July 14, 1998
The images were taken by Galileo's camera during several Ganymede flybys between June 1996 and June 1997. They reveal impact craters with unusual pedestals, dark ejecta haloes, evidence of tectonic activity and possible signs of icy volcanic flows. A crater chain seems to be the result of impacts from a broken-up comet, like the 1994 Shoemaker-Levy impact on Jupiter.
Ganymede is the largest moon of any planet in the solar system. Its distinctive surface is composed half of bright water ice, and half of older, dark, heavily cratered terrain containing ice and rocks. The new images show surface details including abundant remnants of old craters and basins in the dark terrain, thought to date from early in the solar system's history. The images also reveal a complex transition from dark, old terrain to bright, new terrain, where parallel rows of linear mountain-like ridges extend for hundreds of kilometers (or miles). During the early part of Ganymede's history, it is very likely that a global ocean existed below the surface, just as an ocean may exist on Europa today. A complex valley with a lobe-shaped flow appears to be a source of liquid water volcanism. Younger faults are seen cross-cutting older features, which helps scientists piece together the sequence of events in Ganymede's history.
"These new, unprecedented views of Ganymede allow us to address scientific mysteries revealed by earlier spacecraft. By analyzing these images, scientists will lay the cornerstone for interpreting other icy satellites around Jupiter, Saturn, Uranus and Neptune," said Dr. James Head, a Brown University planetary scientist and member of the Galileo imaging team.
Galileo has spent 2-1/2 years orbiting Jupiter and imaging its four largest moons. It wrapped up its primary mission in December 1997, and its current, extended Galileo Europa Mission will continue through December 1999. The mission includes eight Europa flybys, four Callisto flybys, and one or two of Io, as long as the spacecraft remains healthy.
Images will be transmitted on NASA Television on Wednesday, July 15, at 9 a.m., 12 noon, 3 p.m., 6 p.m. and 9 p.m. Pacific time. NASA Television is available on the satellite GE-2, transponder 9C, at 85 degrees west longitude, vertical polarization, frequency 3880 Mhz, audio 6.8 MHz.