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.