initially hot planet cools and contracts, geologic structures begin to form through the buckling and fracturing of the planet’s crust. The cliff-like faults and ridges that result from interior cooling and surface compression look like long ribbons from above. They range from 8 to 800 kilometers long and can be up to 2 kilometers high.
During three flybys in 1974 and 1975, Mariner 10, the first spacecraft sent to explore Mercury, gathered images of less than half the planet’s surface. These older images indicated that, despite cooling over its lifetime, the rocky planet had barely shrunk at all -- maybe 1 or 2 kilometers in radius. But modeling of Mercury’s formation and aging couldn’t explain that finding: after all, as a liquid iron core turns solid over time, it contracts, and the planet should shrink, a lot.
In 2011, NASA’s MESSENGER spacecraft entered Mercury’s orbit, and it’s been collecting data on the entire planet ever since. (It’ll be completing its 2,900th orbit of Mercury later this month.) Using MESSENGER’s high-resolution, low-angle images and topographic data, a team led by Paul Byrne from the Carnegie Institution of Washington created a comprehensive map of the ridges and faults across the entire surface of the planet.
Mercury is basically a raisin. Lobate scarps are cliffs caused by thrust faults that have broken the surface. Wrinkle ridges are caused by faults that don't extend as deep; surface materials from one side of the fault ramp up and fold over, forming a ridge. (Switching metaphors here, imagine a tailor making a series of tucks to take in dress or a pair of pants.) The team mapped a total of 5,934 of these tectonic features.
By adding up the total amount of crust displaced by the ridges and scarps, they found that the total contraction of Mercury’s surface is equivalent to a loss of 4.6 to 7 kilometers of the planet’s radius.
Finally, this greater estimate of shrinkage matches up with models that predict how much a rocky planet contracts as its metal interior cools. “This discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved,” study coauthor Sean Solomon of Columbia University tells Forbes. “It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence.”
In fact, the observed deformation of Mercury’s surface is consistent with a 19th-century theory for a shrinking Earth (now obsolete). We know that’s not true for our planet, since Earth’s outer shell is broken into multiple plates. But Mercury is enclosed by a single plate, so it doesn’t lose its heat through plate tectonics. All planets are cooling over time, but as far as we know, Earth is the only one that has tectonic plates instead of a single, outer shell. The findings provide a framework for understanding how one-plate planets cool -- their thermal, tectonic and volcanic history -- which is especially useful given the increasing number of rocky planets being found around other stars.
The findings were published in Nature Geoscience this week.
Images: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington