FORMATION OF SULFATES ON PARENT BODIES OF CARBONACEOUS CHONDRITES , CERES , EUROPA , AND OTHER ICY BODIES

Introduction: Chemical compounds of S 6+ , sulfates and/or SO3, occur on Earth, Venus, Mars, parent bodies of carbonaceous chondrites, and icy surfaces of Europa and Ganymede. On the terrestrial planets, formation of S 6+ species is related to atmospheric photochemical processes that produce O2 and other O-rich oxidants. Here we explain the formation of sulfates within chondritic bodies through aqueous oxidation of sulfides by O2 and H2O2 released from irradiated water ices and through accretion of S 6+ -bearing ices formed via radiolysis of more reduced sulfur species. Chondrites: Petrographic and chemical studies of CM/CI carbonaceous chondrites indicate the occurrence of Ca, Mg, and Na sulfates and suggest their aqueous deposition on parent bodies [e.g. 1]. Although a low H2 fugacity (f) and H2 escape favor stability of sulfates, H2 gas may not separate and escape from lowfH2 solutions on parent bodies. Our models show that a gas phase may not exist at the low fH2 values needed to stabilize sulfates below ~70 o C. The presence of unaltered Fe-Ni metal in CM chondrites implies that fH2 was not low enough to stabilize sulfates. An elevated Fe 3+ /Fe 2+ ratio in less altered CM and some other chondrites [2,3] is inconsistent with progressive alteration through H2 escape. Low fH2 values needed to stabilize chondritic Fe 3+ hydroxides/oxyhydoxides (e.g. ferrihydrite) could not be achieved through H2 escape. The occurrence of sulfates in mildly altered CM and other chondrites (Tagish Lake, Miller Range 07687 [4,5]) and a coexistence of sulfates with ferrihydrite are inconsistent with a H2 escape scenario. Strong oxidants (e.g. H2O2) could be needed to explain apparent lowtemperature oxidation of aliphatic groups in the insoluble organic matter in carbonaceous chondrites [6]. Although the stability of sulfates in the S-O-H system increases with temperature, mineralogy of metamorphosed chondrites suggests reduction of earlier formed sulfates. Finally, oxidation of sulfides by liquid water (e.g. HS + 4H2O → SO4 2+ H + + 4H2) is inhibited below ~150–200 °C [7]. These inferences imply an action of strong oxidants at the early low-temperature stages of parent body alteration. Plausible formation scenario: Chondritic sulfates could have formed through rapid low-temperature (<0– 30 o C) aqueous oxidation of sulfides by strong oxidants (O2, H2O2, O3, OH ̇, HO2, etc.) [8,9], which are produced through radiolysis and photolysis of water ice before accretion. Numerous data demonstrate the formation the O-rich compounds through UV [10], electron, proton, ion, and X-ray irradiation of water ice. A majority of oxidants could have accreted with irradiated ices, consistent with the abundant (1-10%) O2 in the Jupiter family comet 67/P Churyumov-Gerasimenko [11]. In the presence of 26 Al, additional oxidants could have formed through radiolysis of water within bodies [6]. In addition to the parent body oxidation of sulfides, H2SO4•nH2O and SO3 could have formed through irradiation of SO2, H2S, and Sn in water ices [12,13] followed by accretion on parent bodies. In parent bodies, early acidic fluids formed through aqueous dissociation of H2SO4 and H + formation in reactions of strong oxidants with reduced S and Fe species. Subsequent dissolution of silicates and oxidation of kamacite led to mineral deposition in increasingly alkaline and H2-rich conditions. Although SO4 2-