Influence of a Phase Transition of Ice on the Heat and Mass Balance of Comets

Travel Times of a Descending Melting Probe on Europa

In this study, we calculated the travel times of a thermal probe that descends through Europa's ice shell. The ice column is simplified to a conductive layer. Using a cellular automaton model, the descent of the probe was simulated by tracking temperature changes, with cell interaction dictated by heat conduction and cell state transition rules determined by cell temperatures. Validation tests, including a soil column simulation, and comparison with experimental data, support the reliability of the model. Simulations were performed with 2 different cell sizes, 19 constant probe temperatures, and 5 ice thermal conductivities. A smaller cell size (Δ z = 3   mm) produced shorter travel times (between 22 days for a probe temperature T p = 600 K and ∼4 years for T p = 280 K ) than a larger cell size (Δ z = 1   m), which produced travel times between 27 years (T p = 600K) and ∼103 years (T p = 280K). The ice shell's thermal conductivity has a modest impact on descent times. The results are generally consistent with previous approaches that used more detailed probe engineering considerations. These results suggest that a probe relying solely on heat production may traverse Europa's conductive ice shell within a mission's timeframe.

The internal structure of Eris inferred from its spin and orbit evolution

The large Kuiper Belt object Eris is tidally locked to its small companion Dysnomia. Recently obtained bounds on the mass of Dysnomia demonstrate that Eris must be unexpectedly dissipative for it to have despun over the age of the solar system. Here, we show that Eris must have differentiated into an ice shell and rocky core to explain the dissipation. We further demonstrate that Eris's ice shell must be convecting to be sufficiently dissipative, which distinguishes it from Pluto's conductive shell. The difference is likely due to Eris's apparent depletion in volatiles compared with Pluto, perhaps as the result of a more energetic impact.

Polarized Neutrons Observed Nanometer-Thick Crystalline Ice Plates in Frozen Glucose Solution

Spin-contrast-variation (SCV) small-angle neutron scattering (SANS) is a technique to determine the nanostructure of composite materials from the scattering of polarized neutrons that changes with proton polarization of samples. The SCV-SANS enabled us to determine structure of nanoice crystals that were generated in rapidly frozen sugar solutions by separating the overlapped signals from the nanoice crystals and frozen amorphous solutions. In the frozen glucose solution, we found that the nanoice crystals formed a planar structure with a radius larger than several tens of nanometers and a thickness of 2.5 ± 0.5 nm, which was close to the critical nucleation size of ice crystals in supercooled water. This result suggests that the glucose molecules were preferentially bound to a specific face of nanoice crystals and then blocked the crystal growth perpendicular to that face.

Using MARSIS signal attenuation to assess the presence of South Polar Layered Deposit subglacial brines

Knowledge of the physical and thermal properties of the South Polar Layer Deposits (SPLD) is key to constrain the source of bright basal reflections at Ultimi Scopuli detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. Here we present a detailed analysis of attenuation, based on data acquired by MARSIS at 3, 4, and 5 MHz. We show that attenuation is frequency dependent, and that its behavior is consistent throughout the entire region. This suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and our results are consistent with dust contents of 5 to 12%. Using these values as input, and plausible estimates of surface temperature and heat flux, we inferred basal temperatures around 200 K: these are consistent with perchlorate brines within liquid vein networks as the source of the reflections. Furthermore, extrapolation of the attenuation to higher frequencies explains why SHARAD (Shallow Radar) has thus far not detected basal reflections within the SPLD at Ultimi Scopuli.

MARSIS attenuation and thermal data confirm that liquid brines are the most plausible source for the bright reflections at the base of the South Polar Layered Deposits. Such results also justify why SHARAD does not penetrate to the base of the ice.

Planetary Protection Assessment of Radioisotope Thermoelectric Generator (RTG)-Powered Landed Missions to Ocean Worlds: Application to Enceladus

Landed missions to icy worlds with a subsurface liquid water ocean must meet planetary protection requirements and ensure a sufficiently small likelihood of any microorganism-bearing part of the landed element reaching the ocean. A higher bound on this likelihood is set by the potential for radioisotope thermoelectric generator (RTG) power sources, the hottest possible landed element, to melt through the ice shell and reach the ocean. In this study, we quantify this potential as a function of three key parameters: surface temperature, ice shell thickness (i.e., heat flux through the shell), and thickness of a porous (insulating) snow or regolith cover. Although the model we describe can be applied to any ocean world, we present results in the context of a landed mission concept to the south polar terrain of Saturn's moon Enceladus. In this particular context, we discuss planetary protection considerations for landing site selection. The likelihood of forward microbial contamination of Enceladus' ocean by an RTG-powered landed mission can be made sufficiently low to not undermine compliance with the planetary protection policy.

Formation of Cubic Ice via Clathrate Hydrate, Prepared in Ultrahigh Vacuum under Cryogenic Conditions

Cubic ice (ice I_c) is a crystalline phase of solid water, which exists in the earth's atmosphere and extraterrestrial environments. We provide experimental evidence that dissociation of acetone clathrate hydrate (CH) makes ice I_c in ultrahigh vacuum (UHV) at 130-135 K. In this process, we find that crystallization of ice I_c occurs below its normal crystallization temperature. Time-dependent reflection absorption infrared spectroscopy (RAIRS) and reflection high-energy electron diffraction (RHEED) were utilized to confirm the formation of ice I_c. Associated crystallization kinetics and activation energy (E_a) for the process were evaluated. We suggest that enhanced mobility or diffusion of water molecules during acetone hydrate dissociation enabled crystallization. Moreover, this finding implied that CHs might exist in extreme low-pressure environments present in comets. These hydrates, subjected to prolonged thermal annealing, transform into ice I_c. This unique process of crystallization hints at a possible mechanistic route for the formation of ice I_c in comets.

The presence of clathrates in comet 67P/Churyumov-Gerasimenko

Cometary nuclei are considered to most closely reflect the composition of the building blocks of our solar system. As such, comets carry important information about the prevalent conditions in the solar nebula before and after planet formation. Recent measurements of the time variation of major and minor volatile species in the coma of the Jupiter family comet 67P/Churyumov-Gerasimenko (67P) by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument onboard Rosetta provide insight into the possible origin of this comet. The observed outgassing pattern indicates that the nucleus of 67P contains crystalline ice, clathrates, and other ices. The observed outgassing is not consistent with gas release from an amorphous ice phase with trapped volatile gases. If the building blocks of 67P were formed from crystalline ices and clathrates, then 67P would have agglomerated from ices that were condensed and altered in the protosolar nebula closer to the Sun instead of more pristine ices originating from the interstellar medium or the outskirts of the disc, where amorphous ice may dominate.

Thermal drilling in planetary ices: an analytic solution with application to planetary protection problems of radioisotope power sources

Thermal drilling has been applied to studies of glaciers on Earth and proposed for study of the martian ice caps and the crust of Europa. Additionally, inadvertent thermal drilling by radioisotope sources released from the breakup of a space vehicle is of astrobiological concern in that this process may form a downward-propagating "warm little pond" that could convey terrestrial biota to a habitable environment. A simple analytic solution to the asymptotic slow-speed case of thermal drilling is noted and used to show that the high thermal conductivity of the low-temperature ice on Europa and Titan makes thermal drilling qualitatively more difficult than at Mars. It is shown that an isolated General Purpose Heat Source (GPHS) "brick" can drill effectively on Earth or Mars, whereas on Titan or Europa with ice at 100 K, the source would stall and become stuck in the ice with a surface temperature of <200 K.

Cassini encounters Enceladus: background and the discovery of a south polar hot spot

The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 kelvin are required.

Theory of bulk, surface and interface phase transition kinetics in thin films

We report a theoretical study of phase transition kinetics in confined two-dimensional systems, motivated by recent experimental results on the amorphous-to-crystalline transition in supported, thin amorphous water films [E.H.G. Backus, M.L. Grecea, A.W. Kleyn, and M. Bonn, Phys. Rev. Lett. (to be published)]. We generalize and extend existing theories to simultaneously describe the converted (crystalline) fractions in the bulk, at the sample-vacuum surface, and at the sample-support interface as a function of time. The general approach presented here results in expressions for the time-dependent converted bulk, surface, and interface fractions, for arbitrary desorption rate from the thin film, nucleation and growth rates and also includes finite nucleation grain size. The converted bulk, surface, and interface fractions are calculated for nucleation of the new phase occurring (i) in the bulk, (ii) at the support-sample interface, and (iii) at the sample surface (sample-vacuum interface), resulting in nine expressions. The results demonstrate the advantage of monitoring bulk, surface and interface fractions simultaneously to make definite statements regarding the location of the nucleation, and to reliably determine the values of the relevant crystallization parameters.

The stability against freezing of an internal liquid-water ocean in Callisto

The discovery of the induced magnetic field of Callisto-one of Jupiter's moons-has been interpreted as evidence for a subsurface ocean, even though the presence of such an ocean is difficult to understand in the context of existing theoretical models. Tidal heating should not be significant for Callisto, and, in the absence of such heating, it is difficult to see how this internal ocean could have survived until today without freezing. Previous work indicated that an outer ice layer on the ocean would be unstable against solid-state convection, which once begun would lead to total freezing of liquid water in about 108 years. Here I show that when a methodology for more physically reasonable water ice viscosities (that is, stress-dependent non-newtonian viscosities, rather than the stress-independent newtonian viscosities considered previously) is adopted, the outer ice shell becomes stable against convection. This implies that a subsurface ocean could have survived up to the present, without the need for invoking antifreeze substances or other special conditions.

Controlling the morphology of amorphous solid water

The morphology of amorphous solid water grown by vapor deposition was found to depend strongly on the angular distribution of the water molecules incident from the gas phase. Systematic variation of the incident angle during deposition using a collimated beam of water led to the growth of nonporous to highly porous amorphous solid water. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The ability to control its properties in the laboratory may shed light on some of the outstanding conflicts related to this important material.

The seeding of life by comets

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.

Chemical evolution of interstellar dust, comets and the origins of life

It now appears that the chemical evolution of the pre-solar system interstellar dust ensures that a major fraction of comets is in the form of complex organic molecules at least partially of a prebiotic nature and that the submicron interstellar dust preserves its chemical integrity as result of forming a very tenuous low density comet structure whose solid matter occupies approximately 1/5 of the total volume. This low density micro structure further provides a physical basis for comets bringing a significant fraction of the original interstellar organic molecules to the earth unmodified by the impact event. Finally, the evidence for a large number of comet collisions with the early earth ensured that the major organic molecular budget on the earth's surface was "continuously" supplied along with water well before 3.8 billion years ago which is the earliest date for life. The chemistry and morphological structure of a comet nucleus as an aggregate of interstellar dust is used to provide comparisons with a variety of comet Halley results: the density of the nucleus and of the dust; the dust cloud model and its consequences on the production of C+ and CN in the coma by small organic grains; the surface albedo and the low nucleus heat conductivity and high surface temperature; the appearance of 10(-14) g and 10(-17) g dust particles along with higher masses; the mass spectra of dust and infrared spectroscopy as evidence for complex organic grain mantles and of very small (approximately 0.01 micrometer) carbonaceous and silicate grains; the appearance of small grains resulting from breakup of larger grains. The cosmic ray dosage of a comet nucleus during its 4.5 billion years in the Oort cloud appears to be many orders of magnitude less than the dosage of the preaggregated interstellar dust by ultraviolet photons except perhaps in the outer few meters of the nucleus of a new comet. The heat conductivity calculated for aggregated dust is certainly less than 10(-4) that of crystalline ice. This, in combination with the interstellar dust microstructure, provide a basis for showing that solar heating of the interior of a nucleus is lower than previously estimated.

The formation of a permanent dust mantle and its effect on cometary activity

The growth of a permanent, permeable, dust mantle on the surface of a comet nucleus, composed initially of dusty amorphous water ice, is investigated. Numerical simulations of the evolution of one-dimensional comet nucleus models, in Comet Halley's orbit, are carried out for various parameters, allowing for the crystallization of the amorphous ice. It is assumed that the mantle forms gradually, by the accumulation of a constant fraction (0.001-0.01) of the dust, which is not carried away with the sublimating ice. It is found that an approximately 1-cm-thick dust mantle diminishes the average sublimation rate by a factor of approximately 5, and a further growth of the dust mantle may decrease the surface activity of the nucleus by another factor of 10. Therefore, the activity of a dust-covered nucleus is expected to result mainly from exposed patches of ice and from craters, such as were observed on Comet Halley by Giotto. These are formed by explosions of gas-filled pockets in the crystalline outer layer of the nucleus. The insulating effect of the dust mantle causes the crystallization of the amorphous ice to proceed at a slower rate than in the case of a bare icy nucleus. Thus, the thickness of the outer crystalline shell, overlying the amorphous ice core, is always greater than 15 m, but does not exceed a few tens of meters. This size range is compatible with the amount of gas released in the numerous small explosions which were observed on Comet Halley.

Methods for computing comet core temperatures

General analytic expressions are derived that relate the surface temperature to the temperature deep within the nucleus of a spherically symmetric layered comet in thermal equilibrium. The relation between the average surface temperature and the mean temperature at great depths depends entirely on the temperature dependence of the thermal conductivity. The core temperature is given by the inverse of the anti-derivative of the thermal conductivity, with respect to temperature, operating on the average value of the anti-derivative of the thermal conductivity evaluated at the surface temperature. Using these expressions detailed numerical models of the surface temperature of comets can be used to directly estimate the core temperature. For the special, albeit unphysical, case of an isothermal, low-conductivity comet nucleus, without sublimation, the core temperature can be determined analytically. To illustrate the dependence of core temperature on eccentricity this simple case is solved assuming that the temperatures dependence of the thermal conductivity is given by that of crystalline ice. For an eccentricity of approximately 0.5, the core temperature obtained is 3% colder than the corresponding value obtained assuming constant thermal conductivity an is 11% colder than the result of Klinger's (1981) formula. This method is also applied to a detailed numerical model with a complicated nonintegrable thermal conductivity.

Temperatures within comet nuclei

We have performed a theoretical study of temperatures beneath the surface of a comet's nucleus. We solve the one-dimensional heat conduction equation for the outer portion of the comet. The upper boundary condition of the model is given by energy balance at the surface of the nucleus, including conduction of heat inward, radiation, insolation as modified by the coma, and sublimation. Our coma model assumes single scattering and includes attenuation of direct sunlight by dust grains, scattering of light onto the nucleus, and infrared radiation by dust grains. The lower boundary condition is zero net heat flux around an orbit. The thermal conductivity expression for the nucleus includes direct conduction at grain boundaries, radiative conduction, and Knudsen flow vapor diffusion. The thermal diffusivity of the nucleus and the resultant temperature profiles are shown to be strongly dependent on the physical properties of the material, including porosity, pore size, and compaction. The temperature profiles and the equilibrium temperature deep within the comet also depend on the functional relationship between thermal conductivity and temperature; the highest deep equilibrium temperatures are found for models where the thermal conductivity increases strongly with increasing temperature. The dependence of temperatures on the albedo and thermal emissivity of the nucleus is also calculated, as well as the variation of temperature with latitude for a variety of pole orientations. The effect of a dust mantle on subsurface temperatures is also investigated. All calculations are presented for short-period comets with orbits that make them accessible for exploration by spacecraft rendezvous. In situ measurements of the thermal profile in the upper meter of a comet nucleus can substantially constrain the thermal diffusivity of the material, which in turn can provide significant information about the physical properties of the nucleus.

Probing the presently tenuous link between comets and the origin of life

A unique set of millimeter-wave experiments for future cometary space missions is discussed. These experiments could yield answers to many basic questions about the presently undetermined nature of cometary nuclei and inner comae. This same set of experiments, designed to do fundamental cometary research, could simultaneously provide information on whether the accepted biological requirements necessary for the development of life are met in comets.