The scientific exploration of Mars over the past several decades has resulted in increasing evidence that the martian surface hosted habitable environments early in its history, as well as evidence of the building blocks of life in the form of organic molecules (1). Habitats on Mars that could harbor extant martian life have been hypothesized, such as subsurface environments, caves, and ice deposits (2). Mars is currently recognized as a “paleo-habitable” planet, reflecting its ancient habitability. Fully understanding the evolution of habitability and whether Mars has ever hosted life will be essential to understanding and exploring other extraterrestrial habitable environments and potential life-forms (3). Flagship missions of multiple space agencies in the 2020s will play essential and complementary roles and could finally provide an answer to these long-standing questions.
The planned Mars Sample-Return (MSR) mission of NASA and the European Space Agency should reveal more about the habitability of Mars by helping to determine the geologic evolution of Jezero crater and its surrounding areas, which are believed to be the site of an ancient lake (see the photo). The Mars 2020 Perseverance rover will attempt to collect samples that will allow scientists to explore the evolution of Jezero crater and its habitability over time, as well as samples that may contain evidence of biosignatures. A high-priority science objective for MSR returned-sample science is to understand the habitability of Mars and look for potential signs of both extinct and extant life (4).
Mars is not alone because it has two small moons, Phobos and Deimos. Throughout the history of Mars, numerous asteroidal impacts on Mars have produced martian impact ejecta, and a fraction of the ejected material has been delivered to its moons (5). Phobos is closer to Mars, so it has more martian ejecta than Deimos. Numerical simulations show that >109 kg of martian material could be uniformly mixed in the regolith of Phobos (the resultant martian fraction is >1000 parts per million) (5).
Even if martian life-forms existed and could survive the transport to Phobos without suffering from impact-shock decomposition (with a peak pressure of <5 GPa) (5, 6), the Phobos environment is highly inhospitable (7). Phobos does not have air or water, and its surface is constantly bathed in solar and galactic cosmic radiation. This indicates that martian materials on Phobos’ surface almost certainly do not contain any living microorganisms.
Instead, there may be dead biosignatures on Phobos, which we have called “SHIGAI” (Sterilized and Harshly Irradiated Genes, and Ancient Imprints)—the acronym in Japanese means “dead remains.” SHIGAI includes any potential microorganisms that could have been alive on Mars and were recently sterilized during or after the delivery to Phobos, and the microorganisms and biomarkers that had been processed on ancient Mars before the delivery to Phobos, including potential DNA fragments. The Mars-moon system is an ideal natural laboratory for the study of interplanetary transport and sustainability of SHIGAI on airless bodies in the Solar System.
Should a martian biosphere exist, any biosignatures or biomarkers observed in the samples from Jezero crater could be widespread elsewhere on Mars and possibly occur on the surface of Phobos. Because martian ejecta has been thoroughly delivered to Phobos by impact-driven random sampling, the biosignatures and biomarkers that may be contained in the Phobos regolith could reflect the diversity and evolution of a potential martian biosphere.
Martian Moons eXploration (MMX), developed by the Japan Aerospace Exploration Agency, plans to collect a sample of >10 g from the Phobos surface and return to Earth in 2029 (8). Detection of a “fingerprint” of martian life and SHIGAI should be achievable through comprehensive comparative studies using martian material from the Phobos surface and samples from Jezero crater returned by MMX and MSR, respectively.
The MSR samples have the potential to contain a variety of biomarker molecules (e.g., lipids, such as hopanoids, sterols, and archaeols, and their diagenetic products) (4). The sample could include modern living organisms from Jezero crater, if they are present. Of course, MSR could return samples without any evidence of life because of the focus on a single location. A distinct advantage for MMX is the ability to deliver martian materials derived from several regions. The random nature of the crater-forming impacts on Mars statistically delivers all possible martian materials, from sedimentary to igneous rocks that cover all of its geological eras.
Mutual international cooperation on MSR and MMX could answer questions such as how martian life, if present, emerged and evolved in time and place. If Mars never had life at all, these missions would then be absolutely vital in unraveling why Mars is lifeless and Earth has life. Therefore, the missions may eventually provide the means to decipher the divergent evolutionary paths of life on Mars and Earth.
Acknowledgments: We thank D. W. Beaty, M. Grady, and B. Carrier for comments and discussions on NASA-ESA MSR science. We thank H. Sugahara, M. Fujimoto, K. Kurosawa, and H. Genda for many useful discussions on Japan Aerospace Exploration Agency MMX science. This Perspective was constructed through discussions with them and submitted on their behalf.