Nature © Macmillan Publishers Ltd 1998 8 NATURE | VOL 391 | 22 JANUARY 1998 363 letters to nature Evidence for a subsurface ocean on Europa Michael H. Carr*, Michael J. S. Belton†, Clark R. Chapman‡, Merton E. Davies§, Paul Geisslerk , Richard Greenbergk, Alfred S. McEwenk, Bruce R. Tuftsk, Ronald Greeley¶, Robert Sullivan#, James W. Head ✩ , Robert T. Pappalardo ✩ , Kenneth P. Klaasen**, Torrence V. Johnson**, James Kaufman**, David Senske**, Jeffrey Moore††, Gerhard Neukum‡‡, Gerald Schubert§§, Joseph A. Burns#, Peter Thomas# & Joseph Veverka# * US Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA † National Optical Astronomy Observatory, 950 Cherry Street, Tucson, Arizona 85719, USA ‡ Southwest Research Institute, 1050 Walnut Street, Boulder, Colorado 8030, USA § Rand Corporation, 1700 Main Street, Santa Monica, California 90406, USA k Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA ¶ Geology Department, Arizona State University, Tempe, Arizona 85287, USA # Cornell University, Ithaca, New York 14853, USA ✩ Geology Department, Brown University, Providence, Rhode Island 02912, USA ** Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 911909, USA †† NASA/Ames Research Center, Moffet Field, California 94035, USA ‡‡ DLR-Institut fu ¨r Planetenerkundung, Rudower Chaussee 5, 12489 Berlin, Germany §§ Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA ......................................................................................................................... Ground-based spectroscopy of Jupiter’s moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior 1–4 . In addition, images obtained by the Voyager spacecraft revealed that Europa’s surface is crossed by numerous intersecting ridges and dark bands (called lineae) and is sparsely cratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto 5 . It has been suggested that Europa’s thin outer ice shell might be separated from the moon’s silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heat- ing 6–10 ; in this model, the lineae could be explained by repetitive tidal deformation of the outer ice shell 11–13 . However, observa- tional confirmation of a subsurface ocean was largely frustrated by the low resolution (2km per pixel) of the Voyager images 14 . Here we present high-resolution (54 m per pixel) Galileo space- craft images of Europa, in which we find evidence for mobile ‘icebergs’. The detailed morphology of the terrain strongly sup- ports the presence of liquid water at shallow depths below the surface, either today or at some time in the past. Moreover, lower- resolution observations of much larger regions suggest that the phenomena reported here are widespread. The observations are of a region near 13° N, 273° W, just south of the intersection of two prominent ridges, Asterius Linea and Agava Linea (Fig. 1). These and similar ridges appear in Voyager images as bright lines with dark margins, so were called ‘triple bands’. From Voyager data much of Europa’s surface had been categorized as mottled terrain or plains 15 . The area discussed here is mostly mottled terrain. It is crossed by numerous triple bands mostly trending roughly southeast–northwest, and by discontinuous rays from the crater Pwyll, 700 km to the south. Nested images were taken at 1.2 km per pixel, 180 m per pixel and 54 m per pixel. This terrain can be divided into four components: the general background plains, the lineae, locally disrupted areas, and the large disrupted area south of the main ridge crossing in Fig. 1. The background plains are composed of ridges superimposed on ridges, so that the surface resembles that of a ball of string. The lineae are mostly long, linear bundles of ridges and furrows that stand at a higher elevation than the surrounding plains. Elevations derived from shadows and photometric measurements indicate that the youngest of the prominent ridges have elevations of 100–200 m. Older ridges have much more subdued relief. Our main concern here is with places where the plains are locally disrupted as seen in Fig. 2, and broad areas of disruption as seen in Fig. 1 and in detail in Fig. 3. Around the periphery of the area seen in Fig. 1, the plains are locally disrupted to form quasi-circular spots which are generally darker than their surroundings. The spots are mostly 10–20 km across and appear to be local upwellings of some kind (Fig. 2). In some of the spots, the surface is simply raised to form a low, flat- topped dome on which the original texture of the surrounding plains is preserved. In more advanced states of development, parts or all of the original surface texture are replaced by a fine-scale blocky texture and the disrupted zone is bound by an inward-facing escarpment. The disruptions cut across ridges, even those ridges that are relatively young. Most of the spots have a lower albedo than the unaffected terrain, which gives the terrain a mottled appearance. The mottled terrain covers large areas of Europa, which suggests that the disruptive process just described is widespread. To the south of the prominent ridge crossing in Fig. 1 is a dark, diamond-shaped area, 100 km across, where widespread disruption of the crust has occurred. Within the area the crust has broken into pieces up to 20 km across. The individual crustal blocks are flat- topped and surrounded by cliffs that are 100–200 m high, as indicated by shadows. Preserved on the surface of the blocks is the original plain’s surface texture of criss-crossing ridges. The texture on many of the blocks enables the reconstruction of the Figure 1 Disrupted zone at 13° N, 273° W. The scene is 200 km across. Just south of the prominent ridge crossing at the top of the image, the surface has been broken into blocks that have moved laterally from their original positions, as indicated by the texture of the pre-existing terrain preserved on the block surfaces. Around the periphery of the large disrupted zone are many additional, smaller and roughly circular areas where the original surface texture has been destroyed. Illumination is from the lower right.