Gravity‐Driven Differences in Fluvial Sediment Transport on Mars and Earth.

There is abundant evidence from fluvial landforms and deposits that Mars had rivers that actively transported sediment and shaped its surface. Sediment transport equations are playing a key role in quantifying river processes from these observations, which continue to increase in quality and quantity. In this study, we review sediment transport equations developed on Earth and isolate the effect of gravity for the case of an alluvial channel. We compare 33 formulas used to calculate the sediment transport rate, under transport‐limited conditions, for grain sizes that range from silt to boulders and a lognormal sediment distribution. Results indicate that for a given discharge, channel morphology and grain size, the lower gravity on Mars compared to Earth results in: (a) larger grains mobilized on Mars and transported in suspension, and (b) larger suspended sediment transport rates on Mars and therefore larger total transport rates. Importantly, the effect of gravity is different for bed load and suspended load, with nonlinearity at the bed load‐suspended load transition zone. Therefore, typical total‐load transport relations that do not distinguish between bedload and suspended load are not appropriate for other planets as they simplify the effect of gravity. Gravity‐driven differences in fluvial sediment transport should produce differences in sediment sorting, morphology and stratigraphy between Earth and Mars. Additionally, our results show how Earth‐derived fluvial sediment transport theory can be applied beyond Mars to other planets and moons. Plain Language Summary: There is much evidence that Mars had rivers that actively transported sediment and shaped its surface. Preserved ancient landscapes altered by water provide valuable insights into past processes on the planet's surface and the presence of water. To better understand these landforms, we rely on knowledge gained from systems on Earth. However, is it fair to do so when the gravity on Mars is much lower? How does gravity affect sediment transport and the landforms created by water? In this study, we isolate the effect of gravity on sediment transport by water with an analytical river model. We used 32 sediment transport formulas to compare sediment transport rates on Earth and Mars for the same conditions except gravity. The results show that larger grains are picked up by the flow on Mars and the transport rate of sediment traveling in suspension is higher, and therefore total transport as well. Because grains transported near and on the bed are less affected than the grains in suspension, the effect of gravity varies with the way of transport and hence grain size. Therefore, gravity‐driven differences in sediment transport by water should produce differences in sediment sorting, morphology and stratigraphy between Earth and Mars. Key Points: Fluvial sediment transport rates are higher on Mars than on Earth for the same discharge and channel geometryGravity affects suspended sediment transport more than bed load transport and could hence lead to differences in morphology and stratigraphyTotal load sediment transport equations should be avoided on other planets and moons due to simplified attribution of gravity [ABSTRACT FROM AUTHOR]