P13D-01 INVITED
Enceladus 2008: Cassini Flies Low
Though a small world, Enceladus has become a major focus of the Cassini mission because of its
extraordinary present-day geologic activity, the anomalous heat and carbon-bearing substances emanating
from its 'tiger stripe' fractures, and the likelihood of organic-containing liquid water within. Cassini's 2-year
Equinox Mission extension, which began on July 1, 2008, calls for continued exploration with several more
close Enceladus encounters; four extremely close flybys have occurred in 2008 alone. The latter have seen
closest approach altitudes dipping as low as 25 km above the equatorial region and allowing deep passages
through its vapor/particle plume for in-situ sampling and very high resolution imaging of its fractured southern
polar cap.
This presentation will offer an overview of the flyby geometries and scientific opportunities presented during
these 2008 encounters with Enceladus, previewing the surface reconnaissance imaging obtained by the
Imaging Science Subsystem (ISS), and the most significant results obtained by the other Cassini instruments.
http://ciclops.org
P13D-02
Enceladus South Polar Terrain Geology: New Details From Cassini ISS High Resolution Imaging
The Cassini spacecraft executed a close flyby of Enceladus on August 11 (altitude: 50km); two more are planned for October 9 (altitude: 25 km), and October 31 (altitude: 196 km). High resolution (as fine as 7m/pixel) images of known geologically active features in the South Polar Terrain (SPT) have been returned to investigate how plume eruptions, tectonism, and seismicity alter the surface and to reveal how the SPT has evolved over time. We examined six known eruption sites (Spitale and Porco 2007, Nature 449, 695-697) along Cairo, Baghdad, and Damascus Sulci, as well as inactive portions of the "tiger stripes" and bright fractured terrain in adjacent areas. We also obtained contiguous ISS broadband multi-spectral mosaics of the entire SPT region to refine our geological and digital terrain maps and to search for volcanically and tectonically driven temporal changes. The highest-resolution images show ice blocks up to tens of meters in size that are widely but non-uniformly distributed over a variety of terrain units. The upraised flanks and valley walls of active tiger stripes are mantled in places by smooth fluffy-looking deposits, most likely accumulations of coarse-grained plume fallout. With increasing lateral distance from the stripes, the smooth upraised flank deposits grade into rounded, platy-textured, elongate hills and a conspicuous system of quasi-parallel knobby ridges and grooves that have spacings and dimensions comparable to the tiger stripe flanks themselves. Peculiar narrow lenticular ridges, perhaps emplaced by extrusion or as icy pyroclastic deposits, rise from tens to hundreds of meters along the medial fissures of some tiger stripes. On regional scales, the ends of the tiger stripes are bounded by a complex network of fractured terrain, within which can be found numerous transform faults that lie at high angles relative to the trends of the tiger stripes. Observed offsets along these transforms and an absence of lateral symmetry of the displaced terrains suggest that tiger stripes are not exact analogs to classic terrestrial oceanic rifts. Instead, any possible tectonic divergence is more likely a result of the superposition of many regionally and temporally distributed spreading centers.
P13D-03
Liquid Water and Thermal Activity of Enceladus' South Polar Terrain
The endogenic activity on Enceladus is only located on a specific region at the south pole, from which jets of water vapor and ice particles have been observed. Heat power required to sustain such an activity over geologic timescales remains problematic. In a recent study, we show that only interior models with a liquid water layer at depth can explain the observed magnitude of dissipation and its particular location at the south pole (Tobie et al., Icarus, 2008). Here, we investigate the thermal stability of a localized liquid water reservoir at the rock-ice interface by performing simulations of thermal convection in three-dimensional spherical geometry. Preliminary results indicate that a total power ranging between 2.4 GW and 8 GW, comparable to the typical tidal dissipation value expected in the south polar region, is required to sustain a localized liquid zone. We are currently coupling our 3D viscoelastic tidal dissipation model to the 3D thermal convection code in order to precisely determine the associated tidal dissipation field. Preliminary results will be presented. In parallel, we investigate the likelihood of short resurfacing events by incorporating a self-lubricating, simple damage rheology using a 2D cartesian version of the code. When stress accumulation leads to the rupture of the lithosphere, a heat flow of about 200 mW.m-2, comparable to the values observed over the south pole, is observed during a relatively short period of time (< 500 kyr).
P13D-04 INVITED
Constraints on Tidal Heating in Enceladus
Two constraints for Enceladus seem difficult to dispute: First, that it is emitting more heat than allowed by equilibrium tidal heating models (defined as those in which the tidal heat production and orbital eccentricity are constant with time, cf. Meyer and Wisdom, 2007). Second, the maximum possible instantaneous tidal heat generation is over two orders of magnitude larger than what is observed. (This can be shown independent of any particular rheological model.) There is nothing mysterious about the simultaneous correctness of these two statements since one pertains to the orbital evolution aspect of the problem and the other pertains to the actual mechanism of dissipation. In reality, the maximum heat dissipation would never be approached since it would quench the eccentricity in a short timescale (ten to a hundred thousand years). However, this high upper bound assures us that there is no fundamental difficulty with the tidal heating mechanism, only with its constancy over long periods of time. This suggests that one should seek a model in which the eccentricity fluctuates, possibly mediated by the bounds imposed at the high end by the stress limits for ice fracture and at the low end by the stress at which motion on faults (lubricated or dry) ceases to be possible. I will describe models of this kind that are capable, at least in principle, of producing the desired heat flow in instantaneous equilibrium (i.e., without the need to store heat in the interior). This is a stress-mediated model rather than a thermally mediated model (like that advocated for Io, which has a similar disequilibrium problem). Such a model must (in analogy with successful models for Io and Europa) simultaneously provide the right environment for the desired straining of the outer part of the ice shell and a deeper environment that is sufficiently deformable so that a large tidal Love number (of order a hundred times the elastic value) can be achieved. It is argued that solid water ice (even near the melting point) is unlikely to satisfy this condition, although there continue to be uncertainties in ice rheology that prevent a certain conclusion. This has led previous workers to favor an ocean (not necessarily global) at greater depth, not as the direct source for the plumes but as a decoupling mechanism (conceptually like Europa). Since ice fractures lubricated with very small amounts of water (too small to drain quickly on geological timescales but enough to greatly reduce friction as in pore pressure effects for terrestrial earthquakes) are also capable of providing the desired large, fluid-like deformation. I will argue that there is no necessity of an ocean but probably the necessity of some liquid. It remains an open question as to whether this is liquid water or some other fluid (not pure water) and whether this (or its most volatile components) participates in the plumes. There remain many unanswered issues with how Enceladus works, just as there are for Io, both in respect of orbital evolution and internal properties and evolution.
P13D-05
Constraining the Endogenic Power of Enceladus' Tiger Stripe Region
On 12th March 2008, and again on 11th August 2008, the Cassini spacecraft flew past the South Pole of Enceladus at a distance of a few hundred kilometers. During both encounters the long-wavelength detector (FP1) on Cassini's Composite Infrared Spectrometer (CIRS) took over one hundred 10 to 600 cm-1 (1000 – 17 micron) spectra of the thermal emission from the active tiger stripes. The wavelength region covered by FP1 contains most of the power radiated by the tiger stripes and thus is very valuable for constraining total heat flow. Previous estimates of endogenic heat flow were derived by extrapolation from shorter-wavelength CIRS data. During the August encounter FP1 achieved a maximum resolution of 3 km, over the tiger stripe Arabia Sulcus. Whilst the best spatial resolution during the March encounter was significantly lower (50 km to 200 km) a larger number of spectra were taken, covering single or multiple tiger stripes. Using these measurements and previously determined surface thermal properties of Enceladus the total magnitude of the endogenic power from this region is constrained. An important part of this process is the subtraction of the background emission from re-radiated sunlight, using a passive thermal model, which is constrained by CIRS FP1 observations that do not include the tiger stripes. The good spatial coverage of the tiger stripes has enabled the endogenic heat released by individual tiger stripes to also be determined. It is shown that the release of this heat by the tiger stripes is not uniform, but rather varies between stripes and along the stripes. The magnitude and scale of this variation will be fully explored.
P13D-06
Comparison of March 2008 Composite Infrared Spectrometer (CIRS) tiger stripe observations with shear heating predictions
Cassini's discovery of elevated temperatures [1] apparently centred on the south polar "tiger stripes" [2] prompted the suggestion that tidally-driven shear heating [3] was responsible for the heat production, while tides might also control the timing of vapour release [4]. The recent March 2008 flyby provided an opportunity to test the shear-heating hypothesis. During this flyby, CIRS obtained a map of the 9 - 16 micron thermal emission from all the tiger stripes (except part of Alexandria Sulcus) with a spatial resolution of 4.5 - 9 km, showing substantial variations in thermal emission along the length of the stripes. The shear-heating hypothesis predicts variations in tiger-stripe heat production depending on the local, time-averaged tidal shear stress [3]. Synthetic CIRS images can be obtained by convolving the predicted heat production variations with the CIRS instrument response. At low resolution, the synthetic local brightness temperature is controlled by the total length of tiger stripes within the instrument field of view. The resulting pattern of predicted brightness temperatures differs significantly from the CIRS observations. However, the original model did not take into account the variable tiger-stripe spacing, which results in variable strain rates and shear heating. Taking this effect into account results in a predicted brightness temperature distribution which more closely resembles the observations. [1] Spencer et al. Science 2006 [2] Porco et al. Science 2006 [3] Nimmo et al. Nature 2007 [4] Hurford et al. Nature 2007
P13D-07
Mobile lid convection beneath Enceladus' south polar terrain
Enceladus' south polar region has a large heat flux that is spatially associated with cryovolcanic and tectonic activity. Tidal dissipation and vigorous convection in the underlying ice shell are possible sources of heat, however, prior predictions of the heat flux carried by stagnant lid convection are too low to explain the observed heat flux. The high heat flux and cryovolcanic/tectonic activity in the region suggest that near- surface ice has become rheologically and mechanically weakened enough to permit convective plumes to reach close to the surface. If the yield strength of Enceladus' lithosphere is less than 1 to 10 kPa, convection may occur in the "mobile" lid regime, characterized by large heat fluxes and large horizontal velocities in the near-surface ice. Ice shells convecting in this regime of behavior have heat fluxes comparable to that observed by CIRS. If this style of convection is occurring, the south polar terrain should be spreading horizontally with v ~ 1 to 10 mm yr-1 and should be resurfaced in 0.1 to 10 Myr. This estimated age is comparable to age estimates of 0.5 Myr based on crater counts from Cassini imaging. Maxwell viscoelastic tidal dissipation in such an ice shell is not capable of generating enough heat to balance convective heat transport. Tidal heat is likely generated in the near-surface along faults as suggested by Nimmo et al., Nature, (2007). It is also possible that tidal dissipation within the ice shell occurs by other processes not accounted for by the canonical Maxwell dissipation model.
P13D-08
Opportunities and Challenges in Scientific Exploration of Both Titan and Enceladus by a Single Mission
In 2007 NASA's Science Mission Directorate (SMD), in efforts to start an outer solar system flagship mission in the near future, commissioned studies of mission concepts for four high-priority outer solar system destinations: Europa, the Jovian system, Titan, and Enceladus. Follow-on studies begun in 2008, jointly conducted with ESA and incorporating the selection of the LaPlace and TandEM Cosmic Vision study proposals, combine the Europa and Jupiter system mission concepts into a single mission, and the Titan and Enceladus mission concepts, along with related Saturn system science, into a single mission. The 2007 studies of Titan and Enceladus missions allowed each mission concept to focus on a single destination. The 2008 study must address both Titan and Enceladus exploration with a single mission. This raises significant challenges to mission designers but also presents opportunities. Challenges stem from Titan's and Enceladus' very different locations within Saturn's gravity well, and from the different natures of the two satellites. With current or even envisioned propulsion systems, delta-V requirements prohibit a spacecraft that inserts into orbit at one from subsequently escaping and inserting into orbit at the other. Missions that perform interlaced flybys of the two bodies are limited to relatively high flyby speeds. "Pumping down" flyby speeds at one of them terminates, at least temporarily, flybys of the other, and for Enceladus requires significant time and delta-V. But despite their obvious differences, such as Titan's thick and Enceladus' almost non-existent atmospheres, a science payload designed for Titan can perform excellent science at Enceladus as well. It might be possible to have a single mission target separate orbiters to the two destinations, but this architecture would likely require a launch vehicle significantly larger than is required for a mission that orbits only one body. This paper will discuss these challenges and opportunities.