Investigating Europa's Habitability with the Europa Clipper

Abstract

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

Keywords: Europa; Habitability; Ice; Jupiter; Ocean worlds.

Fig. 1

Jupiter’s moon Europa is one the most promising candidates for hosting life today among ocean worlds in the solar system. In its investigation of Europa’s habitability, the Europa Clipper mission seeks to understand the provenance of water, essential chemical elements and compounds, and energy, and how they might combine to make this moon’s environments suitable to support life. Modified from a pre-Europa Clipper mission study report (Europa Study Team 2012)

Fig. 2

Examples of extremophiles and common environments on Earth for varying solution pH and temperature. The investigation of Europa’s habitability extends beyond the binary of whether or not life can survive there. On Earth, life can be found almost everywhere water is stable as a liquid. How much biomass is present—and thus how easily that life might be detected—depends on the available chemical energy, which varies mainly with conditions of temperature and pH, as shown here. In addition to confirming the presence of liquid water, Europa Clipper may be able to characterize the details of Europa’s habitability by identifying the specific environmental conditions that are present and how long they might have existed. Reproduced from Shock and Holland (2007)

Fig. 3

Europa Clipper investigations, left, will acquire the fundamental data—images, spectra, in situ samples, etc.—that will be interpreted through models and analyses to derive properties of Europa that reveal the types of metabolism that might be possible, and the environmental conditions that might support life’s origin and persistence through time. This synthesis of information is needed in order to assess Europa’s habitability

Fig. 4

Europa is a global system with different interacting regions and processes. Europa Clipper will provide an unprecedented view into those processes, providing clues to the overturning of the ice and ocean, and the chemistry of the deeper interior. Artist credit: D. Hinkle

Fig. 5

Chemical affinity for methanogenesis from CO₂ and H₂ inferred for Enceladus’s ocean, computed from available equilibrium thermodynamic data based on the volatile contents and particle-bound ions in south polar plumes. By similarly constraining the composition and volatile content of materials within Europa, either from plume materials that may be present, or from oceanic materials at the surface, the Europa Clipper mission may be able to constrain the chemical affinity available to support metabolic processes. From Waite et al. (2017). Reprinted with permission from AAAS

Fig. 6

Europa’s ocean salinity is intrinsically linked to its temperature and to the thickness and dynamics of the overlying ice. For a given ice depth—5 and 30 km here—different compositions and amounts of salt require different melting temperatures of the ice, which sets the temperature at the top of the ocean. Convection in the ocean determines how temperature increases with depth as the salty fluids compress adiabatically. The temperature profile of the ocean strongly affects the conductivity of the ocean, which will be inferred from ECM and PIMS observations. Modified from Vance et al. (2021)

Fig. 7

A systems model of Europa showing the interconnected systems of material exchange, thermal inputs, and reservoir state, building from the physical processes depicted in Fig. 4. Such a framework, common in Earth systems science, can be used to understand the overarching dynamics and stability of the system. It also illustrates how an input or perturbation to one part of the system may have wide-ranging impacts to other parts of the satellite system. Sizing of arrows and boxes is not quantitative

Fig. 8

Europa Clipper may reveal the oxidation state of the ocean, which determines what kind of life might be able to exist there. Here, the modeled global fluxes O₂ and H₂ are inventoried on the y-axis, with the x-axis depicting the dependence of H₂ flux on the extent of heating in the silicate layer. Less tidal heating enables deeper thermal fracturing and the potential for more H₂ generated by water-rock chemistry (serpentinization). The bounds on O₂ are broad, owing both to limited constraints on the magnitude and time-dependence of radiolysis of water at Europa’s surface, or of the efficiency of moving oxidants into the ocean. The balance of these reductant and oxidant fluxes determines the ocean’s pH and also the available energy for metabolism (e.g., Waite et al. 2017). Modified from Vance et al. (2016)

Fig. 9

The relative energy available from different metabolic reactions can be constrained by the overall Europa Clipper investigation. The available energy is determined by the chemical disequilibria of materials provided by geological processes. In this figure, reactions of common inorganic substrates are plotted according to their redox potential at 1 atm, pH = 7, and 25 °C (i.e., neutral water at standard conditions). Only redox reaction pairs with a negative net redox potential can provide energy; oxygenic photosynthesis, as on Earth, requires energy (sunlight). The arrows indicate three metabolic reactions, two of which are photosynthetic processes facilitated by complex biomolecules that are not known to exist on Europa. By gathering an inventory of the simpler inorganic materials and constraining their fluxes in Europa’s ocean, ice, and seafloor, the Europa Clipper mission can assess the pathways that might be available for life. More complex biomolecules involved in metabolism, such as the photosystem complexes indicated here (P870 and P680), if detected, might be interpreted as biosignatures if the simpler substrates they mediate were also found. From Gaidos et al. (1999). Reproduced with permission from AAAS

Fig. 10

The composition of Europa’s ocean is a record of how Europa evolved. In some scenarios, an initially H₂-rich ocean would have become more oxidizing over time due to the influence of radiolytically produced oxidants from the surface and/or H₂ escape to space. Some models (upper left; modified from Zolotov ; T = 0 °C, P = 0.1375 GPa, water/rock ratio (W/R) = 1 by mass, 10% of reacted C in carbonaceous chondrites) predict that decreasing $f{\mathrm{H}}_2$ might have driven the ocean composition from one dominated by chlorides to one dominated by sulfates (for log $f{\mathrm{H}}_2<-9$). Alternatively (upper right; also from Zolotov ; T = 0 °C, P = 0.1375 GPa, W/R = 1, log $f{\mathrm{H}}_2=-10$), if the ocean retained significant abundances of inorganic carbon species, concentrations of sulfate ions are compatible with those of chlorides. Another model (bottom; from Melwani Daswani et al. . Adopted with permission from John Wiley and Sons.), that also considered the retention of CO₂ (here, nominal pH of 5.5, in equilibrium with a CM chondrite composition) shows the effects of varying solubility of ions with depth. Differences between the ion abundances are attributed to different input mineral compositions

Fig. 11

A subset of habitability parameters in the ocean that may be inferred from the synthesis of Europa Clipper measurements. The arrows show the directions of data flow to constrain the parameters. For example, ECM, E-THEMIS, and REASON will contribute measurements that will constrain the temperature of the ocean by measuring the temperature-dependent electrical conductivity of the ocean, and by constraining the composition and thickness of the ice, and thus the temperature at the ice-ocean interface

Fig. 12

Top: Bulk accretion temperatures vs object radius, after McKinnon and Zolensky (2003). For a retention factor of $h=0.4$ (which depends on accretion rate and planetesimal size), Europa’s temperature exceeds the dehydration temperature of serpentine minerals (800 K). Bottom: Predicted mantle temperatures in Europa at 2.1 and 4.2 Gyr after accretion, including tides. From Běhounková et al. (2021). Adopted with permission from John Wiley and Sons; blue: minimum, red: maximum, black: average, green: solidus temperature)