Introduction

A world of extremes, Io, in addition to displaying extensive volcanism, supports some of the highest mountains in the Solar System. Yet, in an apparent paradox, very few of the mountains actually resemble volcanoes; the majority of the towering peaks appear to be tectonic structures: uplifted and sometimes tilted blocks, which are bounded by steep scarps and, in many cases, fractured (Figure 6.1). However, there are no obvious tectonic patterns revealed in the global mountain distribution beyond a bimodal variation of number density with longitude (Figure 6.2; Schenk et al., 2001; Kirchoff and McKinnon, 2005) and local associations with paterae, and Io's ubiquitous volcano-tectonic depressions (e.g., Masursky et al., 1979; Whitford-Stark, 1982; Carr et al., 1998; McEwen et al., 2000; Radebaugh et al., 2001; Jaeger et al., 2003; see examples in Figure 6.1). Current evidence indicates that ultimately the mountains do owe their existence to the incredible rate of volcanic resurfacing: the resulting subsidence can induce more than sufficient compressive stresses in the lithosphere, driving mountains up by extensive thrust faulting (Schenk and Bulmer, 1998; Jaeger et al., 2003).

The existence of well over one hundred of these structures imposes constraints on the structure of Io's mechanical lithosphere and their characteristics provide insights into interior and crustal properties. The state of the mountains is intimately related to other geologic processes acting on Io (e.g., both extrusive and intrusive volcanism, Io's internal tidal dynamics, and surficial mass wasting). Therefore, the processes by which the mountains form and evolve have implications for many different aspects of Io. In this chapter we describe the current state of understanding, derived from Voyager and Galileo observations and analyses thereof, of Io's mountains, their formation and evolution, and the implications for Io's tectonics and lithosphere.

Figure 6.1. This moderate-resolution, ~500m per pixel, regional mosaic combined with lower resolution, 1.3 km per pixel, color images acquired by Galileo includes several examples of Ionian mountains and volcanic centers. The mountains are isolated from each other, but a high fraction of those in this region are associated with paterae. The illumination, which is from the left, accentuates the topography and surface textures. This effect is strongest on the eastern side where the solar incidence angle is 21-28° and weakens toward the west where the Sun is higher, solar incidence angle ~37-45°: compare the visibility of ~10 km high Gish Bar Mons, between Gish Bar Patera to the south and Estan Patera to the north; ~6 km high Monan Mons, between Monan Patera to the north and Ah Peku Patera to the south; and ~7 km high Euxine Mons. (See also color section.)

Figure 6.1. This moderate-resolution, ~500m per pixel, regional mosaic combined with lower resolution, 1.3 km per pixel, color images acquired by Galileo includes several examples of Ionian mountains and volcanic centers. The mountains are isolated from each other, but a high fraction of those in this region are associated with paterae. The illumination, which is from the left, accentuates the topography and surface textures. This effect is strongest on the eastern side where the solar incidence angle is 21-28° and weakens toward the west where the Sun is higher, solar incidence angle ~37-45°: compare the visibility of ~10 km high Gish Bar Mons, between Gish Bar Patera to the south and Estan Patera to the north; ~6 km high Monan Mons, between Monan Patera to the north and Ah Peku Patera to the south; and ~7 km high Euxine Mons. (See also color section.)

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