July 30, 1997

Callisto Retains Its Virgin Structure

Jupiter's satellite Callisto as viewed by NASA's spacecraft Galileo in November 1996. The image on the left is crossed by a prominent fault scarp. Several smaller ridges run parallel to it and there are a number of small impact craters. The right image shows a chain of impact craters that is believed to have been formed by an interplanetary object that fragmented into several pieces before it smashed into Callisto. Callisto's surface is the most heavily cratered in the solar system and the oldest, dating to about 4 billion years ago.

Image Credit: NASA/Jet Propulsion Laboratory (JPL).

According to Greek mythology, the nymph Callisto wanted to remain a virgin but was loved by Zeus, the god ruling over all other gods.

The planetary body Callisto has been more successful in resisting Jupiter's influence ("Jupiter" is the ancient Romans' name for Zeus). Gravity measurements made by NASA's spacecraft Galileo during its November 4, 1996, flyby indicate that Callisto is a homogeneous collection of rock and ice much as it was when it was formed roughly four and a half billion years ago. It did not become differentiated into a metallic or rocky core surrounded by lighter material like most large bodies in the solar system. Callisto retains its virgin structure.

Unlike Callisto, its large sister satellites Io, Europa, and Ganymede are differentiated. They were melted early in their formation by tidal heating from Jupiter. This permitted denser material, like metals and rocks, to sink toward the center and less dense material to float to the surface (differentiation happened also on the Earth, although the sources of heating were different). Callisto, which is farther from Jupiter than Io, Europa, and Ganymede, experienced much less tidal heating and thus avoided differentiation. The lack of differentiation is thought to be the reason Callisto has the oldest surface of any planetary body in the solar system.

More Cool Stuff

We obtained the above images and information from press release 97-110 of NASA's Jet Propulsion Laboratory (JPL), which manages the Galileo mission for NASA. When you get to this press release, click on "Galileo's Nov. 4, 1996, flyby of Callisto":

http://www.jpl.nasa.gov/galileo/status970523.html

For additional information on the Galileo mission, go to the project's homepage:

http://www.jpl.nasa.gov/galileo

There are several other NASA-supported sites on the Web that offer information about the Sun's planets, their satellites, and other solar system bodies:

"Eyes on the Sky, Feet on the Ground," a set of pages of the Web site "Using Science and the Internet as Everyday Classroom Tools" from the Smithsonian Astrophysical Observatory. Planets are discussed in Chapter 5:

http://hea-www.harvard.edu/ECT/the_book/

The "Public Connection" Web site, an outreach project of Rice University and the Houston Museum of Natural Science:

http://space.rice.edu/hmns/connect.html

The "Science Information Infrastructure" project of the University of California at Berkeley:

http://www.cea.berkeley.edu/~edsci/sii/sii_sii.html

"Windows to the Universe," an innovative and engaging Web site about the Earth and space sciences:

http://www.windows.umich.edu/

We featured Europa, Io, and the Galileo mission in earlier Observations of the Week. To access them, go to our archive:

http://observe.ivv.nasa.gov/nasa/ootw/1997/oarch_index.html

Click on the Observations of the Week of April 23, 1997 (Europa), May 22, 1996 (Io), and November 29, 1995 (Galileo's arrival at Jupiter).