Insights into complex layered ejecta emplacement and subsurface stratigraphy in Chryse Planitia, Mars, through an analysis of THEMIS brightness temperature data

Abstract

Layered ejecta craters on Mars have been interpreted to indicate the presence of volatiles in the substrate, making them important targets for the investigation of sites of astrobiological significance. If the ejecta are associated with the presence of water in the substratum, specific surface grain size trends are expected. In this study we explore the distribution of grain sizes in the layered ejecta of impact craters located in Chryse Planitia, using Thermal Emission Imaging System (THEMIS) thermal infrared data. Ejecta grain size trends, in conjunction with ejecta mobility and lobateness values, are applied to assess the degree of surface flow of the ejecta, and in turn to constrain the plausible volatile abundance, cohesion, and fine particle content of the target materials. Craters with a larger fraction of small grain sizes in their ejecta showed greater ejecta mobility and lobateness, consistent with a water-rich and/or a low-cohesion target. Craters displaying decreasing grain size with increasing radius had smaller diameters and lower ejecta mobility and lobateness, indicating only a minimal component of surface ejecta flow. Ejecta grain size trends varied with crater diameter, from which the presence of vertical compositional stratigraphy in Chryse Planitia is inferred and interpreted. Our observations are synthesized into a number of plausible geologic scenarios for Chryse Planitia. Martian LE Craters as Probes of Subsurface VolatilesMartian layered ejecta blankets are distinct from the most common ejecta morphologies observed on the Moon and Mercury, which are characterized by ballistic rayed deposits, with the ejecta texture becoming finer with increasing radial distance [Melosh, 1989]. The formation of LE blankets has been attributed to the effects of the target lithology (material strength, grain sizes, volatile content), atmospheric drag, and JONES ET AL.LAYERED EJECTA GRAIN SIZE 986 PUBLICATIONS

Subsurface Sediment Mobilization in the Southern Chryse Planitia on Mars

“…Insets 3 and 4 show different possibilities for the ascent off liquefied sediment and were derived from Mazzini and Etiope (2017) and Skinner and Mazzini (2009), respectively. They show that sediment from several strata in the stratigraphy may have contributed to the extrusions, consistent with the complex stratigraphy in Chryse Planitia (Jones et al, 2016). This is also the case for the spatial distribution of the observed edifices at local scale.…”

“…Such a sedimentary architecture may have favored the release of sedimentary volcanic material with different water content and effusion rates from different depths, perhaps even at the same time (see inset 4 in Figure 7b). A complex stratigraphy of Chryse Planitia has indeed been independently suggested on the basin of layered impact crater ejecta characteristics (Jones et al, 2016). A stack of sedimentary layers containing substantial portions of mud (Jöns, 1985) and some volcanic material (Scott & Tanaka, 1986), possibly hosting aquifers (Rodríguez et al, 2007), and ice layers or lenses (Carr & Head, 2019) would be a plausible source for liquefaction, subsurface sediment mobilization, and sedimentary volcanism.…”

“…An additional problem for an igneous interpretation of the mapped landforms is posed by the presence of subsurface volatiles. The distribution of rampart craters on the highlands and on the smooth plains (marked by blue polygons in Figure 1) is evidence for subsurface water and/or ice across the investigated area (e.g., Jones et al, 2016). Moreover, specific morphological features such as a lobate-shaped ejecta blanket, numerous pits on the crater floor, and the fluvial landforms in the crater interior associated with the young (<5 Ma) Mojave crater suggest that volatiles may persist in the subsurface even in the very recent past (Werner et al, 2014).…”

Fluidized Appearing Ejecta on Ceres: Implications for the Mechanical Properties, Frictional Properties, and Composition of its Shallow Subsurface

“…In this model we treat the FAE as ground-hugging continuous flows similar to landslides, debris flows, granular flows, or snow avalanches, which is consistent with how Martian fluidized ejecta have been interpreted to have been deposited and has been observed in laboratory ejecta experiments (e.g., Jones et al, 2016; Mouginis-Mark & Baloga, 2006; Runyon & Barnouin, 2018). In the model proposed here (equation (3)) we establish upper bound values for the coefficient of sliding friction encountered by FAE on Ceres by ignoring flow within the cohesive mass of ejecta.…”

Testing landslide and atmospheric‐effects models for the formation of double‐layered ejecta craters on Mars

“…In contrast, Jones et al. () reported that several DLE craters exhibited larger particles at the ejecta margins, but our detailed review of these craters suggests that they are unlikely to be DLE craters and require updated classification. We suggest that the presence of surface ice at the time of DLE crater formation is ultimately responsible for the observations discussed above.…”