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Ice in the Solar System Solar and Stellar Physics Through Eclipses ASP Conference Series, Vol. 370, 2007 O. Demircan, S. O. Selam & B. Albayrak Ice in the Solar System Julyan H. E. Cartwright Laboratorio de Estudios Cristalogr´aficos, CSIC, E-18100 Armilla, Granada, Spain. Abstract. Ice is ubiquitous in the solar system; it is also a substance with a fascinatingly complex set of transitions between its multiple forms. Here I review what is known of the various crystalline and amorphous phases of ice, and of their presence on different solar system bodies. 1. Introduction Water exists all around us, as liquid, vapour, and solid; it is the only substance we commonly encounter so close to its triple point. Despite its ubiquity and importance, there remains much to be discovered about this simple molecule. Its peculiar nature is manifest to anyone who has pondered why, unlike the great majority of substances, the solid floats on the liquid. The anomalous properties of water are intimately related with why it has been vital for the emergence of life on Earth, and to why it is often considered that water would be essential to the origin of life anywhere in the universe. Although there are myriad forms of the solid phase of H2O — ice — found on the Earth’s surface, with very different morphologies and bulk properties, at the microscopic level they are all, from snow flakes to icebergs, composed of the same crystalline polymorph, hexagonal ice — ice Ih — which is the stable version at the temperatures and pressures in our biosphere. However, outside this range, ice crystallizes as many other polymorphs (Petrenko and Whitworth 1999). It also appears to have more than one amorphous state; at least two apparently distinct amorphous ices have been described (Mishima and Stanley 1998; Debenedetti & Stanley 2003; Debenedetti 2003). Amorphous ice is of great interest in astronomy (Ehrenfreund et al. 2003), as at the low temperatures of interstellar space, water adsorbing onto dust grains solidifies in an amorphous state (Jenniskens et al. 1995). This ice is built up by ballistic deposition; molecules stick directly where they land, without surface diffusion, owing to the very low temperature (3–90 K) of the interstellar medium. It has been estimated that most of the ice in the universe is to be found on these interstellar dust grains, which implies that the majority of the ice in existence is in this amorphous form. It may even be involved in the origin of life, as amino acids have been created on ice under the conditions of the interstellar medium, suggesting that complex organic molecules, the precursors of life, or life itself, could have arrived here on Earth from space, via comets or meteorites. 265 266 Cartwright 10 12 XI X VII VIII 10 9 VI IX II V III Liquid 10 6 Ih Ic Pressure (Pa) XI Vapour 3 10 Solid 1 0 100 200 300 400 500 600 700 800 Temperature (K) Figure 1. The phase diagram of water. 2. Polymorphs and Polyamorphs of Water Water has a complex phase diagram (Fig. 1); it exhibits an extensive range of crystalline solid phases, or polymorphs, distinguished from each other by the arrangement of water molecules in the crystal lattice. The physical properties of the polymorph, such as density, conductivity, vapour pressure and sublimation rate, are dictated by its crystalline structure. Thirteen ice polymorphs, named ices I to XIII, are known at present (the phases are numbered with Roman numerals in the order they were discovered). Most are thermodynamically stable under some range of pressure–temperature conditions, with some phases also exhibiting metastable zones, and a few having no regions of absolute stability at all. Most of the crystalline phases are formed only under high pressure, as we may note in Fig. 1. If we consider now only those phases stable at normal atmospheric pressure and below, we find that hexagonal ice, Ih, that encountered on Earth, is the thermodynamically stable phase down to approximately 72 K. Below this temperature, ice XI is the thermodynamically stable phase, but in pure ice Ih the molecular relaxation rate is too slow for the transformation process to be observed, and ice Ih continues to be metastable. Ice Ic should also be mentioned here; there is the same relationship between ices Ic and Ih as between cubic and hexagonal close packing structures in metals. It is always metastable to ice Ih. In vapour deposition experiments, ice Ih is formed above ∼150 K, while ice Ic appears between ∼130–150 K. Below ∼130 K, the deposit produced is of amorphous ice. Ice in the Solar System 267 Ice also appears to possess more than one amorphous1 phase, a phenomenon termed amorphous polymorphism, or polyamorphism (Poole et al. 1995). Two phases, low density amorphous (LDA) and high density amorphous (HDA) ices, are seen as glassy versions of two proposed liquid phases, low density liquid (LDL) and high density liquid (HDL), which, in turn, are associated with a putative liquid–liquid phase transition in water (Stanley et al. 1998). This hy- pothesized liquid phase and the associated critical point have not been seen experimentally. The reasons can be seen in the phase diagram of Fig. 1, where the theoretical phase transition curve is marked. It is within the region in which ice Ih is the thermodynamically stable phase, so to obtain this liquid a sample would have to be supercooled with sufficient rapidity to avoid crystal nucle- ation. Furthermore, there would then arise the additional difficulty that the system passes into a glassy state; below the glass transition temperature there is no longer a liquid but an amorphous solid. Amorphous ice was first found in vapour deposition experiments. In 1935 Burton & Oliver announced that below about 163 K, their ice condensate was not crystalline, but rather amorphous (their proposed transition temperature is rather high, and most investigators nowadays take 130 K as more typical). This ice was later termed amorphous solid water (ASW). Much later, it was shown that ASW differs when deposited at lower and higher temperatures. At low temperatures (∼<30 K), a higher density amorphous ice is formed, while at higher temperatures (∼30–130 K) a lower density amorphous ice is deposited (Jenniskens & Blake 1994; Jenniskens et al. 1995). These ices are similar to HDA and LDA produced in solid and liquid state transformations. While some see essential differences, others propose that any variations arise from differing annealing times and microporosities (Jenniskens et al. 1995). There are then two schools of thought as to whether or not all these amor- phous ices can be catalogued as either LDA or HDA. The issue hinges on whether the LDA–HDA transformation is a first-order phase transition or not. The ev- idence in favour appears to be strong (Mishima and Suzuki 2002). But on the other hand, it has been reported that a series of intermediates states can be observed in the transformation (Tulk et al. 2002), implying that LDA and HDA are not polyamorphs, but the extremes of a continuum of states. However, those in the pro-phase-transition camp refute this, pointing out that for a reversible transition between LDA and HDA, a temperature of at least 130 K is required, and the experiments concerned were carried out at lower temperatures than this, so that “these results neither suggest nor require a reinterpretation of the HDA–LDA transformation” (Debenedetti 2003). Recently a distinct form of amorphous ice was reported (Loerting et al. 2001) on isobaric heating of HDA. This ice, being even more dense than that phase, was termed very high density amorphous ice (VHDA). It could be made to transform to HDA on isochoric heating, and to LDA on isobaric heating. It is not yet clear whether VHDA is yet another amorphous phase of ice, or just a kinetically trapped structure. 1By this is meant a solid with no crystalline structure at all, which is a glass, or vitrified liquid; there is much confusion in the literature induced by the use of this term when a material is in fact microcrystalline or nanocrystalline. 268 Cartwright 3. Ice in the Solar System Where are these ices to be found in nature? Let us embark on a journey in our imagination outwards from the Sun. First we arrive at Mercury. There are indications from radar imaging that there may be ice in the permanent shadow within craters near the poles (Slade et al. 1992); this would probably be in a low density amorphous form. Venus does not appear to be capable of harbouring ice. Earth, on the other hand, has ice at its poles, on mountain peaks, and in its atmosphere, all in the form of ice Ih (not even at the base of the thickest ice layers on Earth is the pressure sufficient to produce a high-pressure phase). In the stratosphere there may also be ice Ic (Murray et al. 2005). On Earth, ice plays a very important rˆole in shaping our climate, both via the polar ice caps, and through icy particles in the stratosphere, which are implicated in ozone depletion chemistry (Murray et al. 2005). The presence of ice on the Moon is expected, albeit not yet confirmed, in the same environment as on Mercury: in permanent shadows in craters near the poles (Vasavada et al. 1999). On Mars, there is ice Ih above and below ground in the polar regions (Titus et al. 2003). And in the asteroid belt, between Mars and Jupiter, there are found icy asteroids, the so-called main-belt comets (Hsieh & Jewitt 2006). Beyond what is termed the snow line, near the orbit of Jupiter, ice becomes more and more prevalent (Jewitt et al.
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