Biosignatures & Paleobiology

Phosphates on Mars and Their Importance as Igneous, Aqueous, and Astrobiological Indicators

By Keith Cowing

Status Report

Life

January 12, 2025

Filed under apatites, biophysics, Biosignatures, Gale Crater, geochemistry, Gusev Crater, Jezero Crater, Mars, Mars Sample Return, merrillite, meteorites, Mount Sharp Group, organic chemistry, phosphate, Wishstone

Phosphates on Mars and Their Importance as Igneous, Aqueous, and Astrobiological Indicators — PIXL Red/Green/Blue (RGB) maps of P2O5, FeO, and CaO, respectively, highlighting occurrences of Ca phosphates in pink and Fe phosphates in yellow. White bars represent ~2 mm scale bars for each image. (A) Igneous merrillite in the Séítah abrasion patch Dourbes located within the mesostasis (meso) between cumulus olivine (Ol) and pyroxene (Px). (B) Detrital monomineralic apatite (Mono-Ap) in the Thornton Gap abrasion patch within the delta. (C) Fe phosphates in the Ouzel Falls abrasion patch within the Onahu conglomerate outcrop within the delta, and a close-up of a WATSON image showing the blue/green color of the Fe phosphates (WATSON image: SI1_0789_0736995354_613FDR_N0390926SRLC000 36_000095J01.IMG, enhanced with 1% contrast stretch). — Life

This paper reviews the phosphate phases in meteorites and those measured by landed spacecraft, what they reveal about past igneous and aqueous conditions on Mars, and important implications for potential prebiotic chemistry, past habitability, and potential biosignatures that could be detected in samples returned from Mars.

A review of the 378 martian meteorites as of 2023 indicate that of the two most common phosphate minerals in Mars meteorites, merrillite and apatites, the apatite composition is largely F- and Cl-rich, with shergottites containing more OH.

The phosphate concentrations examined across multiple missions show a relatively narrow range of phosphate, with higher concentrations observed in the Mount Sharp Group in Gale crater and Wishstone at Gusev crater and lower concentrations observed at Jezero crater floor and Jezero fan.

Possible secondary phosphates detected on Mars, including Fe phosphates at Jezero crater and Gusev crater and Ca- and Al-bearing secondary phosphates, temperatures of formation of secondary phases and their dissolution rates and solubilities are reviewed and summarized.

Despite this wealth of information about phosphates on Mars, due to their fine scale and relatively low concentrations, Mars Sample Return is needed to better understand phosphate and its implications for the igneous, aqueous, and astrobiological history of Mars.

Backscattered electron images of apatite and merrillite in martian meteorites. (A) Intergrown merrillite (Merr) and apatite (Ap) associated with maskelynite (Msk) and late-stage minerals, including ilmenite (Ilm), titanomagnetite (Tmt), and pyrrhotite (Po) in shergottite RBT 04261 (adapted from [25]). (B) Subhedral apatite (Ap) associated with clinopyroxene (Cpx), maskelynite (Msk), and late-stage pyroxferroite breakdown products (Pb) in shergottite Los Angeles (adapted from [25]). (C) Apatite (Ap) inclusions in Ca-rich clinopyroxene rims in shergottite Shergotty (adapted from [25]). (D) Anhedral apatite (Ap) associated with alkali-rich maskelynite (Ak-msk) and olivine (Ol) in chassignite NWA 2737 (adapted from [27]). (E) Apatite (Ap) rimming merrillite (Merr) in the matrix of regolith breccia NWA 7034 (adapted from [29]). (F) Anhedral merrillite (Merr) intergrown with carbonate (Carb) and apatite (Ap) in orthopyroxenite ALH 84001 (Opx = orthopyroxene) (adapted from [20]). — Life

Phosphates on Mars and Their Importance as Igneous, Aqueous, and Astrobiological Indicators, Life (open access)

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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