Touring the Surface

When Galileo discovered Europa in 1610, along with three other moons of Jupiter, his telescope showed them as only star-like points of light (Fig. 2.1). But even that low resolution was enough to show that the moons orbited around Jupiter, and to prove that the Earth is not the center of everything after all. Europa is about the same size as our Moon, but it is typically a couple of thousand times farther away from us. So over the following centuries, viewed from telescopes trapped on Earth, Europa remained only a single, unresolved point of light (Fig. 2.2).

Even so, important information came from that point of light. Its wavelength spectrum, what our eyes crudely see as color, depends on the material on Europa's surface. During the mid-twentieth century, instruments were developed that could accurately measure such spectra. Using a spectrograph and including measurements at infra-red wavelengths (too long for the human eye to see), in 1972 MIT graduate student Carl Pilcher inferred correctly that Europa's surface is predominantly water ice.