Assessment of Planetary Protection Requirements for Spacecraft Missions to Icy Solar System Bodies (2012) / Chapter Skim
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4 A Geophysical Perspective and Inventory of Habitable Environments on Icy Bodies
4 A Geophysical Perspective and Inventory of Habitable Environments on Icy Bodies
Pages 23-40
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From page 23... ...
The bulk of the chapter concerns Decision Point 5, discussing transport processes that might operate between the uninhabitable surface and poten tially habitable subsurface environments. The chapter concludes with a survey of icy bodies to delineate areas of concern for planetary protection.
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From page 24... ...
Magnetometer data provides compelling evidence of liquid water for Jupi ter'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)
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From page 25... ...
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)
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From page 26... ...
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.
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From page 27... ...
Cryovolcanism 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, includ ing Europa, Ganymede, Enceladus, Titan, Ariel, and Triton.
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From page 28... ...
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. Near-Surface Melting 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.
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From page 29... ...
This style of convec tion, 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.
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From page 30... ...
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
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From page 31... ...
"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 auto matically indicate contact between surface materials with that liquid.
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From page 32... ...
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 local ized 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.
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From page 33... ...
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.
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From page 34... ...
Therefore, Triton should be approached with caution from a planetary protection standpoint until more information is available. CONCLUSIONS AND RECOMMENDATIONS 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.
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From page 35... ...
R.D. Lorenz, Thermal drilling in planetary ices: An analytic solution with applications to planetary protection problems of radio isotope power sources, Astrobiology, 2012, in press.
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From page 36... ...
Wang, Pressurized oceans and the eruption of liquid water on Europa and Enceladus, Geophysical Research Letters 34:L07202, 2007.
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A.C. Barr, Mobile lid convection beneath Enceladus' south polar terrain, Journal of Geophysical Research 113:E07009, 2008.
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From page 38... ...
Cadek, and C Sotin, Solid tidal friction above a liquid water reservoir as the origin of the south pole hotspot on Enceladus, Icarus 196:642-652, 2008.
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From page 39... ...
A.C. Barr, Mobile lid convection beneath Enceladus' south polar terrain, Journal of Geophysical Research 113:E07009, 2008.
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