Constraining Venus’ convection regime from Baltis Vallis topography
Authors: McGregor Nathan , Nimmo Francis , Gillmann Cedric , Golabek Gregor , Plattner Alain , Conrad Jack 
Published: February 2023

Baltis Vallis (BV) is a 6,800-km long lava channel on Venus with a present-day uphill flow direction. The apparently uphill flow must be a consequence of deformation changing the topography after flow emplacement. The topography of BV thus retains a record of Venus&#8217; convection history, as mantle convection causes time-dependent surface deformation. Venus&#8217; mean surface age is likely in the range 300-500 Ma. The observed deformation of BV indicates that mantle convection was active over the past &#8764;400 Myr and provides constraints on the length scales and vertical amplitudes involved. We place constraints on Venus&#8217; present-day internal structure and dynamics by comparing dynamical topography produced by numerical convection codes with the topography of BV.We simulate time-dependent stagnant-lid mantle convection on Venus with a suite of coupled interior-surface evolution models for a range of assumed mantle properties. We compare the simulated topographies of model BV profiles to the actual topography of BV using two metrics. The first metric is the root-mean-square (RMS) height. A model is considered successful if its RMS height is similar to the RMS height of BV. The second metric is the &#8220;decorrelation time&#8221;. Given a particular model time &#964;, the correlation between model BV topography at a later time &#964;2 and an earlier time &#964;1 is calculated. When this correlation first falls to zero, the decorrelation time is then &#964;2 &#8211; &#964;1. The decorrelation time is inspired by the observation of BV&#8217;s present-day uphill flow and the inference that the present-day topography must be uncorrelated with the original topography when BV formed flowing downhill. We compare this decorrelation time to the surface age of Venus (&#8764;400 Ma). A model is considered successful if the decorrelation time is less than the surface age of Venus.From 14 mantle convection models, each initialized with different parameters, we identified two convection models that best fits our metrics. These models have a viscosity contrast &#8710;&#951; of 108 and 107, respectively, and both have a Rayleigh number Ra of 108. Although Venus&#8217; heat flux is highly uncertain, our model fluxes are consistent with some inferred heat fluxes. Models with higher total surface heat fluxes tend to yield lower decorrelation times; our favored models have some of the highest heat fluxes. We also find that models with a higher Ra tend to have a lower RMS height, in agreement with Guimond et al. (2022).Our favored models have vigorous convection beneath a stagnant lid, and high surface heat fluxes. The viscosity of the lower mantle in these models is &#8764;1020 Pa s, roughly two orders of magnitude lower than that of Earth&#8217;s. The majority of the surface heat flux is due to melt advection, indicating high rates of volcanic resurfacing. While current data are insufficient to test these predictions, once paired with forthcoming observations from several new Venus missions, our work will be able to bring Venus&#8217; interior into sharper focus.