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Enceladus Focus Group Meeting

Date: Sunday, October 8, 2006

Location: Caltech, Pasadena, CA, US,

Caltech, Room TBD

October 8, 2006

8:30 am - 6:00 PM

Organizers: C. Porco, C. McKay

SECTION I: ENCELADUS SCIENCE - Recent results, models and speculations from the Cassini mission

Review of Cassini Observations on the Geysers, Geology and Heat Flow of Enceladus Porco, C.

The results from the suite of Cassini instruments on the geysers and geology/geophysics of Enceladus, and the implications of these findings for the presence of a near-surface liquid water in the south polar region will be reviewed.

Cassini UVIS Observations Amanda R. Hendrix and Candice J. Hansen (JPL/Caltech)

Cassini Ultraviolet Imaging Spectrograph (UVIS) measurements are yielding exciting information on Enceladus' surface, its plumes and gaseous environment. Stellar occultations have led to the discovery of water vapor in the plumes of Enceladus, and UVIS-measured oxygen in the Saturn system can be linked to plume activity. Surface measurements at far-UV wavelengths reveal grain size and compositional information.

Cassini Neutral Mass Spectrometer Observations of the Enceladus Plume J. Hunter Waite on behalf of the INMS Science team

We present updated results on the mass spectrum of the Enceladus gas plume between 1 and 99 Daltons obtained by the Cassini Ion Neutral Mass Spectrometer on the flyby of Enceladus in July of 2005.

The major mass peaks are due to water vapor along with carbon dioxide, methane, and carbon monoxide or molecular nitrogen, as reported in the Science Special issue of Cassini at Enceladus. There are also hints of other trace organics. A more complete analysis has now been carried out to examine the limits on the presence of ammonia and to better determine the trace organic content.


Models of the Enceladus Plumes

Andrew P Ingersoll, C C Porco, P Helfenstein, R A West

The gases in the plumes include H2O, CO2, N2, CH4, and possibly other hydrocarbons, according to the INMS team. The solid particles in the plumes are probably water ice, but the identification is less certain than for the gaseous components. The plumes emanate from warm cracks in the surface near the south pole. The gas has a scale height of 80 km, according to the UVIS team, and the particles have a scale height of 30 km, according to the ISS team. By integrating across the plumes in their images, the ISS team was able to infer the upward flux of particles vs. altitude. Close to the surface, the falloff of density with altitude is much steeper than that of an escaping atmosphere in which both the particles and the gas are moving upward with the thermal velocity of the gas. Thus some of the particles are falling back to the surface and some are escaping. The larger scale height of the gas implies that the escaping fraction is greater for the gas. I will present models that attempt to explain these plume data. The density of the gas and the size of the particles determine the degree of dynamical coupling between gas and particles. There are three models - a sublimating gas that picks up particles as it leaves the surface; sublimating gas that forms particles in flight as the pressure decreases; a boiling liquid that freezes by evaporative cooling as the pressure decreases. Each model has its own range of mass flux, density, particle size, and scale height for both gas and liquid. I will discuss the implications of the observations and models regarding the possibility of liquid water near the surface.

The Microphysical Properties of Enceladus' Regolith Verbiscer, A.

Abstract: The 2005 opposition of Saturn presented the rare opportunity to observe Enceladus at the minimum phase angle (~0.01 degrees). We report results of a coordinated worldwide campaign using ground-based telescopes and HST to use broadband visible and near infrared photometric measurements at the smallest phase angles possible as a tool with which we can effectively probe the optically active layer of Enceladus' surface.

Analyses of viscously relaxed craters and fracture morphology on Enceladus

Elizabeth P. Turtle, Diana E. Smith, Veronica Bray, Jason E. Perry, Paul Helfenstein, Julie Rathbun

We are investigating variations in the distributions and morphologies of geological features, namely viscously relaxed craters and fractures, over the surface of Enceladus. We will present maps and analyses of the craters and fractures and discuss their implications for the properties and history of the lithosphere.

Unstable Extensional Tectonics on Enceladus Michael T. Bland and Adam P. Showman

Cassini images of Enceladus' surface have revealed the existence of networks of sub-parallel, periodically spaced ridges and troughs similar in morphology to Ganymede's grooved terrain. We model the formation of these ridge-and-trough terrains using a fully nonlinear, finite-element model under conditions appropriate to Enceladus. We find that unstable extension in Enceladus' low gravity environment produces relatively strong instability growth, leading to the formation of periodic topography with amplitudes and wavelengths roughly consistent with the ridge-and-trough terrain.

Enceladus: Topography, Cartography, & Craters, Oh My! P. Schenk, LPI; S. Seddio, U. Rochester

A new controlled global image mosaic of Enceladus allows us to map this most unusual satellite. A preliminary geologic mapping and maps of crater distribution shows several episodes of resurfacing. High-resolution topographic mapping is in progress and will be shown.


Because theories of volcanism on Enceladus (and on Io) are beset with some puzzling difficulties, it is appropriate to explore alternative models, such as that the delayed release of photochemical solar energy produces the observed hot spots. A number of mixtures (in gas, liquid, and solid phases) of inorganic chemical species capture radiant energy in photochemical reactions, and these mixtures are quasi-stable at temperatures typical of Io and Enceladus. In the model I propose, one or more energy-bearing molecular species are produced broadly across the illuminated surface of the satellite in photochemical reactions, they condense as LIQUIDS on the satellite surface, they flow downhill under gravity (either running on the surface or permeating through the regolith), and they eventually energetically decompose, in the case of Enceladus in 2006, when they approach the warm regions near the south pole and become increasingly unstable.

Radiolytic Energy Inputs for the Enceladus Plumes from Magnetospheric Electrons and High Energy Cosmic Rays

John F. Cooper and Paul D. Cooper

The energy inputs into the polar cap of Enceladus from magnetospheric and cosmic ray irradiation are significant relative to the power requirements for the observed plumes and thermal emissions. Exothermic interactions of radiolytic oxidants with endogenic reductants, e.g. ammonia, in the near-surface ice of the south polar cap region could provide heat for enhanced water sublimation.. Enceladus may provide a model for surface brightening of large Kuiper Belt objects from outgassing driven by radiation chemistry.

Ion Neutral Sources and Sinks for Saturn’s Inner Magnetosphere: Consequences for Enceladus Neutral Cloud Production

E. C. Sittler Jr., M. H. Burger and R. E. Johnson

Using Cassini CAPS ion-electron observations within Saturn’s inner magnetosphere and using fluid parameters derived from this data set by Sittler et al., 2005, we are able to solve the field-aligned force balance equation along dipolar field lines and compute total flux tube content NL2 for water group ions and protons. The composition is consistent with a neutral source of water molecules. The neutral production is found to peak near Enceladus’ L shell with source strength SW ~ 1028 mol/s, while the ion production tends to peak near Dione’s L shell with ion production SW+ ~ 1027 ions/s. Our results show that Enceladus is the dominant source of neutrals within Saturn’s magnetosphere.

Enceladus Neutral Water Plume and Torus

M. H. Burger, E. C. Sittler Jr. And R. E. Johnson

We present a model of the gaseous component of Enceladus' water plume consistent with the observations by UVIS (Hansen et al. 2006) and INMS (Waite et al 2006) from the 14 June 2005 flyby of Enceladus by Cassini. These observations imply a neutral gas escape rate of 300 kg/s from the south polar region. Much of this material escapes from Enceladus to form a water torus orbiting Saturn that is very closely confined to Enceladus' orbital distance. Charge exchange within this torus forms a larger, secondary torus (Johnson et al., in press) which has been observed by HST (Shemansky et al. 1993) and UVIS (Esposito et al. 2005).

Impact and Thermal Processes in Enceladus Regoliths

Raul A. Baragiola, Mark J. Loeffler and Ujjwal Raut, University of Virginia, Charlottesville, VA

We will discuss experiments in our laboratory that show the ejection of bursts of nitrogen and hydrogen gases accompanied by ice particles when warming radiolyzed water-ammonia ices. We will discuss these and other regolith processes quantitatively in relation to the observation of Enceladus plumes.

Interior Structure of Enceladus Schubert

It is argued that Enceladus is a differentiated body with a silicate core and ice/water shell and that Rhea is an undifferentiated, homogeneous, ice/rock body. It is also proposed that Dione, like Enceladus, might also be differentiated. The internal structure of Enceladus is based on its density, surface characteristics, and signs of endogenic activity, including atmospheric plumes, south polar thermal anomalies, and interaction with incident plasma. Rhea’s internal structure is indicated by its density, quadrupole gravitational coefficients, heavily cratered surface, and lunar-like interaction with incident plasma. Dione’s modified surface, density, and Enceladus-like interaction with impinging plasma suggest a differentiated and internally active satellite. The medium-sized icy Saturnian satellites with the highest densities and silicate mass fractions, Enceladus and Dione, respectively, are the ones that are apparently differentiated and active. This may be explained by early and long term radiogenic heating of the silicates. Tidal heating in the Enceladus-Dione resonance might also be important.

Thermal convection in ice I shell of Enceladus: Implications for the surface structures formation

Giuseppe Mitri and Adam P. Showman Lunar and Planetary Lab., University of Arizona (

Modest internal heat production variations can force an ice shell to switch between a conductive and convective state (Mitri and Showman, 2005; McKinnon, 2006). In the presence of an internal ocean, a conductive-convective transition can produce radial expansion of a cooling ice shell. The rapidity of these switches implies that stress buildup, hence extensive fractures, could occur. We explore this hypothesis for Enceladus. We present results of numerical simulations of convection in the ice I shell, with Newtonian rheology, temperature-dependent viscosity, and tidal internal heating. We demonstrate that the onset of convection in Enceladus ice shell floating on an ocean produces tectonic stress of ~500 bars, and fractures of few tens km depth.

Diapir-Induced Reorientation of Enceladus and Implications for the Subsurface

Robert Pappalardo (JPL) and Francis Nimmo (University of California, Santa Cruz)

The current south polar location of the Enceladus hotspot can be explained by reorientation of the satellite's rotation axis due to the presence of a large low-density subsurface diapir. If the diapir is in the ice shell, then the ice shell must be relatively thick (~90 km) and maintain significant rigidity (elastic thickness greater than ~0.5 km); if the diapir is in the silicate core, then Enceladus cannot possess a global subsurface ocean, because the core must be coupled to the overlying ice. This model suggests that any liquid water within Enceladus may be localized in a south polar sea at the deep ice-rock interface, and surface venting may be due to localized heating along shear-heated fractures.

Internal melting and the shape of Enceladus Collins, G.

If the thermal energy radiating from the south polar area of Enceladus is supplied over a limited area at the base of the ice shell, melting of a localized pool of water is favored over convection or global ocean production if the ice begins in conductive equilibrium. The observed shape of Enceladus can be fit by a differentiated body (core density < 2700 kg m-3) in hydrostatic equilibrium, when the effect of melting the ice shell is taken into account, since this melting produces a large pit centered on the south pole. Reported deviations from the best fit ellipsoid (high at 50°S, low at the south pole) can also be fit by this model, and are sensitive to the shape of the heating profile applied to the base of the ice shell. The large surface pit at the south pole represents a significant negative gravity anomaly, which may serve to reorient Enceladus and place the active region at the south pole.

Geophysical Constraints on the Interior of Enceladus.

Frank Sohl (1), Hauke Hussmann (2), Matthias Grott (1), and Ruth Ziethe (3)

(1) DLR Institute of Planetary Research, Berlin-Adlershof, Germany
(2) Institute of Planetology, University of Muenster, Germany
(3) Physikalisches Institut, University of Berne, Switzerland

Interior structure models of Enceladus are constrained by the satellite's mean density, equilibrium shape, and intrinsic heat flow mainly released in the South Polar Terrain. We discuss the likely range of interior structures, bulk compositions, and present thermal states with implications for future gravitional field measurements.

Tidal Dissipation in Enceladus

Hauke Hussmann (1), Matthias Grott (2), and Frank Sohl (2)

(1) Institute of Planetology, University of Muenster, Germany
(2) DLR Institute of Planetary Research, Berlin-Adlershof

Tidal heating may contribute significantly to the internal energy budget of Enceladus. We calculate global tidal heating rates for various interior structure models, including models with and without internal oceans. Implications for the thermal state of the ocean and its detectability due to tidal deformation will be discussed.

Enceladus' interior dynamics.

Matthias Grott (1), Frank Sohl (1), and Hauke Hussmann (2)

(1) DLR Institute of Planetary Research, Berlin-Adlershof
(2) Institute of Planetology, University of Muenster, Germany

A diapiric upwelling has been speculated to be the cause for Enceladus' south polar activity. It is conceivable that degree-one convection is the reason for Enceladus' asymmetric appearance. We investigate scenarios for large-scale convection patterns and possible plume formation.

SECTION II: EARTH ANALOGS, EXTREMOPHILES, AND LIFE SEARCH - Based on what we know about life on Earth, what kinds of organisms might one expect to find in the habitable zone on Enceladus, and how would we detect evidence of their presence?

Life on Enceladus, an overview
Chris McKay
NASA Ames Research Center

Life in the subsurface and in cold environments on Earth provide models for how life might survive on Enceladus. These ecosystems also provide analogs for a search for life on future missions to Enceladus.

Methanogens: A Model for Life on Enceladus
Tim Kral

Methanogens are microorganisms in the domain Archaea that inhabit a wide variety of anaerobic environments on planet Earth, many of which would be considered extreme. My laboratory has been studying methanogens for the past thirteen years as a model for life on Mars. Characteristics of methanogens and what we know about Enceladus thus far suggest that they might also serve as a model for life on Enceladus.

Searching for Organics and Other Volatile Ices at Enceladus
B. Dalton

Evidence from Cassini indicates the presence of simple hydrocarbons (such as acetylene) in the observed plumes and, by inference, in the near-surface deposits. This and related compounds should be identifiable at the surface if present in sufficient concentrations. Simple, targeted investigations with ISS, VIMS and CIRS, coupled with laboratory measurements of compounds of interest under the relevant conditions, can quickly advance our knowledge of what to look for and how to evaluate astrobiological potential at Enceladus.

Enceladus: Cassini Observations and the Implications for the Search for Life

Chris Parkinson and Yuk Yung

The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in our solar system. With its significant geothermal energy source propelling these plumes $>$80 km from the surface of the moon and the ensuing large temperature gradient with the surrounding environment, it is possible to have the weathering of rocks by liquid water at the rock/liquid interface. On Enceladus, the weathering of rocks by liquid water and any concomitant radioactive emissions are possible incipient conditions for life. If there is CO, CO$_2$ and NH$_3$ present in the spectra obtained from the plume, then this is possible evidence that amino acids could be formed at the rock/liquid interface of Enceladus. The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life. We discuss the search for signatures of these species and organics in the Cassini UVIS spectra of the plume and implications for the possible detection of life.

Survival Lifetimes for Amino Acids Under UV Photolysis in Near-Surface Ice on Enceladus

J. Goguen, G. Orzechowska, P. Johnson, A. Tsapin, I. Kanik, W. Smythe, JPL

Our recent lab experiments on the decomposition of 4 amino acids by UV photolysis in mm-thick crystalline ice at T=100K suggest that their concentration in the top meter of Enceladus ice at 70 S latitude will be reduced by 50% in ~100 years. This result has important implications for what species might be detectable and where these biologically important molecules might survive if these amino acids were present on Enceladus.

The detection of biomarkers in minerals deposited by hydrothermal systems in the search for life on Enceladus.

Stephen A. Bowden1*, Rab Wilson2, Jonathan M. Cooper2 and John Parnell1 1Dept Geology and Petroleum Geology, University of Aberdeen, UK. 2Dept Electronics and Electrical Engineering, University of Glasgow, UK.

Hydrothermal systems on Enceladus could represent a habitat for life and redistribute biomarkers from the subsurface to the surface. We have detected fatty acid and hydrocarbon biomarkers (at the 10s ppm level) in sulphates deposited by an ancient (39 Ma) impact-induced hydrothermal system on Earth. Visible cellular material has not been observed, so the biomarkers likely originated elsewhere in the hydrothermal system, and we conclude that it is reasonable to look for fossil evidence of life in areas that have seen hydrothermal activity. On Enceladus, biomarker extraction could be replicated in a lab-on-a-chip format, then the extracted compounds presented to a spectrometric instrument (routinely carried on probes and landers) for analysis.

Planetary Protection(PP) and Enceladus missions: Don't leave home without it! M. Race SETI Institute

The Outer Space Treaty requires that exploration be done in a way that avoids potential cross contamination of celestial bodies from 'hitchhiker' microbes on spacecraft. Both COSPAR (the international scientific body that oversees planetary protection policies) and NASA have established regulations and implementation controls on outbound spacecraft that are likely to apply to Encealdus because of its biological potential and presence of liquid water. Although specific forward contamination controls have not yet been set for Enceladus, it is highly likely that mission instruments, spacecraft and operations will face a PP review/approval process as well as possible PP controls.

SECTION III. FUTURE MISSION DESIGNS - What future missions to Enceladus, or Enceladus and Titan, might look like, what is technically possible in the next 10 years, what kinds of instruments would be most astrobiologically productive, what kinds of measurements we would like to make, etc.

Titan-Enceladus Cycler Orbits
Ryan Russell (presenting author) and Nathan Strange

This talk will describe and characterize potential cycler orbits that repeatedly shuttle a spacecraft between Titan and Enceladus using little or no fuel. The frequent Titan and Enceladus flybys provide opportunities for remote sensing and in-situ atmospheric science as well as telecommunication relays for surface landers at Enceladus and aerial vehicles at Titan. The potential and limitations of cycler trajectories will be overviewed in the context of future Enceladus missions.

An Enceladus Mission Concept Study

Amy Simon-Miller (NASA GSFC)

We will present the initial plans of an Enceladus mission concept study to be performed by GSFC. In this study will we address the types of missions that are possible, including the options for sample return, orbiter remote and in-situ sensing and shallow ice vent penetrators. Our goal is to design a small flagship or New Frontiers class mission concept.

Monitoring of geological activity on Enceladus from future missions

Author: Cynthia Phillips, SETI Institute

Current geological activity, in the form of plumes of material being ejected from Enceladus, has been detected by the Cassini spacecraft through thermal measurements, magnetospheric measurements, and images of the plumes themselves. A future mission to Enceladus should monitor the satellite, and others in the Saturn system, for changes using these three methods. In addition, a fourth method, systematic comparison of images taken by the Cassini spacecraft to those taken by the new mission, will allow detection of surface geological activity.

Automating the Detection of Enceladus Plumes
Wagstaff, K, Rebecca Castano, Ashley Davies, and Brian Bue

We have developed a method for analying images of spherical bodies, such as Enceladus, to determine whether a plume is present. It is computationally efficient enough to run on a typical spacecrft processor, permitting the spacecraft to automatically flag images containing plumes as high priority for transmission. Onboard analysis can greatly increase the amount of temporal coverage, and therefore also increase the number of positive detections of plume events as they happen.

Penetration of Enceladus Ice Tiger Stripes
Jones, J. and Castillo, J.

Thermal analyses have shown that the thickness of the ice in the warm "tiger stripe" region of Enceladus is less than 40-m thick, and energy analyses show that the ice can be penetrated by means of direct impact or by slow sublimation of ice by a heated probe. Specific examples of these two methods include: 1. Calculations based on unclassified Sandia National Laboratories missile penetration tests show that a steel harpoon (10 cm diameter, 2-m long, 132 kg) traveling at 150 m/sec, would penetrate through 40-m of ice with a maximum g-force of 350 g's, and similar to Sandia sea ice tests, would allow for a tethered antenna to be left on the surface. 2. Another specific example of a means to penetrate through the ice is with a soft-landed spacecraft that deploys a tethered 3-cm diameter probe with a 50-watt heater that sublimates through the ice at the rate of 1-m per day and relays data through optical fibers to the surface craft.

Sensor and Instrument Integrity
Jenson, Jerry

During the Huygens probe descent and landing on Titan, several instruments recorded surprising and puzzling results. Temperatures recorded during the descent were unexpectedly low, accelerometers were flatter than anticipated, and a permittivity probe did not react to surface penetration as designed. In each of these cases it is difficult to ascertain whether the sensors failed, or the data accurately represent Huygens' environment. Future deep space missions must use integrated sensor validation and self-testing concepts to assure data reliability.

EAGLE (Enceladus Astrobiology & Geophysics Landing Expedition): Mission Overview
Frank H. Mycroft

Students in the NASA Academy program at Goddard have completed the conceptual design of a mission consisting of an Enceladus lander and Saturn orbiter to launch in 2023. This talk focuses on the engineering capabilities that significantly influence the feasibility of our science payload. The mission trajectory, launch windows, mass allocation, propulsion systems, communications link, and cost estimate for the EAGLE concept will be presented.

EAGLE: Science Payload
James J. Wray

The EAGLE lander will carry instruments to probe the subsurface structure of Enceladus, and to study the chemical composition of its plume and surface ice. Organic molecules ranging from simple hydrocarbons to amino acids and nucleobases will be prime targets. In this talk, the scientific requirements for orbital and surface operations are presented, along with the suite of instruments selected to meet them.

EAGLE: Innovative Designs for Future Exploration Technologies
Diana Trujillo

By the time the EAGLE mission launches in 2023, technology will have developed exponentially in many areas, thereby opening doors to new exploration capabilities. Several innovative techniques could be used to expand the scope of the mission to explore some of the less accessible regions of Enceladus. Innovative experiments under consideration for the EAGLE mission include exploration of the Tiger Stripes' interiors, analysis of the tidal stresses that drive the South Polar activity, redirecting the lander's final propulsion stage for impact on the icy surface, and the ability to hop to multiple locations, thus broadening our scientific investigation of Enceladus.

Remotely Exploring Astrobiology at Enceladus
Joe Pitman, Lockheed Martin Space Systems

We describe the application of state-of-the-art reconnaissance technology applied to the search for life sign indicators on Enceladus during long-distance observations, short-term flybys, and high-altitude science orbits on future mission concepts. We overview our MIDAS payload technology features and capabilities for providing a suite of complementary passive and active laser remote sensing methods well suited to astrobiology exploration of Enceladus. By applying those capabilities in a coordinated fashion, we can significantly enhance potential scientific return, including effectiveness of probes or landed packages sent to the surface, such as by exquisitely characterizing candidate sites during early mission phases.

Silicon-Germanium (SiGe) Technology
For Europa and Enceladus Lander or Probe Missions

Leora Peltz1, William Atwell2, and Robert Frampton3
1The Boeing Company, Advanced Avionics, Huntington Beach, CA 92605
2The Boeing Company, Space Exploration, Houston, TX 77058
3The Boeing Company, Systems Engineering, Huntington Beach, CA 92605

A Europa mission has been designated by the Decadal Study as the highest priority flagship mission for the next decade, and is likely to be the next large mission to the outer planets after Cassini. An Enceladus mission also has heightened interest following the Cassini close flyby. A mission to Europa or Enceladus would likely include landers and/or surface probes. These surface probes, in order to have a lifetime longer than a few hours, would need electronics designed to operate at the extreme cold surface temperatures. Europa has extreme environmental conditions, with a surface temperature varying from a low of 50 Kelvin up to 125 Kelvin, with a mean temperature of around 100 Kelvin. The surface of Enceladus is dominated with ice blocks at temperatures of 70-80 Kelvin, with hot spots topping 100 Kelvin. Europa also has an extreme radiation environment: 20 Mrad-H2O per month at the surface. Enceladus surface has a more benign radiation environment. These are harsh and challenging environments for electronics circuitry. SiGe technology is particularly suited to meet these demands. The performance characteristics of SiGe devices vary gracefully over this extreme temperature range, with no evidence of abrupt "killer" phenomena. The structure of SiGe devices also confers a "free perk" - multi-Mrad total dose hardness, with no intentional hardening. This paper presents the radiation environments at the surface of Europa, Ganymede, Callisto, and Enceladus. It presents design characteristics for Silicon-Germanium circuitry for operating low temperature (to 40 Kelvin) and high radiation environments, along with preliminary test results. The paper also presents preliminary work with Gallium Nitride electronics.

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