PHYSIOCHEMICAL DIFFERENCES BETWEEN LATE NOACHIAN AND EARLY HESPERIAN VOLCANIC TERRAINS IN THE MARTIAN HIGHLANDS: POSSIBLE CAUSES AND IMPLICATIONS

Volcanic resurfacing in the Martian highlands is best exemplified by early Hesperian (eH)-aged ridged plains, which are thought to be flood basalts erupted from fissures [e.g. 1]. Less areally extensive, but common, are mid- to late-Noachian (mN and lN)-aged volcanic units found in some intercrater plains and crater floors [1-3]. These units are mostly found in eastern Noachis Terra, Tyrrhena Terra, and Terra Cimmeria [2]. Using morphological, spectral and thermophysical observations, we show that volcanic surfaces in these two settings exhibit physiochemical differences. The older units typically exhibit relatively high thermal inertia values consistent with very little sediment cover, enrichments of olivine and/or pyroxene, and polygonally fractured appearances at the decameter scale. Conversely, the eH units typically exhibit thermal inertia values consistent with unconsolidated sediment, and bedrock exposures are rare. Though the eH units are olivine- and pyroxene-bearing [e.g. 4-5], surface materials do not show strong olivine enrichments compared to the mN/lN units, except in crater ejecta. Near the margins of Syrtis Major and Hesperia Planum, however, we observe olivine-enriched bedrock similar to that found in mNh and lNh intercrater units. This, combined with observations of olivine-enrichment in crater ejecta within eH units, suggest that the physiochemical differences between eH and mNh/lNh volcanic units are primarily related to differences in the extent and/or thickness of sediment cover, with more sediment cover on eH surfaces (though compositional differences between older and younger volcanic terrains are also present, e.g. [5-7]). This raises the question of why the younger volcanic surfaces are more heavily mantled than older surfaces. Possible explanations include: inefficient removal of sediment due to lower erosion rates in the Hesperian [e.g. 1], more efficient trapping of sediment, perhaps by wrinkle ridges (?), or differences related to eruption conditions (e.g. subglacial vs subaerial, volatile contents, etc.) that could lead to differences in texture and erodability. These scenarios will be evaluated in detail in future work. [1] Carr and Head, 2010 [2] Rogers and Nazarian 2013 [3] Tanaka et al. 2015 [4] Rogers and Christensen, 2007 [5] Ody et al., 2012 [6] Mustard et al. 2005