Identification of Primary Noachian Crustal Blocks on Mars with Crism

Introduction: The nature, origin and evolution of Mars' Noachian crust is relatively unknown [1]. Detailed analyses of meteorites point to an early differentiation event for the planet but the only rock definitively of Noachian age is the Martian meteorite; ALH84001, comprised of >95% orthopyroxene [2]. Our understanding of the composition of the Martian crust is derived mostly from Amazonian-aged meteorites that are basaltic or cumulates derived from basaltic magmas [1]. Remotely sensed data show a primarily basaltic surface as well for the Hesperian-Late Noachian ridged plains [3,4]. It has been recently shown that phyllosilicates formed by alteration of igneous rocks are common in Noachain terrains [5,6]. Four billion years of impact gardening, aqueous alteration, and resurfacing has left few coherent sections of early Noachian crust exposed at the surface [7]. Apparently what remains exists largely in breccia blocks often embedded in a matrix containing alteration minerals. These remnants of ancient Martian crust offer a significant sampling of the earliest crustal formation on Mars and a starting mineralogy for the widespread aqueous alteration that occurred in the early Noachian. One hypothesis is that Mars initially had a magma ocean surface that crystallized mafic minerals [8]. Alternatively the crust may be an accumulation of extrusive and intrusive magmatism. Subsequent aqueous alteration of the mafics into phyllosilicates seems to be very widespread globally but constrained to the earliest Noachian period [9]. The identification and characterization of the earliest mafic crust will provide critical constraints on the early history of Mars. This work defines the indentifying characteristics of the early Noachian mafic crust and documents the initial search and cataloging of the features with observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument [10]. We consider which regions on Mars are most likely to preserve exposures of the early mafic blocks and search the existing hyperspectral data for mineralogical signatures of the Noachian crustal blocks. Datasets and Methods: The brecciated nature of the remnant early crust requires high-resolution observations to discern individual blocks and the component mineralogy. The CRISM instrument on the Mars Reconnaissance Orbiter (MRO) is capable of 18 m/pixel full resolution images and is used to detect the