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comment The next frontier for planetary and human exploration The surface of 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 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 of extinct life in materials Ekilometres, offers an unprecedented depths4. In the subsurface — shielded accessible on the . 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 ? 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 -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 and 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 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

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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 — the Mars Advanced Sounding/drilling mm–cm for Subsurface and Ionosphere MERs (G) (G) Sounding (MARSIS) on 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, /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. are indicated by colour. While near-surface liquid brines are possible, pure liquid water could only Although we have barely scratched the be thermodynamically stable at depths of about 2–20 kilometres, as restricted by the local thermal Martian surface, we observed a diversity of gradient and surface temperature. In shaded green, we highlight that at a depth of a few metres organic subsurface environments reflected in the molecular biosignatures are thought to be better preserved9. ISRU (in situ resource utilization) is mostly many subsurface sample colours underlying concerned with the first tens of metres of depth, requiring easy access to resources, hence focusing on a homogeneous red surface layer — ice, brines and clays. The search for life is tightly coupled to liquid water and brines. Understanding the suggesting large mineralogical, chemical 14,15 modern-day distribution of subsurface ice calls for better modelling of liquid groundwater flow across and redox variations (see Fig. 1, bottom geologic time, bridging ISRU to life exploration. Images of drill holes with MSL illustrate the abundance left). We also see widespread hydraulic of subsurface environments through the diversity of subsurface colours below a uniform surface fracturing and mineralization in veins, colour14,15. Drill hole images credit: NASA/JPL-Caltech/MSSS/UofA/USGS-Flagstaff. which imply a long and rich history of water in the subsurface16. (4) Various missions — in situ ground- following: (1) the geographic distribution in the first 10–50 m at lower latitudes — a based, but, although more controversial, also and depth of shallow ices and other potential region of particular interest to human space orbital via spectroscopy — have provided resources (for example, brines, hydrated exploration due to the optimal levels of evidence of salts that, when mixed with minerals, clathrates, useful gases and metals) solar insolation and solar power production water, may form brines with low eutectic

Nature Astronomy | www.nature.com/natureastronomy comment temperatures17. For perchlorates specifically, are closer to the surface and where redox devils, or whether ionospheric EM signals experiments suggest frequent formation gradients amenable to sustaining life might reach the surface. If the latter occurs, then during dust storms18. The existence of various be maintained, today and in the past20,26. It is comparison of the using a surface and salts would allow for shallow briny subsurface important to remember that Mars offers the an orbital instrument (magnetic transfer waters locally (see Fig. 1, bottom right). opportunity to obtain data from different function, MTF) may detect groundwater at (5) Measurements at Gale Crater by subsurface depths, locations across the depths below one kilometre. By contrast, an Curiosity related to seasonally variable planet’s surface, and epochs in time, whereas artificial EM source allows the EM response concentrations (including intermittent on Earth plate tectonics has significantly to be measured without relying on ambient peaks about one order of magnitude above diminished very old rocks4. Therefore, in fields. Direct-current-based transient background levels) of methane and the order to validate models with observations, electromagnetics (TEM) is a classical presence of complex organics and nitrates in it is quintessential to model the Martian method that uses a coil on the surface to near-surface drill samples, and calculations subsurface in 4D (3D in space and one generate the necessary external EM field. on the potential availability of dissolved dimension in time across the last 4.5 billion Scaling current terrestrial TEM capabilities

O2 suggest ongoing subsurface processes years). Such modelling will soon profit to achieve groundwater detection on Mars that could provide the electron donors from data obtained with InSight27. InSight is indicate that aquifers as deep as several and acceptors to support subsurface life preparing to explore the large-scale interior kilometres or greater can be detected with in the present19–22. of the planet using seismology, geodesy a small system29. Currently, a collaboration and measuring the surface heat flow with between the Jet Propulsion Laboratory Enabling Mars subsurface exploration HP3. The seismological suite will provide (JPL) and the Southwest Research Institute Mars subsurface missions can today broad-scale seismic data on the structure of is developing a small (~5 kg, ~tens of W) capitalize on recent technological the Martian interior that will help to better TEM prototype called TH2OR (Transmissive achievements for sounding, drilling, cave constrain estimates of crustal thicknesses. H2O Reconnaissance) to search for deep exploration, and in situ sample analysis, on Together, global monitoring and 4D groundwater and characterize its salinity progress in our scientific understanding of subsurface science will facilitate future from the Martian surface. the Martian subsurface, and on commercial mission planning, especially for site selection. Seismology on Mars is ideal for and small opportunities. investigating large-scale interior structures, Sounding the subsurface. Advancements as will be demonstrated by InSight in the Global monitoring, 4D subsurface science and miniaturization in classic radar, next two years. Active-source seismology and site selection. High-resolution orbital synthetic-aperture radar (SAR) with is well suited to image shallow crustal images have already provided numerous polarimetry, multi-static radar, higher features of interest such as buried ice and examples of locations with natural entrances peak power, or array techniques and lava tubes. To detect groundwater at great into the Martian subsurface, such as lava interferometry could help propagate more depth on Mars, however, seismology offers tubes/caves or highly fractured terrain energy into the shallow subsurface and a lower sensitivity while generally needing that could provide outlets for discharge of provide sufficient reflections to map shallow active sources, more mass and power than subsurface fluids and/or gases23,24. Orbital ices in the first ~10–50 m of the subsurface TEM, and is, hence, not the first choice instruments (especially in areostationary in lower latitudes, although scattering in when searching for Martian groundwater. orbit) can be used to further identify or regolith is a challenge that limits the depth Nonetheless, though less effective than monitor these sites for sources and sinks of of exploration. For example, combining an TEM, passive high-frequency seismology trace gases, such as near-surface CH4, He orbital radar sounder at 50 MHz and an can help infer how dry the local crust is, or H2, which might be missed by orbiters orbital SAR with polarimetry (L- or P-band which will be soon tested with InSight. that cannot constantly monitor a specific at ~300 MHz) could enable characterization region. Local subsurface trace gas emissions of ice in the upper 10 m of the subsurface. Accessing the subsurface. In conjunction might be generally difficult to detect from In this context, ice sheet cliffs observed in with sounding, physically accessing and orbit due to large dust loads in the lower mid-latitudes28 and debris-covered glaciers12 sampling the subsurface gives independent atmosphere or interfering surface-gas are of special interest. On the surface, the confirmation of remote-sensing interactions and would benefit from surface future ESA ExoMars and rovers observations while also enabling a wide and subsurface gas exchange sensors (for scheduled for 2020 plan to have ground- range of scientifically critical investigations. example, tunable laser spectrometers, soil penetrating radar reaching down to a The technologies for subsurface resource resistivity measurements, gas accumulation depth of a few metres (sounding depths characterization and extraction are already chambers or equivalent). Areostationary for ExoMars WISDOM GPR of ~3 m and developed for harsh terrestrial environments monitoring and surface trace gas sensors RIMFAX on Mars 2020 <​10 m respectively). (including the low/high temperature and the might be particularly needed in order to Whereas ground-penetrating radar has vibrational environment that the equipment resolve the debate on Martian methane. limitations in sounding great depths, lower- could be subjected to during launch, landing We have also gained a deeper frequency techniques can reach deeper. and drilling operations) but need further understanding of Martian subsurface When searching for deep and shallow development for future Martian missions. variability following the recent exploration liquid water, inductive low-frequency Significant progress has also been made of deep continental and oceanic subsurface electromagnetic (EM) techniques exploit in clean drilling and avoiding/detecting habitats on Earth5,6, and the modelling the much higher electrical conductivity contamination in terrestrial rocks30.Current of Mars subsurface environments with of saline water in comparison to ice and 1–10 m drills have been demonstrated under evolutionary geodynamic tools and Mars dry rock (several orders of magnitude) by simulated Mars conditions, while hundred- general circulation models with variable measuring the EM response to an external metre drills are under demonstration. obliquity20,25. Together, these disciplines EM field. We do not know whether there Drills that can reach depths greater than allow us to better constrain locations where are sufficiently strong naturally occurring 100 m under Mars conditions are still the ice, permafrost and liquid water tables EM signals caused by lightning or dust under development. HP3 on InSight aims

Nature Astronomy | www.nature.com/natureastronomy comment to go to a depth of five metres in regolith27. In situ analysis. In situ analyses may be or from landed devices with active or passive The drill developed and qualified for the aimed at life and/or detection, mobility. The latter would be in the form of ESA ExoMars 2020 mission has proven habitability assessment or mineralogical/ scouts that search for trace gas emissions, capability of acquiring samples at 2-m depth geological characterization and determining shallow ices, and deep groundwater, or in regolith using a few tens of watts, while the potential and feasibility of in situ that monitor surface–subsurface volatile being in line with the mission’s required high resource utilization. Current technologies exchange or sample the shallow subsurface. level of cleanliness. Current drills under for organic, mineral and elemental Moreover, penetrators or networks of fixed development, like the rotary-percussive analysis selected for surface Mars missions stations emplaced by small spacecraft could wireline Planetary Deep Drill (PDD, from (for example, SHERLOC, a deep UV address similar goals not requiring mobility. Honeybee Robotics), the rotary ultrasonic Raman/fluorescence spectrometer on Deployment of such smaller missions may wireline Auto-Gopher-2 (AG2, from a Mars 2020; MOMA, on the ExoMars rover) profitably thrive in the next decade due to collaboration between Honeybee Robotics or those already deployed on the surface the emerging exchange between NASA, and JPL), and the rotary-percussive wireline (for example, the ChemCam Laser Induced other large space agencies, new international drill with deep UV/Raman spectrometer Breakdown Spectrometer on MSL) are partners and commercial space providers. WATSON (from a collaboration between adaptable for borehole assessment, either Given their rapid technical development, Honeybee Robotics and JPL) have been post-drilling or simultaneously while such small missions could deliver scientific deployed under simulated Mars conditions drilling. Deep UV Raman and fluorescence data of great significance to a diverse science and/or in Mars-analogue environments spectroscopy has already been integrated community in the next decade, side by side (PDD reached 13.5 m and AG2 reached with a wireline tool for terrestrial subsurface with a potential Mars sample-return mission 7.5 m in a couple of days in a gypsum analysis of boreholes36. In the case of that would bring back samples to Earth. quarry, while WATSON will be tested in penetration into putative subsurface fluid Additionally, collaboration can be Greenland to 100 m in 2019). The coiled reservoirs, wireline logging instruments extended beyond these companies tubing drill RedWater could be deployed recording temperature, pH, alkalinity, traditionally involved in space exploration from a Curiosity-sized rover and penetrate dissolved oxygen, oxidoreduction potential, to those employed in harsh-environment the subsurface to hundreds of metres of salinity/conductivity/resistivity, turbidity terrestrial resource exploration, depth, whereas next-generation iterations and select biologically relevant chemical characterization, extraction and production, of PDD/Watson/AG2 could reach a depth of concentrations such as exsolved gasses and as they have much to bring to bear in terms 1–2 km (refs. 31–33). Fe2+/Fe3+ ratios, would provide pertinent of the technology, equipment and processes We can also utilize miniaturized wireline information on chemical gradients and needed for subsurface exploration. drilling approaches that could enable boundary conditions capable of supporting drilling from just metres beneath the surface various metabolisms. Gas pumping systems A new deep frontier to kilometre depths without significant enabling sampling and analysis of subsurface Orbiters, landers and rovers, especially changes in payload mass. In this case, in gases, such as methane, can be integrated the two MERs and Curiosity, have 24 situ compressed CO2 harvested from the in situ as well . delivered data that have revolutionized our atmosphere could power the drill and act understanding of ancient Martian surface as a drilling fluid instead of water. The New commercial opportunities environments. Those data support a rich ASGARD (Ares Subsurface Great Access Access to space has become a commercial history of groundwater flow and a diverse, and Research Drill) concept under study at endeavour through companies like SpaceX, and from the surface very different, world JPL is targeting a capability to drill down to Blue Origin, Virgin Orbit, United Launch hiding beneath the oxidized surficial kilometres within one Martian year using Alliance, Northrop Grumman, Firefly regolith. InSight and future missions like a low-mass (<100​ kg) and low-power Aerospace, and Relativity Space among the ExoMars and Mars 2020 rovers will (on average <100​ W) solar-powered system others. Some of these companies, in aim to extend our knowledge of ancient that is consistent with particular SpaceX and Relativity Space, habitable surface environments, to produce protocols. This system would return all have added explicit goals to foster human unprecedented data on global large-scale the cuttings to the surface in approximate settlements or 3D print in situ on Mars interior properties, and to inform us about stratigraphic order, so that a surface in the coming decade to reduce the need the shallow Martian regolithic subsurface. instrument suite could perform a triage for importing resources from the Earth. However, questions, in particular about on the stream of cuttings and pull out Partnerships with such commercial whether there ever was or is still life on samples of special interest. companies may both reduce costs and Mars, how the Martian climate changed over In addition to direct subsurface drilling, increase the frequency of opportunities to long periods of time, whether there still is technologies to access and map subsurface reach Mars.They also provide additional liquid water and whether there are enough voids robotically, in a manner that is safe reasons for studying the Martian subsurface, accessible resources for an extended human and compatible with planetary protection with a focus on resources, a critical presence, will remain unanswered until we concerns, are being developed34. Use of swarm- requirement for human presence and start to ‘go deeper’. ‘Going deep’ and using algorithm-controlled small self-propelled units 3D printing on Mars. Mars as a testbed for subsurface exploration is ideal for the deployment of suites of many Many of the goals of Mars subsurface was recognized as a critical step when units that can sustain high losses and still do exploration could be addressed using a new searching for life by the National Academy the reconnaissance and measurement jobs class of more affordable small spacecraft37. of Sciences Committee on the Strategy for that are needed for precursor missions, for Examples of the observations that could the Search for Life in the Universe38. . example, using hopping microbots to spread be performed with small spacecraft in ‘Going deep on Mars’ is an into a network within a subsurface cavity. orbit include observing the outgassing of interdisciplinary project that calls for Robots that can climb, bounce, crawl, slither trace gases, seeps, fractures, caves, shallow expertise from the whole Mars community. and slink are all possibilities for accessing ices, geomorphological and spectroscopic It does not only bridge , underground terrains35. indicators for salts and shallow liquid water, polar sciences, climate, surface geology,

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Rogers26, *e-mail: Vlada.Stamenkovic@jpl..gov (Te National Academies Press, Washington, DC, 2018). L. Rothschild7, M. Russell1, H. Sapers1, B. Sherwood Lollar27, T. Spohn8, J. D. Tarnas22, Published: xx xx xxxx Acknowledgements We thank the Keck Institute for Space Studies (KISS) 1 28 29 1 https://doi.org/10.1038/s41550-018-0676-9 M. Tuite , D. Viola , L. M. Ward , B. Wilcox for kick-starting this work through a KISS Workshop and R. Woolley1 References held 12–16 February 2018 at the California Institute of 1 Technology, Pasadena, CA, and the Canadian Institute for Jet Propulsion Laboratory, California Institute 1. Grotzinger, J. P. & Milliken, R. E. in Sedimentary of Technology, Pasadena, CA, USA. 2Honeybee 1-48 (SEPM Society for Sedimentary Geology, Tulsa, 2012). Advanced Studies (CIFAR) for allowing this discussion to expand with the Earth 4D workshop. Part of this work was Robotics, New York, NY, USA. 3ESA, European 2. Carr, M. H. & Head, J. W. III Earth. Planet. Sci. 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