The Venusian lowlands: key to unravel mantle dynamics

A global-scale network of tectonic plates like on Earth is currently absent on Venus [1]. However, a rich inventory of tectonic features, including rift systems (chasmata) [2] and mountain belts [3], along with volcanic edifices [4] and coronae [5], indicates diverse modes of past and likely also present localized deformation and associated magmatism. Radar-smooth crustal blocks spanning several hundreds of kilometers have also been identified [6] and were suggested to have rotated and laterally moved relative to one another in the recent past. Geodynamic concepts of a static lid with possible episodic overturning or a plutonic squishy lid are currently favored to describe Venus’ recent evolution [7].

The Venusian surface is dominated by lowland volcanic plains mapped as lower and upper regional plains, shield plains and groove belts [8] that cover approximately 75% of the planet. These plains are in parts very smooth, likely basaltic in composition, and among the youngest landforms on Venus. They may play a key role in understanding geodynamic processes on Venus. Here, we study these lowlands plains and find that they comprise polygonal domains bounded by fractured ridges, which may provide a window into Venus' mantle dynamics.

Remote sensing combined with geostatistical analysis reveals that the entity of the lowlands forms a network of aligned ridges delimiting polygonal lowlands, occurring in four specific patterns: 1) symmetric and 2) asymmetric ridges, 3) flat, strongly fractured belts, and 4) double-ridge-furrow belts. The polygon diameters show a skewed distribution with dominant modes at 800 km and 1,300 km. The polygon centers correlate with Bouguer gravity highs and low inferred crustal thicknesses. Many Venusian coronae occur within the interiors of these polygons.

These observations are consistent with a convective pattern in which broad mantle upwellings define the polygonal domains and their margins. In contrast, the fractured ridges and furrows represent mobile compressive structures that may coincide with sheets of sinking, cooler lithosphere. The polygons of the Venusian lowlands are thus interpreted as surface expressions of mantle convection and can help to constrain rheological and geodynamic parameters of Venus. Recent 2D geodynamic models predicting a bimodal distribution of plume sizes in Veus’ mantle [9] show a good match to the dominant dimensions of the mapped polygons and are therefore consistent with our geodynamic interpretation. Morever, recent 3D geodynamic models of Venus’ mantle convection with GAIA [10] show the development of convection cells that produce a polygonal surface deformation pattern, where mantle upwellings coincide with the polygon interiors and sheets of donwelling lithosphere coincide with their boundaries.

References

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