W. B. Moore

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.

Io's gravity field and interior structure

Radio Doppler data generated by the Deep Space Network (DSN) from four encounters of the Galileo spacecraft with Io, Jupiters innermost Galilean satellite, are used to infer Ios gravitational quadrupole moments. By combining the four flybys into a single solution for the gravity field, the response of Io to the second degree tidal and rotational potentials is accurately determined. This is characterized by the value of the second degree potential Love number k 2 = 1.2924 ± 0.0027. We construct interior models that satisfy constraints imposed by the mean radius R = 1821.6 ± 0.5 km, the mean density p = 3527.8 ± 2.9 kg/m 3 , and the normalized axial moment of inertia C/M R 2 = 0.37685 ± 0.00035. The gravitationally derived figure of Io has principal axes (c < b < a) a = 1830.0 ± 0.5 km, b = 1819.2 ± 0.5 km, and c = 1815.6 ± 0.5 km, consistent with the shape determined by imaging. Gravitational and other data strongly suggest that Io is in hydrostatic equilibrium. In this case, models of Ios interior density show that Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe-FeS core or between 350 and 650 km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the gravitational data.

Timing of the Martian dynamo.

On Mars, the strong magnetization in the highland crust of the southern hemisphere and the absence of magnetic anomalies at the Hellas and Argyre impact basins have been taken as signs that the core dynamo that once drove the planets magnetic field turned off more than 4 billion years (Gyr) ago. Here, we argue instead that the Martian dynamo turned on less than 4 Gyr ago and turned off at an unknown time since then. High spatial resolution magnetometry in both Martian hemispheres is needed to reveal the true history of the Martian dynamo.

The role of rheology in lithospheric thinning by mantle plumes

Lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on Venus, the Tharsis rise on Mars, and several hotspot swells on Earth. We present a suite of time-dependent, three-dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature-dependent viscosity mantle. Efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume-lithosphere boundary layer. These instabilities are non-axisymmetric, time-dependent, and have horizontal scales of a few tens of kilometers. These instabilities depend on the temperature-dependence of viscosity and occur when the plumes viscosity is about an order of magnitude less than the background mantle, as predicted by boundary-layer theory. Thus, in planetary mantles, plumes with excess temperatures of 100 to 200 K will efficiently thin the lithosphere via small-scale convective instabilities.

Heat transport in a convecting layer heated from within and below

[1] The heat transport scaling relationships for fluids heated from within and below are established based on numerical calculations of isoviscous Boussinesq convection at infinite Prandtl number. The internal temperatures scale in a way that is similar to the internally heated case but with an offset equal to the average of the two boundary temperatures, reflecting the underlying temperature structure of bottom-heated convection. The heat fluxes through the boundaries scale as a linear combination of the end-member modes of heat transport, consistent with the effects of internal heating on the interior temperature. The boundary layer thicknesses, however, depend on H and Ra in a nonintuitive way. As the internal temperature increases with the addition of internal heating, the upper thermal boundary layer thickens despite the increased temperature drop across the layer (the reverse is true for the bottom boundary layer). This is inconsistent with the idea that the boundary layer thickness is controlled by a stability condition on the local Rayleigh number, which would predict that the boundary layer would thin as the temperature drop increases. Deriving boundary layer thicknesses from the scalings for heat flux and boundary layer temperature drop provides an excellent fit to the model results and reveals the importance of plumes arriving from the other boundary layer in establishing the boundary layer thickness. This suggests that, although widely used, boundary layer stability analysis is not an accurate description of the processes controlling boundary layer thickness in systems with two active boundary layers at moderate Ra.

Lithospheric thickness and mantle/lithosphere density contrast beneath Beta Regio, Venus

The spatial variation of the geoid/topography ratio over the large Venusian volcanic highland Beta Regio is suggestive of thermal compensation, i.e., support of the highlands topography by lithospheric thinning. Both the thickness of the lithosphere and the density contrast at its base can be inferred from a quadratic regression of suitably filtered (600 km < wavelength < 4000 km) geoid vs. topography data. The regression yields a mean lithospheric thickness of 270 km and a density contrast of magnitude 2.5% to 3.0%. Simple isostatic balance of the long-wavelength topography at Beta Regio requires thinning of the lithosphere by 50–60% beneath the rise.

The formation of Tharsis on Mars: What the line-of-sight gravity is telling us

Line-of-sight (LOS) spacecraft acceleration profiles from the Radio Science Experiment and topography from the Mars Orbiter Laser Altimeter (MOLA) instrument of the Mars Global Surveyor (MGS) are analyzed to estimate the effective elastic thickness (Te) for various regions of Tharsis. We identify a buried basin flanking the Thaumasia Highlands at the southeastern margin of Tharsis. Assuming that this basin results from lithospheric flexure from surface loading by the Thaumasia Highlands, we fit LOS profiles across the feature with a thin-shell, elastic flexure model and find the mountain belt to reflect a value of Te ∼ 20 km consistent with a Noachian formation age. We also determine admittances from LOS profiles for five regions across Tharsis and fit them with theoretical admittances calculated using the flexural model. Crater density, surface density, and predominant surface age are found to vary systematically across Tharsis while Te does not. The highest surface density and lowest Te values are obtained for the western portion of Tharsis where crater densities are lowest. Our results imply the majority of the topographic rise was emplaced within the Noachian irrespective of the surface ages. Topographic loading and resurfacing (i.e., volcanic activity) persisted into the Amazonian while becoming increasingly confined to the western margin where the youngest surface ages are found and the eruptive style transitioned from effusive volcanism to shield-forming volcanism as Te increased.

Stable 1:2 Resonant Periodic Orbits in Elliptic Three-Body Systems

The results of an extensive numerical study of the periodic orbits of planar, elliptic restricted three-body planetary systems consisting of a star, an inner massive planet, and an outer massless body in the external 1 : 2 mean-motion resonance are presented. Using the method of differential continuation, the locations of the resonant periodic orbits of such systems are identified, and through an extensive study of their phase-parameter space it is found that the majority of the resonant periodic orbits are unstable. For certain values of the mass and the orbital eccentricity of the inner planet, however, stable periodic orbits can be found. The applicability of such studies to the 1 : 2 resonance of the extrasolar planetary system GJ 876 is also discussed.

Lithospheric thinning and chemical buoyancy beneath the Hawaiian Swell

The source of the buoyancy that supports the Hawaiian Swell is not well understood. The swell may be supported by replacement of negatively buoyant lithosphere with buoyant mantle or by emplacement of buoyant material beneath lithosphere of normal thickness. These mechanisms can be distinguished by examining the quadratic relationship between geoid height and bathymetry. At the Hawaiian Swell, the curvature of the geoid height vs. swell topography relationship is negative, indicating that the swell is supported by thinned lithosphere. The magnitude of the curvature suggests that the mantle filling in the region of thinned lithosphere is both thermally and chemically buoyant.

Designing science web sites

From a scientist’s viewpoint a web site is one tool used to conduct research. From an artist’s viewpoint web sites are a form of visual composition. From a developer’s point of view a web site is a type of application. While web sites are a relatively new medium with a particular set of constraints, they do adhere to the same basic design principles that apply to other art forms. These design principles are the basic assumptions that affect the arrangement of elements within a composition. A successful design uses the principles and elements to achieve a visual goal in the composition. A web site designed for scientists has unique properties which are not shared by many other types of web sites. These properties influence the overall visual design of the web sites. Recently at the Institute of Geophysics and Planetary Physics at UCLA undertook a re-design of a number of its websites. In the effort, the use of visual design principles combined with the properties of a science web site were put to the test. In all, six different web sites were designed each with a difference science focus. We describe the process used to design the web sites which involve forming teams of designers, scientists and developers. We present example pages from each design and conclude with a discussion of what was learned during the process.