The geophysical context of solar system objects constrains the potential propagation of terrestrial organisms with known minimal nutritional requirements and environmental tolerances as outlined in Chapter 3. The outer solar system contains a broad diversity of icy bodies, ranging from co-accreted satellites bound to their gas giant parent planets to small icy leftovers of planet-like comets, Centaurs (whose orbits cross the giant planets), and the Kuiper belt objects (KBOs). Icy bodies can be divided into categories by size: large icy bodies (radius > 1,000 km, like Europa, Ganymede, Callisto, Titan, Triton, and large KBOs like Pluto and Eris), mid-size icy bodies (200- to 1000-km-radius objects like Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Miranda, Ariel, Titania, Umbriel, Oberon, Charon, most known KBOs, and the asteroid Ceres), and small icy bodies that are small enough to avoid becoming spherical (<200 km, like Phoebe, Hyperion, Nereid, comets, Centaurs, and ring moons).
The cold and inhospitable surfaces of icy bodies in the outer solar system serve as a natural barrier to the forward contamination of their warmer and more hospitable interiors. Here, the committee describes the geophysical “bottlenecks” that separate terrestrial organisms hitchhiking on a spacecraft from entering potentially habitable environments existing within icy bodies. This chapter first outlines the properties and locations of potentially habitable environments, discussing Decision Points 1 through 4 from Chapter 2. The bulk of the chapter concerns Decision Point 5, discussing transport processes that might operate between the uninhabitable surface and potentially habitable subsurface environments. The chapter concludes with a survey of icy bodies to delineate areas of concern for planetary protection.
The reconnaissance of icy bodies in the outer solar system is incomplete, and in many places basic surface and interior properties remain unknown. Scientists have data from only half of the surfaces of the objects in the Uranus and Neptune systems, and they lack close spacecraft observations for all objects beyond the orbit of Neptune. Interior structures of the satellites of Jupiter and Saturn are constrained by the moment of inertia, which has been measured during close flybys. However, the interpretation of the moment of inertia value in terms of an interior density profile produces results that are not unique.1 As a result, the reported depths and densities of interior layers are inferences based on assumed common materials that could make up the interior of the body. For other bodies that lack flyby data, interior states represent well-informed guesses. The chemical composition of most bodies is constrained by infrared spectroscopy, which senses only the top few microns of the surface. The only bodies for
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
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)
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 downwellings 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-
pressure ice phases below would not react with the bulk of the rocky interior.19 Radioactive decay could hydrolyze water on a small scale and provide small amounts of chemical energy.20 Material transport from the surface of a body to an interior ocean could maintain a chemical energy gradient, for example due to oxidants produced by irradiation of Europa’s surface.21 If appropriate energy sources occur on an icy body, terrestrial biology would persist only if active geochemical cycles occurred between the liquid and the surface or the liquid and the deep interior. As described under Decision Point 2, planetary protection considerations for future missions will have the advantage of research initiatives that provide new information, including data on the availability of biologically relevant sources of energy on icy bodies. In the absence of such information about energy sources and the bioavailability of minimal element requirements, it is assumed that any liquid water within poorly characterized icy bodies might have the proper chemistry for supporting terrestrial life.
Decision Point 5—Contacting Habitable Environmemnts
Floating outer ice-I shells may be a frustrating impediment to life-detection experiments, and they serve as a protective barrier from the viewpoint of planetary protection. Therefore, setting planetary protection guidelines requires an understanding of the physical processes that allow vertical transport of material between the subsurface and surface of an icy body and the timescales on which transport occurs. Some of these vertical transport processes operate from the top down, whereas others operate from the bottom up (Figure 4.1). There are usually limits to the vertical range over which the processes operate; for example, impact gardening and radiation transport material vertically over ~1-m scales, comparable to the physical size of a spacecraft. Cracks open beneath the surface and may penetrate to a depth of ~1 km. Lithostatic stress limits the propagation depth of cracks in the top of brittle surface materials. Solid-state convection operates within the solid portion of the ice shell, but on most bodies convection is confined beneath a so-called stagnant lid that is several kilometers thick. The stagnant lid is composed of cold material that is so viscous that it cannot participate in convection.22 If there is no overlap between top-down and bottom-up vertical transport processes, a “no-man’s land” exists in the middle of the ice shell that interrupts exchange of material between the surface and a subsurface ocean.
At the very top surface of an icy body without an atmosphere, where exogenous contaminants are likely to be deposited, impact gardening dominates the mixing of these materials into the subsurface. Gardening refers to the churning of surface regolith driven by the impact of meteoroids and the subsequent burial of neighboring surface materials by impact ejecta. Phillips and Chyba estimated that on Europa’s surface gardening could mix loose surface materials to a depth of ~1 meter over 10 million years,23 although this burial might be episodic rather than continuous, given that about 95 percent of the small craters on Europa may be secondary craters.24 The impact rate on Europa is within a factor of 2 of the highest impact rates of any icy bodies in the outer solar system because of its location deep inside the Jovian gravity well.25 Therefore the other icy bodies, with similar or lower impact rates, will have mixing rates due to gardening that are similar to or lower than those on Europa because of lower impact rate and velocity, both of which are controlled by the size of the parent planet and the planet-satellite distance.26 The effect of impact gardening on the burial of surface materials over short timescales is minimal on the icy bodies of our solar system.
At the surface of an icy body, cold ice behaves as a brittle material. If the surface is subjected to tensile stress, the brittle material can fail, producing open tensile fractures. The abundance of cracks and faults on the satellites of the outer solar system attests to this process playing an important role in their history.27 In theory, loose surface material could fall into open fractures, producing surface-subsurface material transport. Such a surface regolith drainage mechanism has been hypothesized to explain “pit chains” on Mars and Enceladus.28,29
The depth to which open tensile fractures can propagate from the surface is limited by the normal stress on
FIGURE 4.1 Depth of penetration of various vertical transport mechanisms on the surface of a generic icy moon with a ~10- to 100-km-thick ice shell (most applicable to bodies of concern—Europa, Enceladus, Titan, and Triton).
the fracture imposed by the weight of the overlying ice. Below some depth the material fails in shear instead of tension, forming a sloping fault surface rather than an open crack. The exact depth of this transition depends on the strength of the brittle material and the surface gravity. The strength of pure, polycrystalline, unfractured laboratory ice falls within the range of 1 to 2 MPa, whereas ice in the natural environment has more mechanical defects and can be one or two orders of magnitude weaker.30,31 The surface gravity of an icy body scales with its radius and its ice/rock ratio. As a consequence of these factors, open fractures can propagate on the order of hundreds of meters into the surface of a large icy body or a few kilometers into a mid-size icy body.
In cryovolcanic eruptions, watery mixtures move from the interior of an icy body to the surface. Cryovolcanism has no direct terrestrial analog but appears to have occurred on several moons of the outer solar system, including Europa, Ganymede, Enceladus, Titan, Ariel, and Triton. The physical processes that enable the eruption of
cryomagma from the interior of icy satellites remain unknown and may vary considerably from one satellite to the next.32 Unlike terrestrial volcanism, in which the magma is usually buoyant relative to the surrounding crust, the greater density of liquid water relative to ice causes the cryolava to sink instead of rising to the surface. Several mechanisms that might overcome this difficulty include33 gas exsolution following depressurization in fluid-filled fractures that propagate upward from the base of the ice shell;34 explosive eruptions of sprays;35 pressurization of liquid chambers in an ice-I shell;36,37 and pressurization of the entire ocean due to thickening of the ice shell.38
Surface temperatures on icy bodies are very low compared to the freezing temperatures of cryolavas. Although cryovolcanism represents interaction between subsurface liquids and the surface environment, material flows primarily toward the surface. From a planetary protection perspective, contact of a spacecraft with a cryovolcanic flow does not necessarily contaminate the source of the cryolava. Drain-back of surface lava that flows down eruptive fissures, possibly into the source chamber, is a rare event in terrestrial volcanism. It occurs in submarine volcanism in the East Pacific Rise39 and on some basaltic eruptions such as those from Kilauea in Hawaii.40,41 In both of these cases, lava lakes were present. Lava lakes are rare on Earth, and outside Earth they have been observed only on Io,42 although past lava lakes may exist on Venus and Mars. In lava lakes, the subsurface melt conduit is hydraulically connected to the surface melt, so that decreases in pressure can result in a reversal of flow from the lake into the subsurface. There is no evidence that lava that has traveled downhill from a source vent could ever drain back to the interior. No features on icy bodies have been interpreted as cryolava lakes. It is possible that cryolava lakes do not form on icy bodies because the negative buoyancy of cryomagmas within icy crusts precludes the establishment of a stable lava lake. Therefore, the committee considers that drain-back events would be rare by comparison with Earth, and only topographic depressions surrounding active cryolava vents are of concern for planetary protection.
The negative buoyancy of liquid water within the ice crust suggests that if a mechanism existed to produce melt near the surface of an icy body, it could drain downward and provide an effective conduit for surface-subsurface transport. Concentration of tidal dissipation within the weakest, warmest ice in the cores of convective plumes could cause melting within the ice shell,43,44 but not within the stagnant lid (see the next section, “Convection”). Convective plumes within a relatively pure ice sublayer could induce melting within the overlying stagnant lid if other materials that lower the melting temperature, like salts, contaminate the lid.45,46 Nimmo and Giese modeled this process for Europa and could not produce significant melt within 7 km of the surface using convective plumes.47 However, even the production of a small amount of partial melt could contribute to enhanced tidal heating and plastic yielding near the surface48 or infiltration of preexisting cracks in the near-surface brittle ice as the trapped, pressurized fluid follows hydraulic gradients within the ice shell.49
Another mechanism for near-surface melting is the concentration of heat flow from the interior of the body in sufficient amounts to thin the overlying ice shell. Once the shell has thinned beyond the thickness required for convection, heat conduction through the shell can control the thickness. Sufficiently concentrated heat could melt through the ice, thus exposing liquid at the surface,50 but this outcome requires more than 300 W/m2 of heat flow from the interior.51 Whether such a concentration of heat is even possible on an icy body is debatable;52 for reference, this is a factor of 103 higher than the heat flow in the south polar terrain of Enceladus, the most geologically active known region on an icy body.
Like rocks in Earth’s interior, water ice behaves as a fluid over geologic time scales. Radiogenic heating at the bottom or within the ice shells of icy bodies would warm ice at the base and cause it to rise from thermal buoyancy while cold ice sinks. Convection likely occurs in the outer ice shells of the large icy bodies and in smaller, tidally heated satellites.53,54,55 The downward flow of cold ice would provide a pathway for relatively rapid transport to the ocean. Typical flow velocities of centimeters per year would yield a timescale for transport to the base of the ice shell (tens to a hundred kilometers) of ~105 years.56 Because the surfaces of the outer planet satellites have
low effective temperatures (~50 K to 130 K) and the viscosity of ice is strongly temperature-dependent, convective plumes are typically confined to a sublayer of the ice shell beneath a “stagnant lid” of cold ice that is too stiff to participate in convection. Heat must be conducted across the stagnant lid. The stagnant lid serves as a barrier to mass transport between the surface of an icy moon and the convective sublayer. However, endogenic resurfacing processes on icy bodies could conceivably breach the stagnant lid, providing a means of communication between the surface, the convective sublayer, and the ocean.
The surface morphologies of Europa and Enceladus and observations of the high heat flow on Enceladus imply that convective motions have reached the surfaces of these bodies.57,58 If the near-surface ice has an extremely low yield stress of ~0.01 MPa,59 it can be dragged along by the underlying convective motions. This style of convection, dubbed “sluggish” or “mobile-lid” convection, is associated with a very thin layer of cold ice at the surface, which can locally achieve essentially zero thickness and, in some cases, periodically rip and sink to the base of the ice shell.60 The predicted heat flow and resurfacing rates from sluggish lid convection within the Enceladus south polar terrain match estimated values,61 lending support for the existence of this style of convection on tidally flexed icy moons, but perhaps only for short periods of time.
The thickness of the stagnant lid is important for determining the likelihood and timescale of transport across the geophysical “no-man’s land” between the base of the stagnant lid and materials near the surface. A rough estimate of this thickness can be obtained by equating the radiogenic heat flux Fr to the convective heat flux Fconv. For large icy bodies, Fr ~ 5 to 10 mW/m2. Tidal heating on Europa and Enceladus dwarfs radiogenic heating. The surface heat flux from tidal dissipation on Europa is plausibly 10 to 100 mW/m2.62 Estimates of the power output in the south polar terrain of Enceladus from Cassini CIRS data are currently 15.8 ± 3 GW,63 which, spread over the 70,000 km2 area of activity,64 is equivalent to 225 ± 42 mW/m2. The convective heat flux Fconv is related to the physical properties of the ice shell,65,66 and the maximum Fconv will give the minimum thickness of the stagnant lid, δL. The minimum stagnant lid thickness occurs when the ice shell is so strongly heated that the temperature in the convective sublayer is close to the melting point of water ice. This situation likely arises on tidally heated icy satellites where the tidal deformation of the ice shell causes solid friction, which converts to heat in the warm interior of the shell.67 If more heat is pumped into the shell, it will melt. For Europa, the maximum Fconv is 60 mW/m2, within the range of possible tidal heat flows, and δL ~ 6 km. For Enceladus, Fconv = 30 mW/m2, which gives δL ~ 14 km, while for the south polar terrain of Enceladus δL ~ 3 km (assuming Ti = 273 K). These models assume that the viscosity and thermal conductivity of the shell are not significantly modified by non-water ice contaminants. The range of possible contaminants has been insufficiently explored to date and is a topic requiring further research.
Icy satellites have experienced global endogenic resurfacing through tectonics, cryovolcanism, or solid-state flow. Large areal coverage of recent (in the past 1,000 years) resurfacing poses the greatest concern for planetary protection. Special attention must be paid to bodies where the zone of near-surface brittle deformation has joined with the underlying convective motion to drive global resurfacing (e.g., Enceladus, Europa, and, possibly, Ganymede). Assuming that the mission does not require the spacecraft to penetrate to habitable environments beneath the ice, planetary protection must consider whether any active geologic processes might have a 10–4 chance of promoting surface-subsurface exchange of material within 103 years of contacting the surface.
Crater counting provides the only currently available method to assess the ages of geologic features on the surfaces of icy bodies. This method involves some uncertainties because of uncertainty in the flux of impactors over time in the outer solar system. To be conservative, surface ages from the youngest limit are used, according to the fluxes of Zahnle et al.,68 which will provide an upper limit on the rate of current geologic activity on an icy body. Take, for example, a hypothetical icy body with a crater age of 108 years. A surface with this crater age could be produced by either patchy regional resurfacing that wipes out craters over an average period of 108 years or a global resurfacing event that occurred 108 years ago. Assuming that resurfacing processes on this hypothetical body occur randomly in either space or time, there is a 10–5 chance that they would affect any particular area of the body in a 103-year period. Thus, an icy body with a surface age exceeding 108 years provides sufficient confidence
that the geologic timescale for delivery of surface materials into potentially habitable subsurface environments vastly exceeds the timescale of biological exploration. To narrow the field of icy bodies of possible concern, it is assumed that the bodies that have surfaces in which the youngest craters are endogenically resurfaced terrains older than 108 years pose no concerns for planetary protection. Icy bodies that exhibit resurfaced areas younger than 108 years require greater scrutiny.
Icy Bodies with Recent Endogenic Activity
Using the 108 year (100 million years) geologic activity cutoff, the inventory of large and mid-size icy bodies can be divided into bodies that present no planetary protection concerns, and bodies that require closer examination before answering Decision Point 5. Almost all icy bodies are heavily cratered and easily fall into the “no concern” category. A few bodies lie near the border but are still on the “no concern” side, including the following:
• Ganymede, which underwent widespread resurfacing over two thirds of its surface due to tectonism and, possibly, cryovolcanism.69 The youngest limit for the crater age of these resurfaced areas occurred at 1 billion years.70
• Dione, which has areas recently cut by faults but exhibits no evidence of other types of geologic activity. Crater counting on these fractured plains indicates that they could be as young as 260 million years.71
• Miranda, whose coronae appear to have formed primarily by tectonism, although they also exhibit unexplained variations in albedo.72,73 Two of Miranda’s three resurfaced coronae exhibit young crater ages, which could be as young as 100 million years.74
After eliminating the icy bodies with no evidence for resurfacing activity in the past 100 million years, the committee considered only four icy bodies for planetary protection: Europa, Enceladus, Titan, and Triton.
The global average surface age of Europa could be as low as 20 million years or as high as 200 million years.75 Crater ages for individual terrain units are difficult to obtain reliably because there are too few large craters for good statistics and the imaging datasets from Galileo and Voyager are insufficient to map the more abundant small craters, except for a few small target areas. One broad comparison of crater differences across classes of terrain types found that chaos terrain areas, which appear to be some of the youngest geologically resurfaced features based on crosscutting relationships,76 have a higher crater density than do the background ridged plains,77 an apparent paradox. In the absence of reliable age information for any subarea of Europa, the age of the entire surface must be considered as a whole. Phillips et al. set an upper limit of 1 km2/year on the current surface modification rate, based on a lack of observable surface changes over 20 years.78 This upper limit rate, which would lead to a lower limit estimate of 30 million years to resurface Europa, is broadly consistent with the lower limit surface age of 20 million years based on crater density.
If Europa has a relatively constant rate and style of resurfacing, and if any type of resurfacing would lead to the introduction of surface materials into a subsurface habitable environment, then the likelihood that a particular part of the surface undergoes resurfacing within 103 years is less than 5 × 10–5. A consideration of the variation in resurfacing styles may further lower this likelihood. Two main styles of endogenic modification dominate Europa: resurfacing by the formation of ridges and bands and resurfacing by the formation of chaos terrain.
Several proposed models for ridge formation have different implications for near-surface habitability and communication with an underlying ocean.79 At one end of the spectrum, the tidal pumping model posits that water from the ocean constantly travels up and down through cracks as they open and close during the diurnal tidal cycle.80 This model requires a very thin ice shell for sufficient isostatic rise of ocean water into the crack, a feature which is not supported by the record of large preserved impact craters.81,82 At the other end of the spectrum, the linear diapirism model posits that ridges form in the solid state.83 In between these end-members, a recent model of shear heating predicts formation of transient pockets of melt in the near-surface environment.84
Chaos terrain covers approximately a quarter of Europa’s surface,85 and, like ridges on Europa, several pro-
posed models explain its formation.86 The most successful models of chaos formation involve some component of near-surface melting,87,88 but the primary difference is whether convective plumes in the solid state (in which case the melt is trapped near the surface) or massive thinning of the ice shell (in which case near-surface melts may communicate with the ocean) drives the process. In either case, chaos terrain almost certainly signals the existence of greater amounts of liquid water near the surface than does ridge formation. “Plates” of preexisting material that do not appear to have chaotically modified surface materials cover a substantial proportion of the area of chaos terrain.
Even if liquid water is produced within the ice shell as a result of chaos or ridge formation, it does not automatically indicate contact between surface materials with that liquid. For example, in the Nimmo and Gaidos ridge model,89 the melt pockets develop a few kilometers below the surface. In the Schmidt et al. chaos formation model, briny melt is produced 3 or more kilometers below the surface. As the surface topography subsides over a liquid area, hydraulic gradients can push melt closer to the surface.90 The cold, near-vacuum environment of the surface presents a hostile environment for the stability of liquid water, which means that melt that reaches the surface is on a one-way trip. Other than the intentional use of a perennial heat source sufficiently powerful to actively melt through Europa’s ice, it is unclear what processes might be able to bury a spacecraft component deep enough to interact with circulating melt, especially considering that on Europa sputtering erosion of the surface by energetic particles is faster than burial by impact gardening.91 It is also important to note that while the Schmidt et al. model predicts the existence of some melt in areas of the near subsurface, the model also requires that these melt bodies are sealed within the ice shell and do not communicate with the underlying ocean.
The prime concern for forward contamination on icy bodies is by organisms that can propagate at temperatures near the freezing point within the ice shell and the ocean. Europa’s rocky interior may contribute a substantial fraction to the overall heat flow, which may lead to the existence of hydrothermal vents hosting steep temperature gradients suitable for organisms with higher growth optima (including mesophiles, thermophiles, and hyperthermophiles). Many thousands of hydrothermal systems might exist on Europa, distributed around the globe.92 To get to a hydrothermal system, an organism introduced to the top of the ocean would need to sink to the bottom, enter a pore in the ocean floor, and then travel through cracks and pores beneath the ocean floor to an area of heating. Theoretical work on the timescale of vertical overturn in Europa’s ocean is sparse, and may depend on poorly constrained salinity levels in the ocean.93 If vertical mixing is driven by upward mass flux from hydrothermal plumes, it may take centuries for cold water at the top of the ocean to cycle to the bottom, and the time it takes for all of the ocean water to cycle through hydrothermal systems is ~107 years.94 Given the low probability discussed above of accidental contact between materials on the surface and the water at the top of the ocean, the likelihood of non-psychrophilic contaminants reaching and colonizing a putative hydrothermal vent within 103 years is necessarily much lower.
In summary, a spacecraft in contact with a randomly selected portion of the uppermost surface of Europa is near the threshold of planetary protection concern as determined by application of Decision Point 5. That is, the probability of the spacecraft, spacecraft parts, or contents contacting a potentially habitable region within 1,000 years is uncomfortably close to the limit of 10–4. The upper limit likelihood of 5 × 10–5 for chance contact between the surface and subsurface liquids (see above) within 1,000 years assumes constant resurfacing that always involves liquid water in contact with the surface. Such liquids must be cold brines hospitable only to psychrophilic organisms. There is currently active debate over the existence of liquids in contact with the surface, how widespread such liquids are, and whether such liquids communicate with the underlying ocean. Therefore this upper limit appears to be very conservative. However, Europa’s activity may be episodic, in which case the constant resurfacing assumption may severely overestimate or underestimate the current situation. In the case of a mission that is intending to land on an area of suspected current resurfacing activity, the likelihood of chance contact with subsurface liquids is much higher. For a mission designed to penetrate the ice shell, the chance is higher yet. Such lander and penetrator missions would not pass the test imposed by Decision Point 5.
Despite its small size (mean radius of 252 km), Enceladus is one of the most geologically active bodies in the solar system. Active cryovolcanism in the form of plumes occurs at the south pole,95 coinciding with a concentrated thermal anomaly,96 with an estimated thermal emission of 15.8 ± 3.1 GW.97 The south polar terrain (SPT) is disrupted by tectonic features with almost no superposed craters, indicating a resurfacing age within the past million years.98 Active venting of water vapor and icy particles is observed from four prominent fissures (commonly known as “tiger stripes”) in the center of the SPT.99,100,101 The temperature near the source of this vented material exceeds 180 K,102 while the presence of salts within the ejected particles implies that the plumes emanate from a subsurface liquid water source that has been in contact with the rocky interior.103
Nimmo and Pappalardo proposed that a solid-state convective ice under the south pole could explain the localized geologic activity of Enceladus.104 Collins and Goodman showed that the geologic activity could also result from localized thinning of the ice shell over an isolated sea under the south pole, with the remnant ice about 9 km thick.105 Tidal heating localized in a thermal plume could partially melt the ice shell and produce the high surface temperature of the south polar regions.106,107 The observed tidal heating requires a subsurface ocean decoupling the ice shell.108 Convection in the solid-state portion of the ice shell beneath the SPT may be vigorous enough to bring it into the mobile-lid regime, possibly recycling surface materials back into the interior.109
For the purposes of planetary protection, the south polar terrain on Enceladus presents a worst-case scenario with respect to Decision Point 5. Active venting from fissures in the ice may lead directly downward into a liquid water environment. The water is salty, indicating chemical reactions with the rocky interior and possible sources of nutrients. Unlike the other three icy bodies examined in this section, Enceladus’ low gravity leads to very low normal stresses at a given crustal thickness, and so cracks at the surface may stay open to a few kilometers depth. Although the chances of a spacecraft accidentally falling into one of the four narrow active fissures is quite small, caution is demanded because of the overall youth of the SPT, the countless number of tectonic features older than the “tiger stripes” (perhaps formerly active sites), and the possibility for surface recycling through mobile-lid convection. The south polar terrain of Enceladus does not pass the test of Decision Point 5, and so any mission intending to travel to this terrain must meet additional planetary protection requirements, and other missions in the vicinity must assess the probability of accidentally crashing into this terrain.
Areas outside the SPT have higher crater retention ages and do not appear to have been active within the past 100 million years. In particular, the cratered plains (which stretch from the subsaturnian point and over the north pole to the antisaturnian point) appear to have very little resurfacing activity within the past 1 billion years, besides a few narrow tectonic fractures that have permitted loose regolith drainage.110 The cratered plains are of little concern from a planetary protection standpoint.
Titan is the only icy body with a dense atmosphere, and present-day surface-atmosphere interactions make aeolian, fluvial, pluvial, and lacustrine processes important on a scale previously seen only on Earth. Similar to Earth’s water cycle, the activity on Titan’s frigid 95 K surface is driven by a methane cycle of evaporation, precipitation, and runoff. The average crater surface age is between 200 million years and 1 billion years, which would remove Titan from the list of planetary protection concern,111 except that there is evidence of some types of geologic activity in the present day, and so it must be scrutinized.
The highest density of craters on Titan exists in the mountainous Xanadu region, whereas no craters are superimposed on the equatorial dunes,112 indicating that they could be active. Other active processes observed on Titan include rainstorms113 and changing lake levels.114 Surface-atmosphere exchange processes drive all of the unambiguous recent geologic activity on Titan, and these processes do not provide a means of transport to liquid water environments habitable by terrestrial organisms.
From a planetary protection standpoint, the important question is where liquid water environments could exist within Titan and whether transport processes leading from the surface to those environments are currently active. Relevant to this point is the debate about the role that cryovolcanism may play in shaping Titan’s surface, whether in the form of degassing or active flows. Before the Cassini-Huygens mission, various workers suggested that Titan
is probably cryovolcanically active, on the basis of geochemical and geophysical models. Substantial quantities of ammonia in the interior could facilitate the melting that would lead to this cryovolcanic activity. Explanations for the present atmospheric abundance of methane require a replenishing mechanism, and cryovolcanism would provide a way to bring methane from the interior to the atmosphere. Cassini results, such as the detection of 40Ar in the atmosphere,115 support the case of cryovolcanism on Titan, as it implies outgassing from Titan’s interior. Various features observed by Cassini on Titan have been interpreted as evidence of cryovolcanic activity. Putative cryovolcanic features were detected by Cassini’s VIMS and radar instruments,116 including lobate flows117 and a tall mountain adjacent to a deep pit with lobate flow-like features.118,119 However, as Cassini acquired more data, particularly topographic data, the cryovolcanic origin of some of these features has come into doubt. It is often difficult to distinguish between fluvial, mass wasting, and volcanic flows using the relatively low-resolution data that are currently available from Cassini, and a self-consistent picture of Titan’s geology can be constructed without any cryovolcanism.120
For the purposes of this report, the committee assumed that cryovolcanic activity on Titan remains a possibility. The area covered by putative cryovolcanic flows is 0.6 to 1 percent of Titan’s surface, and the area of the source vents, where subsurface-surface transport takes place, is about a factor of 10 smaller. If it is assumed that all of Titan’s surface is cryovolcanically resurfaced over the minimum surface age of 200 million years, and activity is randomly distributed in time and space, then the chance of a spacecraft randomly landing in the immediate vicinity of a source vent active within the next 1,000 years is less than 10–6, comfortably within the bounds set by Decision Point 5. However, the flow of liquid methane across Titan’s surface and through its regolith could pick up small particles from a spacecraft and carry them to lower elevations. Underground flow could move liquid methane from the equator (at higher elevation) to the pole (at lower elevation) in a matter of centuries.121 If a spacecraft were to come into contact with Titan’s surface at high elevations, methane flow could significantly spread out the contamination “footprint.”
The lack of experimental research data on physical interactions that could occur between underground methane (carrying the contaminants) and cryovolcanic liquid water in the subsurface warrants pointing out a few first principles. The temperature difference between these liquids is about 180°C; in terms of homologous temperatures, cryolava encountering a lake on Titan is similar to terrestrial lava pouring into the sea. The water is likely to freeze at the same time that the methane boils. One crucial difference from the terrestrial analogy is that water is not significantly soluble in methane, unlike water interacting with silicate melts. Instead, near-freezing water and methane may solidify together into a more stable clathrate. If a terrestrial microbe were to reach a liquid water body on Titan, the extreme conditions could exceed the organism’s limits of habitability. High concentrations of ammonia are likely to exist in cryovolcanic fluids on Titan, and the ocean on Titan probably does not contact the warm rocky interior due to high-pressure ice phases. The interior may even be cold and incompletely differentiated, although information about Titan’s interior structure is more difficult to obtain than for the other saturnian satellites.122
In summary, despite active exogenic processes operating on the surface, the evidence does not indicate that environments habitable to terrestrial organisms currently exist near the surface of Titan. Therefore, currently conceivable missions to Titan would pass the test imposed by Decision Point 5 and require no further planetary protection measures. However, thorough cleaning of these spacecraft may be desirable for other reasons related to mission science, such as sensitive detection of complex organic molecules in the titanian environment.
Images of Neptune’s satellite Triton from the Voyager 2 spacecraft in 1989 revealed evidence of resurfacing processes,123 possibly by cryovolcanism124 and diapirism.125 Triton’s retrograde orbit around Neptune suggests that it is likely to be a captured satellite126 and represents the best current model for a Kuiper belt object. Images revealed eruptive plumes up to 8 km high blowing dark particles downwind in the thin atmosphere.127,128 Smith et al. suggested that solar heating and subsequent vaporization of subsurface nitrogen may drive the eruptions.129 Other proposed mechanisms for gas venting include solid-state greenhouse130 and convection in the solid nitro-
gen caps.131 Therefore, current eruptive activity on Triton most likely reflects solar heating rather than endogenic cryovolcanism, in the sense of bringing molten material from the interior to the surface.132
The scarcity of impact craters on Triton indicates a surface younger than 50 million years in the oldest areas and only a few million years old in the youngest areas.133 A number of geologic features on Triton suggest that widespread cryovolcanism has occurred in the past. Smooth plains with lobate features cover large parts of the observed surface, and interpretations of circular features termed “cantaloupe terrain” suggest they result from solid-state diapirism.134 A possible explanation for the high heat flow required to drive this massive resurfacing is the orbital capture of Triton into the Neptune system followed by a period of high tidal heating and melting of the interior ices,135 making it likely that Triton has an internal ocean.
A major factor for assessing the forward contamination of Triton is that the composition and the temperature of any subsurface liquids that could provide transport down to the ocean are not known. The surface of Triton is cold enough to host ices of nitrogen, methane, and carbon monoxide, and it is likely that ammonia may be mixed with the water ice in the crust.136,137 There is a strong possibility that near-subsurface liquids on Triton accessible to a crashed spacecraft are uninhabitable by terrestrial organisms. But, in the absence of more information on the liquids, the combination of the very young surface age and active cryovolcanic processes strongly suggests that missions contacting the surface of Triton are likely to fail the test imposed by Decision Point 5. Therefore, Triton should be approached with caution from a planetary protection standpoint until more information is available.
Planetary protection should focus on icy moons in the outer solar system where the preponderance of geophysical and chemical data indicates potential habitability for terrestrial life and where evidence of resurfacing activity in the past 108 years increases the likelihood of surface-subsurface transport to interiors that might be habitable. The requisite chemical species required for terrestrial life include liquid water and the key elements carbon, sulfur, nitrogen, potassium, magnesium, calcium, and phosphorus, but currently available data is not informative about their presence or absence on icy bodies. The physical conditions of the target body (e.g., temperature) must be compatible with extremes tolerated by terrestrial organisms.
Recommendation: Evidence of widespread resurfacing activity within the past 108 years requires that NASA evaluate planetary protection requirements for Europa, Enceladus, and Triton using a hierarchical decision-making framework of the kind presented in Chapter 2 and elaborated on in Chapter 3. Spacecraft designers must demonstrate that their plans for missions to these bodies have less than a 10–4 chance of contacting within the next 1,000 years an area of active surface-subsurface transport.
Finding: The possibility for active transport of contaminants into a habitable portion of Titan’s interior over a 1,000-year timescale is more remote than 10–4, removing Titan from high levels of concern for planetary protection. Titan’s average surface age appears to be older than 108 years, and although putative cryovolcanic features have been found, all firm evidence for current geologic activity on Titan indicates that such activity is driven by exogenic processes involving the methane cycle and wind-blown sediment, none of which provides a habitable environment for terrestrial organisms.
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