Elevated olivine weathering rates and sulfate formation at cryogenic temperatures on Mars

ARTICLE DOI: 10.1038/s41467-017-01227-7 OPEN Elevated olivine weathering rates and sulfate formation at cryogenic temperatures on Mars Paul B. Niles1, Joseph Michalski2, Douglas W. Ming1 & D.C. Golden3 Large Hesperian-aged (~3.7 Ga) layered deposits of sulfate-rich sediments in the equatorial regions of Mars have been suggested to be evidence for ephemeral playa environments. But early Mars may not have been warm enough to support conditions similar to what occurs in arid environments on Earth. Instead cold, icy environments may have been widespread. Under cryogenic conditions sulfate formation might be blocked, since kinetics of silicate weathering are typically strongly retarded at temperatures well below 0 °C. But cryoconcentration of acidic solutions may counteract the slow kinetics. Here we show that cryoconcentrated acidic brines rapidly chemically weather olivine minerals and form sulfate minerals at temperatures as low as −60 °C. These experimental results demonstrate the viability of sulfate formation under current Martian conditions, even in the polar regions. An ice-hosted sedimentation and weathering model may provide a compelling description of the origin of large Hesperian-aged layered sulfate deposits on Mars. 1 Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX 77058, USA. 2 Department of Earth Sciences and Laboratory for Space Research, University of Hong Kong, Hong Kong, China. 3 ESCG, Houston, TX 77058, USA. Correspondence and requests for materials should be addressed to P.B.N. (email: ) NATURE COMMUNICATIONS | 8: 998 | DOI: 10.1038/s41467-017-01227-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01227-7 T he nature of the Martian climate through history remains an extremely important and unresolved question in planetary science. Despite huge amounts of data returned from orbiters and landers on Mars, there still remains uncertainty about whether the Martian surface environment ever supported an Earth-like climate and if so, for how long. Certainly, substantial geomorphological evidence has revealed that water has flowed across much of the Martian surface. However, this evidence is generally insufficient to further constrain the timescales involved, and many of the channels, deltas, lakes, and other features could represent only brief water activity1, 2. Substantial oxidized sulfur in Martian soils was detected by the Viking landers and has been suggested to form via oxidation of sulfide minerals or via reactions with acidic aerosols in the atmosphere3–6. However, more recent rover and orbiter results have revealed the presence of large deposits of layered sulfate-rich sediments (up to 30% SO3) that largely occur within Hesperianaged deposits on the Martian surface. On Earth, large deposits of sulfate-rich sediments typically form from bodies of liquid water and therefore their presence on Mars has been taken as evidence for widespread (though transient) lacustrine environments (~3.7 Ga)7. Yet, massive deposits of relatively young sulfates are also observed around the north pole8, in an environment that is very unlikely to have involved surface water. If it can be shown that sulfates can form under the cold, dry conditions in the Martian polar regions on Mars in the Amazonian, it should force a serious reconsideration of how equatorial sulfate-bearing sediments formed in the Hesperian. Understanding the aqueous Results Acid weathering experiments. We performed laboratory experiments to simulate weathering of olivine by thin films of acid fluids at temperatures between −40 and −60 °C. The experiments subjected fine grained (5–53 µm) olivine particles to small amounts of 0.5 M sulfuric acid mixed with 400 µm silica beads at −40 and −60 °C in order to form a 10 µm liquid film coating each bead. These experiments were intended to simulate the exposure of small dust grains to thin films of sulfuric acid at cryogenic temperatures inside ice deposits on the surface of Mars. The longest duration experiments (12 days) yielded sulfate minerals even at temperatures, as low as −60 °C (Fig. 1). While these experiments intentionally and artificially induce contact between the mineral grains and acidic fluids, it is likely that this will occur naturally on Mars, where dust grains in the atmosphere will serve as nucleation points for ice and acidic aerosols which will ensure close contact between the reactants15. a b environments which produce sulfate minerals on Mars is important to understand the evolution of the Martian climate. In this paper we consider an alternate, more uniformitarian view of the ancient Martian climate, contending that prolonged warm temperatures were never present except in the earliest stages of its history, and that the atmosphere and climate have been similar to modern conditions throughout most of the planet’s history1, 9. In this model, the formation of layered sulfate deposits represents a period of intense volcanic outgassing in a cold, dry climate where outgassed sulfur was concentrated into icy ash/dust deposits10, 11. Mass balance calculations have shown that sufficient SO2 was degassed during this period to form massive sulfate-rich deposits11. In particular, it has not been clear how sulfate minerals could form at temperatures below 0 °C where the very low kinetics might effectively lower the weathering rate of basaltic minerals to a virtual standstill12, 13. Very slow dissolution rates have been measured during experimental dissolution of olivine and basaltic glass at temperatures as low as −19 °C in a CaCl2–NaCl–H2O brine13. However, at temperatures below 0 °C acidic solutions can become increasingly concentrated14 through ice formation, which may actually enhance acid-weathering despite the slower kinetics at such low temperatures suggesting that sulfate formation may be possible at temperatures below 0 °C. We have conducted a set of experiments to test this hypothesis and show that the weathering rates of olivine at temperatures below 0 °C are sufficient to form sulfates in this cold, liquid water limited environment over timescales < 1000 years. c S Counts 2K 1K Mg O Na Si Fe 5 Energy (keV) Fig. 1 SEM secondary electron images and EDS spectra of experimental products recovered after 12 days at −60 °C. Recovery was accomplished by freeze drying rather than quenching with NaOH solution (see Methods: Quenching Procedure and Analysis). a Run products are located on 400 µm glass spheres used in experiments. Scale bar is 200 µm b Magnified view of mineral grain identified with red box in 1A. Red cross indicates location of EDS measurement. Scale bar is 10 µm c Energy Dispersive X-ray Spectroscopy (EDS) data that show substantial enrichments in Mg and S consistent with the presence of a Mg-sulfate mineral 2 Olivine dissolution rates and activation energy. After quenching the experiment with a sodium acetate buffer, the concen (...truncated)