Instrumentation for Testing Whether the Icy Moons of the Gas and Ice Giants Are Inhabited

1. Radiation-Resistant Inorganic Biomarkers

To date, organics on Europa have not been detected. Furthermore, the radiation environment at Europa is so strong that any organics in existence, regardless of whether they are biogenic in origin, would have very short lifetimes (Teodoro et al., 2016). Consequently, the potential to distinguish biologically mediated isotopic fractionation of elements among organic species on Europa's icy surface may not be possible (Cooper et al., 2001; Barnett et al., 2012). For these reasons, we have argued (Chela-Flores and Kumar, 2008; Chela-Flores, 2010) that it is unlikely that forthcoming missions slated for Europa or Ganymede flybys will ever carry instruments capable of detecting biosynthetic molecules unique to known cell biology, namely, amino acids, lipids, polysaccharides, and nucleic acids (either DNA or RNA).

A type of geochemical biomarker that would be immune to destruction by the intense radiation at the surface of outer solar system (OSS) moons, should they be present on Europa and other icy worlds, is inorganic S-bearing compounds that were isotopically fractionated by biological compounds. The inherent resistance of such compounds to radioactive destruction is especially important given that the Galileo Mission detected sulfur in patches at the moon's surface at locations such as Castalia Macula (0°N, 225°W), which is of relatively recent origin (Prockter and Schenk, 2005). The JUICE mission (Grasset et al., 2013) and the Europa Clipper (Phillips and Pappalardo, 2014) carry instruments that could potentially investigate S isotopes in sufficient detail to detect an S biomarker.

Evidence of life beyond Earth may be within reach among these solar system moons. Pappalardo et al. (2013) reviewed the evidence for the existence of an active ocean on Europa, a hypothesis under consideration for some time (e.g., Reynolds et al., 1983) and supported by Na/K measurements in the extended atmosphere of Europa (Cassidy et al., 2009). While isotopically fractionated, S-bearing compounds may be found that are consistent with biological activity; additional biomarkers would be needed to determine definitively whether the source of such biomarkers on Europa is due to a “second genesis” (McKay, 2001).

2. Missions to Ocean Worlds of the OSS

At present, there is interest in developing astrobiology missions to search for evidence of life beyond the gas giants. As we learn more about other planetary bodies in our solar system, it has been recognized that the surfaces of other planets, such as Neptune's moon Triton, also serve as candidates for hosting geochemical biomarkers (Gaeman et al., 2012; Nimmo and Spencer, 2014). Hussmann et al. (2006) considered models of the internal structure of Triton and other moons of the ice giants. These models predict a Europa-like rocky core, a subsurface ocean, and—significantly—an outer icy surface for Triton, Ariel, Iapetus, Umbriel, Titania, Oberon, and Pluto's moon Charon (Rhoden et al., 2015). In light of the geochemical considerations that apply in particular to Europa, it is critical to discuss at this juncture what would be required to facilitate the search for geochemical biomarkers by way of the elemental isotopic content of the icy surfaces and exoatmospheres of more distant unexplored moons in the solar system (Chela-Flores et al., 2015).

Specific missions to the ice giants have not, to date, been endorsed, although proposals for future missions to the ice giants (NASA study, 2015) are currently under discussion. These proposals include origins, dynamics, and interiors of neptunian and uranian systems (ODINUS) (Turrini et al., 2014), OSS (Christophe et al., 2012), an orbital mission to Uranus (Arridge et al., 2014), and a mission to the uranian system (MUSE) (Bocanegra-Bahamón et al., 2015). If any of these mission concepts is flown, data acquired by midcentury would allow for insights into habitability beyond the gas giants and, quite possibly, could address the question as to whether worlds in the realm of ice giants are inhabited.

3. Sulfur as a Measurable Geochemical Biomarker

Mass spectrometers (MSs) have unique capabilities that can be applied to studies of habitability and, perhaps, more importantly, life detection on icy worlds. In this regard, two forthcoming missions to the jovian system have been provided with astrobiologically relevant instrumentation (Table 1).

Possible candidates for biota on the icy worlds are extremophilic microorganisms, which account for a major portion of life on Earth. It is worth noting that remarkable variations in microbial sulfate reduction, which were not previously detected as a product of nonliving processes, have been discovered in pure culture experiments (Sim et al., 2011). These experiments revealed a large S mass-dependent fractionation of δ³⁴S of up to −70‰. This biogenic phenomenon is exclusively observed in a handful of natural environments on Earth, which raises the question as to whether an europan biota would be composed of sulfate reducers and disproportionators that could give rise to measurable geochemical biomarkers. Such a possibility could be tested with the MS approved for the JUICE and Europa Clipper missions.

Geochemical biomarkers on the icy surface of Europa could arise, for example, from mineralogy that is indicative of habitable environments (Tulej et al., 2015). Alternatively, the source of geochemical biomarkers could arise from isotopic fractionation of biological or metabolic processes. At the University of Bern, many instruments capable of addressing these possibilities have an impressive flight hardware development heritage (Riedo et al., 2013a, 2013b), as some are part of the JUICE payload, including the particle environment package (PEP) that includes a neutral and ion gas mass spectrometer (Meyer et al., 2017). The accuracy necessary for testing the presence of biogenicity in S isotope systematics on Europa would be achievable with the PEP instrumentation.

The Europa Clipper, an upcoming NASA mission designed to investigate habitability on Europa, has the capability to measure biogenic stable S-isotope fractionation: The MAss SPectrometer for Planetary EXploration/Europa (MASPEX), which will be included in the Europa Clipper scientific payload, was designed to determine the composition of the surface and subsurface ocean by measuring Europa's tenuous atmosphere and any surface material ejected into space. As a part of the Europa Clipper mission, the SUrface Dust Mass Analyzer (SUDA) will measure the composition of small solid particles ejected from Europa and provide an opportunity to sample directly the surface and potential plumes on low-altitude flybys. Significantly, the latter instrument is capable of identifying biosignatures in the ice matrix of ejecta. A lander has also been approved with a payload that includes a MS, as was the case for previous spacecraft missions such as Cassini, Phoenix, and Curiosity (Pappalardo et al., 2016).

At present, however, it is not completely clear whether the effects of radiation on the detection efficiency of these instruments can be reduced (Tulej et al., 2016). Europa's exoatmosphere is normally considered to be an extension of its surface (Coustenis et al., 2010). If its chemical elements are endogenic, the ultimate source must be regions that have a young surface, where the upwelling of subsurface material may occur (Tobie et al., 2010). Tests of biogenicity are feasible on exoatmospheric particles that arise from surficial patches. This raises the possibility that the chemical elements of the exoatmosphere may be from the subsurface ocean. Given the available accuracy of the instruments, anomalous S isotope ratios can be measured in the forthcoming missions and may provide the first evidence for life in the OSS.