comment The next frontier for planetary and human exploration The surface of Mars has been well mapped and characterized, yet the subsurface — the most likely place to fnd signs of extant or extinct life and a repository of useful resources for human exploration — remains unexplored. In the near future this is set to change. V. Stamenković, L. W. Beegle, K. Zacny, D. D. Arumugam, P. Baglioni, N. Barba, J. Baross, M. S. Bell, R. Bhartia, J. G. Blank, P. J. Boston, D. Breuer, W. Brinckerhof, M. S. Burgin, I. Cooper, V. Cormarkovic, A. Davila, R. M. Davis, C. Edwards, G. Etiope, W. W. Fischer, D. P. Glavin, R. E. Grimm, F. Inagaki, J. L. Kirschvink, A. Kobayashi, T. Komarek, M. Malaska, J. Michalski, B. Ménez, M. Mischna, D. Moser, J. Mustard, T. C. Onstott, V. J. Orphan, M. R. Osburn, J. Plaut, A.-C. Plesa, N. Putzig, K. L. Rogers, L. Rothschild, M. Russell, H. Sapers, B. Sherwood Lollar, T. Spohn, J. D. Tarnas, M. Tuite, D. Viola, L. M. Ward, B. Wilcox and R. Woolley xploration of the Martian subsurface, it would have likely been transported with habitability of Mars and the search for to depths from a few metres to many the receding groundwater towards greater biosignatures of extinct life in materials Ekilometres, offers an unprecedented depths4. In the subsurface — shielded accessible on the Martian surface. In the opportunity to answer one of the biggest from the harmful effects of ionizing search for life, extinct or extant, the Martian questions contemplated by humankind: radiation, reactive chemical oxidants and subsurface likely holds the key for answering was or is there life beyond Earth? desiccation — life could have been sustained the ultimate question of Mars exploration: Simultaneously, Mars subsurface exploration by hydrothermal activity, radiolysis, was there ever or is there still life on Mars? lays the foundation for self-sufficient degassing, and water–rock reactions as human settlements beyond our own planet found in terrestrial subsurface microbial Pristine cores for high-resolution climate and provides an emerging potential for communities5,6. Therefore, the most likely reconstruction. Beyond the search synergistic collaborations with the rising place to find biosignatures of putative modern for evidence of life, direct access to the commercial space sector and traditional day extant life is in the subsurface, where subsurface would help reconstruct the long- mining companies. Our understanding of groundwater (likely in the form of brines term climatic and geochemical evolution the Martian subsurface and the technologies containing pure water mixed with salts) could of Mars, with a level of detail and temporal for exploring it — with a dual focus on still be stable7 (see Fig. 1 for stability depth). resolution that is beyond the reach of the search for signs of extinct and extant Recent results from the Curiosity rover8 surface instruments, which typically have life, and resource characterization and suggest the preservation of complex organic to deal with samples that have been altered acquisition — have matured enough for molecules even in near-surface settings. by damaging atmospheric photochemical serious consideration as part of future However, molecular biosignatures are oxidants or solar/cosmic radiation. robotic missions to Mars. likely best preserved at depths of at least a Extended subsurface cores of lake sediments few metres, where they are shielded from or volcanic deposits would provide an The search for life leads underground ionizing radiation and reactive chemical unprecedented record of geochemical Data collected from orbiters and rovers oxidants that can obscure or destroy conditions and atmospheric composition indicate a once warmer and wetter Mars structural complexity that is indicative of dating back hundreds of millions to several that may have been supportive of life as biogenicity, independent of whether putative billion years. Deep cores of polar ice we know it1,2. Results from the MAVEN ancient Martian organisms once inhabited deposits would help reconstruct orbit-driven mission3 suggest that a significant fraction surface or subsurface environments9. Results climate excursions over shorter timescales of the Martian atmosphere was likely lost from terrestrial cratons 2.7 billion years of tens of millions of years. early in the planet’s evolution — sometime old have recently demonstrated that fluid between the Noachian and Amazonian components can be preserved in subsurface Accessing resources for human periods — which would have led to surface fracture groundwaters for billions of years10. exploration. Human exploration of Mars temperatures dropping, to an increase in The practical challenge we face on Mars is to remains a primary long-term objective for harmful radiation reaching the surface, and identify the subsurface sites that have been NASA. Relative to the Moon, Mars offers to the boundary between cryosphere and least exposed to surface conditions. more in situ resources in the form of ices, liquid groundwater moving to greater depths To date, only the Viking landers — hydrated minerals, and CO2 — enabling below the surface, where the temperature launched over forty years ago — have a more sustainable human presence that and pressure would have been high enough sought direct evidence of extant life, but would not depend heavily on frequent to sustain liquid water. they focused on the Martian surface alone. deliveries from Earth. However, to select Regardless of whether life may have ever Subsequent missions have focused instead the most advantageous site for human emerged on or below the surface of Mars, on the related question of the ancient exploration, we need to better grasp the NATURE ASTRONOMY | www.nature.com/natureastronomy comment 2 3 4 (m) Potential missions 0 0.1 1 10 10 10 10 throughout the year and more benign 1 m–kms temperatures; (2) potential chemical and Next generation particulate hazards in the subsurface; and Under demonstration (3) the local likelihood at the landing site for Drilling Demonstrated extant life (and hence also liquid water) and Penetrator to preserve signs of extinct life, to make sure >10s km MTF (G/O) we minimize possible cross-contamination ~10 km and do not alter a potential ecosystem. TEM (G) 100 m Diverse subsurface environments Sounding Surface GPR (G) While Mars subsurface exploration is still in 1–10 m SAR (O) its infancy, the little data we do have support Impactor (O) the idea of a diverse and exciting Martian So far and planned subsurface. Specifically: 100s m (broadly) (~km for ice/volcanic ash) (1) Gamma-ray spectrometers and MARSIS (O) neutron detectors on Mars Odyssey have SHARAD (O) Metres provided on a global scale the elemental InSight (G) abundances of hydrogen, iron, chlorine, ExoMars (G) silicon, potassium and thorium in the very Rimfax/Wisdom (G) cm–dm shallow Martian subsurface (cm–dm). GRS (O) (2) Orbital radars — the Mars Advanced Sounding/drilling mm–cm Radar for Subsurface and Ionosphere MERs (G) Phoenix (G) Sounding (MARSIS) on Mars Express and MSL (G) the Shallow Radar (SHARAD) on the Mars M2020 (G) John Klein Cumberland Windjana Reconnaissance Orbiter (MRO) — have Ice provided rich datasets for characterizing Confidence Hills Mojave Telegraph Peak the stratigraphy of polar regions to a depth Brine of 1–3 km. MARSIS data were recently used Buckskin Big Sky Greenhorn to establish the possibility of perchlorate- Subsurface H2O(l) containing water beneath the south polar layered deposits at a depth of 1.5 km (ref. 11). 2 3 4 0 0.1 1 10 10 10 10 (m) Data from both radars suggest the presence of relatively shallow ice deposits ISRU Life in a few non-polar regions (for example, Deuteronilus Mensae12). However, both Fig. 1 | Sounding and drilling capabilities on Mars. We plot the sounding (dashed arrows) and drilling instruments are ‘blind’ to the top ~10 m, (solid arrows) depths for missions that have already been delivered to Mars or are scheduled (navy and have poor depth perception beyond blue) versus selected potential instruments that could help explore the Martian subsurface (orange). 200 m other than through ice or volcanic ash The arrows indicate the reach of sounding and drilling (minimum and maximum). For drilling, we overburdens, and hence their effectiveness 13 show current capabilities that have been (~15 m) or are currently being demonstrated (~100 m) under is mainly limited to the poles . Hence, simulated Mars conditions, and next generation drills under development (> 1 km). O and G indicate both instruments have not been able to orbital and ground-based missions, respectively. G/O indicates that orbital and ground-based assets conclusively reveal shallow ices closer to the need to work together. MTF, magnetic transfer function; TEM, transient electromagnetics using equator or subsurface liquid water (Fig. 1). own active EM source; GPR, ground-penetrating radar; SAR, synthetic aperture radar; M2020, Mars (3) Rovers like Curiosity have directly 2020; GRS, Gamma Ray Spectrometer on Mars Odyssey; MERs, Mars Exploration Rovers Spirit and sampled the Martian subsurface down to Opportunity; MSL, Mars Science Laboratory/Curiosity rover. The arrows for MARSIS/SHARAD illustrate a depth of approximately six centimetres. a penetration depth of less than 200 m outside of ice or volcanic ash overburdens, and around 1 km in The Phoenix lander managed to scoop one such zones (mainly poles). Depths where ice (cyan), brines (pink), and pure water (blue) could occur sample from 18 cm beneath the surface.