TY - JOUR
T1 - Large Impact Features on Europa
T2 - Results of the Galileo Nominal Mission
AU - Moore, Jeffrey M.
AU - Asphaug, Erik
AU - Sullivan, Robert J.
AU - Klemaszewski, James E.
AU - Bender, Kelly C.
AU - Greeley, Ronald
AU - Geissler, Paul E.
AU - McEwen, Alfred S.
AU - Turtle, Elizabeth P.
AU - Phillips, Cynthia B.
AU - Tufts, B. Randy
AU - Head, James W.
AU - Pappalardo, Robert T.
AU - Jones, Kevin B.
AU - Chapman, Clark R.
AU - Belton, Michael J.S.
AU - Kirk, Randolph L.
AU - Morrison, David
N1 - Funding Information:
We thank W. B. Banerdt, K. Zahnle, and especially M. J. Cintala for their careful reviews of this manuscript. This investigation was funded by NASA’s Galileo Project.
PY - 1998/9
Y1 - 1998/9
N2 - The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) "classic" impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all <30 km diameter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Callisto, probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on Europa. Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement. Callanish and Tyre do not appear to be larger and even more viscously relaxed versions of the classic craters; rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ~10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that ~10-15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<108years). Hence, it seems likely that our preliminary conclusions about the subsurface structure of Europa apply to the current day.
AB - The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) "classic" impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all <30 km diameter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Callisto, probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on Europa. Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement. Callanish and Tyre do not appear to be larger and even more viscously relaxed versions of the classic craters; rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ~10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that ~10-15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<108years). Hence, it seems likely that our preliminary conclusions about the subsurface structure of Europa apply to the current day.
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U2 - 10.1006/icar.1998.5973
DO - 10.1006/icar.1998.5973
M3 - Article
AN - SCOPUS:0000211515
SN - 0019-1035
VL - 135
SP - 127
EP - 145
JO - Icarus
JF - Icarus
IS - 1
ER -