HR: 11:25h
AN: P11C-10 INVITED
TI: Geology of the Icy Galilean Satellites: A Framework for Compositional Studies
AU: * Pappalardo, R T
EM: pappalardo@brown.edu
AF: Brown University, Box 1846, Providence, RI 02912
United States
AB: Galileo spacecraft imaging of the icy Galilean satellites has provided tremendous insight into their
geological histories and evolution. The geological processes of cryomagmatism, tectonism, cratering, and mass wasting are
each significant, with their relative significance varying among the satellites. These processes and their surface
manifestations have important implications for compositional studies of the icy Galilean satellites.
Much of Callisto's surface is blanketed by smooth dark material, which may be a sublimation-derived lag deposit
overlying an icy substrate, redistributed by impacts and mass wasting. Loss of CO2 "bedice" offers a viable mechanism for
creation of a Callistoan surface lag. High-standing knobs and crater rims are bright, suggesting thermal segregation of the
surface into ice-rich and ice-poor regions. Aside from features associated with its large impact structures, the satellite
apparently had an uneventful tectonic and volcanic history.
Ganymede's ancient dark terrain is extremely heterogeneous in albedo at small scales and has been modified by tectonism,
ejecta blanketing, mass wasting, along with concentration of a thin dark surficial lag. The satellite's bright grooved
terrain is pervasively deformed at multiple scales and is locally highly strained, consistent with extensional faulting of
its ice-rich lithosphere. The role of icy cryovolcansim in grooved terrain remains uncertain, as normal faulting alone
apparently has resurfaced some groove lanes. Charged particles which impinge at high latitudes may redistribute water ice to
produce the satellite's bright polar caps.
Sparse craters suggest that Europa's icy surface is young and potentially geologically active today. Its bright plains
are criss-crossed by enigmatic ridges which probably owe their origin to some combination of intrusion and extrusion of
liquid water and/or warm ice. "Triple bands" are ridges with ruddy margins formed through thermal alteration, cryovolcanism,
and/or mass wasting. Wider pull-apart bands have formed by complete separation and spreading of the icy lithosphere, in a
manner which may be broadly analogous to terrestrial sea-floor spreading. Cross-cutting relationships indicate that Europa's
dark bands brighten and fade with age. Relatively dark and red "mottled terrain" consists of pits, domes, dark spots,
patches of smooth plains-forming material, and regions of chaotic terrain. These features suggest surface disruption along
with localized partial melting; this is consistent with solid-state convection of the icy asthenosphere, with warm ice
diapirs triggering partial melting of a salt-rich ice shell. Thermal models predict that tidal heating might maintain a
DE: 6218 Jovian satellites
SC: P
MN: 1999 AGU Fall Meeting