Discovering Europa

Discovering Europa

The Voyager 2 spacecraft will never return to Earth. But its scientific findings, its epic discoveries, its travelers’ tales do return. Take July 9, 1979, for instance. At 8:04 Pacific Standard Time on this morning, the first pictures of a new world, called Europa, after an old one, were received on Earth.

How does a picture from the outer solar system get to us? Sunlight shine on Europa in its orbit around Jupiter and is reflected back to space, where some of it strikes the phosphors of the Voyager television cameras, generating an image. The image is read by the Voyager computers, radioed back across the immense intervening distance of half a billion kilometers to a radio telescope, a ground station on the Earth. There is one in Spain, one in the Mojave Desert of Southern California and one in Australia (on that July morning in 1979 it was the one in Australia that was pointed toward Jupiter and Europa). It then passes the information via a communications satellite in Earth orbit to Southern California, where it is transmitted by a set of microwave relay towers to a computer at the Jet Propulsion Laboratory, where it is processed.

The picture is fundamentally like a newspaper wirephoto, made of perhaps a million individual dots, each a different shade of gray, so fine and close together that at a distance the constituent dots are invisible. We see only their cumulative effect. The information from the spacecraft specifies how bright or dark each dot is to be. After processing, the dots are then stored on a magnetic disc, something like a phonograph record. There are some eighteen thousand photographs taken in the Jupiter system by Voyager 1 that are stored on such magnetic discs, and an equivalent number for Voyager 1. Finally the end product of this remarkable set of links and relays is a thin piece of glossy paper, in this case showing the wonders of Europa, recorded, processed and examined for the first time in human history on July 9, 1979.

The wonders of Europa are now recorded, processed and examined. Image: ©  Elena

What we saw on such pictures was absolutely astonishing. Voyager 1 obtained excellent imagery of the other three Galilean satellites of Jupiter. But not Europa. It was left for Voyager 2 to acquire the first close-up pictures of Europa, where we see things that are only a few kilometers across. At first glance, the place looks like nothing so much as the canal network that Percival Lowell imagined to adorn Mars, and that, we now know from space vehicle exploration, does not exist at all. We see on Europa an amazing, intricate network of interesting straight and curved lines. Are they ridges – that is, raised? Are they troughs – that is, depressed? How are they made? Are they part of global tectonic system, produced perhaps by fracturing of an expanding or contracting planet? Are they connected with plate tectonics on the Earth? What light do they shed on the other satellites of the Jovian System? At the moment of discovery, the vaunted technology has produced something astonishing. But it remains for another device, the human brain, to figure it out. Europa turns out to be as smooth as a billiard ball despite the network of lineations. The absence of impact craters may be due to the heating and flow of surface ice upon impact. The lines are grooves or cracks, their origin still being debated after the mission.