| Autor: |
Preston, Sarah L.1,2 (AUTHOR) slp7@ucla.edu, Siebach, Kirsten L.1 (AUTHOR), Lapôtre, Mathieu G. A.3 (AUTHOR), Banham, Steven G.4 (AUTHOR) |
| Předmět: |
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| Zdroj: |
Journal of Geophysical Research. Planets. Nov2024, Vol. 129 Issue 11, p1-23. 23p. |
| Abstrakt: |
The Stimson formation is a late‐infilling eolian sandstone in Gale crater, Mars that formed from sand accumulation in a dune field analogous to the modern active Bagnold dune field, enabling a unique opportunity to compare the past to the present dune fields on Mars. Previous work suggested that the Stimson has a coarser grain‐size distribution than the active Bagnold dunes based on three images of the Stimson. We analyze grain size in the Naukluft and Emerson plateaus of the Stimson by observing 115 images throughout the formation to classify textures and quantitatively measuring grains in eight representative individual images. Results indicate that the Stimson has a primary grain size mode at <200 μm. In addition, more than 50% of the observed Stimson rock targets display a coarser grain population with a long‐tailed distribution including grains ∼600–1200 μm. The primary grain size mode is similar to that observed in the Bagnold dunes, but the coarse grain size mode was neither observed in the Bagnold dunes nor in ripples adjacent to the dune field. Models for saltation mechanics indicate that the favored grain size for eolian transport on Mars, ∼100–200 μm, is independent of atmospheric density, though atmospheric density affects the wind speeds at which grains can be transported by winds. We conclude that the source of the Stimson dunes was more proximal and coarser than the source of the Bagnold dunes and that the paleoatmosphere was likely not significantly denser than the modern Martian atmosphere. Plain Language Summary: The Curiosity rover on Mars observed ancient cemented sand‐dune deposits called the Stimson formation near the modern and active Bagnold dunes. These observations provide a unique opportunity to compare past and present environments on Mars. Here, we present measurements of the size of sand grains in the Stimson deposits to compare with previously published measurements of the active Bagnold sands. We minimize bias by using observations from two cameras to find proportionate contributions of texture groups and quantify grain sizes for each texture group. We find that more than half of the images of Stimson deposits contain grains that are coarser than the coarsest grains observed near the modern sand dunes. Previous work hypothesized that these coarser grains could imply that the Stimson dunes formed under either a denser or a thinner atmosphere. We use sediment transport theory and observations to show that the atmosphere was likely similar to modern Mars. Instead, the coarse grains reveal that there must have been a supply of coarse grains nearby when the Stimson dunes formed, likely from eroding riverbeds. Cementation of the Stimson dunes required the presence of liquid water and may thus have required a denser atmosphere, possibly indicating cyclic atmospheric changes through time. Key Points: Comparing ancient and modern windblown sediments enables the study of Mars' eolian environments through timeAbout half of the targets in the Stimson sandstone contain a coarse fraction of grains (>500 μm) that is not observed in the modern Bagnold dunesThe presence of coarse grains in the Stimson necessitates a proximal coarse‐grained source [ABSTRACT FROM AUTHOR] |
| Databáze: |
GreenFILE |
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