Lithospheric Thickness and Heat Flow on Venus: Results from a Global Survey of Flexure at Steep-Sided Domes

Abstract:

The lithosphere sets the boundary condition for the internal evolution of rocky planets. In turn, lithospheric flexure probes elastic thicknesses and surface heat flows. Though Venus’ lithosphere is poorly understood, previous work predicted that elastic thickness influences magma ascent and volcano morphology [1]. Steep-sided domes and coronae may typically form at regions with elastic thicknesses of ~10–40 km and <10 km, respectively. Previous studies of flexure found that coronae are indeed associated with thin lithosphere [2, 3]. Intriguingly, roughly half the surface may have surface heat flows equal to or exceeding the average value for Earth [4].

Our project tests the relationship between elastic thickness and volcano morphology as well as the agreement between regionally and locally derived estimates of elastic thickness. We conducted the first global search for flexural signatures around steep-sided domes. We took 8 topographic profiles at regular increments around all catalogued (75) steep-sided domes. Convincing evidence of flexure was found in 29 profiles from 14 different domes. Recent stereo-derived topography revealed ~40% of the flexural signatures despite only covering ~20% of the surface [5]. We fit topographic profiles to models of elastic flexure in axisymmetric geometry. A yield stress envelope converts elastic to mechanical thickness and then surface heat flow.

Derived elastic thicknesses agreed with those inferred from gravity data [4]. Most steep-sided domes had elastic thicknesses of ~15–45 km—mechanical thicknesses are ~20% larger. Some steep-sided domes located near coronae had inferred elastic thicknesses in the range expected for coronae, ~1–20 km. We recreated a prior finding of thin lithosphere at Narina Tholi, a dome on the margin of a corona [6]. Surface heat flows for domes near coronae were ~60–200 mW/m², compared to ~25–80 mW/m²for most domes. Volcano morphology is thus a clue to internal heat flow. Future missions to gather high-resolution gravity and topography are needed to better understand Venus’s interior and evolution.

[1] McGovern et al. (2013), JGR Planets. [2] O’Rourke & Smrekar (2018), JGR Planets. [3] Smrekar et al. (2019), VEXAG17 #8035. [4] Anderson & Smrekar (2006), JGR Planets. [5] Herrick et al. (2012), Eos. [6]Russell & Johnson (2019), LPSC #2896.