Review of Exchange Processes on Ganymede in View of Its Planetary Protection Categorization

Abstract

In this paper, we provide a detailed review of Ganymede's characteristics that are germane to any consideration of its planetary protection requirements. Ganymede is the largest moon in our solar system and is the subject of one of the main science objectives of the JUICE mission to the jovian system. We explore the probability of the occurrence of potentially habitable zones within Ganymede at present, including those both within the deep liquid ocean and those in shallow liquid reservoirs. We consider the possible exchange processes between the surface and any putative habitats to set some constraints on the planetary protection approach for this moon. As a conclusion, the "remote" versus "significant" chance of contamination will be discussed, according to our current understanding of this giant icy moon. Based on the different estimates we investigate here, it appears extremely unlikely that material would be exchanged downward through the upper icy layer of Ganymede and, thus, bring material into the ocean over timescales consistent with the survival of microorganisms.

Band Formation and Ocean‐Surface Interaction on Europa and Ganymede

“…The origin and formation of these tectonized bands may be tied to processes that link the ice shell and the ocean, including spreading, rifting, subduction, and cryovolcanism (Head et al, 1999;Johnson et al, 2017;Kattenhorn & Prockter, 2014;Prockter & Patterson, 2009). Understanding these material exchange mechanisms is key to evaluating the potential habitability of ocean worlds (Grasset et al, 2013;Hand et al, 2009;Spohn & Schubert, 2003). For consistency, we use the term "band" to describe any tabular extensional zone of ridges and grooves that transects surrounding terrain, including the various band types on Europa and groove lanes (sulci) on Ganymede ( Figure 1).…”

Biogeochemical fingerprints of life: earlier analogies with polar ecosystems suggest feasible instrumentation for probing the Galilean moons

“…In two of the Galilean satellites that the JUICE mission intends to explore in some detail, mass spectrometry either on a lander or inferred from orbital measurements from JGO should yield different results for fractionated sulphur according to our hypothesis: the biogenically processed icy patches of Europa should give substantial depletions of 34 S, whereas Ganymede measurements should give significantly lower values for the depletion of 34 S. In other words, a large minus δ 34 S for Europa and small minus δ 34 S for Ganymede, would test the origin of habitable ecosystems in two of the Galilean moons (Chela-Flores & Kumar 2008; Chela-Flores 2010). The relevance of this result should be seen in the light of more recent research (Grassett et al 2013a), where the authors do not exclude that current knowledge of Ganymede's ocean may possess all the requisites for being habitable. The proposed stable isotope fractionation result would be more relevant for testing the nature of icy surfaces of Galilean moons, where biogenic activity from the internal oceans may have altered a relatively young surface measured in tens of millions of years.…”

“…We have to rely eventually on careful consideration of the exosphere of Ganymede. Fortunately, even though a lander is still not under consideration, JUICE is expected to have a broad mission profile, including several orbits around the largest moon of the Solar System that will begin in September 2032, well before the Mission nominal end a year or so later, when the orbiter will be disposed of on Ganymede itself (Grasset et al 2013b), yet not posing any significant planetary protection risk (Grasset et al 2013a): First elliptic (10 000 × 200 km) 30 days. High-altitude circular (5000 km) 90 days. Second elliptic (10 000 × 200 km) 30 days. Medium-altitude (500 km) circular (102 days) Low-altitude (200 km) circular (30 days). …”

Water and Volatiles in the Outer Solar System

“…Ganymede possesses a high-level of differentiation, meaning that there must be a clear separation between an iron metallic core (which existence is proven by the intrinsic magnetic field of the moon), a silicate mantle, and a hydrosphere about 500 km thick. Grasset et al (2013) showed that all models converge towards an ocean thickness that varies from 50 km at 130 km in depth in the case of pure water, up to 230 km at about 60 km depth if ammonia were added to the liquid.…”

“…Future missions such as the Europa Orbiter mission planned by NASA for the next decade will help retire this uncertainty via a combination of ground penetrating radar and gravity science. If Europa's crust is thinner than a few kilometers, the ESA JUICE mission will also be capable to detect the ocean below (Grasset et al 2013). Finally, the interior structure of Enceladus displayed in Fig.…”