Channels and valleys on Venus.

摘要

Venusian channel meanders properties generally follow terrestrial river trends of wavelength (L) to width (W) ratios, suggesting an equilibrium adjustment of channel form. The unusually low L/W values for some sinuous rilles probably indicate modification of original meander patterns by lava-erosional channel widening. A drainage channel constructed by typical basaltic lavas can be significantly deepened and widened by highly fluid and hot superheated tholeiitic basalt or ultramafic lava to form sinuous rilles. Canali-type channels are unique because of their great lengths (up to 6800 km) and nearly constant channel cross sectional shapes along their paths. Common terrestrial-type tholeiite lava cannot sustain a superheated and turbulent state for the long distances required for thermal erosion of canali within allowable discharge rates. If canali formed mainly by constructional processes, sustained discharge rates might travel the observed distances. An exotic low-temperature, low-viscosity lava like carbonatite or sulfur seems to be required for the erosional genesis of canali. The "Outflow Channel" has a morphology similar to that of terrestrial catastrophic flood channels and Martian outflow channels. Calculated discharge rates and power/unit area imply that the channel was carved by very energetic lava flows. Valley network formation seems to require sapping processes which may be caused by low-viscosity lavas moving through a relatively permeable, fractured, medium. The close associations of many sinuous rilles with coronae leads to the hypothesis that mantle-plume volcanism caused the high-effusion and sustained lava eruptions essential for sinuous rill formation. Canali lengths exceed the lengths of other common volcanic channel types on Venus, implying a large volume of lava and long duration of the eruption. An hypothesized global resurfacing event late in history may be responsible for canali formation. Longitudinal profiles of canali-type channels show clear evidence of deformation by plains tectonism. Deformation occurred at multiple scales as evidenced in the profiles. The range of lava channel morphology on Venus is wider than for other terrestrial planets and the Moon. This is probably attributed to the volcanological conditions unique to Venus and to a lack of intense erosional processes.