Images suggest icy eruptions on Ganymede

Jupiter’s moon Ganymede is a study in contrasts: Bright swaths of pure, frozen water slice through darker, heavily cratered ice. New stereo images of Ganymede suggest these swaths are the aftermath of eruptions of water or slushy ice that gave parts of the moon a facelift a billion or more years ago.

The findings also support the notion that an ocean, which now may lie beneath more than a hundred kilometers of ice, once resided nearer to Ganymede’s surface.

Ever since the Voyager spacecraft photographed Ganymede in 1979, scientists have puzzled over the origin of long, bright tracts of ice, which stretch for tens of kilometers and cover two-thirds of the frozen body. Combining Voyager’s images with the much sharper pictures recorded since 1995 by the Galileo craft, a team led by Paul M. Schenk of the Lunar and Planetary Institute in Houston created paired images of several areas on Ganymede.

The images bring three-dimensional relief to a panoply of fractures, faults, and grooved terrain. They reveal that the smoothest, brightest swaths lie in troughs 0.5 to 1 kilometer or so lower than the most heavily cratered areas. Moreover, depressions resembling volcanic calderas lie at the edges of the swaths.

The low elevation and flatness of the bands strongly suggest that eruptions of water or slushy ice flooded these low-lying regions, filling in old craters and then freezing in place, Schenk asserts. Such eruptions didn’t have enough thrust to breach higher-elevation regions, the scientists report in the March 1 Nature.

The low elevation of the icy bands “is the most persuasive evidence that they could be volcanic” in origin, says Louise M. Procter of the Johns Hopkins Applied Physics Laboratory in Laurel, Md. She cautions, however, that if icy eruptions were once common, there ought to be more visible evidence of them.

Stretching or buckling of the surface in the billion or more years since the eruptions could have masked such evidence, contends Schenk. An analysis of ultrasharp images of Ganymede, taken by the Galileo craft last May 20, may reveal whether he’s right, Procter notes.

The eruptions could be tied to evidence that Ganymede has an ocean beneath a thick layer of ice (SN: 12/23&30/00, p. 404). Researchers theorize that when Ganymede first moved into a special position relative to its sister moons, roughly a billion years ago, Jupiter’s tug flexed the satellite much more than it does now. This flexing would have heated the moon, thinning its icy surface and enlarging its buried ocean.

In this scenario, surface eruptions of slushy ice or water may have been more common than in later times, when Ganymede’s orbit became more circular and less subject to flexing.