ARTICLE IN PRESS S0019-1035(05)00134-X/FLA AID:7648 Vol.•••(•••) [DTD5] P.1 (1-11) YICAR:m5 v 1.39 Prn:2/06/2005; 15:56 yicar7648 by:JOL p. 1 Icarus ••• (••••) •••–••• www.elsevier.com/locate/icarus Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate R.W. Carlson a,∗ , M.S. Anderson a , R. Mehlman b , R.E. Johnson c a JetPropulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA b Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA c Engineering Physics, University of Virginia, Charlottesville, VA, USA Received 18 August 2004; revised 14 February 2005 Abstract Sulfuric acid hydrate has been proposed as an important species on Europa’s surface, the acid being produced by radiolysis of surficial sulfur compounds. We investigated the spectral properties of disordered and crystalline forms of sulfuric acid and suggest that the hydration properties of Europa’s hypothesized sulfuric acid lie between two end members: liquid sulfuric acid and its higher crystalline hydrates. The spectra of these end members are similar except for spectral shifts at the band edges. We measured the optical constants of sulfuric acid octahydrate and used these with simple radiative transfer calculations to fit Europa spectra obtained by Galileo’s Near Infrared Mapping Spectrometer (NIMS). The global distribution of the hydrate that we associate here with hydrated sulfuric acid shows a strong trailing- side enhancement with a maximum fractional hydrate abundance of 90% by volume, corresponding to a sulfur atom to water molecule ratio of 10%. The hydrate concentration spatially correlates with the ultraviolet and visible absorption of the surface and with the sulfur dioxide concentration. The asymmetric global distribution is consistent with Iogenic plasma ion implantation as the source of the sulfur, possibly modified by electron irradiation and sputtering effects. The variegated distribution also correlates with geologic forms. A high spatial resolution image shows resolved lineae with less hydrate appearing within the lineae than in nearby crustal material. The low concentration of hydrated material in these lineae argues against their conveying sulfurous material to the surface from the putative ocean.  2005 Elsevier Inc. All rights reserved. Keywords: Surfaces, satellites; Europa; Radiation chemistry; Spectroscopy; Radiative transfer 1. Introduction Europa’s surface is continuously bombarded by high- energy electrons and ions from Jupiter’s energetic magne- tosphere, and this flux of ionizing radiation profoundly af- fects the surface chemical composition. The energy flux of 7.8 × 10 13 eV s −1 cm −2 absorbed in the stopping depth D = 0.62 mm (Cooper et al., 2001) produces a yearly dose of 640 Mrad. For comparison, the lethal dose for humans is ∼400 rad (whole body exposure) and is ∼15 Mrad for the radiation resistant bacteria Deinococcus Radiodurans (Dose et al., 1996; Duggan et al., 1963; Richmond et al., 1999; * Corresponding author. E-mail address: rcarlson@lively.jpl.nasa.gov (R.W. Carlson). Silverman, 1991). Molecular destruction lifetimes in this radiolysis layer (of depth D) varies from a few years to 4000 years (Carlson et al., 2002). Since the optically sensed layer is comparable in depth to the radiolysis layer and ra- diolytic lifetimes are short compared to geological times, molecules on Europa’s surface that are observed by remote optical sensing have recently suffered radiolysis. The chem- ical composition of the observed surface is, therefore, in radiolytic chemical equilibrium and this composition can be markedly different from the primitive unexposed mixture (Johnson et al., 2004). Three radiolytic species identified on Europa are hydro- gen peroxide (Carlson et al., 1999a), sulfur dioxide (Lane et al., 1981; Noll et al., 1995) and molecular oxygen (Hall et al., 1995, 1998; Spencer and Calvin, 2002; Spencer and 0019-1035/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.03.026