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At least 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Distributed coma sources and the CH4/CO ratio in Comet Halley

Early analyses of the Ion Mass Spectrometer (IMS) data from the Giotto flyby of Comet P/Halley indicated significant abundances of CH(+) sub n (n = 1 to 4). The source of these ions was assumed to be frozen CH4 in the nucleus. An abundance of about 2 percent CH4 was consistent with this interpretation, resulting in a ratio of CH4/CO that is greater than the predicted limits for interstellar clouds or the solar nebula. However, subsequent analyses of data from the Giotto Neutral Mass Spectrometer indicate distributed sources of CO and H2CO in the coma that are most likely associated with organic (CHON) particles, rich in CH-bearing compounds that decay and produce CH(+) sub n species. A model is presented that qualitatively accounts for the measured spatial distribution of CO and H2CO, and indicates that most of the CH(+) sub n deduced from the Giotto IMS data may originate from organic compounds in the coma dust and not from volatiles released directly from the nucleus. As a consequence, the absence of CH4 in the icy phase of the nucleus is consistent with the observations with an upper limit of about 0.5 percent to the ratio CH4/CO. This is an important criterion for theories of comet formation.

Boice, D. C.↗
NASA NTRS1990

Comet dust as a source of amino acids at the Cretaceous/Tertiary boundary

It is suggested here that the large amounts of apparently extraterrestrial amino acids detected recently in rocks at the K/T boundary at Stevns Klint, Denmark were actually deposited with the dust from a giant comet trapped in the inner solar system, a fragment of which comprised the K/T impactor. Amino acids or their precursors in the comet dust would have been swept up by the earth both before and after the impact, but any conveyed by the impactor itself would have been destroyed. The observed amino acid layers would thus have been deposited without an impact.

Zahnle, Kevin↗
NASA NTRS1990

The production rate and spatial distribution of H2O for Comet P/Halley

Spectroscopic observations of the O I 1D lines for Comet P/Halley are used to derive water-production rates; 43 exposures during the period August 1985 to June 1986 are reduced. The reduction procedure involves sky subtraction, ratios with solar analog comparison stars, subtraction of contaminating NH2 emissions, and absolute flux calibration. The flux collected within the 2.5-arcsec-wide x 151-arcsec-long integration aperture is converted to a total comet O I 6300-A luminosity using a cometary gas outflow model. Water-production rates for 10 preperihelion dates and seven postperihelion dates are derived. An error budget yields an estimated uncertainty of + 30 percent in H2O production rates with a maximum error of + or - 50 percent. For the preperihelion period, a good fit of the water-production rate as a function of heliocentric distance can be obtained as Q(H2O) = 4.36 x 10 to the 29th x r exp -2.54. A postperihelion enhancement by about a factor of 1.8, which appears to dissipate by June 1986, is observed. Several values of enhanced water production agree well with the observed light curve of P/Halley. Comparison of H2O production rates with those deduced by other methods shows quite reasonable agreement.

Fink, Uwe↗
NASA NTRS1991

The spectrum of Comet Austin from 910 to 1180 A

A spectrum of Comet Austin (1988 c1) has been obtained from 910 to 1180 A. Three bright emission lines were detected, including a forbidden oxygen line (1128 A), which are attributable to radiative pumping of neutral oxygen by solar Lyman-beta. The relative strengths of the observed features should prove to be a useful diagnostic of the physical conditions and radiation fields in cometary comae. In addition, the absence of strong spectral features from highly volatile species such as He, Ar, or N2 can be used to place constraints on the thermal environment under which the comet was formed and has been processed.

Green, James C.↗
NASA NTRS1991

Organic matter in meteorites and comets - Possible origins

At least six extraterrestrial environments may have contributed organic compounds to meteorites and comets: solar nebula, giant-planet subnebulae, asteroid interiors containing liquid water, carbon star atmospheres, and diffuse or dark interstellar clouds. The record in meteorites is partly obscured by pervasive reheating that transformed much of the organic matter to kerogen; nonetheless, it seems that all six formation sites contributed. For comets, the large abundance of HCHO, HCN, and unsaturated hydrocarbons suggests an interstellar component of 50 percent or more, but the contributions of various interstellar processes, and of a solar-nebula component, are hard to quantify. A research program is outlined that may help reduce these uncertainties.

Anders, Edward↗
NASA NTRS1991

Cometary water-group ions in the region surrounding Comet Giacobini-Zinner - Distribution functions and bulk parameter estimates

The bulk parameters (number density and thermal energy density) of cometary water-group ions in the region surrounding Comet Giacobini-Zinner have been derived using data from the EPAS instrument on the ICE spacecraft. The derivation is based on the assumption that the pick-up ion distribution function is isotropic in the frame of the bulk flow, an approximation which has previously been shown to be reasonable within about 400,000 km of the comet nucleus along the spacecraft trajectory. The transition between the pick-up and mass-loaded regions occurs at the cometary shock, which was traversed at a cometocentric distance of about 100,000 km along the spacecraft track. Examination of the ion distribution functions in this region, transformed to the bulk flow frame, indicates the occurrence of a flattened distribution in the vicinity of the local pick-up speed, and a steeply falling tail at speeds above, which may be approximated as an exponential in ion speed.

Staines, K.↗
NASA NTRS1991

Comet Halley remote plasma tail observations and in situ solar wind properties - Vega-1/2 IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986

General features of the solar wind plasma and the interplanetary magnetic field (IMF) are studied for the time period of December 1, 1985 - May 1, 1986. Characteristics studied include corotating events, shocks, and sector boundaries in the IMF. For the same period, Comet Halley was near its perihelion and many ground-based observations of the plasma tail were made. They show interesting events such as tail substorms and disconnection events where the whole distant plasma tail seems to be disrupted from the part that is still attached to the cometary head. Several mechanisms have been produced to explain the cause of these events. The correlation between the IMF sector boundaries measured by Vega and the observed disconnection events are studied, noting that such a correlation would support the dayside reconnection explanation. It is found that only in 50 percent of the considered events does a correlation between these two phenomena exist. For the other cases, a sector boundary of the IMF sweeping over the comet cannot explain the occurrence of disruptions of the main plasma tail.

Delva, Magda↗
NASA NTRS1991

A comet among the near-earth asteroids?

Using available optical and radar observations, orbital solutions have been computed for 12 near-earth asteroids identified as possible extinct comets. For 10 of these objects, there is no evidence for the nongravitational effects that might indicate residual outgassing activity. For 1862 Apollo and 1566 Icarus, the inclusion of cometlike nongravitational effects in their equations of motion significantly reduces their RMS residuals and flattens their systematic residual trends. For Icarus in particular, the determined radial nongravitational parameter retains the same value when different datasets are employed and the predictive capability of the orbits is improved. While these results do not prove that 1566 Icarus is a comet masquerading as an asteroid, future apparitions of this object should be monitored for possible cometlike activity.

Yeomans, D. K.↗
NASA NTRS1991

OH prompt emission in Comet IRAS-Araki-Alcock (1983 VII)

Following Bertaux's (1986) suggestion that photodissociative OH fluorescence can furnish a direct measure of water-density distribution within about 100 km of the cometary nucleus, if sufficiently high spatial resolution is available, attention is presently given to IUE-obtained UV spectra of two comets which differ only as to effective spatial resolution, in order to search for OH prompt emission. IRAS-Araki-Alcock (1983 VII) and Tuttle (1980 XIII) spectra are compared at comparable heliocentric, but differing geocentric, distances; evidence is thereby obtained for OH prompt emission in Comet IRAS-Araki-Alcock, whose implied water-production rate reflects an instantaneous value which may be compared with that derived from OH solar resonance fluorescence.

Budzien, S. A.↗
NASA NTRS1991

Primordial comet mantle - Irradiation production of a stable, organic crust

The thickness and survivability of a cosmic ray-generated primordial comet refractory mantle, or 'crust', are presently predicted by laboratory data and corrected estimates of cosmic ray dose to be capable of surviving a new comet's entry into the inner solar system over numerous revolutions. It is suggested that, since this mantle may be as much as several meters deep, the probe apparatus of the projected CRAF and Rosetta spacecraft will have to be extended in order to reach the desired, unprocessed cometary material. As things stand, there is a high probability that these missions will sample cometary matter than has been heavily irradiated and reprocessed in the Oort cloud.

Strazzulla, G.↗
NASA NTRS1991

Descriptions of the neutral gas outflow in Comets P/Halley and Wilson (1987 VII) from analyses of velocity-resolved H2O line profiles

The spatial distribution and expansion velocity of the Comets Wilson (1987 VII) and pre- and postperihelion P/Halley are derived on the bases of velocity-resolved H2O spectral line profiles, using a kinematic model which synthesizes line profiles for comparison with observed line shapes. The results thus obtained demonstrate that the spherically symmetric outflow at constant velocity is a poor characterization of cometary neutral-gas outflow. While the radial dependence of the H2O expansion velocity is noted to be consistent with theoretically envisioned trends, the high H2O outflow velocity observed in Comet Wilson resists reconciliation with any existing kinematic model.

Larson, Harold P.↗
NASA NTRS1991

Comprehensive model for the nucleus of Periodic Comet Tempel 2 and its activity

A comprehensive synergistic physical model for the nucleus of Periodic Comet Tempel 2 was developed on the basis of observations carried out in 1988. The model includes the best possible estimates of the comet's bulk properties (including the dimensions and the approximate shape), information on its state of rotation, and the characterization of its activity. The model is shown to be consistent with all lines of evidence that are currently available, including relevant information from earlier apparitions.

Sekanina, Zdenek↗
NASA NTRS1991

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1 is studied in spectroscopic observations indicating that the maximum rise time of the CO(+) gas is 1.2 days. This result implies a maximum creation time-scale for the CO(+) gas of the order of 100,000 sec at 5.8 AU. Evidence is adduced that dust events and gas events observed in this comet are not necessarily connected.

Cochran, Anita L.↗
NASA NTRS1991

Cometary ray closing rates - Comet Kobayashi-Berger-Milon

Cometary ray closing rates are measured for a sequence of a dozen plates of Comet Kobayashi-Berger-Milon (1975 IX) on July 31, 1975. The angular velocity is a factor of three to six higher than previously published in the literature for other comets. A deceleration of one to two deg/hr-squared, with no secular trend, is measured. Comparisons are made between these observations and some existing models.

Moore, E. P.↗
NASA NTRS1991

Observations of solar wind ion charge exchange in the Comet Halley coma

Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) observations of solar wind ions show charge exchange effects and solar wind compositional changes in the coma of Comet Halley. As the comet was approached, the He(2+) to proton density ratio increased from 2.5 percent in the solar wind to about 4 percent about 1 hr before closest approach after which time it decreased to about 1 percent. Abrupt increases in this ratio from 2.5 to 4.5 percent were also observed in the beginning and near the end of the so-called Mystery Region. These abrupt increases in the density ratio were well correlated with enhanced fluxes of keV electrons as measured by the Giotto plasma electron spectrometer. The general increase and then decrease of the He(2+) to proton density ratio is quantitatively consistent with a combination of the addition of protons of Cometary origin to the plasma and loss of plasma through charge exchange of protons and He(2+).

Fuselier, S. A.↗
NASA NTRS1991

Implications of C2H photochemistry on the modeling of C2 distributions in comets

Laboratory studies of the secondary photolysis of the C2H radical are summarized and used to explain some discrepancies between models of C2 emission in comets. These studies show that several states of the C2 radicals produced in the photolysis of C2H2 at 193 nm have bimodal rotational distributions when plotted as a Boltzmann diagram. They also establish that the C2 radicals are formed with varying degrees of vibrational excitation, so that if they are formed in a similar manner in comets, the C2 radicals must start out with this initial vibrational excitation.

Jackson, William M.↗
NASA NTRS1991

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism.

Brandt, John C.↗
NASA NTRS1991

Radar interferometric studies of comets

Our objectives are to use radio interferometry to study composition, velocity distribution, maser excitation, and plasma interactions of cometary gas. Two new cometary chemistry programs were started with radio interferometers: (1) the VLA used to search for HC3N emission from Comet Brorsen-Metcalf at 3.3 cm wavelength; and (2) the BIMA millimeter array used to observe Comet Austin in HCN.

Snyder, Lewis E.↗