¹Thomas M. McCollom,²Sally L. Potter-McIntyre,¹Andres Reyes,³Bruce Moskowitz,³Peter Solheid,⁴Victoria E. Hamilton
Jpurnal of Geophysical Research: Planets Link to Article [https://doi.org/10.1029/2025JE009489]
¹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
²Southern Illinois University, Carbondale, IL, USA
³Department of Earth and Environmental Sciences and Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN, USA
⁴Southwest Research Institute, Boulder, CO, USA
Published by arrangement with John Wiley & Sons

A key early discovery of the Mars Exploration Rover Opportunity on Meridiani Planum was the hematite spherules that are a ubiquitous component of the Burns formation sandstones at the rover’s landing site (colloquially known as “blueberries”). The Meridiani spherules possess a suite of characteristics that are collectively very rare in terrestrial settings, including their gray color, a thermal spectral signature that indicates preferential exposure of the c crystal axis, a spherical shape that is evidently attributable to radially oriented crystallite growth, and high chemical and mineralogical purity. The origin of the Meridiani “blueberries” has remained a matter of considerable debate, but one leading hypothesis is that they formed through the decomposition of iron-rich sulfate minerals from the alunite group, specifically jarosite. To date, however, there has been no described terrestrial analog where the formation of hematite spherules is shown to be directly linked to jarosite decomposition. Here, we report the discovery of hematite spherules in Aztec Sandstone that possess many of the same characteristics as the martian “blueberries,” albeit with substantially smaller size. The spherules occur primarily in narrow gray bands within mineralized fractures where the pore spaces are predominantly occupied by jarosite-alunite solid solutions (JASS). The spherules formed through partial decomposition and release of Fe³⁺ from adjacent JASS, supporting the possibility that analogous processes may have been responsible for the formation of hematite spherules during diagenesis of the sulfate-rich Burns sandstones on Mars. Continued study of the Aztec Sandstone spherules may provide new constraints on near-surface environmental conditions on early Mars.

^1,2Marina E. Gemma,³Katherine A. Shirley,¹Timothy D. Glotch,^2,4,5Denton S. Ebel,^2,5,6Kieren T. Howard
Journal of Geopyhsical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2025JE008963]
¹Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
²Department of Earth and PlanetarySciences, American Museum of Natural History, New York, NY, USA,
³Atmospheric, Oceanic, and Planetary Physics,University of Oxford, Oxford, UK
⁴Lamont‐Doherty Earth Observatory, Columbia University, Palisades, NY, USA
⁵Department of Earth and Environmental Sciences, Graduate Center of the City University of New York, New York, NY,USA
⁶Department of Physical Sciences, Kingsborough College, City University of New York, Brooklyn, NY, USA
Published by arrangement with John Wiley & Sons

aboratory spectral analysis of well-characterized meteorite samples can be employed to more quantitatively analyze asteroid remote sensing data in conjunction with returned extraterrestrial samples. In this work, we examine the combined effects of physical (temperature, particle size) and chemical (petrologic type, metal fraction) variables on visible and near-infrared (VNIR) spectra of ordinary chondrite meteorite powders. Six equilibrated ordinary chondrite meteorite falls were prepared at a variety of particle sizes to capture the spectral diversity associated with asteroid regoliths dominated by various grain sizes. Mineral compositions and abundance were determined from electron microprobe analysis of meteorite thick sections to precisely characterize changes in spectral features due to variations in mineralogy. VNIR spectra of the ordinary chondrites were measured under simulated asteroid surface conditions at a series of temperatures chosen to mimic near-Earth asteroid surfaces. The resulting spectra show minimal variation in both major absorption bands across the simulated near-Earth asteroid temperature regime. Changes in particle size result in variations in band centers and band area ratios for material of the same composition, two key parameters typically used to derive asteroid composition. Unlike previous spectral investigations of ordinary chondrites, we retained the metal fraction in our powders instead of analyzing only the silicate fraction. Metal has a subtle but non-negligible effect on the VNIR spectra of ordinary chondrites. The more petrologically pristine samples from each ordinary chondrite group display relatively weaker absorption bands than their more thermally altered counterparts. The band centers shift to longer wavelengths as grain size and petrologic type increase.

¹S. P. Aithala,¹R. A. Lange,¹M. M. Hirschmann
Journal of Geophysical Research: Planets (in Press) Open Access Link to Article [https://doi.org/10.1029/2025JE009148]
¹Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA, 2Department ofEarth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA
Published by arrangement with John Wiley & Sons

To elucidate the relationship between oxygen fugacities (f_O2) recorded in martian basalts and redox processes in the martian interior, superliquidus 100-kPa furnace experiments on a composition similar to Humphrey (Adirondack basalt) were conducted at variable f_O2 and temperature. Quenched glasses were analyzed by EPMA, Mössbauer spectroscopy, colorimetric wet chemistry, and microbeam X-ray absorption near edge structure (XANES) spectroscopy. The experiments reveal Mössbauer and wet chemical determinations of silicate glass Fe³⁺/Fe^T agreeing within uncertainty, supporting the accuracy of extended-Voigt-based fitting of Mössbauer spectra when recoil-free fraction is considered. Fe³⁺/Fe^T ratios determined from Mössbauer spectroscopy from Humphrey and previously studied martian-relevant glass compositions are combined to calibrate models that characterize the relationship between Fe³⁺/Fe^T, f_O2, temperature, and composition in martian silicate liquids. The models demonstrate, similar to previously investigated silicate liquids, that the correlation between $$ and log f_O2 in martian magmas has a slope less than the value (0.25) expected if ferric and ferrous iron oxide mixed ideally. Martian magma Fe³⁺/Fe^T ratios are more temperature-sensitive compared to non-martian compositions, suggesting that temperature variations may contribute to comparatively large f_O2 variations in martian basalt. The models are applied to demonstrate that the Fe³⁺/Fe^T increases required to explain multiple-log unit changes in f_O2 in shergottite magma would not increase terrestrial magma f_O2 as effectively. To aid in future investigations of martian magma redox, a XANES technique that allows for non-destructive, microanalytical characterization of Fe³⁺/Fe^T in natural martian materials and martian-relevant experiments is introduced.