Don Gurnett

Principal Investigator of Plasma Wave Instrument

Principal Investigator of Plasma Wave Instrument

Fairfax, Iowa

In 1610, when Galileo (Galilei) was the first person in the world to look through a telescope at an astronomical object, he looked at Jupiter, and he saw four moons going around it. The historical importance of that event is it convinced him that Copernicus was right -- the Sun was the center of the solar system, and the planets were revolving around it.

March 5, 1979, was Voyager’s first close flyby of Jupiter's moon Io. It had a strange orangish color, as opposed to all the other moons, which basically looked white. So, picture this: We’re approaching Jupiter, and ever since the discovery of the Galilean satellites by Galileo in 1610, they’d just been dots of light. You know, you can’t see any significant details at all. I remember this quite vividly: Overnight, we closed in on Jupiter, and we went from the point of just seeing an orangish-colored dot, and then all of a sudden, within a matter of hours, we fly by Io, and we get these tremendous pictures.

It was like the historical event when Galileo first looked through a telescope. Now, we finally see what these moons look like, and they were just absolutely stunning. I can put it no other way.

Voyager gave us what was called the pizza picture of Io. I mean, it looked like a pizza. Nobody expected it to look like that. It had what they refer to as ringworms (white splotches) and black spots that looked like olives on a pizza. Io is one of the most unusual, picturesque objects in the solar system.

Then, in a matter of a day or so, people started to realize what some of the features were. All these black dots on Io are volcanoes; it is the most volcanic object in the solar system. And so, it came to be realized that this moon is being heated by tidal forces from the other moons. The gravitational tugs from these other moons -- especially Ganymede because it’s so large -- are flexing the interior of Io and heating it up. And, the moon is entirely molten, except for a thin skin on the surface.

Later NASA missions, such as Galileo, showed you can actually see the surface of Io changing, because this volcanic material is actually coming down on the surface, solidifying and then getting re-melted into the interior. This is probably similar to what happened with Earth when it was first formed.

I was there at the instant of discovery, like I said, kind of like Galileo was. Of course, we’re not looking through a telescope; we’re looking through the eyes of Voyager. In my mind, I always think of that as one of the greatest events that I’ve been eyewitness to. And, there have been many such things with Voyager, but the one with Io just stands out above everything else.