The scientific value of Mars Sample Return

The return of Mars samples to Earth is among the most ambitious goals of the spaceflight programs of the National Air and Space Administration (NASA) and the European Space Agency (ESA). Preparing for and conducting a Mars Sample Return (MSR) campaign has been identified as the highest priority in the last three Planetary Decadal Surveys (1–3)—the means by which the National Academies of Sciences, Engineering, and Medicine provide advice to NASA on its science and exploration programs.

The NASA Mars 2020 Perseverance rover has collected and cached samples in Jezero crater, an impact structure that contains lavas, deltaic sediments deposited by water, and ancient basement rocks. The samples have been carefully selected to address critical science questions identified by the scientific community. A future Sample Retrieval Lander would carry a Mars Ascent Vehicle, into which the samples will be loaded and then launched into Mars orbit. An ESA Earth Return Orbiter would rendezvous with the orbiting sample container and return it to Earth in the next decade.

The science that can be accomplished with appropriately chosen Mars samples is compelling—an important next step in planetary exploration. This Special Feature is intended to provide an overview of the research that is expected from these samples, should the MSR Program be completed. The papers in this Feature are as follows:

• •Perspectives on Mars Sample Return: A critical resource for planetary science and exploration (4) focuses on unanswered questions that can only be addressed by laboratory analyses of returned Mars samples. This paper also explains why the chosen sampling site on Mars is relevant to address these questions. Also considered are the technological “firsts” of MSR and the broader societal benefits of MSR beyond science.
• •Mars Sample Return: From collection to curation of samples from a habitable world (5) considers the challenges for remote sample collection, retrieval, and return. Also considered are sample curation issues once the samples are returned to Earth, including planetary protection from unknown chemical and biological agents.
• •The question of whether Mars has, or ever had, life is central to Mars and planetary science. Organic matter and biomarkers: Why are samples required? (6) considers measurements to assess the possibility of extinct or extant life and the necessary sensitivities and controls.
• •Measurements of multiple radiogenic isotopic systems are necessary for quantifying the ages of samples and for understanding the formation and geologic evolution of the planet. A tale of two planets: Disparate evolutionary models for Mars inferred from radiogenic isotope compositions of martian meteorites (7) compares planetary differentiation models for Mars, Earth, and the Moon and explains why isotope analyses of additional martian rocks are required.
• •Although the chemical composition of the current martian atmosphere has been measured by spacecraft, analyses of elements in trace quantities and of the isotopic compositions of gaseous compounds are beyond the capabilities of remote sensing. The value of a returned sample of the martian atmosphere (8) considers how analyses of noble gases and of other components in a modern atmosphere sample can help us understand its origin and evolution. Also, samples of ancient atmosphere trapped in rocks can potentially be retrieved to further constrain its evolution.
• •Utilizing martian samples for future planetary exploration—Characterizing hazards and resources (9) explores how returned samples can inform future exploration by humans. This paper explores the issues related to crew health and performance, vehicle system design and operations, and planetary protection strategies for human missions.
• •Several hundred rocks have been launched from Mars as meteorites and collected on Earth. Fundamental constraints and questions from the study of martian meteorites and the need for returned samples (10) reviews what has been learned from Mars samples already in hand and explains why the biased martian meteorite collection is not sufficient to address many critical science questions.
• •The sites from which samples have been collected and cached by the Perseverance rover and what is known about those samples are described in Sampling Mars: Geologic context and preliminary characterization of samples collected by the NASA Mars 2020 Perseverance rover mission (11). In addition to rock cores, the sample collection includes regolith (soil) and atmospheric gas.
• •Light element stable isotope analysis of volatiles in returned martian samples: Constraining the history of water and climate on Mars (12) discusses how analyses of the abundances and stable isotopic compositions of volatile elements in samples constrain the evolution of water and climate, and how these data, in turn, relate to planetary habitability.
• •Ancient Mars is known to have had a magnetic field, but little is understood about the dynamo that produced it. Paleomagnetic measurements on oriented rock samples can constrain the planet’s magnetism and the thermal evolution of the planet’s interior. What we can learn about Mars from the magnetism of returned samples (13) explains how such measurements can be made on returned samples.

The first returned samples from another planet would engender a major advance in Mars science, as well as enabling exploration and further understanding of other bodies in the Solar System. Scientists of all disciplines—geologists, biologists, chemists, physicists—will be able to investigate critical scientific questions using these samples. Knowledge gaps relevant to future mission design, operations, and safety can be addressed with firmer information about the compositions and properties of surface materials. And like the carefully curated lunar samples collected by Apollo astronauts, returned Mars samples will be an important resource for scientific research for many decades to come. MSR represents an extraordinary opportunity for science.