Emplacement and composition of steep‐sided domes on Venus

Abstract

Steep‐sided domes on Venus have surface characteristics that can provide information on their emplacement, including relatively smooth upper surfaces, radial and polygonal fracture patterns, and pits. These characteristics indicate that domes have surface crusts which are relatively unbroken, have mobile interiors after emplacement, and preserve fractures from only late in their history in response to endogenous growth or sagging of the dome surface. We have calculated the time necessary to form a 12‐cm‐thick crust for basalt and rhyolite under current terrestrial and Venusian ambient conditions. A 12‐cm‐thick crust will form in all cases in <10 hours. Although Venusian lava flows should develop a brittle carapace during emplacement, only late‐stage brittle fractures are preserved at steep‐sided domes. We favor an emplacement model where early‐formed surface crusts are entrained or continually annealed as they deform to accommodate dome growth. Entrainment and annealing of fractures are not mutually exclusive processes and thus may both be at work during steep‐sided dome emplacement. Our results are most consistent with basaltic compositions, as rhyolitic lavas would quickly form thick crusts which would break into large blocks that would be difficult to entrain or anneal. However, if Venus has undergone large temperature excursions in the past (producing ambient conditions of 800–1000 K [e.g., Bullock and Grinspoon, 1996, 1998]), rhyolitic lavas would be unable to form crusts at high surface temperatures and could produce domes with surface characteristics consistent with those of Venusian steep‐sided domes.