GEOLOGY OF THE ICY SATELLITES TORRENCE V. JOHNSON Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Received: 4 September 2004; Accepted in final form: 26 October 2004 Abstract. In the last 25 years, the explorations of the Voyager and Galileo missions have resulted in an entirely new view of the icy worlds orbiting the giant outer planets. These objects show a huge diversity in their characteristics, resulting from their formation histories, internal processes and interactions with their space environments. This paper will review the current state of knowledge about the icy satellites and discuss the exciting prospects for the upcoming Cassini/Huygens mission as it begins a new era of exploration of the Saturn satellite system. Keywords: Outer planets, icy satellites, cratering, oceans, cryovolcanism, tectonic resurfacing 1. Introduction The outer reaches of our solar system are realms of ice. Beyond the orbit of Mars current models for the formation of planetary materials from the solar nebula four and a half billion years ago require that much of the solid material be in the form of ice. Furthermore, the decreased solar energy available in this region compared with the inner planets results in low surface temperatures that allow ice to be stable on unprotected planetary surfaces for billions of years. The solar abundance of elements dictates that most of the ice formed in the outer solar system will be water ice, H 2 O, with smaller amounts of more volatile condensates, such as NH 3 , CH 4 and their hydrated and clathrated (Gautier and Hersant, 2004) forms as well as CO 2 and N 2 , also possible, particularly at greater distances. The ‘expected’ bulk make-up of a satellite around one of the outer planets is thus a combination of rock (and metals) and water ice, in approximately equal propor- tions in most chemical models. Observations by Earth and space based telescopes and data from spacecraft have confirmed this picture and most of the outer planet satellites indeed appear to by ‘icy satellites’, with the major notable exception of rocky, volcanic Io. One might expect the geology on these frozen worlds to be dominated solely by the continual creation of impact craters from asteroidal and cometary debris over the history of the solar system. Instead, observations by planetary spacecraft, particularly Voyager and Galileo, show a stunning variety in these bodies’ current states and their geological histories. The reasons for this geological diversity can be found in the influences of the satellites’ compositions, planetary energy sources and their interactions with their environments. The next sections discuss the major C Springer 2005 Space Science Reviews 116: 401–420, 2005. DOI: 10.1007/s11214-005-1963-1
GEOLOGY OF THE ICY SATELLITES - |LASP|CU-Boulderlasp.colorado.edu/~espoclass/5830_2013_Spring_homework... · 2013. 11. 4. · GEOLOGY OF THE ICY SATELLITES 403 Figure 2. Upper left
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