The nature and origin of Venusian canali

Abstract

Abstract. Venusian canali have many characteristics of terrestrial rivers, notably cutoff meanders, braiding, point bars, and deltas, which required both erosion and sediment transport. This implies that the canali were not formed by construction but rather by thermal or mechanical erosion. We have evaluated the relative importance of these latter two mechanisms, assuming a basaltic substrate and a surface temperature similar to that currently prevailing, -470øC. In order to have thermally eroded the canali, the liquid must have been turbulent and at a temperature above that of the basalt solidus. The most plausible candidates for this liquid are basalt and komatiite lavas. However, at realistic flow rates and extrusion temperatures, flow of basaltic lava is laminar, and therefore basaltic lavas could not have thermally eroded the canali. Although komatiite flow is initially turbulent, the lava will cool in hours to its solidus temperature, whereas it will take months to thermally erode canali. By elimination, only mechanical erosion can adequately explain canali formation. Based on incision and lateral migration rates for terrestrial rivers, it could take from >5 years (in unconsolidated regolith) to 8 x 105 years (in solid basalt) to mechanically eroc[e a typical Venusian canale. These estimates require that the eroding agent had a solidus temperature close to the Venusian surface temperature and that viscosities remained low until solidification. Only halogen-rich, alkali carbonatite and sulfur lavas meet these criteria, and only the former could have been present in sufficient volumes to form the canali. We propose that the canali were mechanically eroded by such carbonatite lavas and that the latter originated from the fusion of anhydrous recycled crust, which had been altered by interaction with a CO2-, SO2-, and halogen-rich atmosphere.

Cross-sectional profiles of Baltis Vallis channel on Venus: Reconstructions from Magellan SAR brightness data

“…This observation implies that the canaliforming fluid behaved like liquid water; namely, a fluid with a low viscosity or a high discharge rate Kargel et al, 1994). Previous studies have argued that the fluid could have been a low-viscosity lava with a low solidus temperature, such as carbonatite Kargel et al, 1994;Treiman, 1994;Baker et al, 1997;Williams-Jones et al, 1998), sulfate lava Komatsu et al, 1992Komatsu et al, , 1993Kargel et al, 1994;Baker et al, 1997;Williams-Jones et al, 1998), and mafic or ultramafic lava Komatsu et al, 1992;Greeley, 1993, 1994), or liquid water (Jones and Pickering, 2003). Reconstruction of the high-resolution morphology of canali is likely to help solve this problem.…”

“…Proposed mechanisms of canali formation include mechanical erosion Komatsu et al, 1992Komatsu et al, , 1993Kargel et al, 1994;Bussey et al, 1995;Baker et al, 1997;Williams-Jones et al, 1998;Jones and Pickering, 2003), thermal erosion (Bussey et al, 1995;Lang and Hansen, 2006), and construction Gregg and Greeley, 1993;Baker et al, 1997). In the two erosional models a channel-forming fluid erodes the surface or subsurface, resulting in a channel floor that is lower than the surrounding plains.…”

“…Gregg and Greeley (1993) and Williams-Jones et al (1998) estimated the cooling rate of several types of lava and the melting rate of basaltic substrate, and found that thermal erosion is more efficient on Earth than on Venus. Snyder (2002) argued against these estimates; he calculated the absorption and emission of thermal radiation by the CO 2 -rich atmosphere of Venus and found that the cooling rate of lava is slower on Venus than on the Earth.…”

“…Such a high discharge rate is expected to cause a braided pattern, yet such a pattern is not apparent from Magellan SAR images of Baltis Vallis . A turbulent flow condition enables the flow to transport residual, unmelted material (Williams-Jones et al, 1998); however, extremely low viscosity is required for turbulent flows. In conclusion, Baltis Vallis is most likely to have formed by mechanical rather than thermal erosion.…”

Volcanism and tectonism across the inner solar system: an overview

“…Remarkably long individual lava channels, of typical lengths of between 100 and 1000 km in general but in some cases extending to more than 5000 km, are explained by invoking very lowviscosity fluxes such as these expected for exotic lava types such as komatites, carbonatites and sulphur flows (Kargel et al 1994;Baker et al 1997;Williams-Jones et al 1998;Komatsu et al 2001;Lang & Hansen 2006).…”