Observation by Pioneer 7 of He+ in the distant coma of Halley's comet     

《Icarus》1987,71(1):192-197

During the approach of Pioneer 7 to a distance of 12.1 × 10⁶ km from the nucleus of Comet P/Halley on March 20, 1986, features were observed in plasma analyzer data that we interpret as He⁺ produced by charge exchange of solar wind He⁺⁺ with neutral cometary material. The maximum flux of the He⁺ was observed several hours after the closest approach of the spacecraft to the nucleus, and is unexpectedly large. Remarkable large discontinuous flux changes were also observed.  相似文献   

Analysis of the morphological structures of comet 1P/Halley in the perihelion passages in 1910 and 1986     

M. R. Voelzke 《Meteoritics & planetary science》2019,54(4):938-945

This work is based on a systematic analysis of images of comet 1P/Halley collected during its penultimate and ultimate approaches, i.e., in 1910 and 1986. This research has identified, characterized, classified, and compared tail structures of comet 1P/Halley, namely disconnection events (DEs), wavy structures, and solitons. The images of the comet during its 1910 passage, as illustrated in the Atlas of Comet Halley 1910 II (Donn et al. 1986), were compared with those of its approach in 1986 as illustrated in The International Halley Watch Atlas of Large‐Scale Phenomena (Brandt et al. 1992). Two onsets of DEs were discovered after the perihelion passage in 1910 with an average value of the corrected cometocentric velocity (Vc) of 57 ± 15 km s^?1. Ten onsets of DEs were discovered after the perihelion passage in 1986 with an average Vc equal to 130 ± 37 km s^?1. The mean value of the corrected wavelength λc of wavy structures in 1910 is equal to 1.7 ± 0.1 × 10⁶ km, as compared to 2.2 ± 0.2 × 10⁶ km in 1986. The mean value of the amplitude A of the wave in 1910 is equal to 1.4 ± 0.1 × 10⁵ km and 2.8 ± 0.5 × 10⁵ km in 1986. The goals of this research were to report the results obtained from the analysis of the P/Halley's images from 1910 and 1986, to provide empirical data for comparison, and to form the input for future physical/theoretical work.  相似文献   

On the modification of structure of cometary nuclei by the transport of super-volatiles     

Shin Yabushita 《Earth, Moon, and Planets》1996,73(3):215-220

Earlier, a study has been made of the transport mechanism of volatile molecules such as N₂ and CO through cometary nuclei as they are heated by radioactive elements. Coupled equations of heat and gas transport in the presence of gas sublimation and recondensation, as well as a heat source, were numerically solved. And it was shown that supervolatiles such as N₂ and CO are transported through the pores of the nucleus, and consequently the volatile molecules become more abundant near the surface than deep inside the nucleus. Here, the process is investigated for a wider range of paramaters such as porosity and nuclear radius. It is shown that provided the central temperature attains the sublimation point of the super-volatiles, they are transported toward the surface regardless of the values of the parameters.  相似文献   

Energy balance and the gas flux from the surface of comet 46P/Wirtanen     

《Planetary and Space Science》1999,47(6-7):735-744

Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×10²⁸ molecules s⁻¹ at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation.  相似文献   

Photometric investigations of distant comets C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1     

A. V. Ivanova  P. P. Korsun  V. L. Afanasiev 《Solar System Research》2009,43(5):453-462

We present an analysis of the results of photometric investigations of two distant comets, C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1, obtained with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The comets under study demonstrate sufficient activity out of the zone of water ice sublimation (at heliocentric distances longer than 5 AU). In the spectra of the investigated comets, we found the CO⁺ and N₂⁺ emission. The presence of this emission may say that the comets were formed in the outer parts of the Solar System, in a protoplanetary cloud at a temperature ≤25 K. We found that the photometric maximum of the ionosphere (in the CO⁺ filter) of the comet C/2002 VQ94 (LINEAR) is shifted relative to the photometric center of the dust coma by 1.4″ (7.44 × 10³ km) in the direction deflected by 63° from the direction to the Sun. Using special filters to process the images, we picked out active structures (jets) in the dust coma of the 29P/Schwassmann-Wachmann-1 comet.  相似文献   

Representative abundances of a few positive ions in the innermost coma of comet Halley     

P. P. Saxena  Alka Misra 《Earth, Moon, and Planets》1993,60(1):59-68

The production rate of H₂O molecules at a heliocentric distance of 1 AU for comet Halley and the abundance ratio with respect to water (H₂O) of parent molecules at the cometary nucleus from the paper of Yamamoto (1987) have been used to compute the number densities of positive ions viz. H₃O⁺, H₃S⁺, H₂CN⁺, H₃CO⁺, CH₃OH ₂ ⁺ and NH ₄ ⁺ at various cometocentric distances within 600 kms from the nucleus.The role of proton transfer reactions in producing major ionic species is discussed. A major finding of the present investigation is that NH ₄ ⁺ ion which may be produced through proton transfer reactions is the most abundant ion near the nucleus of a comet unless the abundance of NH₃ as a parent is abnormally low. Using the quoted value of Q(NH₃)/Q(H₂O) for comet Halley and the life times of NH₃ and H₂O molecules, the abundance ratio N(NH₃)/N(H₂O) is found to be one-third of that used in the present paper. The consequent proportionate decrease in the NH ₄ ⁺ ions does not, however, affect its superiority in number density over other ions near the nucleus.The number density of the next most abundant ion viz. H₃O⁺ is found to be 4 × 10⁴ cm^-3 at the nucleus of comet Halley and decreases by a factor of two only upto a distance of 600 K ms from the nucleus. The ionic mass peak recorded by VEGA and GIOTTO spacecrafts atm/q = 18 is most probably composite of the minor ionic species H₂O⁺, as its number density = 10² cm^-3 remains virtually constant in the inner coma and of NH ₄ ⁺ , the number density of which at large cometocentric distances may add to the recorded peak atmlq = 18. The number densities of other major ions produced through proton transfer from H₃O⁺ are also discussed in the region within 600 K ms from the nucleus of comet Halley.  相似文献   

Main Belt Comet P/2008 R1 Garradd: Duration of activity     

Konrad J. Kossacki  Slawomira Szutowicz 《Icarus》2012,217(1):66-76

In this work we investigated changes of the water emission from a model comet of the size and orbital elements of Comet P/2008 R1 (Garradd). We performed simulations for model cometary nuclei of different compositions and two different orientations in space. Our simulations indicate, that the emission of water decreases from one orbital period to another one, but in some cases slowly. When the rotation axis of the nucleus lies in the orbital plane the seasonal maximum of water production during the first two orbital periods can be as high as about 10²⁶ mol s^?1, but decreases by two orders of magnitude during only 50 orbital periods. The highest rate of water production after many orbital periods is expected when the rotation axis is perpendicular to the orbital plane – the seasonal maximum of water production can be about 5 × 10²⁵ mol s^?1 during the first two orbital periods after activation of the comet and no more than 0.8 × 10²⁵ mol s^?1 500 orbital periods later. The upper estimate for the production of water derived from observations of P/2008 R1 (Garradd) by Jewitt et al. (Jewitt, D., Yang, B., Haghighipour, N. [2009]. Astron. J. 137, 4313–4321) is 5 × 10²⁵.  相似文献   

Solar wind interactions with Comet 19P/Borrelly     

D.T Young  F.J Crary  J.E Nordholt  D Boice  A Eviatar  J.J Hanley  D.J McComas  D Reisenfeld  R.C Wiens 《Icarus》2004,167(1):80-88

The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm³ at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO⁺/H₂O⁺ is lower than that observed with Giotto at 1P/Halley.  相似文献   

Origin of cometary extended sources from degradation of refractory organics on grains: polyoxymethylene as formaldehyde parent molecule     

Hervé Cottin  Yves Bénilan  Marie-Claire Gazeau  François Raulin 《Icarus》2004,167(2):397-416

The radial distribution of some molecules (CO, H₂CO, HNC, …) observed in the coma of some comets cannot be explained only by a direct sublimation from the nucleus, or by the photolysis of a detected parent compound. Such molecules present a so-called extended source in comae. We show in this paper that extended sources can be explained by refractory organic material slowly releasing gas from grains ejected from the cometary nucleus, due to solar UV photons or heat. The degradation products are produced throughout the coma and therefore are presenting an extended distribution. To model this multiphase chemistry we derive new equations, which are applied to Comet 1P/Halley for the case of the production of formaldehyde from polyoxymethylene (POM), the polymer of formaldehyde (-CH₂-O-)_n. We show that the presence of a few percent of POM on cometary grains (a nominal value of ∼4% in mass of grains is derived from our calculations) is in good agreement with the observed distribution, which so far were not interpreted by the presence of any gaseous parent molecule.  相似文献   

Time Analysis of the CO+ COMA OF COMET P/HALLEY BY IMAGE PROCESSING TECHNIQUES     

Voelzke  Marcos Rincon  Schlosser  Wolfgang  Schmidt-Kaler  Theodor 《Astrophysics and Space Science》1997,250(1):35-51

Photographic and photoelectric observations of comet P/Halley's ion gas coma from CO⁺ at 4250 ? were part of the Bochum Halley Monitoring Program, conducted from 1986 February 17, to April 17 at the European Southern Observatory on La Silla (Chile). In this spectral range it is possible to watch the continuous formation, motion and expansion of plasma structures. To observe the morphology of these structures 32CO⁺ photos (glass plates) from P/Halley's comet have been analysed. They have a field of view of 28°.6× 28°.6 and were obtained from 1986 March 29, to April 17 with exposure times between 20 and120 minutes. All photos were digitized with a PDS 2020 GM (Photometric Data System) microdensitometer at the Astronomisches Institut derWestf?lischen Wilhelms-Universit?t in Münster (one pixel= 25 μm × 25 μm ≈ 46′.88×46′.88). After digitization the data were reduced to relative intensities, and the part with proper calibrations were also converted to absolute intensities, expressed in terms of column densities using the image data systems MIDAS (Munich Image Data Analysis System; ESO – Image Processing Group, 1988) and IHAP (Image Handling And Processing; Middleburg, 1983). With the help of the Stellingwerf-Theta-Minimum-Method (Stellingwerf, 1978) a period of (2.22 ± 0.09) days results from analysis of structures in the plasma-coma by subtracting subsequent images. This method is also compared with the Fourier method. There may be a second cycle with a period of about 3.6 days. The idea behind subtracting subsequent images is that rotation effects are only 10% phenomena on gas distribution. Difference images are than used to suppress the static component of the gas cloud. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Energetics of galactic nuclei     

G. S. Sahakian 《Astrophysics》1997,40(4):311-319

A model of compact galactic nuclei in statistical equilibrium was developed in [L. Sh. Grigorian and G. S. Sahakian, Astrofizika (in press)]. It was shown that they should consist predominantly of neutron stars (pulsars) and white dwarfs. The problem of the energy reserves of galactic nuclei is discussed in terms of this concept. The mechanism of conversion of a white dwarf into a neutron star due to the accretion of interstellar matter is considered. This means that a galactic nucleus has an energy reserve of some 5·10⁶⁰ N₈ erg (N is the number of stars in the nucleus). It is shown that galactic nuclei are powerful sources of hard γ radiation [power L » 2·10⁴⁴µ₃₀N₈(Ω/50)^17/7 erg/sec, where µ is the magnetic moment and Ω is the angular rotation rate of a neutron star ] due to curvature radiation from relativistic electron fluxes flowing along channels of open magnetic field lines of pulsars. The x-ray and ultraviolet emission are due to synchrotron emission from the same electron fluxes in the magnetic field of the galactic nucleus (L » 10⁴²-10⁴⁴ erg/sec). The optical (visible and infrared) and radio emission are due to bremsstrahlung from electrons in the interstellar medium [L » 6·10⁴⁶N ₈ ² (5/R_pc)³ erg/sec, where R is the radius of the galactic nucleus]. An equation is obtained for the magnetic moment of a pulsar: µ ≈ 3.4·10^-5L_γP^17/7, where P is the pulsar’s period and L_03B3; is the luminosity of the pulsar’s y radiation.  相似文献   

Coma expansion and photometry of comet Bowell (1980b)     

David Jewitt 《Icarus》1984,60(2):373-385

Optical and infrared observations of comet Bowell are presented. The optical observations indicate that the solid grain coma is expanding at only 0.9 ± 0.2 m sec^?1. This is two orders of magnitude slower than the local gas sound speed and may suggest that gas drag is not responsible for stripping the grains from the nucleus. The hypothesis of “electrostatic snap-off” is tentatively advanced to account for the ejection of the grains. Alternatively, the grains may have an unusual size distribution. The extrapolated motion of the grains suggests that the bulk of the coma was formed when the comet was at a heliocentric distance $\text{R ? 10}\text{AU}$. Any water ice in the nucleus would be too cold to give rise to the observed grain coma by equilibrium sublimation at this R. Further evidence against the production of the grain coma by equilibrium sublimation of the nucleus is provided by broadband (J) photometric observations. Almost all of the observed photometric variations of comet Bowell can be ascribed to geometric effects. Simple models indicate that the total grain cross section has been nearly constant since the time of the earliest observations. The present observations, which suggest that water ice sublimation does not control either the optical morphology or the near infrared photometric behavior of comet Bowell, are contrasted with reported high OH production rates. It is concluded that the grain coma may be largely a relic of activity occurring on the nucleus at $\text{R ? 10}\text{AU}$ while the OH may indicate sublimation from the nucleus near perihelion and from coma grains near $\text{R ? 4.6}\text{AU}$.  相似文献   

Composition of Comets: Observations and Models     

Huebner  Walter F. 《Earth, Moon, and Planets》2000,89(1-4):179-195

We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N₂, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO₂, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.  相似文献   

Have stellar and supernovae radiations altered the pristine nature of comets?     

S. Yabushita 《Earth, Moon, and Planets》1990,49(3):239-245

Effect of stellar and supernova radiations on cometary nuclei in the Oort cloud is investigated. Radiation dose received by a comet is calculated and compared with the one which Halley's comet receives by one perihelion passage. Stellar radiation provides 10 to 50% of Halley unit over 4 billion years. Inclusion of sublimation of volatile molecules such as CO or N₂ does not allow the temperature to rise to 30 K by irradiation of bright OB stars, contrary to the claim of Stern and Shull. A chance encounter with a SN provides radiation dose which is just sufficient to raise to 30 K the surface layer which is 1 m thick on the assumption that the radiation is wholly communicated to the interior. Thus, the comets remain pristine under the effect of stellar and SN radiations.  相似文献   

Rotational dynamics of subsolar sublimating triaxial comets     

Eirik Mysen 《Planetary and Space Science》2004,52(10):897-907

A model with subsolar water sublimation on a triaxial, ellipsoidal comet nucleus is presented for the calculation of reactive torques. The resulting differential equations describing the comet's rotation are then Hamiltonian, and gravity-gradients are trivial to include. While effects derived from a weak perturbing function are neither able to change the rotational excitation nor the spin magnitude of the nucleus, it is shown how the spin orientation of comets can change significantly over an orbital run. However, of the four comets studied, 1P, 19P, 46P and the Rosetta target 67P, 19P and 46P were the only objects clearly exhibiting this feature, thereby confirming a technique used to derive the consequences of a more elaborate model of sublimation induced torques. In particular, the rotational parameters of 67P were seen to be very stable, indicating that a highly kinematical model of its rotation for the mapping of the comet's gravitational field during the Rosetta mission can be used. The model's hierarchy with 1P/Halley as the object with highest excitation probability, is consistent with observations.  相似文献   

The Influence of Reactive Torques on Comet Nucleus Rotation     

A. I. Neishtadt  D. J. Scheeres  V. V. Sidorenko  P. J. Stooke  A. A. Vasiliev 《Celestial Mechanics and Dynamical Astronomy》2003,86(3):249-275

Reactive torques, due to anisotropic sublimation on a comet nucleus surface, produce slow variations of its rotation. In this paper the secular effects of this sublimation are studied. The general rotational equations of motion are averaged over unperturbed fast rotation around the mass center (Euler-Poinsot motion) and over the orbital comet motion. We discuss the parameters that define typical properties of the rotational evolution and discover different classifications of the rotational evolution. As an example we discuss some possible scenarios of rotational evolution for the nuclei of the comets Halley and Borrelly.  相似文献   

The composition of very fine dust in the dust shell of comet halley     

E. N. Evlanov  O. F. Prilutskii  B. V. Zubkov  M. I. Voiskovskii 《Astronomy Letters》2000,26(7):473-483

An analysis of the spectra from the PUMA dust-impact mass spectrometers onboard the Vega-1 and Vega-2 spacecraft shows that a large number of the observed, unidentified small-amplitude peaks are produced by impacts of very-low-mass (from 10^?17 to 10^?20 g) particles. The mass flux of very fine particles accounts for a few percent of the total dust mass flux from comet Halley. The elemental composition of the finest cometary particles is identical to the composition of large particles (10^?12–10^?16 g), in agreement with present views about the nucleus of comet Halley as an aggregate of interstellar dust.  相似文献   

Thermal modeling of cometary nuclei     

Paul R. Weissman  Hugh H. Kieffer 《Icarus》1981,47(3):302-311

A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed.  相似文献   

The Correct Evaluation of the Sublimation Rate of Dusty Ices under Solar Illumination, and Its Implication on the Properties of P/Halley Nucleus     

J.F. Crifo 《Icarus》1997,130(2):549-551

This note (1) provides a formal derivation of an algorithm proposed precedingly without proof for evaluating the sublimation rate of dusty ice under solar illumination, and (2) illustrates the importance of adopting a correct algorithm for such a purpose, by rediscussing the basic characteristics of P/Halley nucleus activity derived from the 1996 flyby data.  相似文献   

On the production of nitric oxide by cosmic rays in the mesosphere and stratosphere     

Marcel Nicolet 《Planetary and Space Science》1975,23(4):637-649

Nitric oxide is formed in the atmosphere through the ionization and dissociation of molecular nitrogen by galactic cosmic rays. One NO molecule is formed for each ion pair produced by cosmic ray ionization.The height-integrated input (day and night) to the lower stratosphere is of the order of 6 × 10⁷ NO molecules cm^?2/sec in the auroral zone (geomagnetic latitude Φ ? 60°) during the minimum of the sunspot cycle and 4 × 10⁷ NO molecules cm^?2/sec in the subauroral belt and auroral region (Φ? 45°) at the maximum of solar activity. The tropical production is less than 10^?7 NO molecules cm^?2/sec above 17 km and at the equator the production is only 3 × 10⁶NO molecules cm^?2/sec.  相似文献