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NASA NTRS2015

PAHs in the Ices of Saturn's Satellites: Connections to the Solar Nebula and the Interstellar Medium

Aliphatic hydrocarbons and PAHs have been observed in the interstellar medium (e.g., Allamandola et al. 1985, Pendleton et al. 1994, Pendleton & Allamandola 2002, Tielens 2013, Kwok 2008, Chiar & Pendleton 2008) The inventory of organic material in the ISM was likely incorporated into the molecular cloud in which the solar nebula condensed, contributing to the feedstock for the formation of the Sun, major planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Additional organic synthesis occurred in the solar nebula (Ciesla & Sandford 2012). Saturn's satellites Phoebe, Iapetus, and Hyperion open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS) on the Cassini spacecraft at Saturn. Phoebe (mean diameter 213 km) is a former TNO now orbiting Saturn (Johnson & Lunine 2005). VIMS spectral maps of Phoebe's surface reveal a complex organic spectral signature consisting of prominent aromatic (CH) and aliphatic hydrocarbon (=CH2, -CH3) absorption bands (3.2-3.6 micrometers). Phoebe is the source of a huge debris ring encircling Saturn, and from which particles ((is) approximately 5-20 micrometers size) spiral inward toward Saturn (Verbiscer et al. 2009). They encounter Iapetus and Hyperion where they mix with and blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aromatic CH is approximately 10 times as abundant as aliphatic CH2+CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, comet particles, and in carbonaceous meteorites (Cruikshank et al. 2014). A similar excess of aromatics over aliphatics is seen in the qualitative analysis of Hyperion and Phoebe itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 (is) approximately 4, which is larger than the value found in the diffuse ISM ((is) approximately 2-2.5). Insofar as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inventory, it can be key in understanding the content and degree of processing of that nebular material. A dynamical subset of TNOs define the Kuiper Belt, from which the short-period comets originate. Particles collected from comet 81P/Wild contain PAHs with an interstellar signature of deuterium. By inference, the PAHs contained in Phoebe and now dusted on the surfaces of two other Saturn satellites share that interstellar origin. There are other Phoebe-like TNOs that are presently beyond our ability to study in the organic spectral region, but JWST will open that possibility for a number of objects.

Solar nebula↗

NASA NTRS1976

The neutral atmospheres of comets

A comprehensive review of current knowledge about the neutral gas atmospheres of comets is given, with emphasis on the task of deriving the chemical composition of the cometary nucleus. The discussion centers on the following major topics: photometric and spectrophotometric observations of the neutral atmosphere and its constituent neutral radicals, molecules, and atoms; excitation processes responsible for the observed emissions from the neutral coma; the parent molecules of the neutral radicals; the gas-phase chemistry likely to take place in the dense inner coma; dynamic models (both exospheric and hydrodynamic) of the neutral atmosphere; and the chemical compositions of comets. The primary conclusions reached on the basis of all the data considered are that: (1) the chemical composition of the dust component is most likely to be the same as that of the stream meteoroids which burn up on hitting the earth's upper atmosphere; (2) the dominant icy component in most comets is probably H2O; and (3) anomalous abundances of other volatile species are present in some comets.

Mendis, D. A.↗

NASA NTRS1995

Dynamics of cometary dust

This paper summarizes recent progress in our understanding of the morphological diversity of dust comets. This diversity is a product of dust emission from discrete active areas on the nucleus surface and provides information on the comet's rotation state and source function. Advances in computer simulations of dust coma morphology are described and the diagnostic properties of various dusty features are emphasized. Also addressed are some of the issues of dust tail morphology and particle fragmentation. Finally, lessons learnt from investigations of the dust population of comet Shoemaker-Levy 9 are discussed and implications for physical studies of the comet's collision with Jupiter are identified.

comets↗

NASA NTRS2005

Will Deep Impact Make a Splash?

Recent cometary observations from spacecraft flybys support the hypothesis that short-period comets have been substantially modified by the presence of liquid water. Such a model can resolve many outstanding questions of cometary dynamics, as well as the differences between the flyby observations and the dirty snowball paradigm. The model also predicts that the Deep Impact mission, slated for a July 4, 2005 collision with Comet Temple-1, will encounter a layered, heterogenous nucleus with subsurface liquid water capped by dense crust. Collision ejecta will include not only vaporized material, but liquid water and large pieces of crust. Since the water will immediately boil, we predict that the water vapor signature of Deep Impact may be an order of magnitude larger than that expected from collisional vaporization alone.

Sheldon, Robert B.↗

NASA NTRS2001

An Integrated Modeling Study for Coordinated Observations of H, O, OH, and H2O(+) Emissions in the Coma and Ion Tail of the Comet Hale-Bopp

This project has two overall objectives. One objective is to advance our general understanding of both the comet neutral atmosphere and the cometary plasma in the atmosphere and ion tall. The other objective is to obtain specific key information about comet Hale-Bopp that is generally important for Hale-Bopp studies. The primary emphasis in this project is to analyze, in a self-consistent manner, excellent quality high resolution image and line profile observations obtained by the University of Wisconsin for H, O, OH, and H2O+ emissions from the inner coma, outer coma, and ion tail of Hale-Bopp. The information on the spatial and velocity distributions of H2O neutral and ionized photo-products in the inner coma, outer coma, and in the H2O+ ion tail is of substantial and direct importance in the development of an integrated understanding of the complex structure and dynamics of the neutral and plasma species in the atmosphere of Hale-Bopp in particular and comets in general. The H2O production rate of Hale-Bopp is determined and, together with the other information related to the structure and dynamics of the neutral and plasma atmospheres obtained in this study, provide critical information important for a wide variety of research conducted by other groups.

Smyth, William H.↗

NASA NTRS1993

(abstract) Multispecies Gas Flows in the Interior of Comets

A theoretical comet nucleus model has been written which employs 3 volatiles: CO, CO 2 , and H 2 O. It allows variations for equations of state: the Clausius/Clapeyron equation as well as other equations based on the latest thermodynamic data: the Brown/Ziegler relationship for CO 2 and CO at low pressure and the Lowe expression for water vapor at low pressure. It allows for variations in the physics of the outflow, from an essentially throttling mechanism (pure sublimation to space), to the Knudsen regime, to the Dusty-Gas Dynamic model. Variations in several parameters are explored. Results are compared with data from actual comets. None of the variations in parameter space produce a result that compares well with the comet data. All models yield a total gas production between 10&sup27; and 10&sup28; per second (low). This suggests that secondary mechanisms peripheral to this modeling effort, such as the opening and closing of cracks, a conductivity expression which depends upon porosity as well as temperature, etc., may play a more important role in the cometary efflux process than previously thought.

comets↗

NASA NTRS1997

Lunar and Planetary Science XXVIII

The present conference discusses such topics as density crossovers in lunar picrites, the geology of the Cassini impact basin, Mars, nanobacteria in carbonates, the properties of shocked aerogels, a chemical model of Comet Halley, lunar mascons, the impact evolution of icy regoliths, the geology of the Venera 8 landing site, the photogeologic mapping of northern Venus, HST observations of Mars, observational constraints on the rotational dynamics of Mars, and primordial magnetic field measurements from the moon. Also discussed are models of the S2 fluorescence spectra of comets, Martian crater ejecta, the heights of Venusian steep-sided domes, cloud-climate interactions on Venus, the Humorum basin geology from Clementine data, an early Amazonian lake in the Gale crater of Mars, nebular fractionations and Mn-Cr systematics, the Rock Chipper planetary surface sample collection, Mariner 10 stereo images of Mercury, remote and local stresses and Calderas on Mars, the electrostatic charging of saltating particles, SO2 detected on Callisto, the Mars Explorer Planetary Data System, an assessment of explosive venting on Europa, the sequential faulting history of the Mars Valles Marineris, a search for Martian sediments, the composition and internal structure of Europa, long-term and 'diurnal' tidal stresses on Europa, and episodic greenhouse climates on Mars.

Source record↗

NASA NTRS1986

Space observations of comet Halley

Observations of comet Halley by the Solar Maximum Mission, the International Ultraviolet Explorer, the Pioneer Venus Orbiter, the Dynamics Explorer 1, and by sounding rockets are reviewed. Visual images taken through the wide-band blue filter showed an extended coma elongated approximately in the antisolar direction. Preperihelion observations show definite evidence for both a plasma and a dust tail. The ultraviolet observations indicate that an extensive hydrogen cloud extends to well over 10 billion km from the nucleus. The derived water production rate for the comet showed very large changes around the time of perihelion. The rocket observations showed strong H I, O I, and C I emissions, several CO fourth positive bands, and the C II emissions at 1335 A extending into the tail.

Brandt, J. C.↗

NASA NTRS1986

Statics and dynamics of Giacobini-Zinner magnetic tail

The data gathered by the International Cometary Explorer during its traversal of Comet Giacobini-Zinner's tail are subjected to stress balance requirements, yielding estimates of unmeasured quantities. It is noted that the comet's tail is embedded in an ionosheath whose static pressure is nearly equal to the solar wind stagnation pressure, leading to a large lobe field strength. A systematic variation is found in the ion temperature across the tail, implying the variation of the pickup velocity of new ions. Axial stress balance yields an expression for the strength of the lobe field which reveals weak variation with axial distance.

Siscoe, G. L.↗

NASA NTRS1986

Dynamics of cometary plasma tails

Cometary plasma tail data obtained before and after the current appearance of Halley's Comet are reviewed. The relation between the formation of plasma tails and comet/solar-wind interactions is discussed. The ion rays and helical waves of the plasma tail are described. The disconnection events and interaction of bright comets with the IMF, associated with the current appearance of Comet Halley, are examined. Consideration is given to the cometary magnetic barriers observed by the Vega-1 and Vega-2 spacecraft.

Niedner, Malcolm B., Jr.↗

NASA NTRS1975

Dynamical and photometric investigation of cometary type 2 tails

The absolute calibration of the photometric profile of the antitail of Comet Kohoutek 1973 XII on plates taken with the Curtis Schmidt telescope of the University of Michigan at the Cerro Tololo Inter-American Observatory is described in detail. The formula for the determination of the air mass, and the correction for atmospheric absorption and for the loss of light due to vignetting are included. The calibration stars were used to derive the coefficients converting the relative intensity scale to the absolute surface-brightness units. The extensive results of the study of the orbital evolution of vaporizing dust particles are listed in a tabular form. Gradual evaporation from the surface of a particle results typically in its expulsion from the solar system. The properties of the particle and the elements of its orbit at expulsion are given as functions of the particle's properties and orbit before appreciable evaporation commenced. Also given are circumstances at an encounter of an expelled particle with the earth as a function of the particle's properties. A few specific cases are represented graphically.

Sekanina, Z.↗

NASA NTRS1979

Interplanetary dust

Progress in the study of interplanetary dust during the past four years is reviewed. Attention is given to determinations of the relative contributions of interstellar dust grains, collisional debris from the asteroid belt and short-period comets to the interplanetary dust cloud. Effects of radiation pressure and collisions on particle dynamics are discussed, noting the discovery of the variation of the orbital parameters of dust particles at 1 AU with size and in situ measurements of dust density between 0.3 and 5 AU by the Helios and Pioneer spacecraft. The interpretation of the zodiacal light as produced by porous absorbing particles 10 to 100 microns in size is noted, and measurements of the Doppler shift, light-producing-particle density, UV spectrum, photometric axis and angular scattering function of the zodiacal light are reported. Results of analyses of lunar rock microcraters as to micrometeoroid density, flux rate, size distribution and composition are indicated and interplanetary dust particles collected from the stratosphere are discussed. Findings concerning the composition of fragile meteoroid types found as cosmic spherules in deep sea sediments are also presented.

Brownlee, D. E.↗

NASA NTRS1982

The cometary atmosphere and its interaction with the solar wind

The present state of knowledge of cometary atmospheres and their interaction with the solar winds is assessed. Current models of the magnetospheres and ionospheres of comets are based on either an atmospheric chemistry approach, minimizing the effects of atmospheric dynamics and thermodynamics, while the other method takes the exact opposite viewpoint and roles and significance are reversed. A major difficulty in completing the models is a lack of data on the chemical composition of the cometary nucleus, although UV observations have revealed the H, C, O, and S elemental abundance ratios. It is suggested that further studies of collisional ionization by energetic electrons be performed to characterize processes in the cometary atmosphere. Hydronium may be the dominat molecule in the inner coma, if the assumption that the ionosphere is a magnetic field-free cavity separated from the solar wind by an unyielding tangential discontinuity surface is accurate. Particular observations and theoretical attention are recommended for the solar wind interaction in the intermediate range, i.e., 2.5-5 AU, when the cometary atmosphere develops.

Mendis, D. A.↗

NASA NTRS1983

Hydrodynamic models of the cometary atmosphere

Self-consistent transonic multifluid solutions for the dynamical and thermal structure of the two-phase dusty gas atmosphere of a comet containing a dirty H2O-clathrate ice nucleus have been obtained. This was done by solving a simultaneous set of differential equations representing conservation of number density, momentum, and energy, as well as the transfer of solar radiation responsible for the major photolytic processes and nuclear heating. The results presented include velocity and Mach number profiles of heavy species and the velocity of dust within the subsonic and supersonic regions, temperature profiles of the dust and gas in the supersonic region, and radial profiles of electron and ion densities in the collision-dominated region.

Mendis, D. A.↗

NASA NTRS1987

Modeling Halley before and after the encounters

Numerical models developed prior to the 1986 spacecraft flybys at comet P/Halley described anticipated dust and gas environments. Predicted values for gas production, for dynamic ranges of gas and dust experiments, and for likely spacecraft effects were matched during the flybys well within expected margins; in particular the dust-impact-generated attitude disturbance experienced by Giotto several seconds before closest approach was within the envelope of expectations from the models. Three major areas are identified in which 1986 results for Halley might be used to improve the models, namely (1) gas production values from earth-based and in situ observations, (2) dust flux and fluence values in the coma from three spacecraft, and (3) dust size distributions from the in situ data which require the presence of numerous particles at masses less than 10 to the -17th kg.

Divine, N.↗

NASA NTRS2001

Impact and Collisional Processes in the Solar System

In the past year, we have successfully developed the techniques necessary to conduct impact experiments on ice at very low temperatures. We employ the method of embedding gauges within a target to measure the shock wave and material properties. This means that our data are not model dependent; we directly measure the essential parameters needed for numerical simulations of impact cratering. Since then we have developed a new method for temperature control of icy targets that ensures temperature equilibrium throughout a porous target. Graduate student, Sarah Stewart-Mukhopadhyay, is leading the work on ices and porous materials as the main thrust of her thesis research. Our previous work has focused on icy materials with no porosity, and we propose to extend our research to include porous ice and porous ice-silicate mixtures. There is little shockwave data for porous ice, and none of the data was acquired under conditions applicable to the outer solar system. The solid ice Hugoniot is only defined for initial temperatures above -20 C. Our program uniquely measures the properties of ice at temperatures directly applicable to the solar system. Previous experiments were conducted at ambient temperatures soon after removing the target from a cold environment, usually just below freezing, or in a room just below freezing. Since ice has an extremely complicated phase diagram, it is important to conduct experiments at lower temperatures to determine the true outcome of impacts in the outer solar system. This research is complementary to other programs on icy materials. Our work focuses on the inherent material properties by measuring the shock wave directly; this complements the macroscopic observations and immediately provides the parameters necessary to extend this research to the gravity regime. Our numerical simulations of impacts in porous ice under very low gravity conditions, such as found on comets, show that the final crater size and shape is very dependent on the dynamic strength of the material.

Ahrens, Thomas J.↗

NASA NTRS1988

Cometary H2(+) and solar wind He(2+) dynamics across the Halley cometopause

Two Mass/Charge (M/Q) of about 2 distributions were observed by the Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) during the encounter with comet Halley. The first, present throughout the encounter, is identified as solar wind He(2+). The second, detectable only within about 600,000 km of the nucleus, is identified as H2(+) (produced primarily by dissociation and ionization of cometary H2O). When these two distributions are separated, distinct differences in their dynamics are apparent, particularly in the density and velocity profiles near the 'cometopause'.

Fuselier, S. A.↗

NASA NTRS1977

Dust evolution from comets

The studies of the evolution of cometary debris are reviewed. The subject is divided into three major sections: (1) the developments in the immediate vicinity of the cometary nucleus, which is the source of the dust; (2) the formation of the dust tail; and (3) the blending of the debris with the dust component of interplanetary matter. The importance of the physical theory of comets is emphasized for the understanding of the early phase of the evolution of cometary dust. A physico-dynamical model designed to analyze the particle-emission mechanism from the distribution of light in the dust tails is described and the results are presented. Increased attention is paid to large particles because of their importance for the evolution of the zodiacal cloud. Finally, implications are discussed for the future in situ investigations of comets.

Sekanina, Z.↗