The sinuous Baltis Vallis on Venus: potential mechanisms of formation and design of an analogue model

Sinuous channels on the terrestrial planets indicate formation by a fluid. One of the longest continuous channels in the solar system, measuring over 7000 km, is Baltis Vallis on Venus (Fig. 1, Baker et al. 1992, Bray et al. 2007). The sinuous nature of this and many other channels has been used to infer formative conditions in the past through comparison of channel and bend geometry with meandering rivers on Earth and lava channels on Earth and the Moon. The viscosity of the flow is inferred to determine meander properties (Baker et al. 1992). In particular, the radius of curvature increases faster with wavelength for lava channels and rilles on Earth, Venus and Mars than for aqueous channels on Earth and Mars (Bray et al. 2007). This data has been used to infer the viscosity of lava flow on Venus and the formative mechanism of channels such as Baltis Vallis. However, these inferences are based on outdated concepts of meandering rivers on Earth, while the processual analogies between collapsed lava tubes, erosional lava flows and meandering rivers are weak at best. Here we develop testable hypotheses from existing theories and a preliminary design of landscape experiments (analogue models) to test such hypotheses based on earlier channel-forming and meandering experiments.

Recent work on river meandering (see for review Kleinhans et al. 2024) shows that a meandering channel shows evidence of lateral migration, whereas a sinuous channel does not. Meandering channels, such as also found on Mars, erode outer-bend banks, which is matched by sedimentation on the inner bank such that the channel maintains an approximately constant width over a large length. Moreover, the wavelength of the meanders, while highly variable, is correlated with channel width. This is explained by physics-based theory for bar and bend formation as a function of channel width-to-depth ratio, as data compared to numerical models and landscape experiments demonstrate (Kleinhans et al. 2024). Finally, some protection against cutoff through the inner bend is required, which can be any form of apparent cohesion, such as caused by vegetation, mud, permafrost or something else. The absence of any evidence of lateral migration in Baltis Vallis indicates that meandering mechanisms do not explain the bends. We must therefore turn to different mechanisms that can form sinuous channels.

Lava tubes and eroding lava both require a fluid, perhaps even turbulent flow. Moreover, explaining the bends requires either a mechanism analogous to the nonlinear sediment transport processes in meandering rivers that lead to bar and bend growth and cutoff, or a certain topographic or subsurface variability on the planetary surface in which a flow path is formed. To start with the first: there are currently no theories known that can explain lateral ‘bar’ in stability in a flow that is much more viscous than water. Moreover, this would require much wider and shallower channels than found on Venus. Our earlier experiments demonstrate several different options, such as nonlinear erosion by a flow which forms irregular bends (Marra et al. 2014) or selection of a non-random path by a flow where its momentum determines the sharpest bends that are taken around the topographic or subsurface variability (Kleinhans et al. 2009).

This brief sketch of theory and reasoning leads to a first design of experiments enabling testing of formation of bends by viscous flows in analogue models in the laboratory. First, the flow must either contain a variation of particle sizes or be turbulent for bed erosion to occur. Second, the flow must be so viscous, or dense, that its width-to-depth ratio is lower than in meandering rivers. Our work on debris flow experiments shows that this can be accomplished with poorly sorted sediments and water. Alternatively, a flow path forms by fluidization within a plain of static but otherwise the same material. Pilots show that this can be accomplished with hyperconcentrated flows in low-density plastic materials. Third, the flows cannot be strong enough to erode outer banks. This is the most critical point. If successful, experiments with provide morphological indications of flow-bank interactions with different mechanisms, ultimately allowing evidence-based inference of channel-forming conditions on Venus.

Figure 1. A 300 km segment of Baltis Vallis showing a sinuous channel in north-eastern direction. The channel is about 2 km wide. Topographic gradient is from left to right. Source: Magellan radar data, contrast-enhanced for visualization. 

Funded by NWO grant 19823

References

Baker, V. R., G. Komatsu, T. J. Parker, V. C. Gulick, J. S. Kargel, and J. S. Lewis (1992), Channels and valleys on Venus: Preliminary analysis of Magellan data, J. Geophys. Res., 97(E8), 13,421– 13,444.

Bray, V. J., D. B. J. Bussey, R. C. Ghail, A. P. Jones, and K. T. Pickering (2007), Meander geometry of Venusian canali: Constraints on flow regime and formation time, J. Geophys. Res., 112, E04S05, doi:10.1029/2006JE002785.

Kleinhans, M.G., Schuurman, F., Bakx, W. and Markies, H. (2009). Meandering channel dynamics in highly cohesive sediment on an intertidal mud flat in the Westerschelde estuary, the Netherlands. Geomorphology, 105, 261–276, https://doi.org/10.1016/j.geomorph.2008.10.005

Kleinhans, M.G., McMahon, W.J. and Davies, N.S. (2023), What even is a meandering river? A philosophy-enhanced synthesis of multi-level causes and systemic interactions contributing to river meandering. Geological Society, London, Special Publications 540, https://doi.org/10.1144/SP540-2022-138

Marra, W. A., Braat, L., Baar, A. W. and Kleinhans, M. G. (2014), Valley formation by groundwater seepage, pressurized groundwater outbursts and crater-lake overflow in flume experiments with implications for Mars. Icarus 232, 97-117, http://dx.doi.org/10.1016/j.icarus.2013.12.026