Revealing Venus Interior from Coronae Analysis

Rifting and rises are prominent landscape features in the roughly triangular area characterized by the presence of three major rises (Atla, Beta and Themis) and two corona-dominated long chasmata (Hecate and Parga). The coronae population associated with these chasmata represents 35% of all Venusian coronae and 56% of coronae associated with fracture zones (Smrekar et al., 2010). We focused on the spatial analysis of the coronae population associated with Parga chasma for identifying the depth of the main thermal anomaly that fed (and maybe still feeds) them.

We explore a formation mechanism for coronae based on the Rayleigh–Taylor (R-T) gravitational instability (Tackley and Stevenson, 1991) of the lithosphere that may occur when a layer of dense fluid overlies a layer of less dense fluid. The R-T gravitational instability theory can be used to draw a relationship between the spacing of volcanic structures and edifices at the surface and the depth of the source of instability beneath the volcanic fields (i.e. the lithosphere-asthenosphere boundary depth where partial melting is initiated and starts the vertical upwelling of material). We performed the analyses both on the entire population and on two sub-groups obtained from automatic clustering based on point spacing analysis. Overall, the results obtained from the analysis of the entire population can be considered a global average while the information extracted from the analyses of the two clusters are to be interpreted as end members. Hence, the lithosphere-asthenosphere boundary depth results to be located at 117 ± 10 km underneath Parga.

Additionally, we ran geodynamical models using a variable thermal conductivity and expansivity, and reference viscosities between 1e20 and 1e22 Pa s. These models use an extrusive to intrusive magmatism ratio of 0.1, a typical terrestrial value (Crisp et al., 1984). The intrusive melt is assumed to stall at the base of the crust (~20 km depth; James et al., 2013), since the latter represents a density barrier. According to these models,  a mantle reference viscosity of 1e20 Pa s is best compatible with the geologically inferred lithosphere thickness as well as a thin mechanical thickness as suggested by elastic thickness estimates (e.g., O’Rourke & Smrekar 2018).

As future missions will return higher resolution imagery and topographical information, we suggest the area of Parga chasma as a region of high interest for future data acquisitions. In fact, more detailed data can allow the observation of stratigraphic relationships between rises, rifts, coronae, and volcanoes in order to reconstruct the event sequences. By means of R-T analysis and similar techniques, we would thus be able to refine current analyses and perform more detailed estimates from smaller volcanic features and obtain more precise information about magma reservoir distribution in the subsurface.