Depth profiles of venusian sinuous rilles and valley networks

More than 200 venusian channels and valleys have been mapped based on analyses of Magellan SAR images. Sinuous rilles are the most abundant channels among six types of venusian channels, and they are widely distributed on Venus. Morphological characteristics of venusian sinuous rilles include sinuous narrowing reaches, source depressions, and length of several 10s to a few 100s of km. This type of channels is known to exist on the Moon and possibly on Mars. Valley networks on Venus often occur in the vicinity of or in connection to sinuous rilles. Cross-sectional morphologies of sinuous rilles and valley networks are of special importance in discussing their formation processes both qualitatively and quantitatively. We reconstructed cross-sectional profiles of 6 sinuous rilles and 2 valley networks using a new radar clinometric method. Reconstructed cross-sections revealed that floors of the channels and valleys are clearly lower than the surrounding plains. This finding implies that the sinuous rilles and the valley networks have erosional origins. Longitudinal depth profiles of the sinuous rilles show distinct decreasing trends toward the termini. Such decreasing trends of depths are qualitatively in agreement with theoretical models and laboratory experiments of thermal erosion. In order to verify this assertion quantitatively, we conduct simple 1-dimensional model calculations under the assumption that both channel-forming lavas and ground substrate are tholeiitic basalt. For initial lava thicknesses in the range 2-6 m, the model calculations yield good matches to the depth profiles. Estimated duration of lava effusion ranges from several months to a few years. These numerical results support thermal erosion of the sinuous rilles but do not necessarily exclude contributions from mechanical erosion processes. Valley networks seem to have formed under a strong structural control in comparison to sinuous rilles. The valleys vary widely in characteristics of the depth profile and flow directions relative to surface slopes. Therefore valley networks appear to have originated from diverse formation mechanisms.