The smallest planet in our Solar System is getting even smaller

Reported by Giles Sparrow

Mercury can be easy to overlook. As the closest planet to the Sun, it’s hard to spot on its fleeting visits to our skies. Over the past few decades, M however, our view of the innermost planet has been transformed by NASA’s MESSENGER space probe. The first spacecraft to orbit Mercury revealed that this mysterious world has a complex history of its own, with a distant volcanic past, a core much larger than that of any other planet relative to its size and an active magnetic field. But perhaps most intriguing of all is the evidence that this tiny planet has shrunk considerably since it formed.

“Mercury is the least understood of our Solar System’s four terrestrial worlds because until relatively recently it was extremely hard to image or visit,” says Dr Paul Byrne of Washington University in St Louis. “It’s deep in the Sun’s gravity well, and we simply didn’t know how to get a spacecraft into orbit around it until 1985.” As a result of these challenges, most of our information about Mercury came from a single NASA probe that made three flybys of the planet in 1974 and 1975. Mariner 10 flew in a solar orbit that intercepted Mercury’s, but the geometry of the two orbits meant that its encounters only revealed a little less than half of the planet’s surface.

Despite the secrets of reaching Mercury orbit being known from 1985, actually sending a spacecraft on the complex trajectory still presented considerable challenges, and it wasn’t until 1998 that NASA seriously began to consider launching such a mission. Early proposals evolved into the ambitious MESSENGER mission, which launched from Cape Canaveral in 2004 and entered orbit around the scorching planet in March 2011 after a tortuous flight involving one flyby of Earth, two of Venus and three of Mercury itself. “I had the good fortune to be a member of the MESSENGER science team, so I’ve been with the mission pretty much from its early days,” recalls Dr Tom Watters of the Smithsonian’s Center for Earth and Planetary Studies. “We were in a happy position as the second spacecraft to visit Mercury and the first to actually orbit the planet, so in terms of pure discovery we were seeing parts of the planet that had never been seen before by a spacecraft.”

Some of the most distinctive features on Mercury, known from the initial Mariner 10 flybys, were the elongated cliffs that weave their way across the cratered landscape. In places they cut craters in two, creating either a sharp height difference between one side of the crater and the other, or in some cases riding up across and completely burying part of the crater. “It was pretty obvious from Mariner 10 images that these fault scarps were widespread, and that suggested at least the parts of the planet that we could see had contracted,” continues Watters. “It’s like what happens to an apple when its core starts to dry out and shrink, and the skin starts to wrinkle and adjust to it. But we couldn’t be sure the effect was global. When MESSENGER made its first flyby of the unimaged hemisphere in 2008, one of the first things that popped out was another of these very large scarps, which we now call Beagle Rupes. It was really after that we could confirm that we were looking at a global contraction of the planet.”