ARTICLE IN PRESS JID: YICAR [m5G;October 19, 2016;21:30] Icarus 000 (2016) 1–10 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Considering the formation of hematite spherules on Mars by freezing aqueous hematite nanoparticle suspensions M.R. Sexton a , M.E. Elwood Madden a , A.L. Swindle b , V.E. Hamilton c , B.R. Bickmore d , A.S. Elwood Madden a,* a School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd, Norman, OK 73019 b Wichita State University, Wichita KS 67260, United States c Southwest Research Institute, Boulder CO 80302, United States d Department of Geological Sciences, Brigham Young University, Provo UT 84602, United States a r t i c l e i n f o Article history: Available online xxx a b s t r a c t The enigmatic and unexpected occurrence of coarse crystalline (gray) hematite spherules at Terra Merid- iani on Mars in association with deposits of jarosite-rich sediments fueled a variety of hypotheses to explain their origin. In this study, we tested the hypothesis that freezing of aqueous hematite nanoparti- cle suspensions, possibly produced from low-temperature weathering of jarosite-bearing deposits, could produce coarse-grained hematite aggregate spherules. We synthesized four hematite nanoparticle suspen- sions with a range of sizes and morphologies and performed freezing experiments. All sizes of hematite nanoparticles rapidly aggregate during freezing. Regardless of the size or shape of the initial starting ma- terial, they rapidly collect into aggregates that are then too big to push in front of a stable advancing ice front, leading to incohesive masses of particles, rather than solid spherules. We also explored the ef- fects of “seed” silicates, a matrix of sand grains, various concentrations of NaCl and CaCl 2 , and varying the freezing temperature on hematite nanoparticle aggregation. However, none of these factors resulted in mm-scale spherical aggregates. By comparing our measured freezing rates with empirical and theoret- ical values from the literature, we conclude that the spherules on Mars could not have been produced through the freezing of aqueous hematite nanoparticle suspensions; ice crystallization front instability disrupts the aggregation process and prevents the formation of mm-scale continuous aggregates. © 2016 Elsevier Inc. All rights reserved. 1. Introduction 1.1. The surprising form of coarse crystalline hematite on Mars: spherules New and surprising observations have emerged at every step as scientists have continued to refine the resolution of analy- ses used to study hematite on Mars. Earth-based observations of Mars detected the abundant and wide-spread red, dust-borne nanophase hematite that contributes to the distinctive color of Mars (Morris and Lauer, 1990). Later, orbital spacecraft instruments with mapping capabilities such as the Mars Global Surveyor Ther- mal Emission Spectrometer (MGS TES) discovered significant abun- dances (∼10–60%) of coarse crystalline (gray) hematite in Meridi- ani Planum, Mars (Christensen et al., 2000). Thermal infrared (TIR) spectra of this crystalline hematite lack an emissivity minimum at * Corresponding author. E-mail address: amadden@ou.edu (A.S. Elwood Madden). 390 cm -1 (25.64 μm) (Christensen et al., 2001), indicating that the hematite is crystallographically oriented to provide emission pre- dominantly from crystallographic planes perpendicular to the c- axis (Lane et al., 2002). Furthermore, the spectral characteristics and band depths were found to be consistent only with tested ma- terials having grain diameters ∼10 μm or larger, more than 1000 times larger than the nanophase ferric oxide in the worldwide dust (Christensen et al., 2000). Based on the stratigraphic relation- ships of the coarse gray hematite, the favored formation scenario was some type of chemical precipitation involving aqueous fluids (Christensen et al., 2000). The Mars Exploration Rover (MER) Opportunity landed in Merid- iani Planum to examine gray crystalline hematite deposits in situ, and discovered that the hematite occurs not as schistose or loose particles, but as mm-scale spherules inferred to be groundwater concretions (Klingelhofer, 2004; Squyres et al., 2004). The sur- prising occurrence of hematite spherules led to new questions about how such spherules could form and exhibit crystallographic orientation/c-axis emission within the geologic and hydrologic http://dx.doi.org/10.1016/j.icarus.2016.10.014 0019-1035/© 2016 Elsevier Inc. All rights reserved. Please cite this article as: M.R. Sexton et al., Considering the formation of hematite spherules on Mars by freezing aqueous hematite nanoparticle suspensions, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.10.014