Compositional Constraints on Hematite-rich Spherule (blueberry) Formation at Meridiani Planum, Mars

Introduction: Meridiani Planum was chosen as the landing site for the Mars Exploration Rover Opportunity partially based on Mars Global Surveyor Thermal Emission Spectrometer data indicating an abundance of hematite [1]. Hematite often forms through processes that involve water, so the site was a promising one to determine whether conditions on Mars were ever suitable for life [1,2]. Opportunity struck pay dirt; it’s Miniature Thermal Emission Spectrometer (Mini-TES) and Mössbauer Spectrometer (MB) confirmed the presence of hematite in sulfate-rich sedimentary beds and in lag deposits [3]. Meridiani Planum rocks contain three main components: silicate phases, sulfate and possibly chloride salts, and ferric oxide phases such as hematite [2]. Primary igneous phases are at low abundance [4, 5] despite the basaltic origin of the protoliths. Jarosite, an alkali ferric sulfate, was identified by Mössbauer. Some of the hematite is contained in the spherules, and some resides in finer grains in outcrops [2, 5]. Mössbauer and Mini-TES data indicate that hematite is a dominant constituent of the spherules [5-7]. Panoramic Camera (Pancam) and Microscopic Imager (MI) images of spherule interiors show that hematite is present throughout [2]. The exact composition of the spherules is unknown. Mini-TES only identifies a hematite signature in the spherules; any other constituents have an upper limit of 5-10% [7]. The MB data are consistent with the spherules being composed of only hematite [5].