Reduced Surface Weathering and Recent Activity at Hotspots on Venus from VIRTIS Data

Details Reduced Surface Weathering and Recent Activity at Hotspots on Venus from VIRTIS Data Reduced Surface Weathering and Recent Activity at Hotspots on Venus from VIRTIS Data

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p31d..03s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P31D-03

Mathematics

Logic

[5430] Planetary Sciences: Solid Surface Planets / Interiors, [6295] Planetary Sciences: Solar System Objects / Venus

Scientific paper

Analysis of data from the Visible and Infrared Thermal Imaging Spectrometer on Venus Express reveals surface thermal emissivity variations in the southern hemisphere that are interpreted as compositional variations (Helbert et al., 2008; Mueller et al., 2008). We show that most of the major high emissivity anomalies are correlated with stratigraphically young volcanic flows at all three hotspots located in the southern hemisphere: Imdr, Dione, and Themis Regiones. These areas have gravity and topography signatures consistent with one or more hot mantle upwellings. Although a difference in the original composition of the flows could also explain the anomalies, we interpret them as most likely to be areas of geologically recent flows that have not fully reached chemical equilibrium with the atmosphere, implying a surface weathering process that reduces the mafic mineral content. The scale of the flows is consistent with on-going volcanic resurfacing. If we assume global resurfacing rates of 1-10 km/yr from geologic studies, the average age of these flows is of the order of 10,000-100,000 yr. However if hotspots and other geologically young regions not imaged by VIRTIS are also active, this average age is likely to be many times less. Estimating the number of active plumes on a planet also has important implications for interior convection. Venus may be unique in terms of having two scales of plumes, as seen by the ~1500 km diameter hotspots and the ~300 km diameter coronae. Venus has 9 hotspots, and ~500 coronae. Themis Regio shows numerous active coronae, while Imdr and Dione Regiones are more like terrestrial hotspots, with 1-3 active volcanoes but no coronae. Thus the VIRTIS data suggest both scales are active on geologically recent time scales. The formation of hot plumes is driven by the thermal boundary layer at the base of the mantle. We have carried out whole mantle convection simulations in 1/6 of a spherical shell to estimate the number of plumes as a function of different mantle parameters. Initially internal heating has been neglected. Several other parameters are being investigated at the present time. The relationship between the thickness of the lower thermal boundary layer and the width of the plume will provide a means to extrapolate the present results to more realistic values of the Rayleigh number. We will compare the characteristics of the lower thermal boundary layer (thickness, temperature difference and heat flux) to scaling laws issued either from prior studies. We will estimate the amount of melting predicted and examine the shear stress along the conductive lid. Finally, we will begin upper mantle Cartesian simulations to look at the formation of smaller scale plumes capable of producing coronae. The location of recent volcanism on Venus offers important constraints on the type of active features, the scale of on-going resurfacing, potentially the chemistry of the surface weathering products, and the number and scale of active mantle plumes. We will investigate the implications of these constraints in this talk.

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