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

Paul B Niles,D C Golden,Joseph Michalski,Douglas W Ming

doi:10.1038/s41467-017-01227-7

Paul B Niles, D C Golden + Show 2 more

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https://doi.org/10.1038/s41467-017-01227-7

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Abstract

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 cryo-concentration of acidic solutions may counteract the slow kinetics. Here we show that cryo-concentrated 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.

Highlights

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
Mass balance calculations have shown that sufficient SO2 was degassed during this period to form massive sulfate-rich deposits[11]. 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 standstill[12, 13]
At temperatures below 0 °C acidic solutions can become increasingly concentrated[14] through ice formation, which may enhance acid-weathering despite the slower kinetics at such low temperatures suggesting that sulfate formation may be possible at temperatures below 0 °C

Summary

Introduction

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. 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. 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 history[1, 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 deposits[10, 11]. 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

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