Humanity got its first good look at Mercury in the 1970s, when Mariner 10, a NASA spacecraft, embarked on a tour of the solar system’s innermost planets. The mission revealed that Mercury produced a magnetic field, an invisible bubble that protects the surface from the worst of the sun’s radiation. Magnetic fields are generated from within, by a planet’s liquid-metal core, still hot from its creation several billion years ago.
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Mariner also photographed chains of cliffs and ridges, some as tall as Washington State’s Mount St. Helens, stretching for hundreds of miles. The landforms, known as scarps, fascinated scientists. They set about explaining the scarps’ origins, and quickly ruled out several explanations. Mercury bears the marks of volcanism—long channels carved into the rock by flowing lava—but activity ceased a few billion years ago. It also exhibits signs of an ancient bombardment by objects of various sizes, but such onslaughts produce craters, not mountains.
Scientists examined what they knew about planet formation. The iron cores of all the rocky planets in our solar system, including Earth, are still cooling down from their fiery births 4.5 billion years ago. As liquid iron turns solid, it contracts, creating space for the crust above to contract, too. The ground cracks and thrusts upward, redrawing the landscape. If the planet was covered in a uniform crust, like a lid, the effects of this contraction should remain visible. The scarps, scientists predicted, must be a result of this phenomenon.
With this hypothesis in hand, scientists used Mariner images to calculate the length and height of the scarps. They used the measurements to estimate how much Mercury had shrunk in its lifetime.
But they couldn’t be entirely sure of even their best estimates. Mariner had approached Mercury only three times before zooming back out into space, and the encounters had allowed the spacecraft to photograph just 45 percent of the planet’s surface. To test their predictions, scientists needed a spacecraft to visit the planet and stay for a while.
That spacecraft, named MESSENGER, arrived in 2011. The mission provided global views of Mercury for the first time. “I was like a kid on Christmas morning every single day for three years because there’s always something new to see and always something surprising,” recalls Chabot, who worked on the mission’s imaging system.
The photographs showed that Mercury had undoubtedly shrunk. Thousands of scarps snaked along its surface. Scientists studied about 6,000 of the structures and calculated that the planet’s radius of 2,440 kilometers (1,516 miles) has shrunk between 14 kilometers and 20 kilometers (8.6 miles and 12.4 miles) since its creation.
Scientists have detected such scarps on other worlds in the solar system, like the moon and Mars. But the signs of shrinking are more prominent on Mercury, says Paul Byrne, a planetary geologist at North Carolina State University who led the MESSENGER study.