Chuck Bednar for redOrbit.com – @BednarChuck
The smallest planet in the solar system is getting smaller, with the most recent estimates indicating that Mercury’s diameter has shrunk by an estimated 8.5 miles (14 kilometers) during its 4.5 billion years of existence, according to a recent Nature Geosciences study. Is there a giant shrink ray in space? Maybe.
It is just one of the quirks of an unusual planet which, according to National Geographic, could potentially harbor organic materials in the permanently shadowed craters at the its north pole and contains host a significant amount of water ice despite its proximity to the sun.
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Mercury’s shrinkage has long been a fascinating phenomenon. In March 2014, data obtained from NASA’s MESSENGER spacecraft revealed that the planet had contracted into itself and lost more than four miles (7 kilometers) of elevation in some parts, resolving a paradox between thermal history models and previous estimates of Mercury’s contractions.
Why is Mercury shrinking? The answer lies in the rigid, outermost shell of the planet known as the lithosphere. On Earth, the lithosphere is divided into a series of tectonic plates that can slip underneath one another to accommodate this contraction. On Mercury, however, it is one large entity, so when its surface crinkles, it forms long, steep cliffs with curved edges.
Raisin skin surface
These features are known as lobate scarps, and while some of them are small, others can grow to be up to 600 miles long and two miles high, Paul Byrne, author of the study and a planetary geologist at the Carnegie Institution for Science, told Nat Geo. These surface wrinkles are said to be similar to those appearing on a raisin skin as it begins to dry out and shrivel up.
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Water loss isn’t to blame for Mercury shrinking, however. The planet is growing smaller because of cooling and contracting taking place within its iron-rich metallic core, the website noted. As a general rule, Byrne said, more contraction produces scarps that are taller, longer and steeper, and by measuring their length and height, experts can calculate how much shrinking is taking place.
Scarps and ridges
In their new study, Byrne and his colleagues identified and measured nearly 6,000 scarps and ridges on the planet’s surface, then calculated that Mercury’s diameter might have shrunk by as little as 5.7 miles (9.2 kilometers) or as much as 8.8 miles (14.2 kilometers).
These figures are closer to predictions than previous observations, and “are helping resolve a discrepancy between theory and observation that has existed since Mariner 10 first swooped in and took some photos of the little planet in the mid-1970s,” National Geographic reported.
One unexpected discovery, though, is that Mercury is not contracting uniformly. Rather, the wrinkles and scarps are distributed unevenly throughout its surface, and some regions (including the Northern volcanic plains) are showing more evidence of shrinkage than others.
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“I think it’s probably the result of variations in the strength of rocks across and within the planet,” Byrne told the website. “Some parts of the planet’s lithosphere are likely stronger than others, and so the outcome of their response to stresses due to global contraction will probably look different – either in terms of shapes of structure, distribution of structures, or both.”
Scarps continue to form, and as Thomas Watters of the Smithsonian National Air and Space Museum told Nat Geo this week, those features “really are exciting because they’re showing us that new faults are forming on Mercury as a result of the most recent phase of interior cooling and global contraction. These faults are so young that they’re probably forming today.”
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