Volcanoes in Space: JASON Digital Lab

Then John Spencer and his colleagues discovered a new way to pinpoint volcano locations: occultations. When Io passes behind another moon or Jupiter itself, a hotspot disappears at the moment it moves over the horizon.

By measuring the total infrared light from the blurry image of Io and Jupiter as the occultation progressed, we hoped to see sudden jumps in brightness as individual volcanoes winked out or re-emerged from behind Jupiter. The timing of these events would tell us where the volcanoes were, and the size of the jumps would tell us how big and hot they were."

Even better, a second event—an eclipse—occurs immediately before or after the occultation. While passing through Jupiter's shadow, Io's volcanoes glow without the competition of sunlight and are much easier to see.

In December 1989, Spencer and his colleagues first tried viewing Io's volcanoes during an occultation:

"As Io passed behind the disk of Jupiter itself, Loki's glow held steady until it swept behind the planet and suddenly winked out. The time of the disappearance precisely confirmed that the bright volcano was Loki. The occultation idea worked!"

Volcano Hunting

Watching an occultation of Io