Explosive volcanic activity on Venus: The roles of volatile contribution, degassing, and external environment

We investigate the conditions that will promote explosive volcanic activity on Venus. Conduit processes were simulated using a steady-state, isothermal, homogeneous flow model in tandem with a degassing model. The response of exit pressure, exit velocity, and degree of volatile exsolution was explored over a range of volatile concentrations (H2O and CO2), magma temperatures, vent altitudes, and conduit geometries relevant to the Venusian environment. We find that the addition of CO2 to an H2O-driven eruption increases the final pressure, velocity, and volume fraction gas. Increasing vent elevation leads to a greater degree of magma fragmentation, due to the decrease in the final pressure at the vent, resulting in a greater likelihood of explosive activity. Increasing the magmatic temperature generates higher final pressures, greater velocities, and lower final volume fraction gas values with a correspondingly lower chance of explosive volcanism. Cross-sectionally smaller, and/or deeper, conduits were more conducive to explosive activity. Model runs show that for an explosive eruption to occur at Scathach Fluctus, at Venus’ mean planetary radius (MPR), 4.5% H2O or 3% H2O with 3% CO2 (from a 25 m radius conduit) would be required to initiate fragmentation; at Ma’at Mons (~9 km above MPR) only ~2% H2O is required. A buoyant plume model was used to investigate plume behaviour. It was found that it was not possible to achieve a buoyant column from a 25 m radius conduit at Scathach Fluctus, but a buoyant column reaching up to ~20 km above the vent could be generated at Ma’at Mons with an H2O concentration of 4.7% (at 1300 K) or a mixed volatile concentration of 3% H2O with 3% CO2 (at 1200 K). We also estimate the flux of volcanic gases to the lower atmosphere of Venus, should explosive volcanism occur. Model results suggest explosive activity at Scathach Fluctus would result in an H2O flux of ~107 kg s-1. Were Scathach Fluctus emplaced in a single event, our model suggests that it may have been emplaced in a period of ~15 days, supplying 1-2 x 104 Mt H2O to the atmosphere locally. An eruption of this scale might increase local atmospheric H2O abundance by several ppm over an area large enough to be detectable by near-infrared nightside sounding using the 1.18 µm spectral window such as that carried out by the Venus Express/VIRTIS spectrometer. Further interrogation of the VIRTIS dataset is recommended to search for ongoing volcanism on Venus

Emplacement of giant radial dykes in the northern Tharsis region of Mars.

Three distinct sets of graben are associated with the volcano Alba Patera on Mars. One set, approximately circumferential to the edifice, has long been accepted to have formed as a tectonic response to an extensional stress regime associated with the evolution of the Alba Patera edifice. A second set includes mainly linear structures interpreted by many workers to have formed in response to very large-scale regional stresses. We infer that the third set of graben, all of which are relatively linear, none of which are strictly parallel to members of the second set, and many of which contain numerous pit craters, formed above long (∼1000 km), laterally propagating regional dikes emanating from a volcanic center located to the south within the Tharsis region. The expected geometries of such dikes (several hundred meters depth to dike top, ∼20 km depth to dike base, mean dike width ∼30–90 m) are modeled on the assumption that they were fed from a shallow magma reservoir centered on a neutral buoyancy horizon, expected to be present at a depth of ∼10 km on Mars. The volumes of magma in the dikes are consistent with a reservoir similar in size to those inferred to be present under the Tharsis shield volcanoes provided that the dikes were emplaced during caldera collapse episodes. The sizes of the graben associated with these dikes are consistent with the relaxation, during or immediately after dike emplacement, of preexisting regional extensional stresses of a few tens of MPa

Stardust Interstellar Preliminary Examination I: Identification of tracks in aerogel

Here, we report the identification of 69 tracks in approximately 250 cm2 of aerogel collectors of the Stardust Interstellar Dust Collector. We identified these tracks through Stardust@home, a distributed internet-based virtual microscope and search engine, in which > 30,000 amateur scientists collectively performed >9 × 107 searches on approximately 106 fields of view. Using calibration images, we measured individual detection efficiency, and found that the individual detection efficiency for tracks > 2.5 ?m in diameter was >0.6, and was >0.75 for tracks >3 ?m in diameter. Because most fields of view were searched >30 times, these results could be combined to yield a theoretical detection efficiency near unity. The initial expectation was that interstellar dust would be captured at very high speed. The actual tracks discovered in the Stardust collector, however, were due to low-speed impacts, and were morphologically strongly distinct from the calibration images. As a result, the detection efficiency of these tracks was lower than detection efficiency of calibrations presented in training, testing, and ongoing calibration. Nevertheless, as calibration images based on low-speed impacts were added later in the project, detection efficiencies for low-speed tracks rose dramatically. We conclude that a massively distributed, calibrated search, with amateur collaborators, is an effective approach to the challenging problem of identification of tracks of hypervelocity projectiles captured in aerogel