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1.
Surface morphology and related issues for nuclei of three comets: Halley, Borrelly and Wild 2, are considered in the paper. Joint consideration of publications and results of our analysis of the comets’ images led to conclusions, partly new, partly repeating conclusions published by other researchers. It was found that typical for all three nuclei is the presence of rather flat areas: floors of craters and other depressions, mesas and terraces. This implies that flattening surfaces or planation is a process typical for the comet nuclei. Planation seems to work through the sublimation-driven slope collapse and retreat. This requires effective sublimation so this process should work only when a comet is close to the Sun and if on the nucleus there are starting slopes, steep and high enough to support the “long-distance” avalanching of the collapsing material. If the surface had no starting slopes, then instead of planation, the formation of pitted-and-hilly surfaces should occur. An example of this could be the mottled terrain of the Borelly nucleus. Both ways of the sublimational evolution on the nucleus surface should lead to accumulation of cometary regolith. The thickness of the degassed regolith is not known, but it is obvious that in surface depressions, including the flat-floor ones, it should be larger compared with nondepression areas. This may have implications for the in situ study of comets by the Deep Impact and Rosetta missions.Our morphological analysis puts constraints on the applicability of the popular “rubble-pile comet nucleus” hypothesis (Weissman, 1986. Are cometery nuclei primordial rubble piles? Nature 320, 242-244.). We believe that the rubble pile hypothesis can be applicable to the blocky Halley nucleus. The Borelly and Wild 2 nuclei also could be rubble piles. But in these cases the “rubbles” have to be either smaller than 30-50 m (a requirement to keep lineament geometry close to ideal), or larger than 1-2 km (a requirement to form the rather extended smooth, flat surfaces of mesa tops and crater floors). Another option is that the Borelly and Wild 2 nuclei are not rubble piles.In relation to surface morphology we suggest that three end-member types of the comet nuclei may exist: (1) impact cratered “pristine” bodies, (2) non-cratered fragments of catastrophic disruption, and (3) highly Sun-ablated bodies. In this threefold classification, the Wild 2 nucleus is partially ablated primarily cratered body. Borrelly is significantly ablated and could be either primarily cratered or not-cratered fragment. Halley is certainly partially ablated but with the available images it is difficult to say if remnants of impact craters do exist on it.Recently published observations and early results of analysis of the Tempel 1 nucleus images taken by Deep Impact mission are in agreement with our conclusions on the processes responsible for the Halley, Borrelly and Wild 2 nuclei morphologies. In particular, we have now more grounds to suggest that decrease in crater numbers and increase of the role of smooth flat surfaces in the sequence Wild 2?Tempel 1?Borelli reflects a progress in the sublimational degradation of the nucleus surface during comet passages close to the Sun. 相似文献
2.
Harker David E. Woodward Charles E. McMURTRY Craig W. Goetz J. A. Pipher J. L. Forrest W. J. 《Earth, Moon, and Planets》1997,78(1-3):259-264
We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion.
All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from
the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest
radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ)
in all the images closely follows the “steady state” ρ−1 model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant
implying the dust grain properties vary with coma distance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Cristiano B. Cosmovici Vojko Bratina Gottfried Schwarz Gianpaolo Tozzi Michael J. Mumma Roberto Stalio 《Astrophysics and Space Science》2006,301(1-4):135-143
We report here on unique post-perihelion (2.3 AU) measurements of Comet Hale-Bopp in the FUV-range (950–1250 Å) by means of the UVSTAR spectrometer from the space shuttle with the main purpose of searching for argon and other FUV emitters. New methods for separating the strong airglow emission at shuttle altitudes are here discussed in detail. Due to our low resolution (15 Å) and S/N ratio the possible rocket-borne detection of argon near perihelion (0.9 AU) could not be confirmed. New species as N2 are suspected but difficult to separate from the strong airglow emission at shuttle altitudes. From the Lyα brightness (1.30± 0.08 kRy) a water production rate Q = 5.9 ± 0.4 × 1029 molecules s?1 could be derived and compared with other post-perihelion observations. 相似文献
4.
Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images
of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring
outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one
source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion.
The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized
to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows
even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope
in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover
a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures
in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright
structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with
higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the
whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for
the comets previously observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Churyumov K. I. Klesachonok V. V. Mussaev F. A. Bikmaev I. F. Galazutdinov G. H. 《Earth, Moon, and Planets》1997,78(1-3):105-110
We present the results of the preliminary study of the comet Hale-Bopp spectrum obtained April 17, 1997 by K. Churyumov and F. Mussayev with the help of the 1-meter Zeiss reflector and the echelle spectrometer (spectral resolutionλ/Δ λ ≈ 50000), CCD and the long slit, oriented along the radius-vector(“Sun-comet direction”). Energy distributions for three selected regions including the C3, C2 (0-0) and CN(Δ ν = 0) molecules emissions of the comet Hale-Bopp spectrum were built. The rotational lines of the CN(Δ ν = 0) band were identified. The nature of the high emission peak near λ 4020 Å in the C3 band is discussed. The presence of the cometary continuum of the nonsolar origin is assumed. 相似文献
7.
Taban IC Bechmann RK Torgrimsen S Baussant T Sanni S 《Marine environmental research》2004,58(2-5):701-705
The single-cell microgel electrophoresis assay or the comet assay was used to evaluate DNA damage of dispersed crude oil on sea urchins (Strongylocentrotus droebachiensis) and mussels (Mytilus edulis L.). Sea urchins were exposed to 0.06 and 0.25 mg/L dispersed crude oil in a continuous flow system, while the mussels were exposed to 0.015, 0.06 and 0.25 mg/L dispersed crude oil. Sea urchin coelomocytes and mussel haemocytes were sampled after 4 and 5 weeks exposure, respectively. In the sea urchin coelomocytes, there was a significant concentration-related increase in the percentage of DNA in comet tail. In mussel haemocytes, there was a significantly higher percentage of DNA in comet tail for all treatments compared to the control. The responses were concentration-related up to 0.06 mg/L oil. The two highest exposure concentrations of mussels were not significantly different from each other. These results indicate that the comet assay can be used for biomonitoring of DNA damage in marine invertebrates following oil contamination. 相似文献
8.
C. Debi Prasad T. Chandrasekhar J. N. Desai N. M. Ashok Vinod Krishan 《Journal of Astrophysics and Astronomy》1989,10(1):1-20
During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H2 O+ emission was recorded in thecoma at ~ 0h UT on 1986 March 13. Observations were carried out using a special IHW filter for H2 O+ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24?39’N,72? 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ~ 35km s?1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet 相似文献
9.
Kentaro Terada Hitoshi Negoro Kiyoshi Hayashida Shunji Kitamoto Hiroshi Tsunemi Hiroshi Oya Takayuki Ono Akira Morioka Yuzuru Tawara Toshifumi Mukai Masahiro Hoshino Toshio Terasawa 《Earth, Moon, and Planets》1994,66(1):75-81
An ASCA observation of the Jovian impact of the comet Shoemaker-Levy 9 is reported. Four impacts of H, L, Q1 and R were observed and four impacts of B, C, G, and Q2 were observed within 60 minutes after their impacts. No significant flaring of X-ray emission was observed. Upper limit X-ray fluxes of 90 % confidence level, averaged 5 minutes just after the impacts, were 2.4 × 10–13 erg sec–1 cm–2, 3.5 × 10–13 erg sec–1 cm–2, 1.6 × 10–13 erg sec–1 cm–2 and 2.9 × 10–13 erg sec–1 cm–2 for the impacts of H, L, Q1 and R, respectively, in the 0.5(0.7 for H and Q1)–10 keV energy range. However, a hint of X-ray enhancement around Jupiter from July 17 to July 19 was detected with about 2 6 × 10–14 erg sec–1 cm–2 in the 0.5–10 keV energy range. 相似文献
10.
On July 20, 1994, before the Q fragments of Comet Shoemaker-Levy 9 fell to Jupiter, more than 200 spectra of the Jovian features were obtained at the Crimean Astrophysical Observatory in the wavelength range 5700–7600 Å with a 26 s exposure time and a spectral resolution of 20 Å. We found a time-varying Na D line emission in the form of two components with Doppler shifts of about 30 Å. The brightest and most frequent sodium flares were detected when the Q fragments passed through the Jovian inner magnetosphere at a distance of about three the Jovian radii (3RJ) from its center, where they crossed the Io-Jupiter current tube. A frequency analysis of our data revealed a flare recurrence time scale of 1 min. We conclude that sodium was released from the cometary dust and from the surfaces of numerous cometary debris and that its amount was enough to produce the observed emission. The observed high-speed clouds of sodium atoms are assumed to have been formed through ionization, ion acceleration by the bidirectional electric fields of Alfvén waves in the Io-Jupiter current tube, and their neutralization. 相似文献