Infrared observations of Saturn's rings by Cassini CIRS : Phase angle and local time dependence

Since the Saturn orbit insertion (SOI) of the Cassini spacecraft, in July 2004, the Cassini Composite Infrared Spectrometer (CIRS) has obtained a large number of thermal infrared spectra of Saturn's rings. Over the two and a half years of observations to date, ring temperatures were retrieved for a large range of unique geometries, inaccessible from Earth. Understanding their dependencies with phase angle and local time is a clue to understanding the thermal properties and dynamics of Saturn's ring particles.Azimuthal scans of rings, which have been obtained by CIRS at constant radial distance from the planet, have been planned to measure ring temperature variations with local hour angle. Over 47 azimuthal scans for Saturn's main rings (A, B, C and Cassini Division) have been retrieved to date, on both lit and unlit sides, at different phase angles and spacecraft elevations. The first measurements of the transient thermal episode of eclipse cooling in the planetary shadow have also been obtained for all three rings.In this paper, we present an overview of all azimuthal scans obtained by the Cassini/CIRS instrument so far and the dependencies of the temperature and the filling factor with the phase angle and the local hour angle. The ring temperature varies with longitude as the input heating flux coming from Saturn and the Sun changes. The decrease in temperature with the increasing phase angle on both the lit and the unlit sides and for most of the local time also suggests the presence of slowly rotating particles. The crossing of the planet's shadow generates drastic azimuthal variations in temperature, up to 20 K in the C ring. The strong anisotropy of emission observed outside the shadow between low and high phase angles decreases when ring particles cross the shadow, suggesting that particles are almost isothermal in the shadow. This suggests a thermal inertia associated with a rotating rate of particles low enough to have a thermal contrast on their surface.The temperature in the B ring is less sensitive to the phase angle effect on the lit side, suggesting that particles are close enough to form a flat layer at a scale larger than the particle's radius. On the unlit side, particles in the B ring are less sensitive to the lack of solar input than in the C ring or in the A ring. Azimuthal variations of the filling factor in the A ring are also detected with changing ring local time. This effect might be created by the presence of gravitational instabilities (wakes).     

Subsurface heat transfer on Enceladus: Conditions under which melting occurs

Given the heat that is reaching the surface from the interior of Enceladus, we ask whether liquid water is likely and at what depth it might occur. The heat may be carried by thermal conduction through the solid ice, by the vapor as it diffuses through a porous matrix, or by the vapor flowing upward through open cracks. The vapor carries latent heat, which it acquires when ice or liquid evaporates. As the vapor nears the surface it may condense onto the cold ice, or it may exit the vent without condensing, carrying its latent heat with it. The ice at the surface loses its heat by infrared radiation. An important physical principle, which has been overlooked so far, is that the partial pressure of the vapor in the pores and in the open cracks is nearly equal to the saturation vapor pressure of the ice around it. This severely limits the ability of ice to deliver the observed heat to the surface without melting at depth. Another principle is that viscosity limits the speed of the flow, both the diffusive flow in the matrix and the hydrodynamic flow in open cracks. We present hydrodynamic models that take these effects into account. We find that there is no simple answer to the question of whether the ice melts or not. Vapor diffusion in a porous matrix can deliver the heat to the surface without melting if the particle size is greater than ∼1 cm and the porosity is greater than ∼0.1, in other words, if the matrix is a rubble pile. Whether such an open matrix can exist under its own hydrostatic load is unclear. Flow in open cracks can deliver the heat without melting if the width of the crack is greater than ∼10 cm, but the heat source must be in contact with the crack. Frictional heating on the walls due to tidal stresses is one such possibility. The lifetime of the crack is a puzzle, since condensation on the walls in the upper few meters could seal the crack off in a year, and it takes many years for the heat source to warm the walls if the crack extends down to km depths. The 10:1 ratio of radiated heat to latent heat carried with the vapor is another puzzle. The models tend to give a lower ratio. The resolution might be that each tiger stripe has multiple cracks that share the heat, which tends to lower the ratio. The main conclusion is that melting depends on the size of the pores and the width of the cracks, and these are unknown at present.     

Thermal modeling of cometary nuclei

A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed.     

C ring fine structures revealed in the thermal infrared

We analyze data sets obtained with the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft after the Saturn Orbit Insertion (SOI). Using the mid-IR interferometer's FP3 channel (600-1100 cm⁻¹), we derive radial temperature profiles for the C ring with a spatial resolution never achieved before. For the first time, the C ring's plateaus and ringlets can be clearly separated from the optically thinner background and their thermal behavior is studied separately for different viewing geometries. In particular, thermal phase curves derived for the plateaus reveal an interesting surge near 0° phase, not observed in the background. We show that mutual shadowing in the plateaus can explain the existence of the surge but is not sufficient to model the phase curves in detail. By analogy with thermal emission of asteroid surfaces we discuss the possible influence of small scale and large scale roughness of the ring structure itself. Because infrared emissivity cannot be derived without being deconvolved from the ‘structural’ filling factor, we examine temperature and filling factors measurements at opposition where the filling factor is most constrained. The occurrence of higher temperatures in the plateaus than in the background near opposition likely arises from enhanced mutual heating between particles, multiple scattering and surface roughness combined with a higher single-scattering albedo.     

Heat balance in Titan's atmosphere

The recent measurements of the vertical distribution and optical properties of haze aerosols as well as of the absorption coefficients for methane at long paths and cold temperatures by the Huygens entry probe of Titan permit the computation of the solar heating rate on Titan with greater certainty than heretofore. We use the haze model derived from the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008a. A model of Titan's aerosols based on measurements made inside the atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.11.019] to evaluate the variation in solar heating rate with altitude and solar zenith angle in Titan's atmosphere. We find the disk-averaged solar energy deposition profile to be in remarkably good agreement with earlier estimates using very different aerosol distributions and optical properties. We also evaluated the radiative cooling rate using measurements of the thermal emission spectrum by the Cassini Composite Infrared Spectrometer (CIRS) around the latitude of the Huygens site. The thermal flux was calculated as a function of altitude using temperature, gas, and haze profiles derived from Huygens and Cassini/CIRS data. We find that the cooling rate profile is in good agreement with the solar heating profile averaged over the planet if the haze structure is assumed the same at all latitudes. We also computed the solar energy deposition profile at the 10°S latitude of the probe-landing site averaged over one Titan day. We find that some 80% of the sunlight that strikes the top of the atmosphere at this latitude is absorbed in all, with 60% of the incident solar energy absorbed below 150 km, 40% below 80 km, and 11% at the surface at the time of the Huygens landing near the beginning of summer in the southern hemisphere. We compare the radiative cooling rate with the solar heating rate near the Huygens landing site averaging over all longitudes. At this location, we find that the solar heating rate exceeds the radiative cooling rate by a maximum of 0.5 K/Titan day near 120 km altitude and decreases strongly above and below this altitude. Since there is no evidence that the temperature structure at this latitude is changing, the general circulation must redistribute this heat to higher latitudes.     

Heat flow and depth to a possible internal ocean on Triton

The Raz Fossae, a pair of ≈15-km wide trough en echelon interpreted as grabens, can be used to propose an estimation of the depth to the brittle-ductile transition on Triton. This estimation may in turn give an idea of the thermal state of Triton's icy lithosphere when these features formed. Given the young age of its surface, the conclusions obtained could be roughly applicable to the present state of this satellite of Neptune. Considering water or ammonia dihydrate as possible components of the lithosphere and a feasible range of strain rates, it was estimated that surface heat flow is greater than that inferred from radiogenic heating, especially for a lithosphere dominated by water. Also, an internal ocean could lie at a depth of only ∼20 km beneath the surface. The presence over the surface of an insulating layer of ice of low thermal conductivity (e.g., nitrogen) or of regolith would only substantially alter these estimates if the effective surface temperature were considerably higher than the observed value of 38 K.     

Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects

The detection of induced magnetic fields in the vicinity of the jovian satellites Europa, Ganymede, and Callisto is one of the most surprising findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites' outermost icy shells that may contain even more water than all terrestrial oceans combined. The maintenance of liquid water layers is closely related to the internal structure, composition, and thermal state of the corresponding satellite interior. In this study we investigate the possibility of subsurface oceans in the medium-sized icy satellites and the largest trans-neptunian objects (TNO's). Controlling parameters for subsurface ocean formation are the radiogenic heating rate of the silicate component and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer Solar System. Based on the assumption that the satellites are differentiated and using an equilibrium condition between the heat production rate in the rocky cores and the heat loss through the ice shell, we find that subsurface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO's 2003 UB₃₁₃, Sedna, and 2004 DW. Subsurface oceans can even exist if only small amounts of ammonia are available. The liquid subsurface reservoirs are located deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with the surrounding magnetic fields and charged particles and by the magnitude of a satellite's response to tides exerted by the primary. The latter is strongly dependent on the occurrence of a subsurface ocean which provides greater flexibility to a satellite's rigid outer ice shell.     

Almost analytic models of ultramagnetized neutron star envelopes

Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 10¹⁵ − 10¹⁶ G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.     

Amalthea: Implications of the temperature observed by Voyager

Voyager 1 IRIS observations of Amalthea, although initially indicating an unusually high temperature, now give a temperature of only 164 ± 5°K, a value consistent with the Earth-based measurement by G. H. Rieke [Icarus25, 333–334 (1975)] of 155 ± 15°K. We numerically modeled the temperature profile in the satellite's surface layer as a function of location and time of day, assuming a triaxial ellipsoid shape and thermal properties similar to those of the lunar soil. The major heat source is direct insolation, but temperatures are increased slightly by thermal radiation from Jupiter (?9°K), by sunlight reflected from the planet (?5°K), and by charged particle bombardment (?2°K). Maximum calculated temperatures reach 166°K, and we estimate that the temperature that Voyager would have measured under these circumstances is ≈160°K, in agreement with the observed temperature. Possible sources of error in the model are discussed in detail, including satellite shape effects, unusually low emissivity, uncommonly rough surface, abnormal thermal intertia, variability of the charged particle flux, and Joule heating. The IRIS observation strongly suggests that (i) the Amalthean surface has an emissivity near unity; (ii) the charged particle flux on the satellite at the time of observation was no more than 20 times larger than the flux indicated by Pioneer observations; and (iii) Joule heating of the satellite is insignificant (a conclusion also supported by rough calculations). The IRIS observation cannot, however, put any useful limits on the thermal inertia of the Amalthean surface layer.     

The nature and possible origin of lava-like material within cheniér crater

This paper considers the origin of certain tongues of lava-like material in Cheniér Crater, a meteorite crater located about 63 km northeast of the major crater, Tsiolkovsky, on the lunar far side. The author contends that the tongues originated from subsurface movement of magma generated as a result of the meteorite impact which created Tsiolkovsky Crater. The impact produced lines of weakness which were further enhanced by the impact forming Cheniér. Magma then moved from Tsiolkovsky through the zones of weakness to Cheniér Crater, extruding on the surface to form the first stringer. Following the extrusion, magnetic movement stopped and a cap formed over the vent. Enough heat was left in place under Cheniér, however, to cause crustal melting and consequently the extrusion of a second and possible third lava-like stringer before the magma chamber under Cheniér cooled and a cap over the vents permanently formed. Confirmation of the theory depends upon whether magma can move through weak zones in the lunar subsurface. Indications of this possibility have been suggested in findings dealing with the floors of Tycho and Aristarchus craters and in a study of the effects of artificial cratering.     

Waves in Jupiter's atmosphere observed by the Cassini ISS and CIRS instruments

The Cassini Imaging Science Subsystem (ISS) and Composite Infrared Spectrometer (CIRS) reported a North Equatorial Belt (NEB) wave in Jupiter's atmosphere from optical images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] and thermal maps [Flasar, F.M., and 39 colleagues, 2004. Nature 427, 132-135], respectively. The connection between the two waves remained uncertain because the two observations were not simultaneous. Here we report on simultaneous ISS images and CIRS thermal maps that confirm that the NEB wave shown in the ISS ultraviolet (UV1) and strong methane band (MT3) images is correlated with the thermal wave in the CIRS temperature maps, with low temperatures in the CIRS maps (upwelling) corresponding to dark regions in the UV1 images (UV-absorbing particles) and bright regions in the MT3 images (high clouds and haze). The long period of the NEB wave suggests that it is a planetary (Rossby) wave. The combined observations from the ISS and CIRS are utilized to discuss the vertical and meridional propagation of the NEB wave, which offers a possible explanation for why the NEB wave is confined to specific latitudes and altitudes. Further, the ISS UV1 images reveal a circumpolar wave centered at 48.5° S (planetocentric) and probably located in the stratosphere, as suggested by the ISS and CIRS observations. The simultaneous comparison between the ISS and CIRS also implies that the large dark oval in the polar stratosphere of Jupiter discovered in the ISS UV1 images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] is the same feature as the warm regions at high northern latitudes in the CIRS 1-mbar temperature maps [Flasar, F.M., and 39 colleagues, 2004. Nature 427, 132-135]. This comparison supports a previous suggestion that the dark oval in the ISS UV1 images is linked to auroral precipitation and heating [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547].     

On the relationship between ground temperature histories and meteorological records: a report on the Pomquet station

An experimental air–ground climate station is operating in Pomquet, Nova Scotia, monitoring meteorological (surface air temperatures at three heights, wind velocity and direction, incoming solar radiation, precipitation, snow depth and relative humidity) and ground thermal variables (soil temperatures at depths of 0, 5, 10, 20, 50 and 100 cm). Readings are taken every 30 s and 5 min averages are stored, in order to characterize the energy exchanges at the air ground interface. Here, I report on the first year of operation. For spring, summer and fall, we find that soil temperatures track surface air temperatures with amplitude attenuation and phase lag with depth confirming that heat conduction adequately describe the soil thermal field at the Pomquet site. For winter conditions, we find that heat transfer is dominated by latent heat released during soil freezing and to a lesser extent by the insulating affect of snow cover. A numerical model of heat conduction was used in order to estimate the magnitude of the heat released by freezing during the winter months. I also show that there is an inverse correlation for the difference between soil (100 cm) and air temperatures and the incoming solar radiation at the site.     

On the resurfacing of Ganymede by liquid-water volcanism

A long-popular model for producing Ganymede's bright terrain involves flooding of low-lying graben with liquid water, slush, or warm, soft ice. The model suffers from major problems, however, including the absence of obvious near-surface heat sources, the negative buoyancy of liquid water, and the lack of a mechanism for confining the flows to graben floors. We present new models for cryovolcanic resurfacing to overcome these difficulties. Tidal heating within an ancient Laplace-like orbital resonance (Showman and Malhotra 1997, Icarus 127, 93; Showman et al., 1997, Icarus 129, 367) provides a plausible heat source and could allow partial melting to occur as shallow as 5-10 km depth. Our favored mechanism for delivering this water to the surface invokes the fact that topography—such as a global set of graben—causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows (graben) despite the negative buoyancy of the liquid. These eruptions can occur only within the topographic lows; furthermore, as the low areas become full, the pressure gradients disappear and the resurfacing ceases. This provides an explanation for the observed straight dark-bright terrain boundaries: water cannot overflow the graben, so resurfacing rarely embays craters or other rough topography. Pure liquid water can be pumped to the surface from only 5-10 km depth, but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush. A challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows; the resurfacing must occur before the graben topography disappears. We also evaluate alternate resurfacing mechanisms, such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate-contaminated crust that is denser than liquid water, and conclude that they are unlikely to explain Ganymede's bright terrain.     

The thermal history of the moon

A simple analysis shows that the normal assumption of an outward heat flow, together with the normally assumed surface layer of low thermal conductivity, would give rise to microwave emission effects and to local variations in surface temperature which are not in fact observed. It is concluded that either the surface layer must be much thinner than is at present postulated, or that the outward flow of heat must be much smaller than is supposed.     

Origin of ice diapirism, true polar wander, subsurface ocean, and tiger stripes of Enceladus driven by compositional convection

We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an “ammonia feedback” mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer above. We then consider a large scale instability arising from the pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Enceladus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region.     

Thermal history of the Jovian satellite Io

The discovery of large volcanic eruptions on Io suggests that Io is one of the most geologically active planetary bodies. The energy source of this geologic activity is believed to be tidal heating induced by Jupiter. A number of thermal history calculations were done to investigate the effect of tidal heating on the thermal history of Io taking into account solid state convection and advective heat transfer. These simulations show that the total tidal heating energy in Io is almost equal to the advectively transferred heat, indicating that the observed heat flow from Io is nearly equal to the total tidal heating energy. Since total tidal heating energy is dependent on the radius of the liquid mantle and the internal dissipation factor (Q), the radius of the liquid mantle can be estimated for a given value of Q. Some reasonable thermal history models of Io were obtained using a model with Q ≈ 25–50 in which the magma source of Ionian volcanism is at a depth of 100–300 km. The models satisfy the heat flow data and the existence of a thick lithosphere. Using a model with Q = 25 and L = 300 km (thickness of the advective region) as the standard model (model II), we then studied the effect of convective heat transfer and the initial temperature distribution on the Ionian thermal history. In these calculations, the other parameters are the same as in the standard model (model II). These calculations show that although the temperature distribution in the central region reflects the difference in the efficiency of convective heat transfer and initial temperature distribution, the temperature distribution in the outer region does not changes appreciably.     

Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements

Spectra taken by Cassini’s Composite Infrared Spectrometer (CIRS) between 10 and 600 cm⁻¹ (17-1000 μm) of surface thermal emission of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus have been used to derive the thermal inertia and bolometric Bond albedo values. Only an upper limit for the bolometric Bond albedo of Iapetus’ dark leading side could be determined due to the insensitivity of the thermal model to albedo when albedos are very low. The thermal inertia in this region however is better constrained. The CIRS coverage of Enceladus is extensive enough that the latitudinal variation in these values from 60°S to 70°N has been determined in 10° wide bins. The bolometric Bond albedos determined here are consistent with literature values which show the surface of the saturnian icy moons to be covered in ice contaminated to varying degrees. The thermal inertia of the moons is shown to be in the range 9-, approximately 2-6 times lower than that of the Galilean satellites, implying a less well consolidated and more porous surface. The thermal inertias of Iapetus and Phoebe are somewhat higher, suggesting that the very low thermal inertias of satellites from Rhea inwards may be related to their probable coating of E-ring material. Latitudinal variations on the surface of Enceladus show that the bolometric Bond albedo and thermal inertia increase towards the active plume source at the south pole.     

Oceans in the icy Galilean satellites of Jupiter?

Equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of Jupiter—Europa, Ganymede, and Callisto—may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick. A wide range of rheology and heat transfer parameter values and present-day heat production rates have been considered. The rheology was cast in terms of a reference viscosity ν₀ calculated at the melting temperature and the rate of change A of viscosity with inverse homologous temperature. The temperature dependence of the thermal conductivity k of ice I has been taken into account by calculating the average conductivity along the temperature profile. Heating rates are based on a chondritic radiogenic heating rate of 4.5 pW kg⁻¹ but have been varied around this value over a wide range. The phase diagrams of H₂O (ice I) and H₂O + 5 wt% NH₃ ice have been considered. The ice I models are worst-case scenarios for the existence of a subsurface liquid water ocean because ice I has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling. In the context of ice I models, we find that Europa is the satellite most likely to have a subsurface liquid ocean. Even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model. If tidal heating is invoked, the ocean will be much thicker and the ice shell will be a few tens of kilometers thick. Ganymede and Callisto have frozen their oceans in the nominal ice I models, but since these models represent the worst-case scenario, it is conceivable that these satellites also have oceans at the present time. The most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock. Rock increases the density and the pressure gradient and shifts the triple point of ice I to shallower depths where the temperature is likely to be lower then the triple point temperature. According to present knowledge of ice phase diagrams, ammonia produces one of the largest reductions of the melting temperature. If we assume a bulk concentration of 5 wt% ammonia we find that all the satellites have substantial oceans. For a model of Europa heated only by radiogenic decay, the ice shell will be a few tens of kilometers thinner than in the ice I case. The underlying rock mantle will limit the depth of the ocean to 80-100 km. For Ganymede and Callisto, the ice I shell on top of the H₂O-NH₃ ocean will be around 60- to 80-km thick and the oceans may be 200- to 350-km deep. Previous models have suggested that efficient convection in the ice will freeze any existing ocean. The present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant-viscosity convection models. The present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer, if the latter exists at all. The stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant-viscosity convecting layer. We have further modified the parameterization to account for the strong increase in homologous temperature, and therefore decrease in viscosity, with depth along an adiabat. This modification causes even thicker stagnant lids and further elevated temperatures in the well-mixed sublayer. It is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing.     

Morphology, temperature, and eruption dynamics at Pele

The Pele region of Io has been the site of vigorous volcanic activity from the time of the first Voyager I observations in 1979 up through the final Galileo ones in 2001. There is high-temperature thermal emission from a visibly dark area that is thought to be a rapidly overturning lava lake, and is also the source of a large sulfur-rich plume. We present a new analysis of Voyager I visible wavelength images, and Galileo Solid State Imager (SSI) and Near Infrared Mapping Spectrometer (NIMS) thermal emission observations which better define the morphology of the region and the intensity of the emission. The observations show remarkable correlations between the locations of the emission and the features seen in the Voyager images, which provide insight into eruption mechanisms and constrain the longevity of the activity. We also analyze an additional wavelength channel of NIMS data (1.87 μm) which paradoxically, because of reduced sensitivity, allows us to estimate temperatures at the peak locations of emission. Measurements of eruption temperatures on Io are crucial because they provide our best clues to the composition of the magma. High color temperatures indicative of ultramafic composition have been reported for the Pillan hot spot and possibly for Pele, although recent work has called into question the requirement for magma temperatures above those expected for ordinary basalts. Our new analysis of the Pele emission near the peak of the hot spot shows color temperatures near the upper end of the basalt range during the I27 and I32 encounters. In order to analyze the observed color temperatures we also present an analytical model for the thermal emission from fire-fountains, which should prove generally useful for analyzing similar data. This is a modification of the lava flow emission model presented in Howell (Howell, R.R. [1997]. Icarus 127, 394-407), adapted to the fire-fountain cooling curves first discussed in Keszthelyi et al. (Keszthelyi, L., Jaeger, W., Milazzo, M., Radebaugh, J., Davies, A.G., Mitchell, K.L. [2007]. Icarus 192, 491-502). When applied to the I32 observations we obtain a fire-fountain mass eruption rate of 5.1 × 10⁵ kg s⁻¹ for the main vent area and 1.4 × 10⁴ kg s⁻¹ for each of two smaller vent regions to the west. These fire-fountain rates suggest a solution to the puzzling lack of extensive lava flows in the Pele region. Much of the erupted lava may be ejected at high speed into the fire-fountains and plumes, creating dispersed pyroclastic deposits rather than flows. We compare gas and silicate mass eruption rates and discuss briefly the dynamics of this ejection model and the observational evidence.     

Atmospheric control of the cooling rate of impact melts and cryolavas on Titan’s surface

As on Earth, Titan’s atmosphere plays a major role in the cooling of heated surfaces. We have assessed the mechanisms by which Titan’s atmosphere, dominantly N₂ at a surface pressure of 1.5 × 10⁵ Pa, cools a warm or heated surface. These heated areas can be caused by impacts generating melt sheets and (possibly) by endogenic processes emplacing cryolavas (a low-temperature liquid that freezes on the surface). We find that for a cooling cryolava flow, lava lake, or impact melt body, heat loss is mainly driven by atmospheric convection. Radiative heat loss, a dominant heat loss mechanism with terrestrial silicate lava flows, plays only a minor role on Titan. Long-term cooling and solidification are dependent on melt sheet or flow thickness, and also local climate, because persistent winds will speed cooling. Relatively rapid cooling caused by winds reduces the detectability of these thermal events by instruments measuring surface thermal emission. Because surface temperature drops by ≈50% within ≈1 day of emplacement, fresh flows or impact melt may be difficult to detect via thermal emission unless an active eruption is directly observed. Cooling of flow or impact melt surfaces are orders of magnitude faster on Titan than on airless moons (e.g., Enceladus or Europa).Although upper surfaces cool fast, the internal cooling and solidification process is relatively slow. Cryolava flow lengths are, therefore, more likely to be volume (effusion) limited, rather than cooling-limited. More detailed modeling awaits constraints on the thermophysical properties of the likely cryomagmas and surface materials.