The value of CHONS isotopic measurements of major compounds as probes of planetary origin, evolution, and habitability

1Southwest Research Institute, San Antonio, TX 2NASA Goddard Space Flight Center, Greenbelt, MD 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 4University of Maryland College Park, Greenbelt, MD 5University of Arizona, Tucson, AZ 6California Institute of Technology, Pasadena, CA 7The Open University, Milton Keynes, UK 8The Pennsylvania State University, University Park, PA 9Carnegie Institution of Washington, Washington, DC 10Mount Holyoke College, South Hadley, MA 11Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 12Southwest Research Institute, Boulder, CO 13Johns Hopkins University Applied Physics Laboratory, Laurel, MD 14Washington University, Saint Louis, MO 15Aix-Marseille Université, Marseille, France 16Arizona State University, Tempe, AZ

A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

Executive Summary
Carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS) are key ingredients for life, as well as components of volatile compounds that drive geological activity in many environments. CHONS isotopic ratios are a powerful tool for addressing critical questions in planetary science (Table 1). Existing isotopic measurements demonstrate the value of these data for understanding planetary origins and evolution and provide a roadmap for using isotopes to search for biosignatures. To address the science questions outlined in this white paper, we recommend investment in critical technologies ( Table 4) that enable isotopic measurements of extraterrestrial materials with high sensitivity, precision, and accuracy, including: (1) Sample preparation, separation, and introduction technologies for direct sampling and returned sample analyses; (2) Mitigation and reduction of contamination, including low-outgassing materials, maintaining spacecraft cleanliness through launch, and lessons learned from current and past missions; (3) Curational and calibration facilities; (4) Laboratory and modeling studies to develop frameworks for the interpretation of mission data. These capabilities will support investigation of possible mission targets that include Mars sample return, Ocean Worlds, the ice giants, comets, Kuiper Belt Objects (KBOs), and inner solar system targets including the Moon, Mercury, and Venus.

Introduction
Measurement of isotopic ratios is an essential tool for identifying the source and evolution of not only geologically and biologically important compounds but also planetary bodies. The mass difference in two isotopes of a given element (e.g., 13 C and 12 C) can lead to separation ("isotopic fractionation") between phases, among different compounds, through reaction networks, or via physical processes ( Table 2). Measurement of the isotopic ratios of a given compound combined with geological context can therefore provide valuable insight into the compound origins and/or a record of physical, chemical, or biological processes [1].

Origins
What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated? Isotopic ratios provide critical information about the early solar system. Application of oxygen isotope ratios to identify mixing of reservoirs as a significant process in the protoplanetary disk may be the most widely used example [2,3]. In addition, O, C and N isotopes in presolar grains have been used to identify the type of star from which the grains formed, and the high 2 H/ 1 H (D/H) and 15 N/ 14 N ratios of chondritic organics are commonly interpreted as evidence for formation at low temperatures. Further work is needed to understand the relationship between primitive isotopic reservoirs such as presolar grains, chondritic organics, and primordial water, and the current distribution of volatiles throughout the solar system. Future measurements of CHONS isotopic ratios in volatiles from the inner and the outer solar system, including sample return from small body populations, are needed to advance understanding of the processes key to solar system formation.

Evolution
Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth?
Comparative planetology provides context for interpretation of planetary climates. Martian isotopic data provide insight into atmospheric loss processes ( Fig. 1 right panel)

Biotic Processes
On Earth, organisms fractionate isotopes of H, C, N, and S during uptake, cellular development, and metabolism. Large fractionations are observed in H, C, N, and S isotope systems, suggesting that evidence for life could be identified in a planetary system from stable isotope measurements (Fig. 2). Indeed, stable isotopes of carbon and sulfur are often used to support claims of the earliest life on Earth [26], using modern-day observations of biological isotope fractionation. Biological isotope fractionation can be observed in several forms: mineral/rock deposits, organic matter, and trapped or escaping gases.
It is imperative for astrobiology to understand isotope fractionations for chemosynthetic pathways in varied conditions (e.g., temperature, oxidation state) and how such records could change over time. For example, biotic and abiotic processes lead to different C but similar H fractionations in methane [27]. This provides a framework for evaluating methane C and H isotopes on Mars or Titan for possible biosignatures, provided such fractionations are understood in specific planetary conditions, such as the effects of Titan's atmospheric chemistry and escape. The S isotope system is also often used to describe the modern and ancient influence of sulfate-reducing microbes in anaerobic systems. As seawater sulfate is consumed by modern organisms, the fraction of 34 S in unconsumed sulfate increases relative to 32 S [29] whereas H2S produced by these microbes is depleted in 34 S to an extent controlled by microbial species, sulfate concentration, and rate of respiration [30-32]. Such fractionations have been recorded in sulfate minerals or sulfate bound in the crystal lattice of carbonate minerals, as observed in terrestrial rocks > 2 Ga [33,34], with low δ 13 C in associated minerals suggesting a biological source [35]. Sulfate minerals are thus good targets for biosignature evaluation by in situ and sample return missions.
There are challenges in using isotopic biosignatures. First, all abiogenic mechanisms must be ruled out before an isotopic signature can be accepted as biogenic. For example, the Sample Analysis at Mars (SAM) mass spectrometer on the Mars Science Laboratory (MSL) has measured a range in d 34 S of sulfate and sulfide minerals comparable to that produced by biological systems on Earth. However, equilibrium fractionation and photochemistry together provide a feasible abiotic source for these observations [36]. Second, the magnitude of metabolic fractionation is affected by the concentration of electron donors and acceptors, the metabolic and burial rates, and the overall biotic productivity. Third, terrestrial microbial ecosystems typically change with depth due to the type and availability of electron acceptors (e.g., O2 or SO4 2-). Zones of microbes may interact over variable spatial scales, producing complex mixing trends (e.g. Fig.  6 in [37]). Fourth, in situ analysis of organic and mineral samples typically requires significant sample processing. Despite these challenges, biological isotopic trends appear to be predictable, and therefore are prime candidates for biosignatures. Thus, continued advancement of isotope analytical techniques for biosignature detection is crucial. Laboratory analyses of planetary materials offer high sensitivity and, in some instances, spatial resolution at sub-mm or better scales. Spatially resolved isotopic ratios can be obtained using Secondary Ion Mass Spectrometry (SIMS) or nanoSIMS, providing valuable petrologic context. Nuclear magnetic resonance (NMR) can be performed non-destructively, but has reduced sensitivity overall and does not provide petrologic context. Recent developments in laboratory analysis include position specific isotopic measurements, currently made via NMR [53] or pyrolysis-MS. For a molecule such as CH3COOH, acetic acid, such measurements can distinguish the carbon isotope ratio for the CH3 position from the COOH position. Such data show strong promise, but further development is needed to establish rigorous context for interpretation, as well as support more sensitive measurements via MS [54]. Other promising techniques include atomic probe tomography (APT), which can spatially resolve isotopic heterogeneity in nanoscale materials such as presolar grains [55].

Potential Mission Applications
Prioritization of isotope data for many different mission concepts supports the goals of the science community as a whole (Table 1). Notably, while major patterns and trends will emerge with data from multiple environments (Fig. 1), many of the science questions outlined here can be significantly constrained by a single mission platform.