Thermophysical and Compositional Trends of Lithified Bedforms on Mars

Abstract:

Recent work (Chojnacki et al., under review) has examined the widespread presence of lithified bedforms across Mars. These features – known as paleobedforms – resemble modern day dunes and mega-ripples, but also host impact craters and degradation characteristics of lithified rock. This duality implies these ancient bedforms underwent an enigmatic cementation process allowing their surface geometry to be preserved. In this study, we derive thermal inertia (TI) using the KRC model to understand the physical structure of these features and provide a quantitative constraint on their degree of lithification.

Low TI indicates an unconsolidated material like sand, whereas high TIs are consistent with lithified materials such as bedrock. However, the TIs for much of Mars fall within an intermediate range between the two endmembers. Such values are likely due to surficial coatings and not exclusively the degree of lithification. In these cases, the TI of paleobedforms was compared to that of their surrounding terrain. To derive TI, the KRC model uses brightness temperatures from images taken by the Thermal Emission Imaging System (THEMIS). Images are chosen by prioritizing: 1) pre-sunrise data, to ensure surficial thermal stability; 2) warm surface temperatures, to decrease instrument noise; and 3) low atmospheric dust, to decrease atmosphere contributions. Compositional data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) were collected to constrain mineralogy.

Here we present results from 24 of 39 paleobedform sites. Analysis of CRISM data indicates paleobedforms are commonly basaltic in composition. No paleobedform had high TI consistent with well-lithified rock, although one field might be considered weakly cemented. All other values fit into the intermediate range. Thus far, there is no clear relationship between the TI of paleobedforms and their surrounding terrain (i.e., the TI of paleobedforms is not consistently higher or lower than that of its surrounding terrain). Answering the question of how paleobedforms remain preserved without substantial evidence of cementation is critical in understanding the environmental conditions present during their formation.