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Evidence for Deposition of Chloride on Mars From Small-Volume Surface Water Events Into the Late Hesperian-Early Amazonian

Leask, Ellen K. and Ehlmann, Bethany L. (2022) Evidence for Deposition of Chloride on Mars From Small-Volume Surface Water Events Into the Late Hesperian-Early Amazonian. AGU Advances, 3 (1). Art. No. e2021AV000534. ISSN 2576-604X. doi:10.1029/2021av000534.

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We determine the environmental setting and timing of enigmatic large-scale chloride deposits on Mars, examining all available high-resolution imagery globally, building digital elevation models, surveying targeted infrared images intersecting chloride deposits for accessory minerals (e.g., clay, carbonate, and sulfate), and performing crater counting for age-dating when possible. We find that chloride deposits are commonly draped atop underlying topography, often associated with channels, sometimes perched above deep craters in local topographic lows, and span a wide range of elevations. Where measurable, chloride deposit thicknesses are typically <3 m. The deposits range in elevation over tens of meters, inconsistent with previous hypotheses of playa-like environments, which are very flat. Chloride deposit elevations within larger basins are typically asymmetric with chloride deposits much higher on the inlet channel side, indicating that (a) surface water runoff was the most likely water source and (b) the basins were likely not filled completely with water in a deep lake setting but rather chlorides formed within a series of shallow ponds. Mass balance allows local sourcing from mobilization of cations and chlorine from dust/soil. Though prior global-scale studies report chloride deposits superimposed on Noachian terrains, crater-counting of local, stratigraphically-associated datable surfaces shows chlorides superimposed on 3.4–2.3 Ga volcanic terrain. This indicates that surface waters forming chloride deposits continued through the Hesperian and into the Amazonian, making them among the latest-formed, large-scale deposits of water-related minerals on Mars. Collectively, data suggest formation from meltwater from episodic ice/snow deposits continuing until 2.0–2.5 Ga.

Item Type:Article
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URLURL TypeDescription ItemNASA Planetary Data System InCaltech News
Leask, Ellen K.0000-0002-3220-4003
Ehlmann, Bethany L.0000-0002-2745-3240
Additional Information:© 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Issue Online: 07 January 2022; Version of Record online: 07 January 2022; Accepted manuscript online: 27 December 2021; Manuscript accepted: 11 November 2021; Manuscript revised: 08 November 2021; Manuscript received: 17 July 2021. We appreciate the reviews of Mikki Osterloo, Tim Glotch, an anonymous reviewer, and comments from the editor, Francis Nimmo, that improved this manuscript. Thanks to Jay Dickson for assistance with GIS systems and discussion of Antarctic lakes and to Tom McCollom for discussion of water chemistry. We are thankful for support from CRISM co-investigator funds to B. L. Ehlmann, a National Aeronautics and Space Administration (NASA) Mars Fundamental Research grant to B. L. Ehlmann (#NNX12AB42G) and a Engineering Research Council of Canada PGS-D scholarship to E. K. Leask. The authors declare no conflicts of interest relevant to this study. Data Availability Statement: All Mars data used in this paper are publicly available through the NASA PDS node (, and we thank Mikki Osterloo for making the Osterloo et al. (2010) polygons of chloride detections from THEMIS readily available on the Arizona State JMARS site. Derived statistical data compiled to construct figures are available in tabular form at the CaltechDATA site (Leask & Ehlmann, 2021).
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Subject Keywords:chloride deposits; Mars; aqueous alteration; liquid water; paternoster lakes; Amazonian
Issue or Number:1
Record Number:CaltechAUTHORS:20220106-106515500
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Official Citation:Leask, E. K., & Ehlmann, B. L. (2022). Evidence for deposition of chloride on Mars from small-volume surface water events into the Late Hesperian-Early Amazonian. AGU Advances, 3, e2021AV000534.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112747
Deposited By: Tony Diaz
Deposited On:08 Jan 2022 22:27
Last Modified:02 Feb 2022 23:43

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