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VOLCANISM ON OTHER PLANETS 209

magma reservoirs if they have vertical extents of sium sulfate, and with small amounts of rock and
∼15 km and, although impressive, this would be metal dust from the impact of meteorites, but it is
consistent with the effects of the low gravity on clear that it is not a silicate surface. Nor is it a rocky
reservoir geometry. body covered with a thin layer of ice, because there
are no large mountains. There is even a near-total
lack of large and small impact craters. The absence
13.9 Europa of large craters might not be surprising, because
ice is a weak material that flows under stress (as in
Europa was extensively imaged by the Galileo glaciers on Earth), and over hundreds of millions
spacecraft at resolutions as good as 200 m per of years the rims of large craters and basins would
pixel. The high reflectivity of the surface of Europa sink, and the floors rise, until almost nothing was
is consistent with the spectroscopic evidence that visible. However, the lack of small craters, which
the surface consists almost entirely of water ice form much more frequently and deform propor-
(Fig. 13.20). The ice appears to be “contaminated” tionately less, implies that the surface is geolog-
to some extent with dissolved salts such as magne- ically young.

Fig. 13.20 A 100 by 140 km area of Europa showing complex ridges and fracture patterns cutting the ice layer that forms the
surface. The texture in some places seems to imply that the ice has melted and re-frozen. (Image from the Galileo spacecraft’s
Solid State Imaging system courtesy of NASA.)