Volcanism in Northwest Ishtar Terra, Venus

Soviet Venera 15/16 radar images and topographic data for an area in NW Ishtar Terra, Venus (74°N, 313°E) contain evidence for the existence of a complex volcanic center. Located at the intersection of Akna and Freyja Montes (both elevated ∼4 to 7 km above the planetary reference radius of 6051 km), this volcanic center has a complex caldera system, possibly more than one major eruptive vent, and associated lava flows at lower elevations. Evidence for a volcanic center at NW Ishtar Terra includes: a smooth-surfaced unit with moderate radar brightness; adjacent (possibly associated) bright flows extending northwestward more than 300 km; an irregularly shaped depression 200 by 250 km in size and ∼2.0 km deep; arcuate slopes that appear to mark the rims of several semicircular depressions within the larger depression; and 5.5-km-elevation ridge separating volcanic deposits of northern Lakshmi Planum from those of NW Ishtar Terra. The lack of a notable shield construct at NW Ishtar and the similarity between smooth terrain and undivided plains of Lakshmi Planum suggest that volcanic units at NW Ishtar Terra may represent an earlier (perhaps younger) stage of volcanism than that attributed to Colette and Sacajawea Paterae of Lakshmi Planum. A westward migration in volcanic activity from Sacajewea to Colette NW Ishtar is supported by relative youth of Colette as compared to Sacajawea, the relatively young stage of volcanism at NW Ishtar Terra, and the superposition of this volcano on the banded terrain of Akna and Freyja Montes.