Reconstructing river flows remotely on Earth, Titan, and Mars

Abstract

Alluvial rivers are conveyor belts of fluid and sediment that provide a record of upstream climate and erosion on Earth, Titan, and Mars. However, many of Earth’s rivers remain unsurveyed, Titan’s rivers are not well resolved by current spacecraft data, and Mars’ rivers are no longer active, hindering reconstructions of planetary surface conditions. To overcome these problems, we use dimensionless hydraulic geometry relations—scaling laws that relate river channel dimensions to flow and sediment transport rates—to calculate in-channel conditions using only remote sensing measurements of channel width and slope. On Earth, this offers a way to predict flow and sediment flux in rivers that lack field measurements and shows that the distinct dynamics of bedload-dominated, suspended load-dominated, and bedrock rivers give rise to distinct channel characteristics. On Mars, this approach not only predicts grain sizes at Gale Crater and Jezero Crater that overlap with those measured by the Curiosity and Perseverance rovers, it enables reconstructions of past flow conditions that are consistent with proposed long-lived hydrologic activity at both craters. On Titan, our predicted sediment fluxes to the coast of Ontario Lacus could build the lake’s river delta in as little as ~1,000 y, and our scaling relationships suggest that Titan’s rivers may be wider, slope more gently, and transport sediment at lower flows than rivers on Earth or Mars. Our approach provides a template for predicting channel properties remotely for alluvial rivers across Earth, along with interpreting spacecraft observations of rivers on Titan and Mars.

Dimensionless argument: a narrow grain size range near 2 mm plays a special role in river sediment transport and morphodynamics

“…The data for gravel bed and sand bed rivers are bracketed for the most part by the range Cz = 7 to 20. This suggests that the modified sediment entrainment relation should be valid for flow velocities up to the range 1-3 m s −1 , which are reasonable estimates for bankfull velocity in rivers (Parker, 2014;Birch et al, 2023). The implication is that material in the nominal size range ≥ 1 mm is not subject to significant suspension in typical flood flows of alluvial rivers.…”

“…Generalizing from their 2 mm criterion, as the content of grains in the bed with dimensionless size < D * = 51 is increased, the transport of grains with dimensionless size > D * = 51 is enhanced. The formulation is now directly applicable to rivers on Mars and Titan, as well as Earth, in so far as it can be applied to a heavenly body with arbitrary gravitational acceleration g, a fluid with arbitrary kinematic viscosity ν, and a sediment particle of arbitrary submerged specific gravity R. Birch et al (2023) illustrate how the dimensionless number D * transforms into dimensioned numbers for Mars, where gravitational acceleration is significantly lower, and Titan, where gravitational acceleration is even lower, the clasts are ice rather than quartz, and the fluid is a mix of methane and ethane. They show that the discriminator D * = 51 translates to about 2.66 mm on Mars (mafic sediment in water at 20 • C) and 3.16-4.42 mm on Titan (ice particles in liquid methane/ethane at 84-96 K).…”

Signatures of wave erosion in Titan’s coasts

“…First, depositional landforms appear to be rare—only one major coastal depositional feature has tentatively been identified on Titan, a possible river delta on Ontario Lacus at Titan’s south ( 24 ). Second, Titan’s sediment is probably more buoyant in hydrocarbon liquids than Earth’s sediment is in water ( 6 , 8 , 45 – 47 ), so sediment might be removed from the coastal system more easily. Third, Titan’s north polar seas are thought to be in highstand conditions ( 13 ), flooding steep topography.…”

“…1A ) ( 3 , 4 ). Draining into Titan’s largest lakes and seas are actively flowing rivers ( 5 , 6 ) with lower reaches that appear to be flooded, forming embayments along the coasts ( Fig. 1A ) ( 1 , 7 , 8 ).…”