Cometary Water Research Articles

Cometary ion pressure anisotropies at comets Halley and Grigg‐Skjellerup

Cometary water group ions are the dominant population controlling the interaction of comets with the solar wind. The distribution function of this population has important effects on the overall structure and boundaries in the comet‐solar wind interaction and on the plasma wave activity produced. In this paper we present new results on the behavior of the water group ions measured by Giotto at comets Halley and Grigg‐Skjellerup. The pressures perpendicular and parallel to the magnetic field are presented and the implications explored. We show that during the Halley inbound pass, the firehose instability may excite the observed waves with low wavenumber. Also, we find that the energy density of energetic cometary ions is greater than the observed difference between the energy released from the ring‐like distribution and the energy in the pickup‐excited waves. This indicates that another energy source must be contributing to the acceleration, probably related to the deceleration of the solar wind in the comet frame.

Cometary activity and nucleus modelling: a new approach

The phenomena of comet splittings with an average frequency of about one splitting per 100 years and comet (Chen and Jewitt, Icarus 108, 265–271, 1994), and the restriction of cometary activity to well-defined small areas at the almost passive and mantle covered surface (Keller et al., ESA SP-250, Vol. II, pp. 363–364, 1986) are at present driving challenges to models of structure and evolution of comet nuclei. Extending the presently discussed models by incorporating lateral subsurface transport of sublimed volatiles, there appears the possibility that the places of sublimation are different from those of activity (the so-called active areas). Then, there is no necessity to distinguish between different surface properties at active and passive areas, assuming, e.g. an uncovered icy surface at active areas. Active areas are simply the very local “source sites” where the accumulated subsurface flows from distant regions reach the surface. The pressure driven subsurface flows of volatiles may not only leave the comet at its surface, they may penetrate via cracks, etc. also deeply into the nucleus. There they can cause a further growth of cracks and also new cracks. This can be a cause for the observed regular splittings. Furthermore, actual models (Kührt and Keller, Icarus 109, 121–132, 1994; Skorov and Rickman, Planet. Space Sci. 43, 1587–1594, 1995) of the gas transport through porous comet surface crusts can be interpreted as to give first indications for thermodynamical parameters in heat conducting and porous cometary crusts which are appropriate for 1 AU conditions to permit the temporary existence of a layer with fluid subsurface water within these crusts. This exciting result of the possible temporary existence of subsurface warm water in comets which approach the Sun within about 1 AU makes a cometary subsurface chemistry much more efficient than expected hitherto.

Chemical synthesis of lipids and the origin of life

Keeping in mind the importance of amphiphilic lipids for the formation of semipermeable membranes, a review summary of the sources of appropriate precursors, and chemical reactions for the abiotic synthesis of lipids is presented here within the framework of the theory of chemical evolution. It covers the presence in different cosmic environments of precursors for the formation of the biochemical molecules necessary for the emergence of life on Earth. It starts (1) with a short introduction. Then the following matters are briefly reviewed: (2) The circumstellar and interstellar molecules, some of which, could generate straight chain fatty acids through C9. (3) The possible reactions of hydrogenation and hydrolysis of cyanopolyynes which in the presence first of hydrogen and then liquid water could lead to the formation of aliphatic acids. (4) The composition of comets, where the preliminary analysis by mass spectrometry indicate straight chain hydrocarbons through only C5. (5) The organic compounds in carbonaceous chondrites where aliphatic acids through C12 have been identified, although the branched chain isomers are abundant. (6) The synthesis of some biochemical compounds, such as amino acids present in carbonaceous chondrites, which were probably formed by condensation of presolar precursors, aldehydes and ketones, with HCN in the presence of ammonia and liquid water in the meteorite parent body. The isotopic evidence seems to support this interpretation. (7) The formation of the Earth-Moon system by the catastrophic impact of a Mars-size body with the proto-Earth. (8) The subsequent capture of cometary water, organic and inorganic compounds, which must have led to a very reactive primitive Earth's atmospheric environment. The cometary iron-nickel grains could have catalyzed the formation of fatty acids by Fischer-Tropsch reactions. (9) The laboratory synthesis of straight chain fatty acids from C5 through C20 by Fischer-Tropsch processes. The amounts are usually in excess of the yields of aliphatic hydrocarbons. The chemical synthesis of glycerophospholipids including phosphatidylcholine. (10) The formation of liposomes, primarily, from phosphatidylcholine and the encapsulation within them of biopolymers. (11) Speculations on protocellular models of increasing complexity based on liposomes enclosing catalytic biomolecules. (12) Finally, some of the important problems remaining to be solved concerning the experimental approach to the study of the origin of life are briefly considered. It is hoped that in the next century, significant advances will be made in our understanding of the origin of life on Earth.

HCN emission from comet P/Swift-Tuttle 1992t

Abstract Observations are presented of the J = 1 → 0 line of HCN toward P/Swift-Tuttle 1992t with the NRAO 12 m telescope on 2, 3 and 31 December 1992, and of the J = 3 → 2 line on 30 December 1992. Upper limits were obtained for the 1 10 −1 11 transition of HDO. We measure an HCN production rate Q (HCN) = (2.9 ± 1.0) × 10 26 s −1 on 31 December 1992. A cometary water ice [D]/[H] ratio −3 is inferred from the HDO upper limit.

Comet Shoemaker‐Levy‐9 impact with Jupiter: Aeronomical predictions

The fragments of comet Shoemaker‐Levy‐9 will enter the atmosphere of Jupiter during July 20–26, 1994. Significant amounts of water vapor will be injected into the upper atmosphere of Jupiter either from the comet itself or from the lower atmosphere of Jupiter. The photochemistry of both the neutral gas and the ionosphere will be greatly altered by the influx of this water vapor or the atomic oxygen generated by the dissociation of the water. Enhanced abundances of H2O (or other species such as NH3) in the atmosphere above the homopause should persist for at least a year and should be globally distributed. The odd oxygen (i.e., O or H2O or OH) associated with the cometary water influx alters the ion chemistry by removing H+ ions, which also has the effect of reducing the ionospheric electron density because H+ is ordinarily the main ion species in the Jovian ionosphere. The density of H3+, both in the auroral and non‐auroral ionosphere, will also be reduced due to presence of water. This ion species has been detected spectroscopically at Jupiter, and a drop in its abundance should be detectable. The major ion species will become H3O+ which could reach a peak density as high as 104 cm−3 in the non‐auroral ionosphere and 105 cm−3 in the auroral ionosphere. It should be possible to detect this ion species spectroscopically from Earth‐based observatories.

Mass loading and velocity diffusion models for heavy pickup ions at comet Grigg‐Skjellerup

We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg‐Skjellerup (G‐S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity‐space diffusion of the implanted heavy ion population at G‐S using a transport equation including source, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injection, waves are generated at ω ∼ Ωi the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.

Implications of the high D/H ratio for the sources of water in Venus' atmosphere

THE high abundance ratio of deuterium to hydrogen in the atmosphere of Venus (120 times that on Earth) can he interpreted either as the signature of a lost primordial ocean1, or of a steady state in which water is continuously supplied to the surface of Venus by comets or volcanic outgassing, balancing loss through hydrogen escape2,3. New observations4–6 of a water concentration of only 30 parts per million in Venus' atmosphere imply that the residence time of water in the atmosphere, before it escapes to space, is short compared with the age of the Solar System, casting doubt on the primordial ocean hypothesis. But a recent theoretical reanalysis of collisional ejection7 has increased estimates of the deuterium escape efficiency by a factor of 10: this means that if the venusian water budget is in steady state, the D/H ratio of the source water must be 10–15 times higher than that on Earth, ruling out cometary water, whose D/H ratio is thought to be lower than this8. Here I suggest that these observations can be understood either as the result of continuous outgassing from a highly fractionated mantle source (such as might result from severe dessication of the mantle, or massive hydrogen escape early in the planet's history) or Rayleigh fractionation after massive outgassing from catastrophic resurfacing of the planet in the past 0.5–1 Gyr.

Pickup water group ions at comet Grigg‐Skjellerup

The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the ‘bow shock’ at comet Grigg‐Skjellerup. The results show that the distributions of ions are ring‐like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r−2 dependence.

Cometary water group ion fluxes calculated along the Giotto trajectory under observed solar wind conditions

Modelling of the pick-up ion fluxes for the water group (H 2O +, O +, OH +) far upstream of comet Halley during the Giotto fly-by is continued with additional refinements over the earlier attempts. These are: inclusion of the contributions to O from the dissociations of the minor cometary components CO and CO 2; using the in situ measured magnetic field and solar wind data to determine the direction of integration for the ion flux calculation. As in the previous works, the neutral densities that feed the ions are calculated with the Kepler trajectory model. Comparisons of the model results with the ions measured by EPONA on board Giotto demonstrate agreement in general for relative changes, at least on the inbound leg. This implies that much of the structure seen in the EPONA data is a result of changing solar wind conditions.

The effect of electron collisions on rotational populations of cometary water

The e-H2O collisional rate for exciting rotational transitions in cometary water is evaluated for conditions found in Comet Halley during the Giotto spacecraft encounter. In the case of the 0(00)-1(11) rotational transition, the e-H2O collisional rate exceeds that for excitation by neutral-neutral collisions at distances exceeding 3000 km from the cometary nucleus. The estimates are based on theoretical and experimental studies of e-H2O collisions, on ion and electron parameters acquired in situ by instruments on the Giotto and Vega spacecraft, and on results obtained from models of the cometary ionosphere. Thus, the rotational temperature of the water molecule in the intermediate coma may be controlled by collisions with electrons rather than with neutral molecules, and the rotational temperature retrieved from high-resolution IR spectra of water in Comet Halley may reflect electron temperatures rather than neutral gas temperatures in the intermediate coma.

Cometary water on Venus: Impact modelling and possible evidence

The present small amount of water in the atmosphere of Venus, in connection with the estimated short time scale of water loss from this planet, has lead to the hypothesis that the water concentration in the Venusian atmosphere is in a dynamical equilibrium, where the losses are counter-balanced by a suitable water source. The main candidate water sources are: (a) outgassing from the Venusian surface, due to volcanic activity and (b) cometary impacts. The lack of observational evidence of cometary impacts is usually attributed either to the rarity of such events or to the fact that presently their observational signature is not well understood. In this paper we report on a photographic evidence of a short duration dark feature on Venus, which was observed on 18 May 1988. After eliminating the possibilities of a film defect and an interference from an artificial Earth satellite or an interplanetary object, we conclude that this feature was the signature of an event that took place on the upper haze layer of the Venusian atmosphere. We propose that this event was actually the impact of a small (~1010 gr) comet-like object, consisting mainly of water, on Venus. This impact caused the temporary evaporation ot the sol H2SO4 particles of the upper haze layer and, consequently, a decrease of the albedo of the region around the point of entrance of the comet in the Venusian cloud layers. This region of lower albedo appears as a dark feature in the reported photograph. Our model accounts for the short duration of the feature as well as for its shape.

A model of the thermal conductivity of porous water ice at low gas pressures

This study is concerned with energy transfer in porous water ice at large Knudsen numbers of the sublimated gas. We present a formula for the effective thermal conductivity, λ eff, which is found to be valid for a wide range of material parameters, in particular also for high porosity materials, where other expressions for λ eff published in the literature lose their validity. It is found that above ∼ 190 K λ eff increases strongly with temperature and depends mainly on the average grain size, while at low temperatures it is primarily controlled by the value of the Hertz-factor. The influence of the porosity is relatively weak. The reliability of our expression for λ eff has been checked by comparing computed temperature profiles with the temperature recordings of comet simulation experiments. The predicted temperatures are in good agreement with the experimental results. Finally, our model is applied to a typical surface layer of a cometary nucleus. A strong temperature dependence of the effective thermal conductivity in cometary water ice is predicted.

Comparison of magnetosonic wave and water group ion energy densities at comet Giacobini-Zinner

Magnetosonic waves are generated by the instability of the new-born cometary water group ion distribution. We investigate the energy transfer between components of the water group ion velocity distributions measured at comet Giacobini-Zinner (GZ), caused by wave-particle scattering, and compare the thermal energy density of the ions with the energy density of the waves.

Cometary Water on Venus: Implications of stochastic impacts

The short lifetime of water on Venus suggests that the water abundance is in a near steady-state balance between loss by escape and replenishment by infall. In addition, the observed deuterium-to-hydrogen ratio on Venus is consistent with a steady state and does not necessarily imply a past water excess. We present results of a model incorporating a stochastic cometary source and nonthermal escape of hydrogen that produces the observed water abundance and D/H ratio. The stochastic variability of each of these quantities is shown to be large. We conclude that water on Venus is in a quasi-steady state mediated by large comet impacts and that the early history of water on the planet has been obscured by a history of random impacts.

Comment on the Paper “On the influx of small comets into the Earth's upper atmosphere II. Interpretation” by L. A. Frank, J. B. Sigwarth and J. D. Graven

The suggestion that the Earth is being bombarded by cometoids that subject it to an influx of 3×1011 water molecules/cm²‐sec encounters severe aeronomical problems. There is a serious disposal problem for water molecules deposited near 100 km. The implications for Venus and Mars also present serious problems. It is argued that the proposable is untenable.