The surface of Europa Global scale

Europa's surface is composed predominantly of water ice according to reflectance spectra (Pilcher et al., 1972), and thus the moon would appear to the human eye as a nearly uniform white sphere, 1565 km in radius. The gravitational figure measured by spacecraft indicates that an outer layer as thick as ~150 km has the density of H2O (Anderson et al., 1998). Much of that thick layer is probably liquid water according to estimates of tidal heating (Section 15.3.3), while the surface is frozen due to radiative cooling.

Fig. 15.1. A full-disk view of Europa shows large-scale lineaments indicative of tectonic cracks, dark splotches marking thermally created chaotic terrain. Craters are rare, although a prominent rayed crater, Pwyll, is evident to the left. The equator runs across the centre here. The dark wedge-shaped and tightly curved features just south of the equator are mostly dilational bands.

Fig. 15.1. A full-disk view of Europa shows large-scale lineaments indicative of tectonic cracks, dark splotches marking thermally created chaotic terrain. Craters are rare, although a prominent rayed crater, Pwyll, is evident to the left. The equator runs across the centre here. The dark wedge-shaped and tightly curved features just south of the equator are mostly dilational bands.

The surface does contain more than pure water ice. Even global-scale pictures (with resolution >10 km per pixel) taken with Voyager and Galileo spacecraft cameras show (when the contrast is enhanced as in Figure 15.1) orange-brown markings of still-unidentified substances, which likely include hydrated salts (McCord et al., 1998a) and sulphur compounds (Carlson et al., 1999). The patterns include splotches ranging from tens to hundreds of kilometres across and a network of narrow lines. When the splotches and lines are observed at high resolution (as discussed below), they prove to represent the two major resurfacing processes on Europa: formation of chaotic terrain and tectonics, respectively.

The resurfacing has been rapid and relatively recent as evidenced by the paucity of craters. Apparently, impacts have had a minimal role in shaping the current surface. In Figure 15.1, for example, only crater Pwyll, with its extensive white rays, is readily evident at this scale (lower left). The paucity of craters tells us that the current surface must be very young. It is continually reprocessed at such a great rate that most of the observable terrain, structures, and materials have probably been in place < 50 My (Zahnle et al., 2003).

Even from the global appearance we see indications that the surface has been reworked by stress and heat, forming the tectonic and chaotic terrain, respectively. This chapter shows how these processes modified the surface and have provided, in diverse ways, access to the ocean. The relatively dark, orange-brown material that marks sites of these processes is indistinguishable whether it appears along a tectonic lineament or around a patch of chaotic terrain. It may represent concentration of impurities by thermal effects due to the near exposure of warm liquid or be the most recently exposed oceanic substances. All this activity is rapid, recent, and thus likely on-going.

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