Aqueous alteration revealed by diverse mineralogy at Amazonian-aged Lyot crater, Mars

Located close to the hemispheric dichotomy, Lyot crater is the largest (with diameter 220 km) and deepest impact crater in the northern lowlands of Mars. The impact event could have released substantial amount of volatiles and strongly influenced the evolution of Amazonian Mars climate [1]. Previous works have shown that the impact event probably induced overland flow that formed the channels north of Lyot [2] and within the crater, kilometer-long valleys have formed in micro-environments that post-date the impact [3]. These features suggest that groundwater may have been mobilized in the subsurface, and there was melting of ice-rich deposits after the impact event. In a recent mineralogy survey [4], diverse hydrated minerals in and around Lyot crater have been revealed by data acquired from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). The concentration of hydrated minerals in this region is much greater than average northern plains, suggesting substantial aqueous alteration directly associated to the Lyot impact or in the regional stratigraphy. We have found 7 Fe/Mg phyllosilicate detections, likely smectite or mixed-layer smectite-chlorite, in the central ring region; 10 chlorite/prehnite detections (identified with a typical 2.35 µm absorption, in which 2 have an additional 1.48 µm absorption that matches prehnite, confirming [5]) in the central ring, crater floor, outer rim, and ejecta blanket; and 9 locations with absorptions at 2.21 µm with/without the coexistence with the 2.35 µm feature, which can be explained by illite/muscovite or a mixture of hydrated silica and chlorite/prehnite. The unit with 2.21-µm absorptions is often associated with the smooth mantling deposit, superposed on the valleys within the crater and on the crater rim. Further morphological analysis over these units with distinct mineralogy will shed light on the aqueous activity that formed these minerals. Combined with geological map of the region, we will establish the timeline of aqueous activities associated with the impact and provide insights into the climatic influence of Lyot impact in the Amazonian age. [1] Toon et al. (2010) Ann. Rev Earth Plan. Sci 38 [2] Harrison et al. (2010) GRL 37(21) [3] Dickson et al. (2010) GRL 36(8) [4] Pan, Ehlmann, Carter, Ernst, JGR Planets, submitted [5] Carter et al., (2010) Science.