USING THE MORPHOLOGY OF IMPACT CRATERS AS A RELATIVE AGE INDICATOR FOR FLUVIAL ACTIVITY AT XANTHE TERRA, MARS

Sedimentary fans, sourced by fluvial channels on Xanthe Terra, Mars, are currently being investigated as landing sites for Exomars and Mars2020. However, the age of these channels and fans is poorly constrained. Both missions hope to sample Late Noachian to Early Hesperian-age (> 3.6 Ga) sedimentary material to identify evidence for past life. Here, we present new relative age constraints from Xanthe Terra. Unlike previous attempts to directly date fans by crater counting, we estimate the relative age of the channels that source fans through an analysis of the superposition relationships between channels and craters. We suggest that the degradation state of craters is a proxy for time. Craters that are near pristine, in terms of the ratio of their depth (d) to diameter (D), are younger than more modified basins. For this analysis, we measured the d and D for 318 craters larger than 2 km using High Resolution Stereo Camera (HRSC) digital terrain models (100 m). From these data, we determined that the power law relationship between d and D for both simple and complex craters is most similar to the Tornabene et al. (2013) model. We therefore use the Tornabene et al. (2013) relationship to define our pristine morphologic class which includes 110 craters that have a depth that is within 80% to 100% of the model. The size frequency distribution (SFD) of the pristine class, plotted against martian chronology functions, provides a model age of 3.6 Ga. This implies that any > 2 km size crater that impacted Xanthe Terra within the last 3.6 Ga (Early Hesperian or younger) still exhibits a near pristine morphology. In contrast, the SFD of all craters provides a model age of 3.8 Ga (Late Noachian). This result is consistent with the hypothesis that the majority of surface denudation on Mars occurred during the Late Noachian to Early Hesperian. It also provides an important age metric to compare against the channels. For example, several channels like Nanedi Valles cross-cut the most pristine class of craters implying a relative age of < 3.6 Ga. However, channels such as Hypanis Valles and Sabrina Valles, both sources of fans, are superposed by craters that are modified at or near 80% from the model pristine depth. We suggest that this constrains these fans to at least the Early Hesperian which is within the age constraint for both ExoMars and Mars2020.