The Attributes and Formation Mechanisms of Kallistos Vallis, Venus

Abstract:

Kallistos Vallis is a 1200-km-long Venusian channel system that commences in a region of chaotic terrain and terminates at distal plains mantled by lava flows. Some channel reaches are tens of kilometers wide and in places are characterized by the presence of streamlined erosional residuals about which the system complexly anastomoses. The system has an average longitudinal slope of less than 0.1 degrees. The region of chaotic terrain at the head of the system is interpreted as the surface expression of a deeply-rooted igneous plumbing system that conveyed large volumes of mafic or ultramafic magma to the surface from subcrustal depths. The flow conditions that might have been associated with development of component channels were estimated partly on the basis of known constraints of the Venusian environment and of the nature of volcanic analogs located on other solar system bodies. Lava flows with depths of 5 m and 20 m and viscosities of 1 Pa s would have been fully turbulent on essentially all channel slopes, and could have reached velocities of tens of meters per second and discharges of up to tens of millions of cubic meters per second on longitudinal slopes no greater than one degree. Flows with viscosities of 1 Pa s are expected to have had a capacity for thermomechanical incision of bedrock substrates at rates of several meters per day, facilitated by the relatively high gravitational acceleration and hot surface temperatures of Venus. In general, erosion should have been most significant along reaches located closer to volcanic sources, and constructional channel development should have been most favored along distal reaches. Realistic conditions could have allowed for formation of the entire channel system in as little as tens of days, though development during multiple discrete eruptive episodes separated by geological time is also possible. A total erupted volume of thousands to tens of thousands of cubic kilometers of lava is estimated to have been required for development of Kallistos Vallis, depending on how much of the system was formed through erosion. The eruptive and flow conditions predicted for this system are broadly aligned with those previously determined for large ancient volcanic channels of other rocky bodies of the inner solar system.