Photoelectron fluxes in cometary atmospheres

The photoelectron fluxes in cometary atmospheres are calculated by a Monte Carlo method. This is the first quantitative model calculation of this kind. A pure H₂O atmosphere is assumed with a sublimation rate of 10³⁰ molecules sec^?1 at 1 AU. Discussions of the energetics of electron gas and of the elementary collisional processes in determining the fluxes largely concern this water atmosphere. Influences on the photoelectron fluxes are also investigated for CO, another possible constituent. The excitation rate of the ${}^1\text{D}$ level of atomic oxygen in electron impacts is evaluated. It is highly improbable that the photoelectrons are responsible for the observed 6300 Å emission of the order of 1 kR at a heliocentric distance of 1 AU. The structure of the heat equation for thermal electrons is analyzed and a drastic change of the plasma behavior within the coma region is expected.     

The structure of cometary atmospheres

The temperature distributions in cometary atmospheres at various heliocentric distances for comets of Bennett and Encke types have been calculated by taking into account heating due to the absorption of solar ultraviolet radiation, cooling by H₂O far infrared emission, and various dynamical processes (expansion, advection, and thermal conduction). The agreement of the results with the observations is in general satisfactory. The conversion of CH₄ and NH₃ to CO and N₂ through thermochemical reaction with H₂O is concluded to be impossible, since the temperature is too low at a heliocentric distance 1.5 AU where CO⁺ ions begin to be observable.     

The structure of cometary atmospheres

The distributions of various kinds of molecular ions in the atmospheres of new and old comets made up from dirty ice of the second kind (H₂O ice and hydrate clathrates of CO and N₂) have been computed at various heliocentric distances, by taking into account photoionization, ionmolecular reactions, electron-ion recombinations, and some transport effects. The results have been compared with observations and other computations. It is argued that dirty ice of the second kind model will impose a restriction on the theory of the origin of the solar system.     

Organic component of cometary ice

Investigations of the luminescence of frozen hydrocarbon particles of icy cometary halo have been made. The process of luminescence of icy particles in shortwavelength solar radiation field is considered. The comparative analysis of observed and laboratory data leads of 72 luminescent emission lines in the spectrum of 153P/Ikeya-Zhang comet. Several aspects of the problem are discussed.     

Possible nature of cometary atmosphere particles

The polarimetric and spectrophotometric data of observations, the results of laboratory simulations, and theoretical calculations are considered as evidence in favor of the presence of large irregular particles in cometary atmospheres. The attempt is made to define more precisely the particle parameters. In particular, observations of some comets at small phase angles can be interpreted by light scattering on large icy grains. The results of laboratory experiments with ice at low temperatures and pressures are adduced; this can be explain the formation of a large icy grain cloud near the cometary nucleus. Changes of these particles under the effects of solar radiation are considered.     

Hydrodynamic instability of convectively unstable atmospheres in shear flow

Results are presented concerning the interaction between regions of convectively unstable fluid, bounded above and below by stable fluid, with a basic horizontal flow field, sheared in a vertical direction. The analysis is conveniently based on the definition of the mechanical energy flux associated with wave motion in a stratified compressible fluid, and enables bounds to be placed on the real and complex phase velocities of overstable modes, in addition to some general results on the net upward wave energy flux. It is shown that purely exponentially growing modes (with horizontal wavevectors spanwise to the shear) do not exist. A known sufficient condition for the stability of stable atmospheres is reproduced here with an interesting modification, and details of energy-flux discontinuities at certain singular points of the equations are given. The work is relevant to any astrophysical and geophysical situations in which convectively unstable regions and shear flows are likely to be together present, but the special motivation here is that of describing some aspects of the interaction between supergranular flow and granular convection.     

Experimental study of the degradation of polymers: Application to the origin of extended sources in cometary atmospheres

Abstract— This paper presents some preliminary results concerning the degradation of refractory nitrogenated polymers, which could be responsible for the CN extended source in comets. We are studying hexamethylenetetramine (HMT) and HCN polymers. Both compounds have been irradiated or heated to simulate the degradation processes they undergo in the cometary atmosphere. We show that, even if both compounds are quite stable under photolysis, the heating leads to a much more efficient degradation with the formation of HCN, NH₃, and other heavier compounds. Moreover, the thermal degradation of HCN polymers appears to be more efficient than that of HMT. Thus, the HCN polymers seem to be better candidates for the CN extended source. We are now developing a new reactor to quantify the production of gaseous molecules and to detect in situ CN radicals.     

Dust ring formation due to ice sublimation of radially drifting dust particles under the Poynting-Robertson effect in debris disks

In a disk with a low optical depth, dust particles drift radially inward by the Poynting-Robertson (P-R) drag rather than are blown out by stellar radiation pressure following destructive collisions. We investigate the radial distribution of icy dust composed of pure ice and refractory materials in dust-debris disks taking into account the P-R drag and ice sublimation. We find that icy dust particles form a dust ring by their pile-ups at the edge of their sublimation zone, where they sublime substantially at the temperature 100-110 K. The distance of the dust ring is 20-35 AU from the central star with its luminosity L_??30L_⊙ and 65(L_?/100L_⊙)^1/2 AU for L_??30L_⊙, where L_⊙ is the solar luminosity. The effective optical depth is enhanced by a factor of 2 for L_??100L_⊙ and more than 10 for L_??100L_⊙. The optical depth of the outer icy dust disk exceeds that of the inner disk filled with refractory particles, namely, the residue of ice sublimation, which are further subjected to the P-R effect. As a result, an inner hole is formed inside the sublimation zone together with a dust ring along the outer edge of the hole.     

Acceleration mechanism of particles in the Type-I cometary plasma

In the paper, the accelerated effect of ions has been discussed. The transversal magnetic disturbance is able to bring about the magnetic annihilation and merge in some cometary area. The non-steady-state reconnection process can transform the magnetic energy of some cometary area into the kinetic energy of plasma. In addition, the two stream instability caused by both solar wind and cometary plasmas exists in Type-I tail, it can also lead the paticles to be accelerated and heated in the plasma tail.     

Properties of filamentary sublimation residues from dispersions of clay in ice

During vacuum sublimation experiments on simulated Martian polar deposits and cometary dirty ices, a fluffy filamentary sublimate residue material with unique physical properties was produced. The silica-to-silica bonds that we believe join the particles together are the result of conditions that may exist in some Martian polar deposits and on some cometary surfaces. Submicron particles of montmorillonite clay thinly dispersed (1: 1000 clay/water) and not contacting one another in water ice can form very-low-density structures (density as low as 10⁻³ g cm⁻³) during sublimation of the ice. The lightweight constructs, when viewed in scanning electron microscopy micrographs, are composed of long network chains of the clay particles. The material is sufficiently electrically conductive to drain away the scanning electron microscopy charge. It is also resistant (no change in electronic properties are apparent) to scanning electron microscopy electron-beam heating for hours in vacuo. Infrared spectra and X-ray diffraction patterns of the sublimate residues show little difference from spectra and patterns of the original minerals. Heating in an oven, in air, to 370°C produces little change in the structure of the sublimate residual material. The particle bonding forces are strong and produce a resilient, elastic lightweight material. The particle bonding will allow vapors to diffuse through it, and its thermal conductivity is very low. These properties produce a high-performance vacuum insulation. This material may have applications for insulating ice bodies (solid cryogens) in space. The incoming heat is partially carried away by the out-flowing water vapor.     

Modeling sublimation of ice exposed by new impacts in the martian mid-latitudes

New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the “Dodo-Goldilocks” trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible.     

The heterogeneous condensation of interstellar ice grains

Processes of heterogeneous condensation are investigated. We consider the situation that vapor and solid grains of which material is other than that of the vapor coexist and the saturation of the vapor increases with time. If the contact angle of the condensate to the solid is small (?90°), the condensate will be formed as mantles coating the solid grains but only a fraction of the solid grains will be coated by mantles. The ratio of core-mantle grains to bare solid grains in number will be smaller as the density of the vapor and the time scale of saturation increase are large. Actual calculations are made by adopting H₂O ice as the mantle material and magnesium silicate as the core material.     

Stability of ice on Mars and the water vapor diurnal cycle: Experimental study of the sublimation of ice through a fine-grained basaltic regolith

In order to advance our understanding of the long-term stability of subsurface ice, the diurnal martian water cycle, and implications for liquid water, we determined diffusion coefficients and adsorption kinetics for the water vapor produced by the sublimation of ice buried beneath various layers of fine-grained (<63, 63-125, and 125-250 μm) basaltic powder under simulated martian conditions. Sublimation rates at shallower depths, <10 mm, were determined to be affected by mass transfer through the atmosphere in addition to the basalt layer. For greater depths, the measured diffusion coefficients for water vapor moving through basalt grains were 1.56±0.53×¹⁰⁻⁴, 2.05±0.82×¹⁰⁻⁴, and for the <63, 63-125, and 125-250 μm basaltic layers, respectively. Through the Brunauer, Emmett and Teller (BET) isotherm, which assumes multiple molecular layers of adsorbed water, we determined the adsorption constants of 52.6±8.3 at 270 K for <63 μm, 39.0±6.4 at 267 K for 63-125 μm, and 54.3±9.3 at 266 K for 125-250 μm, resulting in surface areas of 2.6±0.1×¹⁰⁴, 1.7±0.3×¹⁰⁴, , respectively. These results suggest that while diffusion is too rapid to explain the purported diurnal cycle in water content of the atmosphere, adsorption is efficient and rapid, and does provide an effective mechanism to explain such a cycle. The present diffusion data suggest that very thin, <50 pr μm, shallow, 10 mm, ice deposits would last for >10 h at ∼224 K, just above the freezing point of saturated CaCl₂. Temperatures can remain above ∼224 K over most of the planet, which means that water, even as saturated brine, will sublimate before the freezing point is reached and liquid could be formed. On the other hand, 1 m ice layers below 1 m of fine-grained basaltic regolith at 235 K and 10 Pa of atmospheric water could last 600 to 1300 years. At deeper depths and lower temperatures, ice could last since the last major obliquity change 400,000 years ago.     

A thermal modelling of cometary activity with a crystallized water ice nucleus

Arguments are presented to suggest that surface layers of the nuclei of periodic comets consist of crystallized rather than amorphous water ice and thermal modelling of such nuclei is presented. The rate of sublimation of water from a rotating nucleus is found to be greater than that from a uniformly heated nucleus. When the model is applied to P/Halley, the sublimation rate at perihelion is found to be 8.1 × 10²⁹ mol s⁻¹ for a nucleus rotating with a period of 50 hours and 7.6 × 10²⁹ for a uniformly heated nucleus on the premise that the effective radius of the nucleus is 2.5 km. The total sublimation of water per revolution is 5.38 × 10³⁶ molecules for P/Halley and 3.91 × 10³⁶ molecules per P/Crommelin. The result so obtained is discussed in relation to the observational data.     

Mechanistic studies on the decomposition of carbon suboxide in a cometary ice analog

A cometary ice analog sample consisting primarily of carbon suboxide ice (C₃O₂) was produced from the irradiation of its precursor, carbon monoxide. This carbon suboxide sample was subjected to irradiation with energetic electrons at 10 K to simulate the interaction of carbon suboxide-rich cometary analog ices with ionizing radiation. The destruction of carbon suboxide as well as the production of the primary degradation products, dicarbon monoxide (C₂O), and carbon monoxide (CO), were monitored quantitatively by infrared spectroscopy in situ; the gas phase was simultaneously sampled via quadrupole mass spectrometry. A kinetic model was produced to help explain the decomposition kinetics of carbon suboxide in cometary ices and to infer the underlying reaction mechanisms.     

Evaporation of ice in planetary atmospheres: Ice-covered rivers on mars

The evaporation rate of water ice on the surface of a planet with an atmosphere involves an equilibrium between solar heating and radiative and evaporative cooling of the ice layer. The thickness of the ice is governed principally by the solar flux which penetrates the ice layer and then is conducted back to the surface. These calculations differ from those of Lingenfelter et al. [(1968) Science161, 266–269] for putative lunar channels in including the effect of the atmosphere. Evaporation from the surface is governed by two physical phenomena: wind and free convection. In the former case, water vapor diffuses from the surface of the ice through a lamonar boundary layer and then is carried away by eddy diffusion above, provided by the wind. The latter case, in the absence of wind, is similar, except that the eddy diffusion is caused by the lower density of water vapor than the Martian atmosphere. For mean Martian insolations the evaporation rate above the ice is ～ 10^?8 g cm^?2 sec^?1. Thus, even under present Martian conditions a flowing channel of liquid water will be covered with ice which evaporates sufficiently slowly that the water below can flow for hundreds of kilometers even with quite modest discharges. Evaporation rates are calculated for a wide range of frictional velocities, atmospheric pressures, and insolations and it seems clear that at least some subset of observed Martian channels may have formed as ice-choked rivers. Typical equilibrium thicknesses of such ice covers are ～ 10 to 30 m; typical surface temperatures are 210 to 235°K. Ice-covered channels or lakes on Mars today may be of substantial biological interest. Ice is a sufficiently poor conductor of heat that sunlight which penetrates it can cause melting to a depth of several meters or more. Because the obliquity of Mars can vary up to some 35°, the increased polar heating at such times seems able to cause subsurface melting of the ice caps to a depth which corresponds to the observed lamina thickness and may be responsible for the morphology of these polar features.     

On the sublimation of ice particles on the surface of Mars; with applications to the 2007/8 Phoenix Scout mission

Experimental studies related to the sublimation of ice, in bulk or as small particles, alone or mixed with dust similar to that expected on the surface of Mars, are reported. The experiments, a cloud physics particle sublimation model, and a convection model presented by Ingersoll, all indicate a strong dependence of sublimation rate on temperature, and this appears to be the dominant factor, assuming that the relative humidity of the air is fairly low. In addition the rate of loss of water vapour appears to depend primarily on exposed surface area and less on particle size and the total mass of the sample, or the mass of ice in the sample. The 2007/8 Phoenix Scout mission plans to obtain and analyse samples of sub-surface ice from about 70° N on Mars. A concern is that these samples, in the form of ice chips of size about 1 mm diameter, could be prone to sublimation when exposed for prolonged periods (many hours) to a relatively warm and dry atmosphere. Our laboratory simulations confirm that this could be a problem if particles are simply left lying on the surface, but also indicate that samples kept suitably cold and collected together in confined piles will survive long enough for the collection and delivery (to the analysis instruments) procedure to be completed.     

Frozen hydrocarbon particles of cometary halos as carriers of unidentified emissions

The possible nature of unidentified cometary emissions is under discussion. We propose a new model of the ice particles in cometary halos as a mixture of frozen polycyclic aromatic hydrocarbons and acyclic hydrocarbons. We describe principal properties of frozen hydrocarbon particles (FHPs) and suggest interpreting some of the unidentified cometary emission lines as the photoluminescence of FHPs. The results of comparative analysis are present.     

Phase functions of polarization and brightness and the nature of cometary atmosphere particles

The dependence of brightness and polarization of cometary on the phase angle is studied. The similarity between the phase curves of comets, minor planets, and the zodiacal cloud is pointed out. The dependence found correspond to dielectric particles with dimensions greater than 1 μm.