TIMING OF FLUVIAL ACTIVITY IN THE XANTHE TERRA REGION OF MARS: IMPLICATIONS FOR THE HYPANIS DELTA AND OTHER POTENTIAL LANDING SITES FOR MARS2020 AND EXOMARS

The Xanthe Terra region of Mars exhibits multiple fluvial networks that are of interest for future exploration. The timing of these channels is poorly constrained and it’s not clear whether they formed at the same time or spanned a broader period of history. Furthermore, sedimentary fans that are sourced by some of the channels are currently under consideration as landing sites for ExoMars and Mars2020. It is therefore critical to understand their timing within the chronostratigraphic context of Mars. Current estimates, based on crater counts from the fans, places the fluvial activity within the Hesperian to Early Amazonian epochs (Hauber et al. 2013), relatively late in Mars history. Here, we present a new map for Xanthe Terra which includes (1) impact craters > 2 km in diameter, (2) fluvial networks, and (3) secondary crater rays. The map was constructed at 1:40,000 and is based on a near complete Context Camera (CTX) image mosaic (6 m/pixel). Here, we highlight one example of a cross-cutting relationship that is most relevant to the Hypanis Valles and Nanedi Valles fluvial networks. Hypanis sources a deltaic-like fan that is in consideration for both ExoMars and Mars2020. The continuous ejecta and secondary rays of a large (65 km) impact crater (named 15 N here) interact with both valley systems. Crater statistics derived from craters that superpose the ejecta blanket indicate a model age of 3.7 Ga, at the Late Noachian to Early Hesperian boundary. This count includes 2495 craters that are larger than 200 m over an area of 7018 km². We used High Resolution Stereo Camera (HRSC) digital terrain models (100 m) to determine the radial thickness variation of the ejecta to insure that all craters in our model age fit superpose the ejecta. Observations from our mapping indicate that the headwater channels of Hypanis Valles are partially destroyed by the distal toe of 15N’s ejecta blanket. Where the valley system extends beyond the ejecta, rays from 15N create large gouges on the interior valley walls. This suggests that Hypanis Valles pre-dates 15N, constraining its formation to > 3.7 Ga. By contrast, the well-preserved Nanedi Valles post-dates and incises the ejecta of 15N. Thus, 15N represents an unconformity between the events that formed these two independent rivers implying an episodic history of fluvial activity in this region.