J. B. Plescia

Voyager 2 in the Uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70� to -27�), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20�. Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 102 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earths moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Mirandas surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Origin of planetary wrinkle ridges based on the study of terrestrial analogs

Wrinkle ridges (linear, asymmetric topographic highs) are common physiographic features on the Moon, Mars, and Mercury. We describe here some terrestrial features of similar morphology and scale that have formed under compressional stress systems similar to planetary wrinkle ridges. All of the terrestrial analogs are produced by the anticlinal folding of rocks above reverse faults that shallow and typically break the surface. Characteristics common to both terrestrial and planetary ridges include (1) a linear, asymmetric antiformal shape; (2) tension cracks and grabens along the hinges of the antiform; (3) overlapping, en echelon lobes; (4) reversals of the sense of asymmetry along strike; (5) occurrence in compressional environments; and (6) similarity to features produced during compression of various materials in laboratory scale models. These similarities strongly suggest that planetary wrinkle ridges result from deformation associated with shallow thrust faults that either break or come close to the surface. Kinematic models capable of explaining the development of wrinkle ridges include (1) anticlinal folding of rocks over a buried thrust fault that may subsequently propagate to the surface and (2) fault-bend folding of upper-plate rocks over the surface-flattening bend of a thrust fault.

Recent flood lavas in the Elysium region of Mars

Abstract A vast area of smooth plains in southeastern Elysium, here referred to as the “Cerberus Formation,” is interpreted to be of volcanic origin. Surface morphology, lobate edges of the unit, the identification of vents (low shields), and the embayment relation of the unit with adjacent older units all suggest a volcanic origin. The plains were formed by the eruption of low-viscosity lavas that filled a topographic depression in southeast Elysium and then flowed into western Amazonis Planitia. Flow into western Amazonis was facilitated by a series of channels, carved by an earlier fluvial episode, which were exploited by lavas as they moved down slope. Crater frequencies and stratigraphic relations indicate that the unit is very young—Upper Amazonian. The presence of such young flood lavas indicates; (1) late in Martian history, large-scale eruptions of low-viscosity lava was still possible; (b) sufficient heat remained in the mantle to mobilize large quantities of magma; and (c) flood volcanism either continued or resumed in Elysium after activity of the large central volcanoes had ceased. Each of these points are important considerations in understanding the Martian thermal and volcanic history.

The geology of Dione

Dione is one of the most geologically complex of the Saturnian satellites. Crater counts and surface morphology indicates the geologic units observed are of variable age and origin. In an attempt to understand the processes which have affected Dione, a geologic map was prepared. Several geologic units were identified; ancient heavily cratered terrain, two plains units: cratered plains and lightly cratered plains, lobate deposits, crater rim deposits and bright wispy materials.

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz

The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, ∼6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are pres ent: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c (0001), ξ{11\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{2}\) \end{document}2}, and r / z {10\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{1}\) \end{document}1}. The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450–400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

Rocky 7 prototype Mars rover field geology experiments 1. Lavic Lake and sunshine volcanic field, California

Experiments with the Rocky 7 rover were performed in the Mojave Desert to better understand how to conduct rover-based, long-distance (kilometers) geological traverses on Mars. The rover was equipped with stereo imaging systems for remote sensing science and hazard avoidance and 57Fe Mossbauer and nuclear magnetic resonance spectrometers for in situ determination of mineralogy of unprepared rock and soil surfaces. Laboratory data were also obtained using the spectrometers and an X ray diffraction (XRD)/XRF instrument for unprepared samples collected from the rover sites. Simulated orbital and descent image data assembled for the test sites were found to be critical for assessing the geologic setting, formulating hypotheses to be tested with rover observations, planning traverses, locating the rover, and providing a regional context for interpretation of rover-based observations. Analyses of remote sensing and in situ observations acquired by the rover confirmed inferences made from orbital and simulated descent images that the Sunshine Volcanic Field is composed of basalt flows. Rover data confirmed the idea that Lavic Lake is a recharge playa and that an alluvial fan composed of sediments with felsic compositions has prograded onto the playa. Rover-based discoveries include the inference that the basalt flows are mantled with aeolian sediment and covered with a dense pavement of varnished basalt cobbles. Results demonstrate that the combination of rover remote sensing and in situ analytical observations will significantly increase our understanding of Mars and provide key connecting links between orbital and descent data and analyses of returned samples.

Wrinkle ridges in Lunae Planum Mars: Implications for shortening and strain

Photoclinometric data for Lunae Planum wrinkle ridges indicate average relief of {approximately}130 m and distinct elevation discontinuities across the ridge of 55 m. Modeling ridges as the result of thrust faulting and associated upper-plate folding indicates shortening across individual ridges of {approximately}131 m (90% faulting). Total shortening at 20{degree}N across Lunae Planue is {approximately}1,840 m corresponding to a regional compressive strain of 0.29%. Strain appears uniform across Lunae Planum, although it is accommodated by larger and few structures in the west than in the east.

Cratering history of Miranda: Implications for geologic processes

Abstract Mirandas surface is divisible into cratered terrain and coronae. The cratered terrain is the most heavily cratered of the terrains and presumably is the oldest. The frequency of craters in the cratered terrain is variable and related to position on the satellite. The coronae are also variably cratered. Elsinore and Arden Coronae have similar crater frequencies and may have formed simultaneously. They are of intermediate agompared to the cratered terrain and to Inverness Corona, which is the youngest major terrain. Graben formation appears to have occured both before and after the formation of the coronae reflecting periods of global expansion. Mirandas surfaces are, in general, the least cratered and therefore inferred to be the youngest within the Uranian system.

Impact melt in small lunar highland craters

Introduction: Impact-melt deposits are a typical characteristic of complex impact craters, occurring as thick pools on the crater floor, ponds on wall terraces, veneers on the walls, and flows outside and inside the rim [1]. Studies of the distribution of impact melt [2-6] suggested that such deposits are rare to absent in and around small (km to sub-km), simple imapct craters. [6] noted that the smallest lunar crater observed with impact melt was ~750 m in diameter. Similarly, theoretical models [7-10] suggest that the amount of melt formed is a tiny fraction (<1%) of the total crater volume and thus significant deposits would not be expected for small lunar craters. LRO LROC images show that impact-melt deposits can be recognized associated with many simple craters to diameters down to ~200 m. The melt forms pools on the crater floor, veneer on the crater walls or ejecta outside the crater. Such melt deposits are relatively rare, and can be recognized only in some fresh craters. These observations indicate that identifiable quantities of impact melt can be produced in small impacts and the presence of such deposits shows that the material can be aggregated into recognizable deposits. Further, the present of such melt indicates that small craters could be reliably radiometrically dated helping to constrain the recent impact flux. Data Collection: The LROC image data base [11] was searched for fresh craters and those craters were then examined for the presence of visible impact melt. The total archive of images is enormous and so random 10o x 10o latitude / longitude blocks of highland terrain were examined. Morphometric data were collected for those craters having recognizable melt. Melt Recognition: A critical aspect of such an analysis is an accurate identification of actual impact melt. While most melt-containing craters have flat floors, not all flat-floored craters have impact melt (in some cases melt may be present but buried). Characteristics of impact-melt pools on the floor include: smooth surface, low albedo, tension cracks, festoons and swirls, and anomalous small-diameter impact crater morphology. Not all of the examples exhibit all characteristics and some of the larger pools have a complex morphology. In addition to well-defined melt on their floors, the walls of some craters appear be covered with a veneer of material. This may be impact melt or it may be clastic debris moving downslope. In some cases the material has coalesced into flows that extend down the lower wall and onto the floor. This adds credence to the interpretation that the material is melt, but clastic material can also behave in this manner (e.g., North Ray Crater). Figures 1 and 2 illustrate two examples of impact melt on the floors of simple lunar craters. In both of these cases the melt apparently formed a shallow pool on the crater floor. Boulders (up to 10-15 m) are only partially covered along the shallower margins of the pools, and are either not present or completely covered in the pools’ deeper centers. In these cases, the melt is smooth and somewhat darker than the surrounding material. These crater also locally exhibit a veneer on the wall which may also be impact melt.

Graben and extension in Northern Tharsis, Mars

A morphometric analysis of north–south trending graben at 35°N in northern Tharsis is used to provide the first quantitative measurements of crustal extension in and strain for areas of the Martian crust that have undergone both extreme and mild deformation. These estimates also demonstrate the viability of the Viking data for making such quantitative assessment Graben in the region accommodate ∼8 km of post Early Amazonian east–west extension. This extension corresponds to a net regional strain of 0.45 – 0.61%; local strains vary from <1% to 5%. Extension is nonuniform and localized near 110° (south of Alba Patera and north of Ceraunius Fossae) because of reactivation of preexisting fractures in underlying basement and possibly due to superposition of stress fields from Tharsis and Alba Patera. A major mechanical discontinuity in the shallow crust, probably representing the base of or a strength contrast within the megaregolith, is observed. It occurs at a depth of 2 km on the profile flanks and is depressed to about 7.5 km near 110° due to loading of the crust by the volcano. A number of more shallow discontinuities occur within the construct and probably represent lithologic contacts.