Sulfur Abundances and Distribution on the Moon and Mercury: Insights from Sample Analysis and Remote Sensing

Core Concepts

This chapter reviews the current understanding of sulfur's abundance, distribution, and behavior on the Moon and Mercury, highlighting key similarities and differences between these two celestial bodies.

Abstract

• Bibliographic Information: Renggli, C.J., Steenstra, E.S., Saal, A.E. (2024). Sulfur in the Moon and Mercury. In D. Harlov & G. Pokrovski (Eds.), The Role of Sulfur in Planetary Processes: From Cores to Atmospheres. Springer Geochemistry.

• Research Objective: This chapter aims to provide a comprehensive overview of sulfur on the Moon and Mercury, synthesizing data from lunar samples and remote sensing of Mercury to understand the factors controlling sulfur distribution and behavior on these bodies.

• Methodology: The chapter reviews and analyzes data from over 50 years of lunar sample analysis, including S concentrations and isotopic compositions from various missions. For Mercury, the chapter relies on remote sensing data from the MESSENGER mission, particularly XRS and GRS measurements.

• Key Findings:

  • Lunar mare basalts exhibit a narrow range of δ34S compositions, suggesting limited S degassing during eruption.
  • Pyroclastic glasses, on the other hand, show evidence of significant S degassing, with lighter δ34S values and surface coatings enriched in sulfur.
  • Lunar soils are enriched in heavy S isotopes, while picritic glasses are depleted.
  • Mercury's surface exhibits surprisingly high sulfur abundances (up to 4 wt.%), pointing to a sulfur-rich interior and potentially distinct sulfur cycling processes compared to the Moon.

• Main Conclusions:

  • The contrasting sulfur abundances and isotopic signatures in different lunar sample types reveal diverse sulfur behavior during lunar magmatism and volcanic processes.
  • The high sulfur content on Mercury's surface suggests a sulfur-rich planetary building block or a unique differentiation history that concentrated sulfur in the crust.
  • Future missions like BepiColombo are crucial for refining our understanding of sulfur on Mercury and its implications for the planet's formation and evolution.

• Significance: This research contributes to our understanding of the geochemical evolution of airless terrestrial bodies, highlighting the importance of sulfur as a tracer for magmatic processes, volatile degassing, and planetary formation conditions.

• Limitations and Future Research:

  • Limited sample data from the Moon, particularly for specific lithologies like ferroan anorthosites, restricts a complete understanding of lunar sulfur geochemistry.
  • The lack of direct sampling on Mercury necessitates reliance on remote sensing data, which provides limited information about the mineralogy and isotopic composition of sulfur-bearing phases.
  • Future lunar sample return missions and in-situ analysis on Mercury by missions like BepiColombo will be crucial for addressing these limitations and refining our models of sulfur behavior on these bodies.

Stats

The high-Ti basalts contain on average 1758 ppm S, ranging from 1174 to 2770 ppm.
Low-Ti basalts contain approximately 820 ppm S on average, ranging from 396 to 1700 ppm.
The average S content retained in the high-Ti orange glasses is 351 ppm (100 to 750 ppm).
The S content in the low-Ti and very low-Ti green glasses is 476 ppm (289 to 816 ppm) and 267 ppm (67 to 2317 ppm) respectively.
Apollo 16 soils have the lowest S abundances at 536 ± 170 ppm.
Apollo 16 impact melt breccias have S abundances of 534 ± 310 ppm.
The most S-rich soils sampled by Apollo are from Apollo 11 (833 ± 174 ppm on average) and Apollo 17 (874 ± 246 ppm).
The most S-rich breccias are regolith breccias from Apollo 11 (1120 ppm) and soils (949 ± 86 ppm) as well as impact melt breccias from Apollo 17 (722 ± 195 ppm).
High-Ti basalts have an average δ34S composition of 0.77‰ (-0.2 to 2‰).
Low-Ti basalts have an average δ34S composition of 0.31‰ (-1.6 to 1.2‰).
Lunar mare basalts have a mean isotopic composition of δ34S = 0.58 ± 0.05‰, Δ33S = 0.008 ± 0.006‰, and Δ36S = 0.2 ± 0.2‰.
The δ34S isotopic compositions of troilites in Chang’e-5 basalts show significantly more variations from 2 to -1.6‰, with a calculated bulk isotopic composition of δ34S = 0.35 ± 0.25‰.
The highest δ34S in lunar samples were measured in soil samples, with the highest values measured in Apollo 15 and 16 soils (13.3‰ and 13.5‰, respectively).
Numerical modeling of diffusive degassing of very-low-Ti Apollo 15 glasses suggests pre-eruptive water contents of up to 745 ppm, where 98% of the initial water content was lost.
Sulfur is less degassed compared to H2O, Cl, and F, at a loss rate of 19%.
The 74220 orange glasses have a S concentration of 345 ± 64 ppm and the olivine hosted melt inclusions a S concentration of 573 ± 159 ppm.
The orange glasses lost 40 ± 10% of their S concentration by degassing upon eruption.
MESSENGER constrained the surface abundance of FeO on Mercury to 0.6-2.4 wt.%.

Quotes

"The oxygen fugacities of planetary interiors, and the magmatic and crustal rocks derived from them, are important for the understanding of the behavior of S during planetary processes due to the polyvalent nature of the element, with a valence range from 2- to 6+."
"The mantles of the Moon and Mercury are so reducing that sulfur likely only exists in the reduced state S2-, in sulfides, silicate melts, or volcanic gases."
"In contrast, Mercury is the least investigated terrestrial planet in the inner solar system."

Key Insights Distilled From

Sulfur in the Moon and Mercury

by Christian J.... at arxiv.org 10-25-2024

https://arxiv.org/pdf/2410.18599.pdf

Deeper Inquiries

How might the differences in sulfur abundance and distribution on the Moon and Mercury inform our understanding of the formation and evolution of other airless bodies in the solar system, such as asteroids?

The contrasting sulfur stories of the Moon and Mercury offer valuable comparative planetology insights that can be extrapolated to understand the formation and evolution of other airless bodies, particularly asteroids:

Accretion and Differentiation: The stark difference in sulfur abundance between the Moon (relatively S-poor) and Mercury (S-rich) suggests distinct accretion histories and possibly different starting materials in the protoplanetary disk. This resonates with the diversity observed in asteroid compositions, with some being sulfur-rich (e.g., carbonaceous chondrites) and others sulfur-poor. Studying sulfur abundance and isotopic ratios in asteroids can help decipher their formation locations and potential connections to specific planetary bodies.
Volcanic Degassing and Surface Processes: While volcanic degassing played a role in shaping the sulfur distribution on both the Moon and (possibly) Mercury, the extent and style of volcanism differed significantly. On airless asteroids, where volcanic activity might have been limited, impact heating and vaporization could be a dominant factor influencing sulfur distribution. Comparing and contrasting the sulfur signatures in lunar pyroclastic glasses with those potentially found in asteroidal regolith can shed light on the relative roles of these processes.
Space Weathering: The lack of a substantial atmosphere on both the Moon and Mercury, as well as on asteroids, exposes their surfaces to space weathering processes like solar wind implantation and micrometeorite impacts. These processes can alter the surface composition and isotopic signatures of sulfur. Understanding how space weathering affects sulfur on the Moon and Mercury can help us develop models to decipher the intrinsic compositions of asteroids and extract valuable information about their histories.
Sulfur-Bearing Minerals: The mineralogy of sulfur on the Moon and Mercury, primarily as troilite (FeS) in lunar mare basalts, provides a reference point for interpreting remote sensing data of asteroids. Identifying similar or different sulfur-bearing phases on asteroids can provide clues about their geological history, thermal evolution, and potential for hosting water ice or other volatiles.
By applying the lessons learned from the Moon and Mercury, we can use sulfur as a powerful tracer to unravel the diverse origins, evolutionary paths, and potential resource availability of airless bodies throughout the solar system.

Could alternative mechanisms, besides volcanic degassing, contribute to the observed sulfur isotopic variations in lunar soils and breccias?

While volcanic degassing is a primary factor influencing sulfur isotopic variations on the Moon, other mechanisms can contribute to the observed signatures in lunar soils and breccias:

Impact Processes:

Vaporization and Condensation: High-energy impacts can vaporize target rocks, including sulfur-bearing phases. During cooling, this vapor can condense, leading to isotopic fractionation as heavier isotopes are preferentially incorporated into the condensates. This process can create localized sulfur isotopic anomalies in impact melt breccias and surrounding regolith.
Mixing of Target Rocks: Impacts excavate and mix materials from different depths and geological units, each potentially having distinct sulfur isotopic compositions. This impact-induced mixing can create heterogeneous sulfur isotopic signatures in breccias and soils.

Space Weathering:

Solar Wind Implantation: The solar wind, a stream of charged particles from the Sun, can implant sulfur ions into the lunar surface. This implanted sulfur will have a distinct isotopic composition compared to indigenous lunar sulfur.
Sputtering: Micrometeorite bombardment can sputter (eject) atoms from the lunar surface, leading to preferential loss of lighter isotopes and relative enrichment of heavier isotopes in the remaining material.

Photochemical Processes: Ultraviolet radiation from the Sun can induce photochemical reactions on the lunar surface, potentially leading to sulfur isotopic fractionation. However, the efficiency of these processes in the lunar environment is not well-constrained.
Distinguishing between these processes requires careful analysis of sulfur isotopic compositions in conjunction with other geochemical and geological data, such as the presence of impact-related features, space weathering signatures, and the distribution of other volatile elements.

If we were to discover life on other planets, would we expect to find sulfur playing a similarly crucial role in their biochemistry as it does on Earth?

While it's challenging to predict the exact biochemistry of extraterrestrial life, sulfur's chemical versatility makes it a strong candidate for playing a vital role:

Abundance and Availability: Sulfur is a relatively abundant element in the universe, often found in volcanic environments and hydrothermal systems, which are considered potential cradles for life. Its presence in various oxidation states makes it accessible for biological uptake and utilization.
Essential for Key Biomolecules: On Earth, sulfur is a critical component of two essential amino acids (cysteine and methionine), which are the building blocks of proteins. It's also crucial for several enzyme cofactors involved in vital metabolic processes like energy transfer and redox reactions.
Alternative Biochemistry: Even if extraterrestrial life doesn't rely on the same DNA-RNA-protein system as Earth-based life, sulfur could still be essential. For instance, it could be incorporated into alternative biomolecules or play a role in energy metabolism in non-oxygen-based biochemistries.
Extremophiles as Analogs: The existence of extremophiles on Earth, thriving in environments rich in sulfur compounds (e.g., hydrothermal vents, volcanic lakes), demonstrates life's adaptability to utilize sulfur in various forms. This suggests that extraterrestrial life could have evolved similar strategies in sulfur-rich environments.
However, it's also plausible that life could arise using different elements altogether, depending on the specific environmental conditions and available resources.
In conclusion, while we cannot definitively say that sulfur is a universal requirement for life, its abundance, chemical properties, and importance in terrestrial biochemistry make it a prime candidate for playing a significant role in extraterrestrial biology. The discovery of life on other planets would provide invaluable insights into the diversity of biochemistry and the potential for sulfur-based or alternative life forms in the universe.