which deeper knowledge is available are Saturn’s moons Enceladus (where active plumes spew water and other materials from its interior;2 and Titan (where the Huygens probe obtained in situ data about the composition of volatiles in the atmosphere and the upper centimeters of the surface).3

POTENTIALLY HABITABLE ENVIRONMENTS

Decision Point 1—Liquid Water

Terrestrial life has a requirement for liquid water. Because water ice serves as the “bedrock” on an icy body, the existence and the location of liquid water within the body are key to gauging its habitability. Recent exploration in the outer solar system has revealed that many icy moons have liquid water oceans buried beneath several kilometers or tens of kilometers of ice. Magnetometer data provides compelling evidence of liquid water for Jupiter’s moons Europa, Ganymede, and Callisto.4 Oceans are suspected to be present within Saturn’s moons Titan and Enceladus.5,6 Radiogenic heating from the rocky interiors of large and mid-size icy bodies is theoretically sufficient to melt ice at depths greater than 100 km.7 Once melted, internal oceans may also dissipate enough heat to prevent them from freezing.8 These subsurface oceans are gravitationally and thermodynamically stable over time because liquid water is denser than water ice, the low-density phase present on the surface.

Mechanisms for generating liquid water on an icy body include contact with rocky material warmed by tidal heating, shock heating in a hypervelocity impact, tidal heating within the ice, contact of pure water ice near its melting temperature with contaminated ice mixtures that melt at lower temperatures,9 and warming of ice by a perennial heat source (e.g., a radioisotope power system) delivered to the target by the spacecraft. Liquid water may exist in intimate association with the ice; for example, terrestrial organisms in sea ice can survive below-freezing conditions within microscopic brine pockets at ice grain boundaries.10 Except for Titan, icy bodies lack a significant atmosphere. On these airless bodies, direct warming of surface ice will lead to sublimation instead of melting, and liquid water that becomes exposed at the surface will not just pool sedately and freeze, but rather will undergo rapid freeze-boiling.

Localized melting of ice by a radioisotope power system (RPS) is not likely to present a serious concern for future missions to the outer solar system. Studies were conducted at the Jet Propulsion Laboratory in the late 1990s and early 2000s in support of efforts to design an RPS-powered, ice-penetrating probe for application on a future mission to Mars and Europa.11 In addition to the need to seal the heat source within Europa’s ice so as to raise the vapor pressure to a sufficiently high value to initiate melting, the study revealed the critical power needed if any melting were to take place at all. The study team reported the following: “0.6 kW thermal input did not provide enough energy to raise the ice temperature (−170°C) sufficiently to initiate melt. The Europa ice is so cold it acts as an infinite heat sink and the heat is transmitted into the heat sink so quickly that localized phase change at the vehicle shell is impossible. Melt was initiated at 0.8 kW, but with no margin for error on the actual ice temperature. At 1 kW, phase change at the vehicle shell interface was sustainable with the creation of about 1-mm melt-water jacket around the vehicle.”12 A more recent study produced similar conclusions.13

Current outer solar system missions, such as the New Horizons mission to Pluto and the Cassini Saturn orbiter, are equipped with the so-called General Purpose Heat Source–Radioisotope Thermoelectric Generators (GPHS-RTG), each of which has a thermal output of 4.5 kW (at the beginning of the mission). It is thus conceivable that a single GPHS-RTG could initiate local melting if its plutonium-238 heat sources remained sufficiently intact following impact with an icy body. However, future plans for missions (see Appendix B) to objects of concern for planetary protection (e.g., Europa and Enceladus) envisage the use of the Advanced Stirling Radioisotope Generator (ASRG). Each ASRG has a thermal output of only 0.5 kW (at the beginning of the mission), and so ASRGs are unlikely to initiate local melting, except in the unlikely case that multiple ASRGs survived impact while maintaining intimate contact with each other.

In contrast to large or mid-size icy bodies that might contain liquid water in their interior, the nonspherical geometry of small icy bodies indicates that the vast majority of their interiors have remained cold, stiff, and completely solid. Such objects are small enough that they do not contain enough energy (e.g., from radiogenic heating)



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