spaces between the water molecules,4 with gas/ice ratios as high as 3 to 3.5 (by number) at 30 K. As the temperature of condensation increases above 30 K the amount of gas that can be trapped drops quickly by 6 orders of magnitude at the temperature of 100 K, and different species are preferentially trapped, resulting in fractionation. When the ices are warmed, the ice structure reorganizes and the spaces between the molecules get smaller (annealing), and volatiles can be released. In addition, the amorphous ice will undergo an exothermic phase transition to its crystalline form, peaking near 137 K, also resulting in the release of trapped volatiles. Once the phase transition has occurred, the ice will not revert to the amorphous form if the temperature is lowered, unless there is some process (such as cosmic-ray irradiation, or vaporization and recondensation at low temperature) that destroys the structure in the ice.
The compositions of the primitive, low-albedo bodies are only broadly understood, whereas researchers have more information for the gaseous comas (the cloud of gas and dust that extends from the nucleus) of comet nuclei. Comets have been measured photometrically in the optical and spectroscopically in the near-infrared, and volatiles escaping from comet nuclei have been observed at radio wavelengths. In addition, several space missions have made in situ measurements. The materials expected on the surfaces of these bodies include minerals, and with increasing distance from the Sun water-ice, other volatiles, and solid organic materials.
While optical spectroscopy has been used to identify dissociation fragments (daughters) in cometary comas for nearly 100 years, it is often difficult to deduce their parent species. The inherent difficulty is that the volatiles sublime from the nucleus and shortly after entering the coma can experience photolysis and photodissociation. Many species can be seen as both a nuclear (parent) and a dissociation product. Models are then used to infer from the observed spectroscopic band strengths the abundance of various parent or original species coming from the nucleus. One of the surprising things learned from spectroscopic observations of recent bright comets, such as C/1995 O1 (Hale Bopp), is that it is very likely that chemical reactions may have been occurring in the inner coma, and thus these types of observations may not reflect the primordial composition in the cometary interior and may not be entirely useful indicators of the preservation of interstellar material. With recently observed bright comets such as C/1995 O1 (Hale Bopp), C/1996 B2 (Hyakutake), and C/1999 H1 (Lee), it was possible to make direct observations of the parent molecules in the infrared and radio wavelengths.
Spectroscopy of the solid bodies is more difficult owing to their faintness, making it very expensive (i.e., requiring considerable time) in terms of telescope time. In the optical wavelength region the most prominent spectral features come from mineral assemblages, whereas the signatures of organic material lie in the near-infrared region between 1 and 5 µm. Water ice also has strong solid-state absorption bands in this region, as do other volatiles of interest. Good near-infrared spectrometers have been installed only recently on large telescopes to facilitate the search for organic signatures in this wavelength region. The identification of organic species in the spectra requires the use of spectral models that apply Hapke scattering theory using the complex refractive indices of minerals, volatiles, and organic materials in combination to match the features in the spectrum. One of the problems is that the models are very complex and non-unique. In addition, optical constants (complex refractive indices), which must be measured in the laboratory, are known for only a relatively small number of the compounds that are likely to be present.
Organic molecules can be both synthesized and destroyed on outer solar system solids by irradiation. The surface materials on small bodies or grains are typically exposed to charged-particle and ultraviolet radiation, producing radiolysis and photolysis. There is abundant laboratory evidence that carbon-containing frozen mixtures will form complex organics when exposed to radiation and that complex organics break down under irradiation. Under the influence of radiation, these surfaces will grow progressively redder and darker as hydrogen is lost.5-7 However, there is as yet no definitive observational evidence that radiation processing plays an important role in forming organics on solar system surfaces (note that there is abundant evidence now for radiation processing).