AMMONIA WATER VOLCANISM ON ICY SATELLITES - PHASE-RELATIONS AT 1-ATMOSPHERE

Icy satellites exhibit an amazing variety of terrains, many of which were formed by extrusions of aqueous solutions. Aqueous ammonia has been implicated on the basis of theoretical and morphological evidence as a probably cryovolcanic agent on some resurfaced icy satellites. Considering the solvent and caustic properties of ammonia-water (pH up to 13) with respect to chondritic rock, and that many other ammonia-water-soluble substances are probably present in satellite ices, it is unlikely that ammonia-water magmas would be pure. Comets and carbonaceous chondrites represent two possible components of icy satellites. Several comets contain ∼ 1 to several percent each of methanol, formaldehyde, and carbon dioxide relative to water, and C1 and C2 carbonaceous chondrites contain up to 10% magnesium sulfate. Carbon dioxide, magnesium sulfate, and formaldehyde sequester ammonia as ammonium carbonate, ammonium sulfate, and the organic salt hexamethylenetetramine (HMT). If these reactions leave excess ammonia, the resulting ices may include water ice, ammonia dihydrate, and methanol monoammoniate in addition to the reaction salts. This mixture melts near 153 K yielding an extremely viscous solution of water, ammonia, methanol, and small amounts of salts. This liquid may resemble the viscous lavas extruded on Triton, Ariel, and Miranda. However, if these reactions sequester all available ammonia, or if ammonia was not initially present in an icy satellite, then melting produces comparatively low-viscosity, ammonia-deficient saline or methanol solutions. Formation of ammonia-rich liquids is possible only if the molar abundance of ammonia NNH3 > (2NCO2 + 2NMgSO4 + sol2 3NH2CO). For instance, direct melting of pure Comet Halley, even though it may contain some ammonia, probably would not yield an ammonia-water liquid. Sulfur-and carbon-bearing constituents in ammonia-water lavas would alter to spectrally important chromophores on irradiated satellite surfaces, possibly explaining geologically correlated variations in the colors and albedos of icy satellites. Many cryovolcanic surfaces have nearly uniform spectrophotometric properties and may be explained as compositionally invariant eutectic or peritectic mixtures. Other cryovolcanic terrains have stark color and albedo contrasts and may be explained as compositionally distinct melt products. Small but important quantities of dissolved potassium-and rubidium-bearing salts are probably sufficient for isotopic dating. © 1992.