Lowell Observatory

1400 Mars Hill Road

Flagstaff, Arizona 86001

Media contact:

Cynthia Webster

Public Information Officer

Lowell Observatory

Phone: 520/774-3358

On November 25, 1999, Jupiter expert Dr. John Spencer of Lowell
Observatory, working with astronomer Glenn Orton of the Jet Propulsion Laboratory at NASA’s
Infrared Telescope Facility at Mauna Kea, Hawaii, discovered a large volcanic eruption on Io, one
of Jupiter’s satellites. Their infrared photo of the eruption was taken just hours after a close Io
flyby by NASA’s Galileo spacecraft. Spencer’s Earth-based observations over many years show
that this type of eruption occurs only about 20% of the time anywhere on Io, but by amazing
good luck, Galileo caught a close-up view of the eruption in its narrow field of view as it flew

The Galileo spacecraft observed the fiery lava fountain shooting more than a mile above the
moon’s surface. The images show a curtain of lava erupting within a giant volcanic crater.
“Catching these fountains was a one-in-500-chance observation,” said Galileo scientist Dr. Alfred
McEwen from the University of Arizona in Tucson.

Spencer was delighted but not entirely surprised by what Galileo saw. He and former Lowell
Observatory astronomer John Stansberry had inferred the existence of lava fountains on Io
from their earlier Earth-based observations, but, he said, “it’s incredible to see that idea
confirmed so spectacularly by Galileo.” The fact that the same eruption was recorded by
spacecraft as well as by earth-based observing is a boon to the research being done on Io. By
combining the data, scientists have their best chance ever to pin down temperatures of the
extremely hot lava on Io.

The attached is a false-color infrared image of the sunlit disk of Jupiter’s moon Io
The bright spot at the 1 o’clock position is the same lava fountain seen close-up by Galileo’s
camera, but in this case it is seen from Earth at a distance of 630 million kilometers (390 million

[NOTE: The Galileo images are available at