Robert John Sullivan

Geological evidence for solid-state convection in Europa's ice shell

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today. Moreover, models of the satellites interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean,. Here we report on the morphology and geological interpretation of distinct surface features—pits, domes and spots—discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

Galileo's First Images of Jupiter and the Galilean Satellites

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiters Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on Io. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

The Imager for Mars Pathfinder experiment

The imager for Mars Pathfinder (IMP), a stereoscopic, multispectral camera, is described in terms of its capabilities for studying the Martian environment. The cameras two eyes, separated by 15.0 cm, provide the camera with range-finding ability. Each eye illuminates half of a single CCD detector with a field of view of 14.4×14.0° and has 12 selectable filters. The ƒ/18 optics have a large depth of field, and no focussing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 512×512 CCD. The resolving power of the camera, 0.98 mrad/pixel, is approximately the same as the Viking Lander cameras; however, the signal-to-noise ratio for IMP greatly exceeds Viking, approaching 350. This feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes IMP from Viking. Specially designed targets are positioned on the Lander; they provide information on the magnetic properties of wind-blown dust, measure the wind vectors, and provide radiometric standard reflectors for calibration. Also, eight low-transmission filters are included for imaging the Sun directly at multiple wavelengths, giving IMP the ability to measure dust opacity and potentially the water vapor content. Several experiments beyond the requisite color panorama are described in detail: contour mapping of the local terrain, multispectral imaging of the surrounding rock and soil to study local mineralogy, viewing of three wind socks, measuring atmospheric opacity and water vapor content, and estimating the magnetic properties of wind-blown dust. This paper is intended to serve as a guide to understanding the scientific integrity of the IMP data that will be returned from Mars starting on July 4, 1997.

Mass movement slope streaks imaged by the Mars Orbiter Camera

Narrow, fan-shaped dark streaks on steep Martian slopes were originally observed in Viking Orbiter images, but a definitive explanation was not possible because of resolution limitations. Pictures acquired by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft show innumerable examples of dark slope streaks distributed widely, but not uniformly, across the brighter equatorial regions, as well as individual details of these features that were not visible in Viking Orbiter data. Dark slope streaks (as well as much rarer bright slope streaks) represent one of the most widespread and easily recognized styles of mass movement currently affecting the Martian surface. New dark streaks have formed since Viking and even during the MGS mission, confirming earlier suppositions that higher contrast dark streaks are younger, and fade (brighten) with time. The darkest slope streaks represent ∼10% contrast with surrounding slope materials. No small outcrops supplying dark material (or bright material, for bright streaks) have been found at streak apexes. Digitate downslope ends indicate slope streak formation involves a ground-hugging flow subject to deflection by minor topographic obstacles. The model we favor explains most dark slope streaks as scars from dust avalanches following oversteepening of air fall deposits. This process is analogous to terrestrial avalanches of oversteepened dry, loose snow which produce shallow avalanche scars with similar morphologies. Low angles of internal friction typically 10–30i for terrestrial loess and clay materials suggest that mass movement of (low-cohesion) Martian dusty air fall is possible on a wide range of gradients. Martian gravity, presumed low density of the air fall deposits, and thin (unresolved by MOC) failed layer depths imply extremely low cohesive strength at time of failure, consistent with expectations for an air fall deposit of dust particles. As speed increases during a dust avalanche, a growing fraction of the avalanching dust particles acquires sufficient kinetic energy to be lost to the atmosphere in suspension, limiting the momentum of the descending avalanche front. The equilibrium speed, where rate of mass lost to the atmosphere is balanced by mass continually entrained as the avalanche front descends, decreases with decreasing gradient. This mechanism explains observations from MOC images indicating slope streaks formed with little reserve kinetic energy for run-outs on to valley floors and explains why large distal deposits of displaced material are not found at downslope streak ends. The mass movement process of dark (and bright) slope streak formation through dust avalanches involves renewable sources of dust only, leaving underlying slope materials unaffected. Areas where dark and bright slope streaks currently form and fade in cycles are closely correlated with low thermal inertia and probably represent regions where dust currently is accumulating, not just residing.

Evidence for non-synchronous rotation of Europa

Non-synchronous rotation of Europa was predicted on theoretical grounds, by considering the orbitally averaged torque exerted by Jupiter on the satellites tidal bulges. If Europas orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e ≈ 0.01 (ref. 2), generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data suggest that there may be a permanent asymmetry in Europas interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice. Non-synchronous rotation was invoked to explain Europas global system of lineaments and an equatorial region of rifting seen in Voyager images,. Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.

Episodic plate separation and fracture infill on the surface of Europa

Images obtained by the Voyager spacecraft revealed dark, wedge-shaped bands on Europa that were interpreted as evidence that surface plates, 50–100u2009km across, moved and rotated relative to each other. This implied that they may be mechanically decoupled from the interior by a layer of warm ice or liquid water,. Here we report similar features seen in higher resolution images (420 metres per pixel) obtained by the Galileo spacecraft that reveal new details of wedge-band formation. In particular, the interior of one dark band shows bilateral symmetry of parallel lineaments and pit complexes which indicates that plate separation occurred in discrete episodes from a central axis. The images also show that this style of tectonic activity involved plates < 10u2009km across. Although this tectonic style superficially resembles aspects of similar activity on Earth, such as sea-floor spreading and the formation of ice leads in polar seas, there are significant differences in the underlying physical mechanisms: thewedge-shaped bands on Europa most probably formed when lower material (ice or water) rose to fill the fractures that widened in response to regional surface stresses.

Aeolian features and processes at the Mars Pathfinder landing site

The Mars Pathfinder landing site contains abundant features attributed to aeolian, or wind, processes. These include wind tails, drift deposits, duneforms of various types, ripplelike features, and ventifacts (the first clearly seen on Mars). Many of these features are consistant with formation involving sand-size particles. Although some features, such as dunes, could develop from saltating sand-size aggregates of finer grains, the discovery of ventifact flutes cut in rocks strongly suggests that at least some of the grains are crystalline, rather than aggregates. Excluding the ventifacts, the orientations of the wind-related features correlate well with the orientations of bright wind steaks seen on Viking Orbiter images in the general area. They also correlate with wind direction predictions from the NASA-Ames General Circulation Model (GCM) which show that the strongest winds in the area occur in the northern hemisphere winter and are directed toward 209°.

Europa: Morphological characteristics of ridges and triple bands from Galileo data (E4 and E6) and assessment of a linear diapirism model

Galileo solid-state imaging (SSI) images of Europa provide high-resolution data on the morphological characteristics of ridges and permit the development of nomenclatural schemes for their description and classification. Key observations are that ridges (1) are remarkably consistent in their along-strike linearity, width, and height, (2) form long linear features in which preexisting structures can sometimes be traced up the outer slopes of the ridges and in other cases appear to be buried, (3) contain narrow apical zones of small-scale, ridge-parallel faulting, (4) are sometimes flanked by narrow troughs and ridge-parallel fractures, and (5) often display associated color variations. On the basis of the characteristics and associated features of ridges, we find that a process in which initial fracturing (most plausibly related to tidal deformation) of a brittle layer overlying a buoyant ductile substrate leading to linear diapiric upwelling provides a consistent explanation for the observed features. In this process the upwelling linear diapir causes flexure (bending and faulting) of the region marginal to the fracture, the deformation and uplift of adjacent plains material and its preexisting structures to form the apical part of the ridge, the exposure of the inner walls of the crack, and the mass wasting of the inner and outer walls of the ridge to modify, but often not completely destroy, the preexisting structure of the adjacent plains. Specifically, this mechanism can account for many characteristics of the ridges, including their linearity, their consistent and regular morphology over their great lateral extent, their positive topography, the presence of preexisting structure on the outer ridges (caused by upbowing of background ridged plains), the formation of marginal troughs (as diapiric rim synclines), the detailed nature of their outer and inner slopes (caused largely by faulting and mass-wasting processes), and their sequential formation with multiple orientations (related to tidal deformation processes). Linear diapirism also provides a possible explanation for color and albedo characteristics, related to thermal effects of the upwelling warm ice (e.g., inducing volatile migration and grain-size variations). As the vast majority of deformation is vertical in this scenario, this mechanism minimizes the necessity for complementary compressional deformation required by some other models.