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Saturday, December 16, 2000

University of Hawaii

An ocean of life
may lie within a
moon of Jupiter

UH scientists find possible
evidence of water beneath
the surface of Ganymede

By Helen Altonn

Jupiter's moon Ganymede appears to have an ocean beneath its icy crust, suggesting an environment that possibly could sustain life, says University of Hawaii planetary scientist Thomas McCord.

"What we think we discovered is liquid water -- a salty ocean or body of water," he said.

NASA/Jet Propulsion Laboratory
View of Ganymede from the Galileo spacecraft during its first
encounter in June 1996. The dark areas are the older, more
heavily cratered regions and the light areas are younger, tectonically
deformed regions. The brownish-gray color is due to mixtures of rocky
materials and ice. Bright spots are geologically recent impact craters.

McCord, with the Hawaii Institute of Geophysics and Planetology, disclosed findings of salt minerals on Europa, another of Jupiter's moons, three years ago at an American Geophysical Union meeting in San Francisco.

He's at an American Geophysical Union meeting there today with other scientists reporting similar evidence uncovered on Ganymede. NASA's Galileo spacecraft provided a detailed look at the moon with close approaches in May.

McCord's team, including UH researchers Gary Hansen and Karl Hibbits, used Galileo's infrared spectrometer to identify surface materials on Ganymede, one of Jupiter's four largest satellites.

It's more complicated to see them on Ganymede than on Europa, McCord said in an interview before heading to San Francisco, "but we've done it."

Since discovery of salt minerals on Europa, he said, "We've done more work to show they do come from brines -- salt water exposed to the surface."

Also on Ganymede, he said, the salt minerals may result from brine making its way to the surface by eruptions or through cracks.

Two other lines of evidence were presented at today's meeting to support the discovery of an ocean on Ganymede.

Margaret Kivelson, a planetary scientist at the University of California, Los Angeles, and principal investigator for Galileo's magnetometer, found magnetic clues to a possible saltwater layer.

"She announced that they see changes in the magnetic field as the spacecraft flies by which are explained by the existence of a conducting fluid very near the surface," McCord said.

It was difficult to determine because Ganymede, unlike Europa, has a strong magnetic field of its own instead of a secondary field induced by Jupiter's magnetism, he said. "You have to discern a magnetic field on a magnetic field."

However, he added, "Her evidence is pretty clear."

Galileo images of Ganymede also suggested that water or slushy ice may have surfaced through the fractured crust, similar to features on Europa.

Planetary scientists Robert Pappalardo and James Head III of Brown University in Providence, R.I., analyzed the images with researchers at the German Aerospace Center in Berlin.

The photos, taken within 503 miles of Ganymede on May 20, show details of "a tumultuous past," the Brown scientists said.

"Bright, broken swaths, disrupted dark plains and the astounding Arbela Sulcus suggest Ganymede may be more similar to Europa than previously believed," Pappalardo reported.

Arbela Sulcus is a relatively smooth, bright band in a more cratered, older landscape. Possibly it was formed by a complete separation of Ganymede's icy crust, "like bands on Europa, but unusual for Ganymede," Pappalardo said.

McCord works with the Near Infrared Mapping Spectrometer team as leader of the icy satellite investigation. Io has volcanic activity while Europa, Ganymede and Callisto are known as icy satellites because of icy or partly icy surfaces, he said.

The complex situation on Ganymede delayed getting spectacular findings, he said.

"We are seeing salt minerals through the ice, so we have a signature superimposed on another signature. When we disentangle it ... we're left with a salt signal which looks very similar to that for Europa."

In many cases, the ice is large-grained rather than simple frost, he said. "We don't quite understand the significance of that yet."

Kivelson said a melted layer several miles thick, beginning within 120 miles of Ganymede's surface, would fit the data if it were about as salty as the Earth's oceans.

McCord said the discoveries on Ganymede are surprising because most people tended to think it was largely ice on the surface. "Without a closer look and the spatial resolution the Galileo spacecraft gave us, the salt mineral signature was not immediately evident."

The photos and salt mineral findings don't prove that the ocean exists today -- only that it did exist, McCord added. But liquid must exist today because of the magnetic findings, he said.

"The magnetic field is the smoking gun," he said.

When the spacecraft flew by Ganymede last summer, it saw magnetic signatures, so conducting fluid had to be there, McCord said.

"It means there is a heat source -- some mechanism that is providing heat necessary to keep this melted,"he said.

"And it means that there is a habitat, perhaps very similar to that for Europa, which again perhaps may sustain life."

Galileo, orbiting Jupiter since Dec. 7, 1995, will fly past Ganymede again Dec. 28, but it won't be as close as it was in May.

Hawaii Institute of Geophysics and Planetology

University of Hawaii

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