Exploring, Sampling, and Interpreting Lunar Volatiles in Polar Cold Traps

Academic Background

The permanently shadowed regions (PSRs) at the lunar poles, along with other surface volatile deposits, are critical sampling targets for future human missions to the Moon. These volatiles hold significant scientific value and could potentially shift strategies for sustained human activity on the Moon. However, sampling these extremely cold deposits, returning them to Earth, and interpreting their volatile records pose substantial challenges for the Artemis program. Volatiles in these reservoirs may consist of various species whose stable isotope characteristics could elucidate their sources and formation processes. For example, the hydrogen isotope (δD) of potential contributors to the deposits can be used to identify a uniquely light solar wind component. Due to the exceptionally low temperatures of these volatile deposits, sampling, preserving, curating, and analyzing these samples are far more complex than for other sample return missions. Collecting and preserving samples at cryogenic temperatures dramatically increases scientific yield but is technologically demanding and poses increased risks during transport.

Source of the Paper

This paper was authored by Charles K. Shearer, Zachary D. Sharp, and Julie Stopar, affiliated with the Institute of Meteoritics and the Center of Stable Isotopes at the University of New Mexico, and the Lunar and Planetary Institute, respectively. Published on December 16, 2024, in PNAS, the paper is titled “Exploring, Sampling, and Interpreting Lunar Volatiles in Polar Cold Traps.”

Research Process and Results

1. Mechanisms of Volatile Trapping on the Lunar Surface

The lunar poles are unique environments where topography and solar angles combine to create ultracold regions favorable for the long-term retention of volatiles near the surface and the formation of metastable surface deposits. Key scientific objectives for the Artemis and South Pole surface missions include investigating the locations, compositions, ages, sources, trapping mechanisms, loss mechanisms, cycles of deposition and loss, distribution, and volumes of volatiles, as well as any interactions and chemical processes occurring within the regolith and near-surface locations containing volatiles. These investigations will likely be accomplished through in situ surface measurements and eventual sample return.

2. Permanently Shadowed Regions (PSRs)

PSRs are areas without direct solar illumination, resulting in very low surface radiative temperatures, sometimes below 40 K. PSR locations are highly dependent on topography and are particularly found within craters at both lunar poles. The low temperatures in many polar PSRs allow for the sequestration of various volatile species, such as water ice and CO2 ice, over relatively long timescales. Nonpolar PSRs are too warm to serve as persistent cold traps for volatiles.

3. Transiently Shadowed Regions (TSRs)

TSRs are areas that receive partial solar illumination temporarily or seasonally. Similar to PSRs, TSRs can occur outside the polar regions, such as on the night side or adjacent to boulders. However, at the poles, the continuously low sun elevation results in many shadowed areas. These shadows are highly influenced by the shape of nearby terrain and vary over time. Only a few areas near the poles receive illumination for more than 50% of the time.

4. Ice Stability Regions (ISRs)

An ISR is a region where temperatures are low enough for a particular volatile phase to persist for long durations. On the Moon, an ISR is an area where temperatures are ~100 K or less, sufficiently cold to preserve water ice on a million-year timescale. ISRs can occur at the surface or at depth within the regolith. While ISRs can theoretically harbor water ice, further measurements and observations are needed to confirm its presence.

5. Ages of Volatiles

The formation history and ages of volatile deposits are relevant to many scientific objectives addressing the sources of water in the solar system. Studies suggest that polar ISRs and the persistence of ice within them have varied over time, with some areas possibly preserving larger deposits within the subsurface. Ancient ice needs to be protected by burial under impact ejecta or regolith to avoid significant loss and destruction mechanisms.

6. Interactions Between Volatiles and Regolith

While the presence of surface water ice has been proposed for parts of the polar regions and PSRs, water ice is thought to be much more stable and long-lasting if buried by at least a thin regolith covering. Temperature variations between the surface and subsurface can drive processes such as sequestration, cold trapping, diffusion, and chemical reactions. Surface volatiles are also subject to modification by radiation, solar wind influx, and changes in illumination and temperature over time.

7. Sources of Lunar Volatiles and Stable Isotope Characteristics

The isotopic composition of water and other volatiles provides a fingerprint for their sources and the timing of their contribution. Molecular water (in the form of ice) is likely to occur in the PSRs of the poles, sourced from comets, hydrous chondrites, or solar wind. Other forms of water exist as structural or trace hydroxyl (OH−) in minerals or as H+ or OH− resulting from solar wind bombardment of the lunar regolith.

8. Strategies for Collecting Volatiles During Artemis Program Surface Activities

The collection, preservation, curation, and allocation of cold volatile-rich samples will evolve during the growth of the Artemis program. Artemis III is likely to have volatile sampling capabilities similar to the Apollo program. During Apollo, special samples were isolated from the crew cabin and terrestrial atmosphere using various sample containers. For Artemis III, samples placed in sealed containers will contain regolith with a higher expected percentage of volatiles, which has profound implications for the preservation of regolith stratigraphy, volatile–regolith interactions, and crew safety.

Conclusions

Volatile-rich regolith associated with PSRs, TSRs, and ISRs provides important sampling targets for future human missions to the Moon (Artemis). The integration of observations from orbit, on the surface, and of samples returned to the lab will answer many fundamental scientific questions concerning the nature of volatile reservoirs on the Moon and their origins. Stable isotopes of the various volatile components captured in these cold traps provide a fingerprint for their origin. However, numerous mass, technology, and safety issues must be addressed to retrieve this information. On one hand, collecting, preserving, curating, and analyzing samples at the extreme conditions of large PSRs is a significant technological challenge. On the other hand, returning the samples to ambient conditions poses a challenge to the fundamental science advocated for exploring these environments. Potential solutions, such as using technologies already documented in ISS activities, may preserve significant science if sample selection sites are strategically identified. Simply keeping samples slightly below the freezing point of H2O (at one atmosphere) would reduce the potential scientific value of the returned Artemis samples.

Research Highlights

1. Key Findings: Volatile deposits in the lunar poles hold high scientific value and could shift strategies for sustained human activity on the Moon.
   
2. Innovative Methods: The paper explores the technical challenges and solutions for sampling, preserving, and analyzing lunar volatiles at cryogenic temperatures.
   
3. Unique Research Subjects: The permanently shadowed regions and ice stability regions at the lunar poles provide unique environments for studying the sources and evolution of water in the solar system.
   

Significance and Value of the Research

This study not only provides scientific support for the Artemis program but also offers technical guidance for the sampling and preservation of volatiles in future lunar exploration missions. By delving into the volatiles at the lunar poles, we can better understand the sources and evolution of water in the solar system, supporting sustained human activity on the Moon.