Morphologic differences in radial grooves on martian layered (fluidized) ejecta: Implications for emplacement processes and conditions

Abstract

Detailed morphologic and morphometric analysis of the radial grooves on Martian layered ejecta indicates that the grooves fall into three groups that correlate with different types of layered ejecta craters. We collected over 16,000 width and length measurements of 2402 radial grooves on the inner ejecta layers of 36 relatively fresh, layered ejecta craters. These include seven single layered ejecta craters (SLE), seven double layered Type-2 (DLE Type-2), ten double ejecta layer Type-1 (DLE-Type-1), and twelve multiple ejecta layer (MLE) test craters. These test craters are in the diameter range 8.3 km – 38.8 km for latitudes ranging from 45oS to 54oN. Radial grooves resembling longitudinal grooves in landslides throughout the solar system occur on all ejecta layers of SLE, DLE Type-2, and MLE craters (i.e., SDM grooves). This leads us to propose that SDM grooves are structures formed in flowing ejecta by the same processes that produce longitudinal grooves in landslides (e.g., shear and divergent flow). In contrast, the radial grooves on the inner and outer ejecta layers of DLE Type-1 are morphologically and morphometrically different from one another and from SDM grooves. The only type of geophysical flows that form grooves similar to grooves on the inner ejecta layers of DLE Type-1 craters are ones produced by high velocity (i.e., supersonic) blast surges associated with explosive volcanic eruptions (e.g., that occurred during the May 18th1980 eruption of Mount St. Helens USA). This leads us to propose that these Martian grooves are produced by high-speed outflow of particles and gas towards the last phase of crater formation. Furthermore, we propose that this high speed flow is generated when volatile-rich target materials slide into the crater cavity to mix with hot impactite. This generates a fuel-coolant explosion that creates a blast surge that subsequently produces the grooves on the inner ejecta layer. In addition, as the velocity of the outflow drops to subsonic outward of the inner ejecta layer, both erosion and deposition processes operate that result in deposition of a thin layer of material that contains channel-like grooves that are unique to this ejecta layer of this type of crater. This thin deposit is found on the outer ejecta layers, and beyond (on the terrain surrounding fresh DLE Type-1 craters). The production of these grooves on DLE Type-1 ejecta layers likely destroys and/or masks SDM grooves that may have initially formed on DLE Type-1 ejecta layers when they were initially emplaced as dense ground hugging flows.