FORMATION OF LINEAR GROOVES ON MARTIAN DOUBLE LAYERED EJECTA IMPACT CRATERS BY EROSIVE VORTICES

The inner ejecta layers of large, fresh (> 5 km diameter) Double Layered Ejecta (DLE) impact craters on Mars exhibit a spectacular pattern of closely-spaced radial grooves. These grooves cut straight across preexisting, low-relief topography, as well as all other types of flow features (e.g., flow lobes, roll waves) on this ejecta layer, suggesting that the grooves formed after ejecta flow halted, and its surface had stabilized. In a few cases, these grooves transition onto the outer ejecta layer where they are more sinuous, appear to be depositional, and quickly become indistinguishable from other flow features there suggesting that they formed penecontemporaneous, with the formation of flow features on the outer ejecta layer. Individual grooves can coalesce or disappear along their length, and are frequently variable in their cross-sectional shape. Typically the grooves are linear, narrow (<50-~200 m), shallow (<5-~10 m), and extend over distances of up to 10 km from the rim crest of the parent crater. Since their discovery 40 years ago, no hypothesis (e.g., landslides over ice-rich terrain, vortex ring expansion from a collapsing column of impact crater ejecta, or late-stage base-surges) advanced to date offers even a semi-quantitative model for these enigmatic features. Here, using ideas from meteorite impact, volcanic, and subaerial and subaqueous processes on Earth, we propose and quantify a model that a late-stage base surge of particle-laden fluid produced the grooves by vortex-scouring. This particle-laden fluid is inferred to have originated from within the crater cavity, probably late in the impact event when hot ejecta interacted with volatiles on the crater floor. Erupting mixtures of particle-laden fluids flowing along the ground are subject to a vortex-producing instability due to differences in concentration and velocity boundary-layer thicknesses. We formally refer to this mechanism as the “erosive channeling instability” (and informally as “the groovy instability”). Görtler vortices induced by concavity in the substrate are also likely to play a role. By analogy to grooves formed during the 1980 lateral blast at Mount St. Helens, we calculate that it is likely the grooves most likely formed in about 10 minutes, but potentially hours to days after the impact event.