This pair of volcanic features on Jupiter's moon Io represents the longest active lava flow known to exist in our solar system. The volcanic features, Amirani (right side of image) and Maui (to the left, just below the center of the image), were originally thought to be two separate volcanoes. Photo: NASA/JPL

The amount of lava gushing from individual volcanoes on Jupiter's moon Io dwarfs earthly comparisons, and the pace at which lava is repainting Io's surface suggests a novel technique for determining the relative ages of surface regions there.

The latest research about Io, much of it based on data from NASA's Galileo spacecraft, was reported Wednesday at the Lunar and Planetary Science conference in Houston, Texas.

Knowing the relative ages of surface features on a planet or moon is crucial for understanding the processes shaping that world. The favorite age gauge of planetary scientists is counting impact craters. The more impacts still showing, the older the surface. However, volcanoes are resurfacing Io so fast, not a single impact crater has been found there.

"It appears that the same process that destroys the traditional way of dating surfaces is going to provide a new way to date surfaces," said Dr. Dennis Matson, a research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "On Io, surface temperature can give us an indication of surface age."

Calculations of Io's total heat flow suggest that virtually everywhere on Io, even the oldest surfaces were produced by fresh lava so recently that they are still cooling off, Matson said. With some allowances for differences in composition and thickness of the flows, colder surface areas should be older surface areas.

"If we get this technique worked out, we look forward to combining it with other types of information to read the history of Io's surface," he said. "We hope to be able to tell whether lava flows that are far apart on the surface happened at approximately the same time."

Some of the first calculations were reported Wednesday for how fast lava is being produced by individual volcanic features on Io. One thermally steady volcano that produces broad flow fields named Amirani and Maui churns out about 100 cubic meters (about 3,500 cubic feet) of lava every second, said Dr. Ashley Davies, a JPL volcanologist. That's about 200 times as productive as the Kilauea volcano in Hawaii. It's fast enough to overflow two Olympic size swimming pools every minute or three Houston Astrodomes every day.

Amirani's big lava flow on Io. Photo: NASA/JPL

Ionian volcanoes Monan, Tupan, Prometheus, Culann and Zamama each produce one-third to one-half as much lava as Amirani-Maui does, Davies calculates.

Estimates of the volume rate of an eruption are important for understanding what is happening under the surface and at the surface of each volcano. "Flow rate is tied to the source of the magma and the conduits the magma follows to the surface," Davies said. "It strongly influences how the lava landscape forms and how heat is lost from the lava as time passes."

The volume-rate estimates also help crosscheck estimates of the thickness of flows where Galileo's repeat flybys have provided information about how rapidly the flows increase in area. Davies used infrared spectral measurements from Galileo plus modeling of cooling rates to calculate surface coverage rates and eruption volumes. The calculations suggest typical lava-flow thicknesses on Io of one to two meters (three to six feet), he said.

JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.