Is Mercury Still Shrinking? Astronomers Confirm The Littlest Planet Is Getting Littler
Mercury isn't what it used to be. That is, it used to be about 8.6 miles wider.
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A team of scientists says it's for sure: Mercury is shriveling like a grape in the sun. By analyzing images of the solar system's tiniest planet, the researchers could see characteristic ridges along the surface, evidence that the entire planet is contracting. They published their findings this week in the journal Nature Geoscience.
"We show that Mercury's global contraction has been accommodated by a substantially greater number and variety of structures than previously recognized, including long belts of ridges and scarps where the crust has been folded and faulted," the authors wrote in the paper. "We find that Mercury has contracted radially by as much as 7 kilometers, well in excess of the 0.8-3 kilometers previously reported from photogeology and resolving the discrepancy with thermal models."
According to the Los Angeles Times, the previous reports were off because the photos scientists worked with didn't cover the entire planet. The images came from NASA's Mariner 10 spacecraft, which flew by Mercury in 1974 and 1975. More recently, NASA launched MESSENGER, a spacecraft designed to photograph Mercury and enter into its orbit, which it did in 2011. MESSENGER beamed back images of the uncharted half of the planet, giving Paul K. Byrne, of the Carnegie Institution of Washington, D.C., and his colleagues a comprehensive view of the planet's wrinkles.
To understand how the shrinkage works, you have to understand that Mercury isn't like Earth. Aside from the fact that it's much closer to the sun — only about 36 million miles — and is less than half Earth's diameter, Mercury lacks the shifting tectonic plates that float around the Earth's molten innards. Tectonic theory emerged relatively recently, in the middle of the 20th century. It states that the surface of the Earth is actually a handful of massive rocky plates. Where the plates meet, volcanoes, mountains, and earthquakes form.
But geologists didn't always see it that way. Before tectonic theory, scientists believed the Earth's mountainous topography and deep oceans were the result of the molten hot core cooling — and shrinking as it cooled. The crust, then, was shriveling around the shrinking core. They were as wrong as the geocentrists. As a result of tectonic plates melting into the magma below and the formation of new crust, the size of the Earth stays relatively constant.
Not so for Mercury, a planet encased in a single tectonic plate. And in fact, the study authors write, "The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth — now obsolete — that pre-date plate tectonics theory." In other words, the mistaken early 20th century geologists at least predicted the "shrivel rate" accurately, and Mercury is right on schedule.
The moon also has the telltale scarps, and NASA estimated in a 2010 study that the moon had shrunk by about 300 feet for the same reason: a cooling core. The planets and moons used to be hot to the touch as a result of asteroid bombardment during the formation of the solar system. They've cooled over the last 4.5 billion years, and eventually they'll cool completely — and stop shrinking.
Above photo courtesy of Shutterstock
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