TECTONICS ON ICY WORLDS

The satellites of the outer planets display a plethora of interesting structural features. Comparison of tectonics on these bodies to the terrestrial planets can help us to understand the differences in driving forces underlying tectonic activity on these bodies, as well as the role of different crustal materials in the structural expression of strain on their surfaces. Ice is mechanically weaker than most rocks, making it easier to propagate fractures in response to low stresses. Though ice is a brittle material at the frigid surface temperatures in the outer solar system, the amount of thermal energy required to bring the near subsurface to a ductile state near its melting point is lower than for most rocky surfaces. Ice near its melting point is orders of magnitude less viscous than rock at an equivalent near-melting temperature. One possible consequence of this material difference is that a few satellites with youthful surfaces (and likely high heat flow) exhibit bizarre tectonic features, such as ridge complexes on Europa, Enceladus, and Triton. Other outer satellites with apparently thicker lithospheres exhibit structural features that are clearly recognizable from terrestrial experience, such as the ubiquitous sets of normal faults on the surfaces of Ganymede, Dione, Tethys, and Miranda. Studies of strain on various icy bodies has shown copious evidence for extension, some strike-slip motion, and very sparse evidence for features formed by contraction. The driving mechanism for tectonics on many icy bodies appears to be dominated by changes in their tidal and rotational figures. Phase changes within the ice may also play a role on some bodies. A more detailed picture of structural geology and tectonics on the icy satellites remains tantalizing, as many of the necessary tools, such as close-up fieldwork, subsurface electromagnetic sounding, high-order gravity fields, and global altimetry data, have not been gathered for any object in the outer solar system. A proposed future mission to Europa and Ganymede would fill in many of the gaps in our current understanding of tectonics on icy worlds.