Digital elevation map of Enceladus
Clues to Enceladus’ interior: Deep basins on Saturn’s moon Enceladus are seen as dark dimples marked in white letters A through G in this 2009 digital elevation map made from data from NASA's Cassini spacecraft. The basins are about 55 to 110 miles across (about 90 to 175 kilometers). The largest, basin “A,” is about one mile (1.5 kilometers) deep. The heavy lines bounded by black letters show the locations of topographical profiles for basin “A,” seen in the second figure, below. The map has sparked continuing research into Enceladus’ active past.

Image credit: NASA/JPL-Caltech/Lunar and Planetary Institute-USRA/Washington University
Currently, 310 mile- (500 kilometer-) diameter Enceladus shoots an enormous plume of icy particles and vapor into space. Scientists are eager to determine the thermal sources and mechanisms that have made this small moon so active. They also want to know whether the plume is a short-lived occurrence or long-lasting phenomenon. Research led by Jonathan Besserer of the University of California Santa Cruz reported recently in the Journal of Geophysical Research used results derived from Cassini’s visible- and infrared-wavelength observations to look for signs of past interior activity that left readable clues on older areas of Enceladus’ surface.

There are no visible geological clues on the surface that hint at why the depressions formed, Besserer said. But the depressions themselves help tell the story of what may lie beneath -- past, and possibly current, thermal activity hidden underground. Some of these depressions are up to a mile deep and more than 50 miles (80 kilometers) across, and they tend to lie in old, cratered regions of the moon. The fact that the depressions are old strengthens theories that thermal activity, possibly even a local or global ocean, has been shaping Enceladus’ surface for some time.

Besserer and his colleagues have determined that deep upwellings of tidally heated ductile ice would warm surface ice above it, allowing pores and/or fractures to close, or anneal. Over time, as the surface features closed, ice would become more compact and sink relative to the rest of the local surface. Hence, unlike the hot spot on Earth that creates the rise of the Hawaiian Islands, Enceladus' putative past icy upwellings would probably have led to the topographic depressions.

The analysis of the surface depressions provides one more tool for determining past interior activity on a moon where no spacecraft has yet landed, Besserer said. “Even old and currently inactive regions of Enceladus may record signs of one or several episodes of past thermal activity, thus further helping us to unravel the complex history of the satellite.”

Graph showing topographical profiles
The 1-mile (1.5-kilometer) depth of basin “A” is obvious in these topographical profiles of the feature.

Figure credit: NASA/JPL-Caltech/Lunar and Planetary Institute-USRA/Washington University

This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:

1) “Convection-driven compaction as a possible origin of Enceladus’s long wavelength topography,” Jonathan Besserer, Francis Nimmo, James H. Roberts, Robert T. Pappalardo, Journal of Geophysical Research, May 2013

2) “One-hundred-km-scale basins on Enceladus: Evidence for an active ice shell,” Paul M. Schenk, William B. McKinnon, Geophysical Research Letters, Aug. 21, 2009

3) “Geophysical implications of the long-wavelength topography of the Saturnian satellites,” F. Nimmo, B. G. Bills, P. C. Thomas, Journal of Geophysical Research, November 2011

4) “Enceladus: An Active Cryovolcanic Satellite,”
John R. Spencer, Amy C. Barr, Larry W. Esposito, Paul Helfenstein, Andrew P. Ingersoll, Ralf Jaumann, Christopher P. McKay, Francis Nimmo, J. Hunter Waite, a chapter of “Saturn from Cassini-Huygens,” 2009, pp 683-724

-- Mary Beth Murrill, Cassini Science Communication Coordinator