Thermal state and complex geology of a heterogeneous salty crust of Jupiter's satellite, Europa

Abstract

The complex geology of Europa is evidenced by many tectonic and cryomagmatic resurfacing structures, some of which are “painted” into a more visible expression by exogenic alteration processes acting on the principal endogenic cryopetrology. The surface materials emplaced and affected by this activity are mainly composed of water ice in some areas, but in other places there are other minerals involved. Non-ice minerals are visually recognized by their low albedo and reddish color either when first emplaced or, more likely, after alteration by Europan weathering processes, especially sublimation and alteration by ionizing radiation. While red chromophoric material could be due to endogenic production of solid sulfur allotropes or other compounds, most likely the red substance is an impurity produced by radiation alteration of hydrated sulfate salts or sulphuric acid of mainly internal origin. If the non-ice red materials or their precursors have a source in the satellite interior, and if they are not merely trace contaminants, then they can play an important role in the evolution of the icy crust, including structural differentiation and the internal dynamics. Here we assume that these substances are major components of Europa's cryo/hydrosphere, as some models have predicted they should be. If this is an accurate assumption, then these substances should not be neglected in physical, chemical, and biological models of Europa, even if major uncertainties remain as to the exact identity, abundance, and distribution of the non-ice materials. The physical chemical properties of the ice-associated materials will contribute to the physical state of the crust today and in the geological past. In order to model the influence of them on the thermal state and the geology, we have determined the thermal properties of the hydrated salts. Our new lab data reveal very low thermal conductivities for hydrated salts compared to water ice. Lower conductivities of salty ice would produce steeper thermal gradients than in pure ice. If there are salt-rich layers inside the crust, forming salt beds over the seafloor or a briny eutectic crust, for instance, the high thermal gradients may promote endogenic geological activity. On the seafloor, bedded salt accumulations may exhibit high thermochemical gradients. Metamorphic and magmatic processes and possible niches for thermophilic life at shallow suboceanic depths result from the calculated thermal profiles, even if the ocean is very cold.

Introduction

Europa is a prime target for space exploration because of its striking geological/geophysical indications of a water ocean Carr et al., 1998, Pappalardo et al., 1999, Greenberg et al., 1999, Kargel and Consolmagno, 1996, Kivelson et al., 2000 and the ocean's potential astrobiological implications Chyba et al., 1998, Kargel et al., 2000, Gaidos et al., 1999, Abyzov et al., 2001. Highly debated is the total thickness of the icy layer of Europa and the mean depth of the possible ocean. Several models assuming different thermal structures have been proposed. The thick crust models include a stagnant lid and warmer convective part as well as brittle tectonic responses of a thick shell to convective stresses and the large Europan tides Pappalardo et al., 1998, McKinnon, 1999, Hussman et al., 2002, Nimmo and Gaidos, 2002, Ruiz and Tejero, 2003, Spohn and Schubert, 2003. Thin-crust models involve primarily brittle tectonic responses to tidal forces and melt-through (O'Brian et al., 2002). Separate discussions of the composition and thermal state of the crust have not yet linked these factors to one another or coupled both to Europa's geology.

The mineralogical composition, thermal state, and geological activity of a planet's crust are interdependent due to specific physical–chemical properties of major rock types. As occurs in the Earth between oceanic and continental crust or between granitoid shields and sedimentary basins, contrasts in mineralogy and petrology are related to differences in magmatic activity, tectonics, and other processes, but these relationships go two ways; it is a classic “chicken and egg” relationship. Unless the differences in mineralogy seen on Europa can be shown to be strictly superficial, we must consider the possibility that mineralogical/petrological heterogeneities directly influence contrasting magmatic and geological behavior, as well as being the products of these contrasts. This is true of all major silicate bodies having magmatically and tectonically active histories. There is no obvious reason to suppose that Europa and other icy bodies would somehow be immune to this aspect of planetary evolution, unless the crust is so pure and inert or isolated with respect to the rocky interior that there is no impurity in the crust and ocean sufficient to influence melting temperature, rheology, or heat transport.

Thermal models of Europa's crust have considered only properties of water ice until now with the exception of a study by Spohn and Schubert (2003), who have considered the presence of ammonia phases, although they have not been observed at the surface. However, there is observational evidence from Galileo's Near Infrared Mapping Spectrometer (NIMS) of two major classes of minerals on Europa, water ice and hydrates. The hydrates probably are dominantly the higher hydrates of sulfate salts or sulfuric acid, though specific identities remain unknown. These hydrates have been predicted theoretically from the composition of the most altered carbonaceous chondrites Kargel, 1991, Kargel et al., 2000, observed spectroscopically McCord, 1998a, McCord et al., 1998b, McCord et al., 1999, Fanale et al., 1999, Carlson et al., 1999, and studied under Europan conditions thermodynamically Zolotov and Shock, 2003, Marion, 2002 and in the laboratory Fanale et al., 2001, McCord et al., 2001. McKinnon and Zolensky (2003) have considered an alternative means by which magnesium sulfate and other sulfates can form from an initial sulfidic ocean (oceans formed by either “hot-press” or “cold-press” alteration of the bulk mineralogy) by progressive global loss of hydrogen from the ocean. Their model yields upper limits of 8.5–10% MgSO₄ (depending on the details of the accretion and alteration model), which they consider far upper limits due to likely incomplete aqueous reaction of the rock related to the low permeability of altered serpentinized rock. Nevertheless, they consider sulfates a likely important part of Europa's story, though the amounts and origin differ from that considered here.

A close relationship of surface salts or sulfuric acid on Europa to a liquid ocean is unproven, but is suggested by a confluence of geological observations Fanale et al., 2001, McCord et al., 2001, cosmochemical theory Kargel, 1991, Kargel et al., 2000, and magnetometer observations and modeling Khurana et al., 1998, Kivelson et al., 1999, Kivelson et al., 2000. The Galileo magnetometer results show the existence of induced magnetic fields caused by electrical induction currents flowing in each of the icy galilean satellites, explained by the likely existence of an electrically conducting layer, probably a briny liquid ocean Kivelson et al., 1999, Kivelson et al., 2000, Khurana et al., 1998, whose magnetic response was predicted by Kargel and Consolmagno (1996).

The characteristic red color of Europa's impurities is probably caused by partial radiative alteration to sulfuric acid and metastable short-chain sulfur allotropes Carlson et al., 1999, Carlson et al., 2002 from a S-bearing material source. The source materials may be: (a) endogenic brines which could vary in composition from acid to low alkaline Kargel et al., 2000, Marion, 2002 depending on the minerals assumed, or (b) exogenic by ion implantation (Carlson et al., 1999). If sulfates are part of the sulfur cycle in Europa, they would be involved in the endogenous processes and rapidly altered in the radiation and high-vacuum environment of Europa. Some sulfates would be decomposed in less than 3800 years after reaching the surface according to Carlson et al. (2002). However, McCord et al. (2001) point out the differential stability of various sulfate salts, with magnesium sulfate hydrates being highly resilient on geological time scales and sodium sulfates far less resilient to radiative alteration and sublimation.

The spectral signal of the hydrated minerals are locally pure where the crust has been torn open by impacts or tectonic activity or where upwellings or eruptions of deep materials have occurred McCord et al., 1999, Fanale et al., 1999 so we support the endogenic hypothesis for emplacement of these materials. The situation is, however, locally just the opposite: some possible piercement diapirs or viscous domes appear bright against a dark/reddish aureole or background plains (Fig. 1), but in any case, there are strong geologic correlations of bright/bluish and dark/reddish features. There is a possibility that erupted material is initially intrinsically red due to inclusion of sulfur allotropes and other red solids, but more likely reddening is due to geologically prompt radiative alteration of endogeneous S-bearing brines and crustal ices. Hence, while the red material may be a thin alteration coating, it appears that the endogenous precursor material is a deep-seated, widespread, and abundant substance in Europa's crust and ocean.

Geophysical and geological process modeling must consider properties of the non-ice materials because the observations imply that they are not merely trace contaminants or an inconsequential façade. Taking the approximation of the sulfate content for Europa and the possible layer differentiation paths in Kargel (1991) and Kargel et al. (2000), we can assume some salt-enriched structural levels in the crust. It has been proposed that the differentiation processes on Europa, starting from a volatile-rich carbonaceous chondrite (CM2 for instance), could yield a hundred kilometers of hydrated salts. Thus, we have investigated some thermal properties of three candidate hydrated salts (epsomite, mirabilite, and natron) at low temperatures, and we have modeled heat conduction through hypothetical salt enriched layers. Compared to a baseline model of conduction through pure ice, thermal gradients are steeper with salts included. Salt-rich layers and regions likely would exhibit greater geological activity than ice-rich ones. Salt enrichment and crustal heterogeneities in salt and ice content might explain some classes of features on Europa.