WHAT’S MISSING FROM THE SNCS? ESTABLISHING A WISH LIST FOR RETURNED IGNEOUS SAMPLES FROM MARS

One of the first suggestions that the enigmatic group of meteorites known as the SNCs was indeed derived from Mars was based on their compositional similarity to Viking lander-analyzed soils (McSween & Stolper 1980, Sci. Am.), a fact later established though the finding of trapped atmospheric gases within the SNCs (Bogard & Johnson 1983, Science). Since that time, work on these meteorites as a petrologically diverse suite of basalts and ultramafic rocks from Mars has provided insights into the comparative planetology of Mars and Earth (e.g., McSween 1994, Meteoritics). The geochemistry of these samples, especially the ≤ 0.6 Ga shergottite subgroup – suggests that they are derived by partial melting of mantle reservoirs which were established by 4.5 Ga through the crystallization of a Martian magma ocean (e.g., Borg and Draper 2003, M&PS; Debaille et al. 2009, Nat. Geosci.). These reservoirs differ in long-term incompatible element enrichment and redox conditions (see review by McCoy et al. 2011, PNAS), which explains the observed differences in bulk REE patterns and variations in oxygen fugacity in the shergottites (e.g., as noted by McSween et al. 1996, GCA).

While the Martian meteorites provide the only samples of Mars currently available for laboratory study, there are limits to their interpretation. These limits arise because the impact-driven meteorite delivery mechanism (Head et al. 2002, Science), biases in favor of more competent samples – specifically, young igneous rocks (e.g., Walton et al. 2008, GCA), and their source craters are unknown. As such, most of the Martian meteorites are not representative of the whole Martian crust (McSween et al. 2009, Science). Whether the Martian mantle as viewed from the meteorites is applicable to all of Mars is also unknown (McCoy et al. 2011).

Important insights have been gained into the geology of Mars from spacecraft data and studies of the meteorites (e.g., McSween 2015, Am. Mineral.); however, elucidating the petrologic evolution of Mars will only be accomplished through the judicious selection, collection and return of igneous samples from the Martian surface, e.g., with the Mars 2020 sample-caching rover and follow-on sample return missions (Beaty et al. 2016, this meeting). This study outlines the desired characteristics of such samples (a.k.a. “Hap’s Wish List”).