Lada Terra: A 'new' hotspot on Venus

On Earth, areas called 'hotspots' form above mantle plumes and are defined by their topographic swells, volcanism, and large positive gravity anomalies. Hawaii is a classic example. Venus has ~10 such highland regions that are analogous to terrestrial hotspots and are approximately 2000 km in diameter. Modeling of their gravity and topography provides evidence for compensation beneath the thermal lithosphere, which is interpreted to indicate the presence of a hot mantle plume. Analysis of data from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS, see Drossart et al. 2007) on the Venus Express mission reveals surface thermal emissivity anomalies in the southern hemisphere that are interpreted as compositional variations (Helbert et al., 2008; Mueller et al., 2008). Most high emissivity anomalies occur in regions previously interpreted as hotspots: Imdr, Themis, and Dione Regiones. The evidence for plumes at depth, the correlation of high emissivity anomalies to stratigraphically young volcanic flows, and analysis of the likely emissivity of weathered and unweathered basalt on Venus lead Smrekar et al. (2010) to interpret the high emissivity flows as evidence of geologically recent, relatively unweathered volcanic flows. Lada Terra also contains volcanic flows with high emissivity anomalies. This region had not been studied using gravity data previously due to the locally low resolution (65-70 spherical harmonic degree and order). Further, the radar data have a high look angle, making it harder to interpret. The high emissivity anomalies inspired Ivanov and Head (2010) to reexamine this region. Their analysis of the geologic setting along with the high emissivity anomalies and the positive gravity anomaly provide evidence that Lada Terra is likely to be recently active. Here we use the spherical harmonic gravity and topography data from Magellan to calculate a geoid-to-topography ratio (GTR) of 23.5±1.6 m/km. This value is in the range of the GTRs previously found for highlands interpreted as hotspots on Venus and provides evidence of a mantle plume at depth. This brings the number of likely hotspots in the southern hemisphere to 4. Although VIRITS did not acquire 1 micron data in the northern hemisphere, the gravity and topography data for the 7 northern hemisphere hotspots indicate active plumes. The similarity of geologic and gravity signatures between northern and southern hemisphere hotspots suggests that all of them may be sites of currently or recently active volcanism. This brings the hotspot count on Venus to 11. Such hotspots could be plausible sources of lower atmospheric water (Smrekar and Sotin, 2012). The presence of ~10 plumes from the core-mantle-boundary on Venus suggests that the mantle may be heating up (Sotin and Smrekar, 2012 - this meeting). (Drossart et al., PSS, 2007; Helbert et al., GRL, 2008; Mueller et al., JGR-P, 2008; Smrekar et al., Sci., 2010; Ivanov and Head, PSS, 2010; Smrekar and Sotin, Icarus, 2012) This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.