Large Impact Features on Europa: the Galileo View
J.M. Moore, E. Asphaug, D. Morrison (NASA Ames), R.J. Sullivan, R. Greeley, K.C. Bender (ASU, Tempe), P.E. Geissler, A.S. McEwen, B.R. Tufts (LPL, Tucson), J.W. Head III, R.T. Pappalardo (Brown Univ.), M.J.S. Belton (NOAO, Tucson), R.L. Kirk (USGS, Flagstaff), Galileo SSI Team
Voyager images of the surface of Europa revealed several circular features identified to various degrees of certainty as impact features. The arrival of the Galileo Orbiter in the Jupiter system has offered an opportunity to examine several impact features on Europa at considerably better resolution and greater spectral coverage than was possible by Voyager. Utilizing these new data, the objectives of this study are to: (A) use impact features as probes into the crust of Europa, and (B) evaluate the role that varying target rheologies play in the resulting morphology of impact features. The Galileo survey has, to date, observed impact features on Europa that generally fall into the two categories first developed by Lucchitta and Soderblom. That is we observe impact features that; (A) grossly resemble "classic" impact craters (e.g., Pwyll), and (B) features which lack obvious continuous rims or central peaks, are very flat at the scale of the whole feature (though show some high frequency relief), and largely owe their identification as impact features to the field of secondaries radially splayed about them that we will refer to as "ringed maculae" (e.g., Tyre Macula). We will present the observations of "classic' impact craters first and then those of ringed maculae, going in order of increasing size and morphologic complexity. Our initial findings and conclusions are: (A) Ringed maculae are probably impact features that have morphologies that are the consequence of impacts into fluid-rich target materials; (B) The thickness of the brittle, strong bright upper layer of Europa's crust is substantially variable (at least between 1 and 3 km); (C) The rheology of the low albedo "red" layer excavated by impacts from 1 or more kilometers depth may sometimes be strong and brittle if the large massif on Pwyll's floor is a "classic" central peak (Material with the same spectral properties as the dark "red" layer is commonly seen elsewhere on Europa in association with endogenic landforms and fluid-rich deposits.); and (D) "Pedestal" ejecta facies may be produced by the down-slope movement of plastically deforming ice that is warm at the time of emplacement.