ALBERT

Description: Observed geology, photometry, and geophysical data are used to examine various processes and properties that may have contributed to Maranda's evolution. Global tectonics and surface flow features constrain the possible heating mechanisms and materials. Statistics on impact craters and comparisons with other satellites suggest that the impactor-source population evolved through time and that ejecta mantling has resurfaced significant portions of the surface. It is proposed that the coronae, which are unique to Miranda, were formed by relaxation of topographic highs, by lithospheric stress driven by intensity anomalies in the asthenosphere, or by diapirs either breeching the surface or feeding large-scale volcanic flooding through preexisting crack structure.

Description: A geological analysis of six of the Uranus satellites observed in detail by Voyager 2 is presented. All of the satellites except the smallest, Puck, show evidence of cryovolcanic resurfacing: global on the largest four satellites, local in the spectacular coronae on Miranda. The cryovolcanic materials exhibit a range of albedos and morphologies, which are interpreted to reflect a variety of compositions and conditions of eruption at least as complex as those which occur on earth. Eruptions are predominantly large fissure flows that produce extensive flood deposits. Possible evidence of small circular vents and cryoclastic volcanic activity is seen on Miranda and Ariel. All of the satellites except Puck also have extensive sets of grabens and riftlike canyons that show remarkable similarity of pattern: intersection sets trending roughly NW-SW and NE-SW in the low latitudes grading into E-W trends near the poles. As a group, the Uranian satellites are somewhat more active geologically than similarly sized Saturnian satellites.

Description: The concept of the Maxwell time of a viscoelastic material (4.5) is used in conjunction with calculated thermal profiles to evaluate the significance of tectonic estimates of lithospheric thickness. Thermal lithospheric thicknesses provide fundamental constraints on planetary thermal histories that complement the constraints provided by dateable surface deposits of endogenic origin. Lithospheric constraints are of particular value on the icy satellites where our understanding of both rheology and surface ages is considerably poorer than it is for the terrestrial planets. Certain extensional tectonic features can and have been used to estimate lithospheric thicknesses on Ganymede and Callisto. These estimates, however, refer to the depth of the elastic lithosphere defined by the zone of brittle failure. The relation between the elastic lithosphere and the thermal lithosphere (generally defined by the zone of conductive heat transport) is not straightforward, because the depth of brittle failure depends not only on the thermal profile, but also on rheology and strain rate (or the characteristic time over which stresses build towards failure). Characteristic time considerations are not trivial in this context because stresses generating brittle failure on the icy satellites may be produced by impacts, with characteristic times of seconds to days, or by geologic processes with time scales of hundreds of millions of years.

Description: The unusual morphology of the Valhalla multiple or ripple-ring basin in Callisto was totally unexpected in light of the morphologies of large impact structures on the terrestrial planets. Two other ripple-ring basins (RRB's), Asgard and a smaller structure near the crater Adlinda are also described. Several additional RRB's were found on Callisto, an example of which is shown. A previously unrecognized RRB on Ganymede was also found. An image and geologic sketch map of this RRB are shown. Morphometric and positional data for all known RRB's are given.

Description: Ganymede and Callisto, the two giant icy satellites of Jupiter, have very nearly the same size, composition, and location in the solar system, yet their surfaces are profoundly different. A new scenario of their geologic histories indicates that the differences may be only skin deep.

Description: Recent impact crater counts on the Voyager 2 high resolution images of Triton have resulted in a more accurate crater size/frequency distribution down to about 3 km diameter. These counts reveal a size/frequency distribution characterized by a differential -4 slope. This is consistent with the observation that there are no craters larger than 27 km diameter on the 20 percent of Triton viewed at resolutions capable of detecting them. A -4 slope is deficient in large craters and at the very low crater density on Triton no craters larger than about 30 km are expected on just 20 percent of the satellite. The Triton size distribution is significantly different from the differential -3 slope of the fresh crater population on Miranda, but both show leading/trailing asymmetries. Since Miranda is in prograde orbit this crater population is probably due to objects in heliocentric orbit, i.e., comets. If this crater population is due to comets, then the significantly different crater population on Triton is probably due to some other population of impacting objects. The most likely origin of these objects is planetesimals in planetocentric orbits. Because Triton is in retrograde orbit, objects in prograde planetocentric orbits will also produce a leading/trailing asymmetry. If the Triton craters are largely the result of objects in planetocentric orbit, then where are the comet craters that should be there if they have a differential -3 distribution function as inferred from the Miranda fresh crater population? The most likely answer is that they are there, but at such a low density that they can not be distinguished from the planetocentric population. An upper bound on this density can be estimated by determining the density of a crater population with a differential -3 slope where no craters larger than 27 km would be expected on the 20 percent of Triton viewed by Voyager at resolutions sufficient to detect them. This density is at the density of the largest crater. At this density the number of craters in size bins greater than 27 km is less than 1 for a -3 distribution function. The observed size distribution, the upper limit of the hypothetical comet crater size distribution, and the difference between the observed and the hypothetical comet crater populations is shown.

Description: The morphology and morphometry of terrace provide information on the mechanical conditions of failure and enlargement of impact craters. A simple, perfect plasticity model for terrace formation was proposed, but the preliminary terrace width measurements used to support the model were insufficient to provide a rigorous test. Lunar crater terraces were studied morphometrically. Roughly 1,000 terraces in 53 fresh lunar craters were measured using Apollo and Lunar Orbiter vertical stereo photography. Terrace widths and their order inward from the rim were measured along eight equally spaced radials in each crater. A list of the craters including rim diameters and geologic data is presented.

Description: Families of lineaments in the Gilomesh and Ninki basins of Ganymede imply post impact tectonic activity. The grooves, previously considered the youngest tectonic features, are estimated to have formed between 3.8 and 3.1 Gyr ago. One rayed crater however, is probably less than 1 Gyr old, implying tectonic activity on Ganymede has extended nearly to the present. Like the grooves, the lineaments appear to be extensional. The parallel trends and nearly contiguous associations of the lineaments with the grooves imply that both are products of the same stress systems. The young inferred age of the lineaments implies that they (and presumably also the grooves) are not associated with stresses in the cooling of fresh deposits of bright terrain, but are probably associated with underlying convective stress patterns, the long implied duration of an extensional stress regime in Ganymede's lithosphere is consistent with the stress models of derived assuming a differentiated interior.

Description: The hypothesis that palimpsests and anomalous pit craters are essentially pristine crater forms derived from high-velocity impacts and/or impacts into an ice crust with preimpact temperatures near melting is explored. The observational data are briefly reviewed, and an impact model is proposed for the direct formation of a palimpsest from an impact when the modification flow which produces the final crater is dominated by 'wet' fluid flow, as opposed to the 'dry' granular flow which produces normal craters. Conditions of 'wet' modification occur when the volume of impact melt remaining in the transient crater attains a volume comparable to the transient crater. The normal crater-palimpsest transition is found to occur for sufficiently large impacts or sufficiently fast impactors. The range of crater diameters and morphological characteristics inferred from the impact model is consistent with the observed characteristics of palimpsests and anomalous pit craters.

Description: The paper reports on a series of low-velocity impact experiments performed in ice and ice-saturated sand. It is found that crater diameters in ice-saturated sand were about 2 times larger than in the same energy and velocity range in competent blocks of granite, basalt and cement, while craters in ice were 3 times larger. It is shown that if this dependence of crater size on strength persists to large hypervelocity impact craters, then surface of geologic units composed of ice or ice-saturated soil would have greater crater count ages than rocky surfaces with identical influx histories. Among the conclusions are that Martian impact crater energy versus diameter scaling may also be a function of latitude.