P. E. Geissler

Evidence for a subsurface ocean on Europa

Ground-based spectroscopy of Jupiters moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior. In addition, images obtained by the Voyager spacecraft revealed that Europas surface is crossed by numerous intersecting ridges and dark bands (called lineae) and is sparsely cratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto. It has been suggested that Europas thin outer ice shell might be separated from the moons silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heating; in this model, the lineae could be explained by repetitive tidal deformation of the outer ice shell. However, observational confirmation of a subsurface ocean was largely frustrated by the low resolution (>2 km per pixel) of the Voyager images. Here we present high-resolution (54 m per pixel) Galileo spacecraft images of Europa, in which we find evidence for mobile ‘icebergs’. The detailed morphology of the terrain strongly supports the presence of liquid water at shallow depths below the surface, either today or at some time in the past. Moreover, lower-resolution observations of much larger regions suggest that the phenomena reported here are widespread.

Does Europa have a subsurface ocean? Evaluation of the geological evidence

It has been proposed that Jupiters satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

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.

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.

Habitability of Europa's crust: The role of tidal-tectonic processes

Jupiters satellite Europa has been identified as one of the most likely sites for life in the solar system. The tidal-tectonic processes that appear to have governed Europas geology seem to require interaction with an ocean under only a very thin crust, providing a variety of evolving environmental niches. The mutually dependent relationship between orbital evolution and tidal processes in turn controls Europas rotation, heating, and stress. Surface lineaments are correlated with global stress patterns, demonstrating that they form by crustal cracking, but only if a substantial ocean is present to give adequate tidal amplitude. Tidal driving of strike-slip faulting indicates that cracks penetrate to a fluid layer, which is possible only with a very thin ice crust. The characteristic ridge sets that cover tectonic terrain are likely built by tidal pumping of fluid and slush to the surface on a daily basis. Widespread tectonic dilation creates new surface as material rises from below. Chaotic terrain has morphology and other characteristics indicative of melt-through from below. Surface colorants correlate with locations, such as along large-scale ridge systems and around chaotic terrain, where ocean water reached the surface. This model implies that as a result of tides, liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry, creating a variety of habitable environments. The processes were recent and thus most likely continue today. Longer-term evolution of environmental conditions provided the need for adaptation and opportunity for evolution.

Stratigraphy and erosional landforms of layered deposits in Valles Marineris, Mars

The complex stratigraphy of layered deposits suggests a diversity of origins, ages, and post-depositional modification histories. The complexities within some layered deposits indicate changes in the dominant source materials in space and time. The stratigraphy of layered deposits in the isolated Martian chasmata Hebes, Juventae and Gangis is not well correlated. This indicates that at least these chasmata had isolated depositional environments resulting in different stratigraphic sequences. Separated layered deposits in Ophir-Candor and Melas Chasmata might have been a single continuous deposit in each chasma. Chaotic terrains are found in conjunction with layered deposits in Juventae, Gangis and Capri-Eos Chasmata. In these chasmata, layered deposits unconformably overlie chaotic terrains. Chaotic terrain formation may have provided water to form paleolakes, and lacustrine deposition of thick layered deposits may have occurred if the canyons were closed. A very thick sequence of the layered deposits has been exposed by erosion. A combination of gradual processes such as evaporation of ice and eolian and fluvial transport in addition to structural processes may be responsible for this erosion. Another alternative is that catastrophic water release under the layered deposits disrupted and initiated erosion of the layered deposits. Newly identified units of anomalous color are confined to the depressions or reentrants in western Candor Chasma. The difference in color between these units and the surrounding terrain is most consistent with a somewhat greater content of bulk crystalline hematite in these anomalous units. The presence of the Candor units is a result of original and/or secondary deposition which is different from the primary and dominant formation of the layered deposits.

High‐temperature hot spots on Io as Seen by the Galileo solid state imaging (SSI) Experiment

High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileos CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiters shadow) via the SSI clear filter (∼0.4–1.0 µm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io‧s atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io–s surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.

Distribution of chaotic terrain on Europa

The locations of chaotic terrain are mapped over all regions of Europa for which we have adequate resolution (∼200 m or better) and appropriate lighting geometry (Sun angle <35° from the local horizontal), comprising 9% of the total surface of the satellite. Nearly 30% of the mapped area is occupied by chaotic terrain recognizable at 200 m resolution, and sampling at higher resolution suggests that at least 10% more may be covered by small chaos features. The largest contiguous area of chaotic terrain is ∼1300 km across. Chaotic terrain displays variations in freshness of appearance, probably because of aging by fine-scale tectonics. Resurfacing of previous chaotic terrain by larger-scale tectonics or disruption by newer chaos are common. Chaos formation is not necessarily recent relative to tectonics; both types of process appear to have gone on diachronously. The size distribution shows no dominant or characteristic size and appears to have been controlled by competition from tectonics, which has created terrain that occupies more than half of the surface of Europa.

Distribution of strike-slip faults on Europa

Study of four different regions on Europa imaged by the Galileo spacecraft during its first 15 orbits has revealed 117 strike-slip faults. Europa appears to form preferentially right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere. This observation is consistent with a model where diurnal tides due to orbital eccentricity drive strike-slip motion through a process of “walking,” in which faults open and close out of phase with alternating right-and left-lateral shear. Lineaments that record both left-and right-lateral motion (e.g., Agave Linea) may record the accommodation of compression in nearby chaotic zones. Nearly all identified strike-slip faults were associated with double ridges or bands, and few were detected along ridgeless cracks. Thus the depth of cracks without ridges does not appear to have penetrated to the low-viscosity decoupling layer, required for diurnal displacement, but cracks that have developed ridges do extend down to such a level. This result supports a model for ridge formation that requires cracks to penetrate to a decoupling layer, such as a liquid water ocean.

Galileo Multispectral Imaging of the North Polar and Eastern Limb Regions of the Moon

Multispectral images obtained during the Galileo probes second encounter with the moon reveal the compositional nature of the north polar regions and the northeastern limb. Mare deposits in these regions are found to be primarily low to medium titanium lavas and, as on the western limb, show only slight spectral heterogeneity. The northern light plains are found to have the spectral characteristics of highlands materials, show little evidence for the presence of cryptomaria, and were most likely emplaced by impact processes regardless of their age.