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NASA NTRS1992

Geophysical models for the formation and evolution of coronae on Venus

The proposition that Venusian coronae form over sites of mantle upwelling and are modified by subsequent gravitational relaxation is examined using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the Magellan altimetry data and radar images. It is shown that mantle diapirism can produce the domical topography of novae, which may be coronae in the earliest stage of formation. The model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae. The novae dimensions indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick. A flattened diapir at the top of the mantle is modeled and shown to result in plateaulike uplift. The volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae.

Janes, Daniel M.↗

NASA NTRS1993

The tectonic and volcanic evolution of Venus: Catastrophic or gradual?

Radar imaging and altimetry data from the Magellan mission have yielded important new constraints on the tectonic and volcanic history of Venus and on its internal dynamics. The planet lacks global plate tectonics, but a number of chasm systems and corona moat structures have arcuate planforms, asymmetric topogrpahic profiles, and relief analogous to deep-sea trenches on Earth and may be products of limited lithospheric underthrusting or subduction. Several lines of evidence point to a crust and upper mantle stronger than would be predicted by simple extrapolation from Earth and the 450 K greater surface temperature; these include the unrelaxed depths of impact craters, apparently large values of elastic lithosphere thickness, and large ratios of gravity to topography. The density of impact craters indicates an averate crater retention age of about 500 My, but not more than 5% of the recognized craters have been volcanically embayed. This last observation has led to the proposal that Venus has been subjected to one or more global resurfacing events, the latest about 500 My ago, and that the volcanic flux during intervals between such events has been low. That more recent tectonic activity has been widespread, however, is indicated by the high relief and slopes of mountains, chasm walls, and plateau margins; the significant fraction (0.3) of impact craters deformed by younger faults; and the postformational vertical deformation of long channels. Interior dynamical scenarios advanced to account for episodic volcanic resurfacing include catastrophic overturn of a global lithosphere thickened by cooling or compositional buoyancy and strongly time-dependent mantle convective heat flux. Outgassing considerations and analogy with Earth and other terrestrial planets, however, suggests that such catastrophic models are unlikely. If the mantle of Venus cooled more efficiently than that of Earth because of, say, different boundary conditions, a different flow law, or a different degree of layering, then the planet may in the last 500 My have attained lesser mantle temperatures, lower mantle heat flux, and a significantly lesser rate of magma production than Earth yet still display evidence for ongoing convection and active tectonics. Such a 'cold Venus' scenario would be broadly consistent with observations yet be characterized by a gradual volcanic and tectonic evolution.

Solomon, Sean C.↗

NASA NTRS2005

Lithospheric Thickness on Venus from Magellan Gravity and Topography

This final report summarizes work carried out during my PGG funding for the period 3/1/02-2/28/05. Research under this award has focused on the areas described below and is represented in the publications list, invited departmental lectures and presentations at professional meetings. The grant has provided partial support for 1 graduate student, Renee Bulow, and provided 1 month per year of my summer salary. The linking theme of the research performed under this award is the manifestation of the thermal history of terrestrial planetary bodies through the existence and evolution of internally-generated magnetic fields (martian magnetism research, and beginnings of lunar magnetism research), mantle dynamical processes and their resulting surface expression (studies of Venusian coronae task) and the crust and upper mantle structure of a planetary body (lunar seismic structure task). The investigations build upon and extend my previous work supported by the PGG program.

Johnson, C. L.↗

NASA NTRS2005

Evolution of large shield volcanoes on Venus

We studied the geologic history, topographic expression, and gravity signature of 29 large Venusian shield volcanoes with similar morphologies in Magellan synthetic aperture radar imagery. While they appear similar in imagery, 16 have a domical topographic expression and 13 have a central depression. Typical dimensions for the central depression are 150 km wide and 500 m deep. The central depressions are probably not calderas resulting from collapse of a shallow magma chamber but instead are the result of a corona-like sagging of a previously domical volcano. The depressions all have some later volcanic filling. All but one of the central depression volcanoes have been post-dated by geologic features unrelated to the volcano, while most of the domical volcanoes are at the top of the stratigraphic column. Analysis of the gravity signatures in the spatial and spectral domains shows a strong correlation between the absence of post-dating features and the presence of dynamic support by an underlying plume. We infer that the formation of the central depressions occurred as a result of cessation of dynamic support. However, there are some domical volcanoes whose geologic histories and gravity signatures also indicate that they are extinct, so sagging of the central region apparently does not always occur when dynamic support is removed. We suggest that the thickness of the elastic lithosphere may be a factor in determining whether a central depression forms when dynamic support is removed, but the gravity data are of insufficient resolution to test this hypothesis with admittance methods.

Herrick, Robert R.↗

NASA NTRS1993

Flexure and the role of inplane force around coronae on Venus

Large coronae on Venus, such as Artemis and Latona, are rimmed by conspicuous trenches and associated outer rises. Sandwell and Schubert have observed that these systems resemble terrestrial subduction zones in planform and have succeeded in fitting an elastic plate bending equation to the inferred flexural topography. However, the first zero crossing bending moments required are -2.5 x 10(exp 17) N for Artemis and -5.0 x 10(exp 16) N for Latona. Since these moments are similar in magnitude to those of subducting slabs on Earth, a rollback subduction mechanism was proposed to explain the flexure around the largest coronae, although a differential thermal subsidence model is sufficient to account for the topography around some coronae. The purpose is to investigate the effect of inplane force as a possible alternative to large applied moments in producing flexure at Artemis and Latona. The close correlation of gravity to topography on Venus implies the absence of a low viscosity zone and the strong coupling of the lithosphere to mantle convection. If coronae are the surface manifestations of mantle plumes, they may be the sites of active convective stress coupling. As the upwelling reaches the lithosphere, it spreads radially outward, inducing shear tractions on the base of the plate. In addition, the hot, expanding corona may load the surrounding plate horizonally. Both the basal shear stresses and radial loading can be treated as an equivalent compressive inplane force in the mechanical lithosphere, which contributes to the bending of the outlying plate. Using a model that relates inplane force to the measured gravity anomalies, a rough value of the inplane force at Artemis was calculated. Recent Pioneer Venus spherical harmonic gravity models indicate a geoid anomaly of about 75 m over Artemis, which corresponds to an estimated inplane force on the order of -1x10(exp 13) N/m. The gravity model is unable to resolve Latona, but an inplane force of similar dimensions is assumed. The maximum possible inplane force based on the expected rheology can be constrained by using the approximate 5 K/km thermal gradient inferred from the best fit 30 km elastic plate at Artemis and Latona. For a dry olivine flow law in the upper mantle, the compressional load limit of the 60 km thick mechanical lithosphere is -4 x 10(exp 13) N/m. This value is equivalent to a load of -8 x 10(exp 13) N/m on a 30 km thick elastic plate.

Brown, C. David↗

NASA NTRS1993

The spatial distribution of coronae and related features on Venus

Coronae are large quasi-circular geologica features that are common on Venus. They appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere. We have investigated the spatial distribution of 335 coronae and related features identified in Magellan radar data. It is more clustered than a Poisson distribution, with a statistical certainty of more than 99%. It is dominated by a single large cluster centered near the equator at about 245 deg longitude. The features are preferentially found at elevation and geoid values close to the planetary mean, with a paucity at both the highest and lowest levels of topgraphy and geoid. Some coronae appear aligned in quasi-linear chains. We attribute the clustering of coronae and related features to preferential formation of these features above regions of broad-scale mantle upwelling, and suggest that a major mantle upwelling underlies the one large cluster. We suggest that coronae are rare at the lowest elevations because these may be regions of mantle downwelling. The shortage of coronae at the highest elevations may result both from obscuration by other intense tectonism there and from suppression there of their formation by an unusually thick crust. Corona chains may be produced by enhanced passive mantle uplift below failed or incipient rifts.

Squyres, Steven W.↗