THE NATURE AND ORIGIN OF VENUSIAN CANALI

Venusian canali have many characteristics of terrestrial rivers, notab ly cutoff meanders, braiding, point bars, and deltas, which required b oth 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 mechan isms, assuming a basaltic substrate and a surface temperature similar to that currently prevailing, similar to 470 degrees C. In order to ha ve thermally eroded the canali, the liquid must have been turbulent an d at a temperature above that of the basalt solidus. The most plausibl e candidates for this liquid are basalt and komatiite lavas. However, at realistic flow rates and extrusion temperatures, flow of basaltic l ava 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 t ake 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 10(5) years (in solid basal t) to mechanically erode a typical Venusian canale. These estimates re quire that the eroding agent had a solidus temperature close to the Ve nusian surface temperature and that viscosities remained low until sol idification. Only halogen-rich, alkali carbonatite and sulfur lavas me et these criteria, and only the former could have been present in suff icient volumes to form the canali. We propose that the canali were mec hanically eroded by such carbonatite lavas and that the latter origina ted from the fusion of anhydrous recycled crust, which had been altere d by interaction with a CO2-, SO2-, and halogen-rich atmosphere.