Possible recent volcanoes and coronae on Venus: Emissivity anomalies, lithospheric thickness, and resurfacing

Kurzfassung

Extensive calibration of the Venus Express near infrared imaging data, which currently covers most of the southern hemisphere, shows regions of anomalously high or low thermal emission (Mueller et al., 2008; Helbert et al., 2008). Typically high thermal emission anomalies are associated with volcanoes or corona, and lows associated with tessera. Mueller et al. (2008) and Helbert et al. (2008) interpret these anomalies as likely to be associated with either compositional variations or relatively recent volcanic flows that are less weathered. They believe that thermal variations are unlikely to persist over the duration of data collection. In this study we focus on anomalies associated with coronae and volcanoes, and examine additional data that could distinguish between compositional variations, lack of weathering on recent flows, or potentially some combination of the two. Mueller et al. (2008) describe 6 volcanoes or coronae with high emissivity anomalies. Of these, 3 are in Themis Regio, 2 in Dione Regio, and one in Imdr Regio. All of these areas have been identified as likely hotspots with possible current mantle upwelling. The estimated elastic thickness is quite small (0-10 km) at Themis and Dione, consistent with an interpretation of active volcanic centers. The resolution of the gravity data at Imdr is too low to get a reliable elastic thickness estimate, but the large depth of compensation does suggest the presence of a plume. We also examine some flows associated with Atete Corona in Parga Chasma that appear to have low emissivity. Atete is 600 km in diameter and has an elastic thickness of ~45 km. We interpret this region as an area of unusual composition, possibly due to melting in a region of delamination (Elkins-Tanton et al., 2007). Further evidence for the interpretation that these areas of anomalous emissivity may be recent comes from analysis of the resurfacing history of Venus. Phillips and Izenberg (1995) examined the distribution of craters with and without haloes (thin ejecta blankets deposited by the wind) and the degree of modification of haloes. They found two regions with both low spatial crater densities and a large proportion of embayed and tectonized craters. They interpret these as areas of relatively recent resurfacing. One is the Beta-Atla- Themis triangle, which is known for its concentration of volcanic features, and the other is the Lavinia Plantia region. Themis and Dione Regios and Atete Corona lie within these regions, as do some of the areas of low emissivity tessera. Idunn volcano in Imdr is the lone exception, but it does lie on the transition from possibly older areas to younger areas. These lines of evidence strongly suggest a relatively recent origin for the emissivity anomalies associated with coronae and volcanoes.