to generate interior melt during their long-term thermal evolution. Thus small satellites, ring particles, comets, and Centaurs can be eliminated from being bodies of concern for planetary protection.
Decision Point 2—Key Elements
In addition to abundant oxygen and hydrogen on icy surfaces, the key biological elements carbon, surlfur, and nitrogen might also occur in some icy surfaces in the form of ice, clathrates, or simple organics. The elements potassium, magnesium, calcium, iron, and phosphorus can dissolve in liquid water that has been in contact with rocky materials. However, in extraterrestrial environments, the bioavailability of compounds containing these elements may limit their use by terrestrial microorganisms. For example, chemical modeling by Pasek and colleagues predicted that phosphine instead of phosphate will account for available phosphorus on Titan.14 Researchers cannot yet constrain the cycling and bioavailability of different chemical forms of individual elements important to life or their occurrence on icy bodies in our solar system. Knowledge of chemical composition for satellites other than Enceladus and Titan comes mostly from spectroscopy, which senses only the outer few microns of the surface. Volatile frost deposits on the surfaces of icy bodies may not represent their interior chemical composition, making it difficult to assess the abundance of dissolved elements within icy bodies. Therefore, this decision point currently contributes to intellectual completeness rather than serving as a key hinge point for planetary protection decisions. However, someday this decision point may play a more important role in planetary protection policy in response to new information about the chemistry of icy bodies and about minimal requirements for elements for the propagation of microorganisms.
Decision Point 3—Physical Conditions
The range of possible temperatures of liquid water environments within icy bodies is more tightly constrained than the chemical composition. Reservoirs of liquid water within icy bodies always remain in contact with ice, and thus the temperatures within these liquids hover near the freezing point of pure water (which at a minimum is -20°C at a depth of ~100 km in a large icy body) or that of mixed ice plus salts or ice plus ammonia (plausibly as low as -97°C). A source of energy within an ice shell will generally melt the surrounding ice while maintaining the liquid body at the freezing point. In a subsurface ocean overlain by a floating ice shell, the tendency of warm liquid to rise and cool liquid to sink will pin the entire ocean temperature near the freezing point. Heating within such an ocean will cause melting in the overlying ice but will not change the temperature of the water. Under special circumstances, such as in a freshwater ocean15 or if warm saline fluids were injected into the bottom of the ocean,16 a subsurface ocean might become stratified so that the lower layers of the ocean could warm to above freezing but not above 4°C (or 6°C if adiabatic compression at the bottom of a large icy satellite ocean is taken into account).
The only place where the water temperature might rise above this upper limit lies beneath the base of a subsurface ocean in contact with rocky materials. Cracks within a rocky ocean floor would permit infiltration of water, and contact with warmer rocks at depth can lead to porous convection. Such convection is typified by broad down-wellings into the porous rocks balanced by focused upwellings of warm water at hydrothermal vents. The spacing and power output of these hydrothermal systems depend on multiple uncertain assumptions about the nature of the seafloor and the energy source driving the activity.17,18 Compared to Earth, the mass flux of fluid transport for a given change in fluid temperature is lower on icy bodies because lower gravity leads to slower convective velocities. Once emitted from the ocean floor, hydrothermal fluids mix rapidly with the surrounding ocean, such that the water temperatures are within a degree of the temperature of the surrounding ocean within tens of meters from the vent.
Decision Point 4—Chemical Energy
Current knowledge of available redox couples that can provide chemical energy for terrestrial organisms suffers from greater uncertainty than does knowledge of available chemical elements. For icy bodies with liquid water in contact with a rocky interior, water-rock chemical reactions can provide the energy for life. On the largest icy bodies (Ganymede, Callisto, and Titan), ocean water lying between low-pressure ice-I shell above and denser high-