BRINE VOLCANISM AND THE INTERIOR STRUCTURES OF ASTEROIDS AND ICY SATELLITES

Cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites. Volcanic brine petrogenesis in ammonia-deficient satellites should mainly involve eutectic melting in relevant salt-water systems. Carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids. C1 chondrites contain about one-fifth by mass of salts in various states of hydration. Many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system H2OMgSO4Na2SO4. Minor components include sulfates of K, Ni, Mn, and Ca. Chondrites also contain abundant carbonates, but these are probably not very important in brine magmatism due to their low solubilities under expected conditions. Chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of Cl/S. Soluble salts may have profound effects on the geology and structure of icy satellites and asteroids. In some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate-rich mantle and crustal layers. In other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities. Europa probably has a layered crust composed of anhydrous MgNa sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated Mg and Na sulfates near the surface. Ganymede may have a crust about 300 km thick composed of a 10:1 ratio of ice: mirabilite, and a mantle 500 km thick composed of 50% ice phases plus 50% hydrated Mg and Na sulfates. © 1991.