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NASA NTRS1974

Optical image of a cometary nucleus: 1980 flyby of Comet Encke

The feasibility was investigated of obtaining optical images of a cometary nucleus via a flyby of Comet Encke. A physical model of the dust cloud surrounding the nucleus was developed by using available physical data and theoretical knowledge of cometary physics. Using this model and a Mie scattering code, calculations were made of the absolute surface brightness of the dust in the line of sight of the on-board camera and the relative surface brightness of the dust compared to the nucleus. The brightness was calculated as a function of heliocentric distance and for different phase angles (sun-comet-spacecraft angle).

Wells, W. C.↗
NASA NTRS2016

Abundant Solar Nebula Solids in Comets

Comets have been proposed to consist of unprocessed interstellar materials together with a variable amount of thermally annealed interstellar grains. Recent studies of cometary solids in the laboratory have shown that comets instead consist of a wide range of materials from across the protoplanetary disk, in addition to a minor complement of interstellar materials. These advances were made possible by the return of direct samples of comet 81P/Wild 2 coma dust by the NASA Stardust mission and recent advances in microscale analytical techniques. Isotopic studies of 'cometary' chondritic porous interplanetary dust particles (CP-IDPs) and comet 81P/Wild 2 Stardust samples show that preserved interstellar materials are more abundant in comets than in any class of meteorite. Identified interstellar materials include sub-micron-sized presolar silicates, oxides, and SiC dust grains and some fraction of the organic material that binds the samples together. Presolar grain abundances reach 1 weight percentage in the most stardust-rich CP-IDPs, 50 times greater than in meteorites. Yet, order of magnitude variations in presolar grain abundances among CP-IDPs suggest cometary solids experienced significant variations in the degree of processing in the solar nebula. Comets contain a surprisingly high abundance of nebular solids formed or altered at high temperatures. Comet 81P/Wild 2 samples include 10-40 micron-sized, refractory Ca- Al-rich inclusion (CAI)-, chondrule-, and ameboid olivine aggregate (AOA)-like materials. The O isotopic compositions of these refractory materials are remarkably similar to their meteoritic counterparts, ranging from 5 percent enrichments in (sup 16) O to near-terrestrial values. Comet 81P/Wild 2 and CP-IDPs also contain abundant Mg-Fe crystalline and amorphous silicates whose O isotopic compositions are also consistent with Solar System origins. Unlike meteorites, that are dominated by locally-produced materials, comets appear to be composed of materials that were formed across a wide swath of the early protoplanetary disk.

Messenger, S.↗
NASA NTRS1995

Dust as the cause of spots on Jupiter

The long-lived spots caused by the impact of fragments of Comet S-L 9 on Jupiter can be understood if clouds of dust are produced by the impact. These clouds reside in the stratosphere, where they absorb visible light that would ordinarily reflect from the cloud deck below, and reflect radiation at infrared wavelengths that would ordinarily be absorbed by atmospheric methane. Here we show that, provided that the nucleus of a fragment is composed substantially of silicates and has a diameter greater than about 0.4 km, dust in the required amounts will condense from the hot gas composed of cometary and Jovian material ejected from the site where the fragment entered, and the dust will be suspended in the stratosphere for long periods. Particles about 1 micron in radius can explain both the optical properties and longevities of the spots. According to our model, a silicate band should be present in the 10 - micron spectra of the spots.

Field, G. B.↗
NASA NTRS2017

Search for Primitive Matter in the Solar System

Recent astronomical observations and theoretical modeling led to a consensus regarding the global scenario of the formation of young stellar objects (YSO) from a cold molecular cloud of interstellar dust (organics and minerals) and gas that, in some cases, leads to the formation of a planetary system. In the case of our Solar System, which has already evolved for approximately 4567 Ma, the quest is to access, through the investigation of planets, moons, cometary and asteroidal bodies, meteorites, micrometeorites, and interplanetary dust particles, the primitive material that contains the key information about the early Solar System processes and its evolution. However, laboratory analyses of extraterrestrial samples, astronomical observations and dynamical models of the Solar System evolution have not brought yet any conclusive evidence on the nature and location of primitive matter in the Solar System, preventing a clear understanding of its early stages.

Libourel, G.↗
NASA NTRS1999

Optical Studies of Active Comets

This grant was to support optical studies of comets close enough to the sun to be outgassing. The main focus of the observations was drawn to the two extraordinarily bright comets Hyakutake and Hale-Bopp, but other active comets were also studied in detail during the period of funding. Major findings under this grant include: (1) Combined optical and submillimeter observations of the comet/Centaur P/Schwassmann-Wachmann 1 were used to study the nature of mass loss from this object. (2) We modelled the rotation of cometary nuclei using time-resolved images of dust jets as the primary constraint. The fortuitous apparition of comet Hale-Bopp allowed us to see the full complexity of this. (3) We obtained broad-band optical images of several comets for which we subsequently attempted submillimeter observations, in order to test and update the cometary ephemerides. This step is essential to the success of submillimeter observations, which are taken by instruments with narrow fields of view and without benefit of optical confirmation of the pointing. (4) Broad-band continuum images of a set of weakly active comets and, apparently, inactive asteroids were obtained in BVRI using the University of Hawaii 2.2-m telescope. These images were taken in support of a program to test the paradigm that many near-Earth asteroids might be dead or dormant comets. We measured coma vs. nucleus colors in active comets (finding that coma particle scattering is different from, and cannot be simply related to, nucleus color). (5) We obtained spectroscopic observations of weakly active comets and other small bodies limits to outgassing from these bodies, aided by the high (40,000) spectral resolution of HIRES.

Jewitt, David↗
NASA NTRS1981

On the electrostatic charging of the cometary nucleus

The paper demonstrates that a water-dominated cometary nucleus is not shielded from solar UV radiation or solar wind by protective atmospheres beyond a heliocentric distance of 5 AU. It is also shown that submicron dust particles can be electrostatically levitated and blown off the cometary surface. An upper limit on the total mass that can be lost from the sunlit hemisphere is derived using two solar wind models, and the dust mantle erosion is estimated at 5-12 cm. It is noted that the erosion is comparable to the mass of dust blown off a typical medium bright comet by the viscous drag of the sublimating gases during perihelium passage between 3.0 and 0.5 AU, and it is therefore proposed, that the surface dust blow-off be included in the current balance equation.

Mendis, D. A.↗
NASA NTRS2000

Recognizing Leonid Meteoroids Among The Collected Startospheric Dust

Three chemical groups of primary "silicate" spheres <30 micron in diameter of cometary origin were collected in the lower stratosphere between 1981 May and 1994 July. The "silicate" sphere abundances represent an annual background from contributions by sporadic meteor and weak annual meteor shower activities. During two collection periods, from 06/22 until 08/18, 1983 (U2015), and from 09/15-12/15, 1981 (W7027/7029), a higher number of spheres was collected compared to other periods of the year represented by the other collectors studied here. This study links two different data sets, viz. the NASA/JSC Cosmic Dust Catalogs and peak activities of annual meteor showers, and identified high-velocity cometary sources for collected stratospheric "silicate" spheres. The majority of spheres on flag U2015 may originate from comet P/Swift-Tuttle (Perseids), while the majority of spheres on flags W7027/7029 could be from comet P/Halley (Orionids) or comet P/Tempel-Tuttle (Leonids). Variations in relative proportions of the Mg,Si,Ca +/- Al, Mg,Si +/- Fe and Al,Si,Ca spheres may offer a hint of chemical differences among high-velocity comets. Proof for the findings reported here might be obtained by targeted cosmic dust collections in the lower stratosphere including periods of meteor shower and storm activity.

Rietmeijer, Frans J. M.↗
NASA NTRS1972

The gegenschein and distribution of interplanetary dust

Investigations to determine the source of the gegenschein indicated: (1) The gegenschein is an effect, not a thing, and is due to an increase in the reflectivity, near opposition, of dust particles in heliocentric orbits, and is not due to an accumulation of material near the earth. (2) The density of the reflecting particles increases outside the earth's orbit; therefore, it can be explained by dust produced by collisions between asteroids. A cometary origin for the material is considered unlikely.

Roosen, R.↗
NASA NTRS1977

Monochromatic brightness variations of comets. II - Core-mantle model

A class of comets, of which Comet Kohoutek (1973f) is typical, show total as well as monochromatic brightness asymmetries about perihelion. They are fainter after perihelion than before at the same heliocentric distance. A model of the cometary nucleus consisting of a growing nonvolatile dust mantle surrounding a volatile icy core is used to discuss this phenomenon. Numerical results are obtained for Comet Kohoutek (1973f). It is found that dust mantles of thickness in the range of 10-75 cm can be grown by perihelion passage for various values of the thermal conductivity of the dust if there is no substantial dust blow-off by the sublimating volatiles. The thermal conductivity of the dust mantle is quite small and is dominated by 'radiative conductivity' for heliocentric distances less than about 2 AU. Since the radiative conductivity is larger for larger grain size, the thickest mantle corresponds to coarsest matrix. The production rate of the 'parent-molecules' as well as the monochromatic brightness of their 'daughter' products increases less steeply than in the mantleless case, as the comet approaches perihelion.

Mendis, D. A.↗
NASA NTRS1986

Detection of CN emission in Comet P/Halley

The first detection of the CN 3883 A band in emission in Comet P/Halley (1982) in February 1985 is reported. The detection technique, which is described, used blind offsets from secondary reference stars to the comet's predicted position, and involved plotting the comet's track on an enlargement of an appropriate section of the Palomar Sky Survey E plates. The strong solar continuum in the resulting cometary spectrum demonstrates the dominance of the dust reflection spectrum and the reflection from the nucleus. The 3883 A feature is identified with the v = 0 band sequence of the blue system of CN; no other significant features were identified in the spectrum.

Wyckoff, S.↗
NASA NTRS1986

Comet Halley - Chapter I in cometary exploration

The information gained on the Comet Halley by the international probe studies is presented. The new information includes data on the true size and shape of the cometary nucleus and the mass of its dust grains, the chemical composition of the nucleus, and the characteristics of the 'bow wave' of the comet. The requirements of future missions for solving the many questions that are still open are discussed.

Newburn, Ray L., Jr.↗
NASA NTRS1998

The Solar Wind

The first evidence of the solar wind was provided through observations of comet tail deflections by L. Biermann in 1951. A cometary ion tail is oriented along the difference between the cometary and solar wind velocities, whereas the dust tail is in the antisunward direction; the ion tail directions demonstrated the existence of an outflow of ionized gas from the Sun (the solar wind) and allowed estimates of solar wind speed. Spacecraft observations have now established that at 1 AU the solar wind has a typical ion number density of about 7 /cc and is composed by number of about 95% protons and 5% Helium, with other minor ions also present. The solar wind as observed at 1 AU in the ecliptic has speeds typically in the range 300-700 km/ s. At such speeds ions travel from the Sun to 1 AU in from 2.5 to 6 days. The impact of the solar wind on planets with magnetic fields (Earth, Jupiter, Saturn, Uranus, Neptune) causes phenomena such as magnetospheres, aurorae, and geomagnetic storms, whereas at objects lacking magnetospheres (Mars, Venus, comets), atmospheric neutrals undergo charge exchange and are picked up by the solar wind flow. The solar wind also shields the Earth from low energy cosmic rays, and is responsible for the existence of the anomalous component of the cosmic rays a low energy component that is created locally rather than in the galaxy. Presented here is a brief introduction to the solar wind and a description of some current topics of research. Solar wind properties vary a great deal due to the changing magnetic structure on the Sun.

Goldstein, B. E.↗
NASA NTRS1988

The 2.5-12 micrometers spectrum of comet Halley from the IKS-VEGA experiment

The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an "imaging channel" designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5-5 and 6-12 micrometers, respectively, equipped with circular variable filters of resolving power approximately 50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence (D approximately 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D-1. The 2.5-5 micrometers IKS spectra definitely show strong narrow signals at 2.7 and 4.25 micrometers, attributed to the nu 3 vibrational bands of H2O and CO2, respectively, and a broader signal in the region 3.2-3.5 micrometers, which may be attributed to CH-bearing molecules. All these signals present the expected D-1 intensity variation. Weaker emission features at 3.6 and 4.7 micrometers could correspond to the nu 1 and nu 5 bands of H2CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H2O and CO2, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of approximately 10(30) sec-1 for H2O, approximately 2.7 x 10(28) sec-1 for CO2, approximately 5 x 10(28) sec-1 for CO, and 4 x 10(28) sec-1 for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is approximately 9 x 10(29) sec-1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 micrometers emission is attributed to C-H stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 micrometers, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 micrometers, as well as a narrow emission at 3.15 micrometers, which follow well the D-1 intensity variation, might be due to water ice. Emission at 2.8 micrometers is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6-12 micrometers spectrum does not show any molecular emission, nor emission in the 7.5-micrometers region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 micrometers, which suggests the presence of olivine.

NASA Discipline Number 52-10↗
NASA NTRS2009

Evidence for Changes in 81PIWild 2 Organic Matter Since Collection and Comparison of 82PIWild 2 and IDP Organic Matter to Access the Thermal Effects of Aerogel Capture

NASA s Stardust spacecraft collected cometary material during its passage through the dust coma of comet 81P/Wild 2 on January 2nd, 2004 and delivered this material to Earth on January 15th 2006. The first fragment we analyzed during the preliminary examination was partially vaporized by the X-ray beam. The carbonaceous material that survived was re-analysis approx.2 months later and the carbon spectrum for this material had significantly changed from what we first observed.. We have observed similar changes to the carbonaceous matter in some interplanetary dust particles ( IDPs). Some of the 81P/Wild 2 organic matter volatilized upon impact with the aerogel as observed using IR spectroscopy where IR spectra were collected several mms away from sample tracks [1]. The time-temperature profile experienced by any particular 81P/Wild 2 grain during aerogel capture is not known, although Brownlee, et al. suggest that fine-grained materials, <1 micron in size, fragmented and then partially vaporized during collection, while particles much larger then 1 micron in size were captured intact [2]. Nearly all organic matter is subject to thermal alteration. To assess the heating and alteration experienced by the 81P/Wild 2 organic matter during capture we are comparing 81P/Wild2 organic matter with IDP organic matter where we have evidence of heating in the IDP [3,4].

Wirick, S.↗
NASA NTRS2013

C/N and other Elemental Ratios of Chondritic Porous IDPS and a Fluffy Concordia Micrometeorite

Chondritic porous interplanetary dust particles (CP-IDPs) may be cometary in origin [1], as may ultracarbona-ceous (UCAMMs) [2] and 'fluffy' [3] micrometeorites from the Concordia collection. They are all rich in organics, which can rim grains and may have helped glue grains together during accretion [4]. The organics also contain nitrogen the input of which to Earth has potential biological importance. We report C/N ratios, and other properties of CP-IDPs and a Concordia fluffy microme-teorite.

Smith, T.↗
NASA NTRSDate not supplied

Bulk Oxygen Isotopic Compositions of Anhydrous Interplanetary Dust Particles: Indication of an 16O-Poor Reservoir In The Outer Solar System

The oxygen isotopic ratios of ancient solar system materials reflect the isotopically distinct reservoirs in which they formed or were altered. These compositions are generally explained by mixing of material derived from an 16O-rich reservoir and material from an 16O-poor reservoir thought to be isotopically heavy H2O. Evidence of this 16O-poor reservoir is scarce and includes cosmic symplectite (COS), probable silicates in an interplanetary dust particle (IDP), and sulfates in a cometary xenolith. These materials have 17O and 18O enrichments between ~150–200 ‰, which agree with predictions for self-shielding and photodissociation of CO. The ‘cometary’ 16O-poor materials indicate alteration or formation in the outer solar system. The bulk O isotopic compositions of IDPs thought to have cometary origins may also harbor evidence of this primordial 16O-poor reservoir. Here we report the bulk O compositions of anhydrous IDPs and discuss the nebular reservoirs from which they derived

A N Nguyen↗
NASA NTRS2004

Astrobiology of Comets

We review the current state of knowledge concerning microbial extremophiles and comets and the potential significance of comets to Astrobiology. We model the thermal history of a cometary body, regarded as an assemblage of boulders, dust, ices and organics, as it approaches a perihelion distance of - IAU. The transfer of incident energy from sunlight into the interior leads to the melting of near surface ices, some under stable porous crust, providing possible habitats for a wide range of microorganisms. We provide data concerning new evidence for indigenous microfossils in CI meteorites, which may be the remains of extinct cometary cores. We discuss the dominant microbial communities of polar sea-ice, Antarctic ice sheet, and cryoconite environments as possible analogs for microbial ecosystems that may grow in sub-crustal pools or in ice/water films in comets.

Hoover, Richard B.↗
NASA NTRS2017

Ion Microprobe Measurements of Comet Dust and Implications for Models of Oxygen Isotope Heterogeneity in the Solar System

The oxygen isotopic compositions of anhydrous minerals in carbonaceous chondrites reflect mixing between a O-16-rich and O-17, O18-rich reservoir. The UV photodissociation of CO (i.e. selfshielding) has been proposed as a mass-independent mechanism for producing these isotopically distinct reservoirs. Self-shielding models predict the composition for the CO gas reservoir to be O-16-rich, and that the accreting primordial dust was in isotopic equilibrium with the gaseous reservoir [1, 2]. Self-shielding also predicts that cometary water, presumed to represent the O-17, O-18-rich reservoir, should be enriched in O-17 and O-18, with compositions of 200 -1000per mille, and that the interaction with this O-17, O-18-rich H2O reservoir altered the compositions of the primordial dust toward planetary values. The bulk composition of the solar nebula, which may be an approximation to the 16O-rich gaseous reservoir, has been constrained by the Genesis results [3]. However, material representing the O-17, O-18-rich end-member is rare [4], and dust representing the original accreting primordial dust has been challenging to conclusively identify in current collections. Anhydrous dust from comets, which accreted in the distal cold regions of the nebula at temperatures below approximately 30K, may provide the best opportunity to measure the oxygen isotope composition of primordial dust. Chondritic porous interplanetary dust particles (CP-IDPs) have been suggested as having cometary origins [5]; however, until direct comparisons with dust from a known comet parent body were made, link between CP-IDPs and comets remained circumstantial. Oxygen isotope analyses of particles from comet 81P/Wild 2 collected by NASA's Stardust mission have revealed surprising similarities to minerals in carbonaceous chondrites which have been interpreted as evidence for large scale radial migration of dust components from the inner solar nebula to the accretion regions of Jupiter- family comets [6]. These studies have been largely focused on the coarse-grained terminal particles extracted from aerogel collectors; hypervelocity capture into aerogel resulted in fine-grained material that was melted and intimately mixed with the SiO2 capture medium. Hypervelocity impacts into Al foils surrounding the aerogel tiles produced impact craters that captured material from the impactor without significant oxygen contamination, allowing for analysis of both the coarse and fine-grained components of the Wild 2 dust. To date, no particles with definitive hydrated mineralogy have been observed in Stardust samples, though this may be a result of alteration due to hypervelocity capture. High-carbon hydrated CS-IDPs have been suggested as resulting from the aqueous alteration of CP-IDPs [7], and may retain evidence for interaction with O-17, O-18-enriched "cometary" water predicted by CO self-shielding. Here we present results of oxygen isotope measurements of twelve Stardust foil craters and four C-rich hydrated IDPs [8, 9], and discuss implications for models of oxygen isotope heterogeneity in the early solar system.

Snead, C. J.↗