Return to the Iron Planet
IT’S SMALL AND GREY and covered with craters — and, no, it’s not the Moon. At first glance, Mercury looks a lot like Earth’s natural satellite. Both have lava plains and rugged cratered terrain, with bright streaks radiating out from powerful impact scars on their drab, airless surfaces. But the Sun-scorched innermost planet is a very different place, from its inside out.
In contrast to the Moon’s puny core, Mercury’s iron heart takes up some 80% of the planet’s radius. It’s still at least partially molten and generates a weak, global magnetic field. Moreover, the planet’s ongoing, slow solidification affects its surface: As the core freezes, it shrinks, and the crust wrinkles like a drying plum, creating ridges that cut right across big craters.
Look closely at those craters, and you’ll find some have inexplicably dark rays, rather than bright ones. The floors and peaks of others appear to be moth-eaten, with ‘hollows’ of missing rock. Mercury’s dayside might be baked by the Sun to temperatures reaching 427°C, but even that isn’t hot enough to evaporate its surface rock.
These hollows, like so many other features of Mercury, remain an enigma. All of what we know about this world comes from Earth-based observations and a couple of spacecraft. More than 50 years into the age of interplanetary travel, only two missions have targeted Mercury: NASA’s Mariner 10, which flew past the planet three times in 1974 and 1975, and NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (Messenger) spacecraft, which also zoomed by three times before settling into a highly productive orbital mission from 2011 to 2015. Messenger’s data upended scientists’ theories for how the planet formed and left scientists grappling with its results. In October or November of this year,
