THE UPS, DOWNS, INS, AND OUTS OF TWO COMPLETE VENUSIAN CHANNEL SYSTEMS: WHAT CAN THEY TELL US ABOUT THEIR FORMATION?

Constraining channel formation processes is critical for unraveling Venus’ geologic history. To that end, we have examined the morphology of two channels – Tawera Vallis (channel origin ~ 67.5° E, 10.8° S) and an unnamed channel (UC; channel origin ~ 116.8° E, 21.9° S) – that can be traced from their origin to terminus and show minimal evidence of post-formation deformation. They therefore may provide unique insight into formation conditions of at least some venusian channels. Tawera originates as an incised channel at a collapsed pit and extends for >180 km before it breaks into a ~140 km long distributary-like fan; width and depth decrease down channel and range from >1 km wide, >400 m deep near the origin to ~200 m wide, <100 m deep near its terminus. The UC has multiple sources that feed into a central channel; most feeder channels appear to be constructional and the main channel itself exhibits a transition from constructional to incised. The UC ends with a divergence of channels that feed into Morongo Vallis. Some sections of the UC (114.6° E, 21.9° S) show evidence of a narrower sinuous channel set within the larger channel and may be the result of waning flow within the system. Other sections (115.3° E, 21.6° S) contain stream-lined islands, which are in the widest segments and lowest channel gradients (<0.1°). Longitudinal profiles of both channels are concave-up with broad convex-up sections (knick zones) present. Channel breakouts and sinuosity increases (1.04 to 1.08; 1.03 to 1.1) are associated with the two knick zones at Tawera. At the UC, the relationship between knick zones and sinuosity is less obvious, but channel width increases at the start of each knick zone. When compared, both channels suggest channel morphology (sinuosity, width) may be influenced by gradient changes; fluid properties (composition, flow rate), as well as substrate lithology and duration of use, are also likely significant factors for channel formation. The fact that some channels provide evidence of post-emplacement deformation while others do not suggests channel formation is asynchronous and widespread in time. SAR and altimetry data from the upcoming VERITAS mission will likely reveal a more complex character to these (and other) channel systems and may be able to further clarify channel characteristics (shape, slope) providing tighter constraints on formation conditions.