Exogeoscience and Its Role in Characterizing Exoplanet Habitability and the Detectability of Life

The search for exoplanetary life must encompass the complex geological processes reflected in an exoplanet's atmosphere, or we risk reporting false positive and false negative detections. To do this, we must nurture the nascent discipline of"exogeoscience"to fully integrate astronomers, astrophysicists, geoscientists, oceanographers, atmospheric chemists and biologists. Increased funding for interdisciplinary research programs, supporting existing and future multidisciplinary research nodes, and developing research incubators is key to transforming true exogeoscience from an aspiration to a reality.


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The field of exoplanetary science is booming. To date, we have confirmed more than 4,100 extrasolar planets, with that number set to quickly rise thanks to the Transiting Exoplanet Survey Satellite mission.
An ever-growing fraction of these exoplanets is rocky. Yet these rocky bodies reveal a radical diversity of types with no counterparts in the Solar System, from Earth-mass worlds with much more water than Earth, such as some in the TRAPPIST-1 system ( ; ) to 55 Cancri e-a super-Earth with a surface temperature greater than the melting point of most rocks ( ). As the number of known rocky exoplanets grows, exoplanetary science is diligently working to better understand the frequency, composition, and, increasingly, the geological nature of these planets-including their potential to be habitable over geological (>1 Gyr) timescales.
A major challenge to that better understanding, however, is the limited available observations for individual exoplanets. In the optimum case, we are at present capable of measuring only an exoplanet's mass, radius, orbital parameters, temperature/phase curves (in rare cases), and the composition of any atmosphere present. Some surface spatial patterning may be observable in the near future ( ), and the surface of at least one world, LHS 3844b, has been glimpsed ( ). The gulf between planetary and exoplanetary science is narrowing, but it is still a gulf.
Happily, in the coming decades several new instruments capable of observing exoplanet atmospheres and surfaces will see first light, including the James Webb Space Telescope (JWST), the Extremely Large Telescope, the Giant Magellan Telescope, and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey. Measuring the composition of a planet's atmosphere will thus grow more routine, coming to be one of the primary observables for understanding the nature of rocky planets. These secondary atmospheres-generated primarily by volcanism and degassing, either from partial melting of the interior or during an early magma ocean phase-offer us a probe of a planet's prospective habitability, including direct searches for biosignatures (e.g., ; ; ; ). But interpreting these measurements in terms of detecting life is challenged not only by the presence of clouds, aerosols, and ash plumes, but by the nature and history of the atmosphere itself-and particularly the role of geology in shaping the abiotic background from which we must distinguish prospective biosignatures.
Planets orbiting within the habitable zone of their host star are prime observational targets in our search for exoplanetary life. But are they the only places to search for this life? Life on Earth is not enabled solely because of its stable orbit at 1 AU around a G-type star. Rather, the geological characteristics of the planet play a central role in its habitability, chiefly by facilitating a relatively stable climate over the long term through C, N, and H2O cycles that exchange material from the interior to the surface and back again via volcanism and subduction (e.g., ). Other geological aspects are important, too, including the cycling of nutrients into the oceans as a function of the fraction of exposed land and degree of orogeny ( ). An intrinsically generated magnetic field may also play a role in protecting the surface from harmful XUV radiation and minimizing atmospheric stripping ( ), although the extent to which magnetic fields operate on exoplanets and contribute to habitability remains to be determined ( ; ).
Geological processes themselves can lead to false negative or false positive observations for individual biosignature-relevant atmospheric species (e.g., O2, CH4). Abiotic CH4 can be produced by water-rock interactions (e.g., ), and abiotic O2 can build up from water photolysis ( ) or because of runaway greenhouse effects ( ), both of which might result in a false positive detection of life on an exoplanet. Similarly, biologically produced O2 could react with gases released from a reduced mantle (e.g., CO, S), to form CO2 and SO2, in turn leading to a false negative detection ( ; ).
Indeed, the abiotic production and modification of secondary atmospheres represents a confluence of astronomical, geological, and geophysical properties and processes taking place perhaps even before a planetary body's formation has ended. The initial composition of the body, inherited from the portion of the protoplanetary disk in which it formed, sets its mineralogy and volatile content ( ; ; ). During its magma ocean phase, geochemistry and mineral physics set the stage for segregation of the Fe-rich core, which permanently sequesters biocritical elements such as C, P, and O from the mantle as it grows ( ; ; ).
As the body solidifies, incompatible elements are partitioned into the crust, leaving behind a depleted mantle that will chemically evolve further by melting and volcanic degassing. Of note, the timing of magma ocean crystallization and devolatilization can vary by hundreds of millions of years because of the effect of tidal heating from companion planets or moons ( ), so the evolutionary history of an exoplanet need not follow too closely that of worlds in the Solar System.
Through atmosphere-surface interactions, volatiles lock into rocks and are transported potentially all the way to the deep mantle via tectonic processes and mantle convection ( ; ). In parallel, the atmosphere undergoes chemical processes that create and obliterate both biotically and abiotically derived molecules, and interactions with the stellar environment potentially destroy biosignature gasses (e.g., ). All of these phenomena have their own time dependence and likely co-evolve with any life present on these worlds, only to be boiled down to a single, instantaneous atmospheric spectral measurement-with which we must deduce the abiotic history of the planet, its potential habitability in a geological context, and whether life is even detectable in the first place above the abiotic background of that atmosphere.
Studying Earth itself is central to understanding the abiotic background of atmospheric gasses, but only to a point-our atmosphere is indelibly marked by the signature of life. To understand the abiotic context of rocky exoplanets, then, we must look to models that are informed by experiments, theory, models and, where available, observations. But the creation of these coupled interior-surfaceatmosphere models clearly cannot be the purview of any one discipline, and cannot only encompass those planets with which we are familiar. Thus, as we move toward quantifying the detectability of life, Building holistic, systems-level models requires fostering this interdisciplinary approach with the ultimate goal of distinguishing the abiotic evolution of an exoplanet from a detected potential biosignature (cf.
). Yet, because of the known diversity of exoplanets (e.g., ), the recognition that Earth-size worlds in orbit about other stars may not in fact be anything like Earth ( ), the possibility of planetary migration, and perhaps even the rarity of truly Earthlike worlds, it is increasingly clear that such models cannot simply be variations on a theme of Earth.
We must devise new experiments and models to better understand and characterize the planetary diversity we now know to exist, and explore the broader parameter space of rocky exoplanets to be able to make sense of those not yet known to exist.
The creation of such experiments and models represents a major interdisciplinary undertaking where observers, modelers, and experimentalists must understand the application, and limitations, of each other's contributions to this shared endeavor. Currently, the constituent communities needed to fully investigate interior-surface-atmosphere interactions through time are fragmented, even within the individual geoscience and astronomical disciplines. And although the "traditional" Earth, meteoritic, and planetary science communities sporadically interact, as they collectively seek to understand the origins of Earth and other Solar System bodies, each of these communities has their own preferred journals, conferences, workshops, data formats, and even Decadal Surveys or equivalent flagship professional reports. (To wit, the recent "Earth in Time" National Science Foundation Earth Sciences 2020-2030 consensus study report explicitly listed planetary science as "not in its purview" (p. 23).) Both geoscience and astronomy are filled with jargon, sometimes having two substantially different meanings for the same word. For example, to one researcher the word "core" denotes the central, ironrich core of a planetary body, but to another it can mean the entire planet beneath an atmosphere.
Similarly, few astronomers and geoscientists use the term "metal" in the same way. And "Earth-like" is often used to describe planets with some property similar to Earth, but for which the surface and atmospheric conditions may be anything but. Indeed, these fields can have very different science priorities, with planetary geoscience seeking to understand aspects such as composition, density, geodynamics, volatile cycling, and surface features, whereas exoplanetary astronomy focuses on characterizing a body's orbital parameters, its formation, and the age and properties of its host star.
There has been, to date, insufficient interaction between exoplanetary astronomers and planetary geologists, whose knowledge of surface processes, petrology, and volcanism will be critical to helping link planetary properties to atmospheric composition and so develop exogeoscience as a viable discipline, especially given the anticipated deluge of such data in the coming decades.
Developing complex, planetary-scale models requires considerable infrastructure development, as well. Crucial experiments on atmospheric parameters, melting behavior, thermoelastic mineral properties, etc., call for major investment in laboratory space. These investments are necessarily expensive and labor intensive, often having to accommodate high pressures and temperatures. Highperformance computing (and associated expenditure) is just as important. Without such infrastructure, the vital interior-surface-exosphere models we need going forward will simply not be possible.
In the absence of a truly interdisciplinary exoplanetary field, scientists may not be comfortable branching out beyond their established expertise, especially if they feel that interdisciplinary projects are seen as high risk by funding agencies already facing severe selection pressures. Compounding this view is that interdisciplinary projects can also be perceived as having a low or, at best, uncertain impact in the absence of robust observables, especially for exoplanetary research-despite the enormous work, by definition interdisciplinary, required to obtain those observables in the first place.
These concerns are likely amplified for early career researchers and those without the security of a permanent position. Funding sources such as NASA's Habitable Worlds (HW) and Exoplanets Research (XRP) programs have sought to encourage interdisciplinary research by being cross divisional.
Obtaining appropriate reviews can be difficult, however, in part because of bias (or at least preference) toward proposals for which the data needed for the validation of project results are already available.
Nevertheless, there is a pressing need to partner planetary geoscientists and exoplanetary researchers together, or we will not be able to fully understand the ever increasing number of rocky Astronomy can offer other critical information on exoplanets. For example, the ages of Sun-like stars are estimated with activity diagnostics, gyrochronology, and lithium depletion ( ), which, by extension, also yields an upper limit for the age of any exoplanets present-with implications for a planet's thermal and geological evolution (e.g., ; ).
Observations of extrasolar systems can also return information on orbital dynamics. Such observations bear on the prospect for tidally induced volcanism, and whether planetary migration influenced the final distribution of bodies within a given system of interest, with attendant implications for composition, volatile inventories, etc. ( ). When acquired, measurements of atmospheric composition (e.g., from transit photometry/spectroscopy or even direct spectroscopy) can bound estimates of a planet's total volatile inventory, with implications for bulk composition, core chemistry, interior processes, and degassing history. And by considering the characteristics of the host star, it may be possible to understand the UV environment to which the planet is subjected, and even its atmospheric loss rates ( )-helping place constraints on interior degassing models and whether we should expect to find an atmosphere for a given exoplanet ( ).
For instance, computer modeling and laboratory experiments can simulate the effects of variable bulk composition, oxygen fugacity, etc., on mantle chemistry, core formation, and interior structure (e.g., ), with important implications for the chemistry of a planetary surface and even its possible volcanic and tectonic characteristics (e.g., ). For instance, showed how oxygen fugacity played a major role in the thermochemical evolution of Mercury, a highly reduced planet close to its parent star and, perhaps, a high-density exoplanet archetype ( ).
Laboratory and numerical studies can also help extend our understanding of Solar System bodies, We can work to increase the scientific diversity of our conversations by introducing Earth scientists, biologists, ecologists, etc., to the problems and promises of exogeoscience, and by partnering with Meeting and the 2020 AAS Winter Meeting, and the "Exoplanets in Our Backyard" meeting in February 2020 ( )-are exciting steps towards this discourse. Exogeoscience will further benefit from tightly focused workshops to build a systems-level understanding of planetary interior-surfaceatmosphere interactions, featuring organizing committees diverse both scientifically and in actuality.
Geoscientists and astronomers can write papers together. Such partnerships will enhance the reach and utility of exogeoscience studies, with co-authors helping each other to draw reasonable inferences from the limited observables to hand and make testable predictions. These efforts will not only grow an interdisciplinary pool of reviewers for both manuscripts and proposals, but will help to reduce jargon and ultimately aid these disparate groups in better communicating with one another. Equally, we can encourage journals to support interdisciplinary papers through, for example, recruiting editors and reviewers who understand interdisciplinary work, and by hosting special issues and coordinated papers in affiliated journals-for example, the "Exoplanets: The Nexus of Geoscience and Astronomy" special issue in the Journal of Geophysical Research: Planets. And promoting efforts by the geoscience community to archive journal articles with online repositories (including, but not limited to, arXiv.org and essoar.org) will help increase the reach of geoscience research to other disciplines.
We face a choice: to continue exoplanetary geoscience and astronomical research as two separate disciplines, or to build toward a shared understanding of our universe by working together.
Will you join us?