When Voyager 1 trained its camera on the moons of Jupiter, scientists back on Earth had no idea what to expect. Turned out they were right. Voyager was snapping photos of geological features unlike anything anyone had ever seen before. Which meant it was time to make up some new sciency words!
Last week, we learned about the word macula (plural, maculae): a special term for dark spots on the surface of a moon or other planetary body. Now if you’re going to invent a special term for dark spots, you really ought to have a term for bright spots too. And that term is facula (plural, faculae).
To an ancient Roman, facula meant “little torch.” To a modern planetary scientist, it refers to a surface feature that looks brighter than the surrounding terrain. The term was first used this way to describe bright, circular features seen on Ganymede.
If you think Ganymede’s faculae look a little like craters, you’d be on the right track. Like most moons in the outer Solar System, Ganymede is composed of a mixture of rock and ice, and it may have a layer of liquid water beneath its surface.
So the craters left by asteroid impacts on Ganymede sometimes get filled in with icy slush. The slush freezes, and the crater is virtually erased. Only the crater rim remains, and you can see a color difference between old and new surface ice.
The term facula can be used to describe almost any bright spot on a planet-like surface, not just resurfaced craters. For example, there are faculae on the dwarf planet Ceres. Ceres’s faculae are still being investigated by the Dawn spacecraft, but the current best guess is that they’re salt deposits—perhaps salt left behind after very briny water boiled into space.
For next week’s edition of Sciency Words, we’ll conclude our visit to the moons of Jupiter with a quick trip to Io.
Bonus Sciency Word: An impact crater that gets filled in and smoothed over, like the craters on Ganymede, is also called a palimpsest.