Icy satellite

Among places where condensates accreted into significant solid bodies, such as planets, habitable realms have always been rarer than places that were either too cold or too hot for life to exist. Much of our Solar System s mass is still far too hot for life. Most of the deep interiors of the gas giants and rocky planets are too hot, as is, of course, the Sun itself. Most of the surface area of solid bodies in the Solar System are too cold - the icy satellites of the outer planets and the myriad comets and Kuiper Belt Objects on the far outer fringes of the Solar System. In this sense, places like the surfaces of Earth and Mars and Europa s subsurface ocean are indeed very rare places. 

Kargel JS (1991) Brine volcanism and the interior structures of asteroids and icy satellites. Icarus 94 368-390... 

Kargel JS, Consolmagno GJ (1996) Magnetic fields and the detectability of brine oceans in Jupiter s icy satellites. Lunar Planet Sci 27 643-644... 

In a parallel set of discoveries, a reflectance band in the visible, similar to that for solid 02, was seen at low latitudes on Ganymede. In addition, a UV feature associated with O3 was seen on Ganymede and on the icy satellites of Saturn. Coupled with these observations was the much earlier discovery of a band indicative of SO2 in ice at Europa and Callisto and the recent discovery of CO2 trapped in the icy surfaces. The SO2 was initially assumed to be due to sulfur ions originating at lo implanted into the ice at Europa, " but the SO2 is also a radiation decomposition product like the O2, as discussed below. " The CO2 source is probably internal as carbon ions have not yet been seen in the plasma. 

Belitsky and Lane used such studies to outline the chemical pathways that might be relevant on these moons. However, detailed quantitative information for the separate pathways is needed. Earlier, Haring and coworkers, among others, measured ejected molecule mass spectra and ejecta energy spectra for a number of mixtures. These studies indicate that a variety of products are formed and ejected into the gas-phase. However, absolute yields for desorbed species from mixed ices were not given but are needed to understand the ambient gas at each of the icy satellites. ... 

Models of the icy satellites also suggest that hydrated Mg SO4 should also be present. Decomposition of this species could again lead to hydrated H2SO4, in which case MgO or Mg(OH)2 should be present in the surface (Appendix) since Mg is desorbed much less efficiently. However, Mg has not yet been seen spectrally in the surface, ambient gas or plasma. 

Observations have identified the same condensed oxygen signature as seen for the other icy satellites (Spencer and Calvin, 2002). In addition, a tenuous molecular oxygen atmosphere has been identified from Hubble Space Telescope spectra... 

Neptune s largest moon, Triton, was discovered within weeks of the discovery of the planet itself. It is one of the most distant objects in the solar system. Even the outermost planet, Pluto, and its moon, Charon, spend considerable time on their eccentric orbits closer to the Sun than Triton. Its nature remained a mystery until the advent of new astronomical methods in the 1970s and 1980s and the flyby of the Voyager 2 spacecraft in 1989. In many ways, it is a planetary body on the edge —on the outer edge of the main part of the solar system, and the inner edge of the realm of comets and the recently discovered Kuiper belt objects. As such, it shares some of the characteristics of the icy satellites of the rest of the outer solar system with some of the nature of the colder, more distant, cometary bodies. 

In the Jovian system, the namre of the putative subsurface oceans of the icy satellites, particularly Europa, needs intensive future study. Issues include the thickness of liquid layers and ice crusts, the history of communication between liquid layers and the surface, and the brine composition of the liquid. The next mission planned to address these issues is the Jupiter Icy Moons Orbiter, which is designed to orbit each of the icy satellites and explore their surfaces and subsurfaces in detail. 

Consolmagno G. J. and Lewis J. S. (1977) Preliminary thermal history models of icy satellites. In Planetary Satellites (ed. J. A. Burns). University of Arizona Press, Tucson, pp. 492-500. 

Kargel J. S. (1992) Ammonia-water volcanism on icy satellites phase relationships at one atmosphere. Icarus 100, 556-574. 

Surface compositions of icy satellites can be observed by reflection spectroscopy (UV to near-IR 0.2-5 pm) [5J. The physical properties and surlace compositions of icy bodies are summarized in Table 9.4. Major satelhtes of Jupiter (except for lo), Saturn, and Uranus show clear evidence of H2O ice (ice I) on Oieir surfaces. Altliough physical properties, such as density, radius, albedo, and surface structure of tlrese satellites differ greatly, they share basic compositional similarity. It is thus assumed that the ice of these satellites is pure H2O when we discuss Oieir internal structiue. The large spectral differences among satellites reflect surface modification histories by impact, volcanism, tectonics, and solar radiation. It is noted that SO2, O2, and O3 are trace components and contaminants. No NH3 or CH) ices, predicted by the equilibrium condensation theory, have yet been detected on these satellites. 

There is a great diversity of surface morphology of icy satellites (Fig. 9.1), Callisto and Mimas are ratlier heavily cratered witli a paucity of tectonic features. On the other hand, Europa displays global tectonic features a network of fine, curving fissures covers its surface. Ganymede and Tethys are of an inter-... 

Figure 9.2. Size dependence of the bulk density of icy satellites. Solid and broken lines show the compressed density curves for pure ice and an ice-rock mixture (60 % ice + 40 % rock by mass) calculated from Lupo and Lewis f6]. Bulk densities of icy satellites belonging to each planet are shown for comparison.

We find many craters on the surfaces of icy satellites as well as on the moon. This geological structure is a quite general feature found on the solid bodies, which means tliat high-velocity impacts happen frequently in the solar system. Therefore, many experiments Irave been conducted to investigate crater formation in sand and rocky materials, especially those related to lunar geology [16]. Since many icy solid bodies have been discovered in the outer solar system. 

Figure 9.1. Surface images showing geological stnictures of icy satellites taken by Gallileo and Voyager 1. (a) Europa (scale 86 km for the window width), (b) Ganymede (86 km), (c) Callisto (43 km), (d) Mimas. Basic data for these satellites are given in Table 9.4. (Pictures from NASA.)...

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