Our investigations of Earth processes shed light on processes taking place on other planets and moons. In turn, discoveries off Earth have forced us to look at our home planet with fresh eyes. For example, life exists in seemingly impossible places on Earth: in the crushing depths of the ocean, the dry valleys of Antarctica, and acid volcanic springs, to name a few. What can these life-forms tell us about conditions on other worlds that might also be capable of sustaining life?
Hutton and Lyell’s founding principles of Earth science, put forth in the late 1700s and early 1800s, stated that observable processes are the key to unlocking the past and deciphering the future. A key principle of their uniformitarian view of geology stated that the processes that alter Earth are uniform through time. This invokes an image of Earth as a well-understood planet that can be used as an analogue for comparative planetology and exploration of the rest of the universe.
But our understanding of the physical, biological, and chemical processes on our own planet has, in fact, continued to evolve in step with our exploration of the solar system and beyond. Discoveries on the Moon, Mars, and Venus; the satellites of Jupiter and Saturn; and, no doubt, exoplanets will continue to force revision of our understanding of the origin and evolution of Earth processes and vice versa. Evolution in our understanding of biological processes in particular has driven a surge in interest in astrobiology: the study of life in the universe.
A report published last year by the National Academies of Sciences, Engineering, and Medicine , An Astrobiology Strategy for the Search for Life in the Universe, was a major review of the astrobiology landscape, synthesizing recent discoveries, technological advances, and programmatic themes. The report’s goals were to identify conceptual and intellectual developments emerging from the community and to highlight the most promising research questions and technology challenges likely to shape the field over the next 20 years.
The report, which presented a wealth of findings and recommendations, organized its findings into three overarching themes. One of those themes—“Go Deep”—focused on the increasing impact research on subsurface processes and environments is having on planetary exploration and astrobiology. What is so game-changing about this new “subsurface frontier” that puts a whole new perspective on the term “deep space”?
Early years of space exploration and the search for life beyond Earth arguably focused largely on the question of surface life. As late as the 1970s, life on Earth was typically thought of as a thin veneer on the surface of the Blue Planet where the foundation of ecological energy was entirely dependent on accessing the Sun’s energy via photosynthesis.
A new world emerged when deep-sea submersibles explored the mid-ocean ridge hydrothermal vents. There, tube worms more than 2 meters long and clams as big as dinner plates were sustained by an ecosystem based not on photosynthesis but on chemosynthesis. The foundation of the food chain was driven by chemolithotrophic (literally rock eating) microorganisms capable of capturing the chemical energy of reactions between water and rocks to sustain life, even in the dark depths of the ocean floor, far from the reach of sunlight. The shock wave of these discoveries is still affecting research on Earth and beyond.
Life Not as We Know It
Although no evidence of a second origin of life or a shadow biosphere [Cleland and Copley, 2005] has even been hinted at, discoveries from subsurface organisms and ecosystems have, nonetheless, forced us to confront forms of life as we didn’t know them before. Even within the canonical framework of life’s DNA blueprint, the diversity and mysteries of life continue to surprise.
From Carl Woese’s 1977 revolutionary findings of a third kingdom of life (the Archaea) to the discovery of horizontal gene transfer, which does an end run around parent-to-offspring inheritance, new findings that force us to revise our understanding about the nature and evolution of life continue. For example, recently found archaeal lineages from hydrothermal vents, such as the Lokiarchaea, are providing the first tangible evidence to address one of the long-standing issues in evolutionary biology: How did some organisms (the eukaryotes) develop complex systems of organelles and membrane-wrapped genetic material within each of their cells [Spang et al., 2015]?
The discoveries of life sustained by chemosynthesis raise another tantalizing proposition: Could extant life, or signs of past life, be preserved deep in the subsurface of Mars? If life ever arose on a planet like Mars around the same time it did on Earth, that life would have been graced with a warmer, wetter climate and thicker atmosphere. Could water and any associated life have taken refuge in the subsurface as the planet evolved to become the cold, dry desert it is today [Michalski et al., 2017]? Elsewhere in our solar system and beyond to the exoplanets, might chemoautotrophy—synthesizing one’s own food from chemical compounds in the environment—be a more universal strategy for life than photosynthesis?
Go Slow and Keep It Cool
We don’t need the Twitterverse to show us that we are drawn to dramatic, fast-moving phenomena, sometimes out of all proportion to their overall significance. What might we be missing by ignoring small or slow biological ecosystems in the energy-poor deep subsurface [Trembath-Reichert et al., 2017]? What discoveries lie in low-temperature processes that seem “negligible” until the volume of the geosphere they affect is considered or the timescale over which they have occurred is integrated?
Slow processes can have global effects if the long timescales of a planet’s history are involved. A notable example, only recently discovered, is the role that subsurface radiation plays in breaking apart water molecules, providing a vast and unanticipated source of hydrogen for life [Lin et al., 2006]. This hydrogen-generating process is so slow that the global importance of these processes had been completely neglected. Their significance came to light only when the effects of accumulating these reaction products over the vast volume of Earth’s subsurface on a million- to billion-year timescale were calculated [D’Hondt et al., 2009; Sherwood Lollar et al., 2014].
Similarly, over the past decade, science has evolved to recognize that energy-producing processes of water-rock reactions are not confined to high-temperature settings such as vents, hot springs, and volcanoes. Discoveries of hydrogen-producing water-rock reaction processes such as radiolysis and serpentinization occur over a range of temperatures and timescales [Mayhew et al., 2013; McCollom and Donaldson, 2016; Schrenk et al., 2013]. These processes are currently transforming our understanding of the habitability of our planet, expanding our focus beyond the “deep, hot biosphere” to the vast areas and depths of the continental crust where a deep, cool biosphere may reside [Sherwood Lollar et al., 2014; Onstott, 2016].
An expanded understanding of the diversity of water-rock reaction processes on Earth that can sustain habitability is rapidly feeding forward, influencing current and future mission planning for exploration of the planets and moons beyond Earth. Scientists are reexamining the postulate that planetary habitability requires plate tectonics. Perhaps there is not just one model of planetary energy production, distribution, and dissipation capable of sustaining life.
Other planetary-scale processes, such as fluid-rock interactions, or processes relevant to other planets and moons (e.g., tidal heating on Enceladus) may provide alternate and potentially more universal mechanisms to sustain the driving energy that supports life [Meyer and Wisdom, 2007]. Discoveries from Earth’s subsurface are fueling investigation of planetary processes that might explain how organic compounds such as methane could be produced on Mars [Webster et al., 2018] and on Titan [Niemann et al., 2005; Atreya et al., 2006]. Hydrogen detected in the plumes of Enceladus [Waite et al., 2017] has fueled thinking about hydrogen production on Mars [Vance et al., 2011; Tarnas et al., 2018], Europa [Vance et al., 2016; Bouquet et al., 2017], and potentially even Pluto and small icy satellites [Vance et al., 2007].
Earth 3-D and Earth 4-D
Subsurface environments allow us to investigate beyond three dimensions, providing information on the critical fourth dimension—time—required to address fundamental questions regarding the preservation of biosignatures and the record of geological processes. These questions are at the core of understanding planetary evolution, the history of habitability, and the validity of biosignatures. The Curiosity rover’s discovery of organic carbon preserved in 3-billion-year-old Martian mudstones at Gale Crater [Eigenbrode et al., 2018] highlights the importance of analogues in some of the oldest rocks on Earth, also preserved for close to 3 billion years.
On the Canadian Shield and ancient cratons of Australia and South Africa, subsurface samples protected from the impacts of more recent surface carbon cycles provide a testing ground for understanding preservation processes on extreme planetary timescales. Many of the Archean terrains on Earth have been so highly metamorphosed and deformed that their geological record has been obscured. Other intriguing localities such as Kidd Creek on the Canadian Shield have undergone such relatively moderate thermal events and deformation that the “pillow lava” textures that formed when the rocks were first extruded onto the ocean floor 2.7 billion years ago remain visible in the wall rock [Li et al., 2016]. Discoveries at Kidd Creek revealed not only groundwaters with mean residence times of 1–2 billion years [Holland et al., 2013; Warr et al., 2018] but noble gas isotopic signatures reflecting the composition of Archean atmosphere [Holland et al., 2013; Avice et al., 2017].
Understanding the timescale and processes of preservation of water, habitability, extant life, and biomarkers of past life in Earth’s subsurface will challenge and inform planetary exploration. The marine lithosphere provides this understanding on short-term geologic cycles (less than 200 million years), and the Precambrian-aged rocks that constitute more than 70% of the continental lithosphere (600 million to more than 3 billion years old) reveal the longer planetary timescales. Studies on Earth remain vital to exploration of rocky planets like Mars, as well as the ocean worlds and icy moons, and the atmospheric composition and potential habitability of exoplanets. Deep subsurface Earth is the one environment identified to date that is, in some locations, “isolated” from the photosynthetic-driven surface cycles. This environment can provide a test case of how (and what) life might be sustained on bodies where high radiogenic activity and chemical processes are capable of sustaining chemoautotrophic life.
Thinking Beyond “Drill, Baby Drill”
Exploring the kilometers-deep regions of Earth is not easy! How, then, can we access the subsurfaces of other planets and moons in the solar system, let alone extend the Go Deep approach to exoplanets that are impossibly beyond even the farthest-reaching probes and orbiters? The Curiosity rover (and current plans for the Mars 2020 rover) can penetrate the Martian surface only on the centimeter scale. The Roscosmos and European Space Agency joint ExoMars 2020 rover will carry a drill able to reach 2 meters below the surface of the planet [Vago et al., 2017]. Stamenković et al.  recently reviewed strategies in development or under consideration (including sondes, probes, and new drilling technologies) potentially applicable not only to rocky bodies such as Mars but to the ocean worlds.
A vital aspect of the Go Deep philosophy, however, is its focus on subsurface processes, not simply subsurface access. The approach encompasses a multiplicity of strategies to gain information about subsurface processes and environments. Noninvasive geophysical approaches via ground-penetrating radar enabled Orosei et al.’s  discovery of subsurface water on Mars. The heat probe on NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) Mars lander is designed to penetrate the subsurface to several meters. Coupled with InSight’s seismometer, these instruments provide information on subsurface heat, transport, and seismic processes [Banerdt et al., 2013].
In a myriad of geomorphological environments, the subsurface interacts with and brings its information to the surface. Scarps, impact craters, and fractured terrains, as well as potential seeps, lava tubes, and ice caves, may provide alternative means of accessing samples that originated in the subsurface [Boston, 2010; Oehler and Etiope, 2017]. The jets of material from the tiger stripe fractures on the south pole of Enceladus are a prime example of the urgent need to understand subsurface processes. On exoplanets, subsurface processes may also play a role in contributing to the global atmospheric signals gathered by space- and ground-based telescopes in the near future. Go Deep, a wide-ranging conceptual strategy for the investigation of subsurface processes, is neither limited to nor constrained by drilling capabilities.
The Stars and Planets Beneath Our Feet
Poets and storytellers know that humankind strains not only toward the horizon and the stars but to the mysteries of the deep. When we explore planetary processes and search for life in the universe, we must not only look out to the stars—we must also look down to the subsurface.
The many discussions with the National Academies of Sciences, Engineering, and Medicine Committee on the Astrobiology Science Strategy for the Search for Life in the Universe and contributors to the Canadian Institute for Advanced Research (CIFAR) Earth 4D proposal are gratefully acknowledged. This work was supported by the Natural Sciences and Engineering Research Council’s John C. Polanyi Award to B.S.L., with additional support from the CIFAR Exploratory Workshop program and the Cluny Writing Centre.