Synthetic (Hydrothermal) Hematite-Rich Mars-Analog Spherules from Acid-Sulfate Brines: Implications for Formation and Diagenesis of Hematite Spherules in Outcrops at Meridiani Planum, Mars

The Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) orbiter discovered a large area at Meridiani Planum (MP) covered with the Fe-oxide hematite (alpha-Fe2O3) [1,2]. This discovery and favorable landing site characteristics led to selection of MP as the landing site for the Opportunity Mars Exploration Rover (MER) [3]. The Athena science payload onboard the Opportunity rover identified hematite-rich spherules (mean spherule diameter approx.4.2+/-0.8 mm) embedded in S-rich outcrop rock and also as lag deposits of whole and broken spherules [4,5,6,7,8,9]. Although the chemical and mineralogical compositions of spherules are not fully constrained, Moessbauer spectrometer (MB) Miniature Thermal Emission Spectrometer (Mini-TES) and chemical analyses from the Alpha Particle X-Ray Spectrometer (APXS) are consistent with a hematite mineralogical composition and an oxide bulk chemical composition consisting of Fe2O3. MGS-TES, also provides an important constraint that emission from the hematite-rich spherules is dominated by emission along the crystallographic c-axis [1,2,10,11]. The formation of hematite-rich spherules with similar chemical, mineralogical, morphological, and crystallographic properties to the MP spherules is rare on Earth, to date, only two natural analogs have been proposed; one from Utah (Navaho Concretions) and the other from Mauna Kea, Hawaii [12,13]. In this study, we synthesized in the laboratory hematite-rich spherules using conditions that may have existed on Early Mars [14] and compared their properties to those for MP hematite spherules of Mars and the analog spherules from Utah and Mauna Kea in order to assess their relative merit as MP hematite spherule analogs. Such comparisons yield clues to the formation pathway for MP spherules.