Phase dependence, comet

For comets, the spectral phase dependence of the degree of linear polarization is similar to that of atmosphereless solar system bodies. Moreover, for spectral... 

Figure 3. The dashed line shows a typical shape of the polarization phase dependence. Polarization measured in the narrow-band blue filter for comet C/1989 XI (Austin) is shown by ( ) [20].

We have carried out a number of molecular dynamics (MD) simulations on small hydrocarbon molecules in the solid phase using the AREBO potential. The studies were motivated, at least in part, by a desire to understand the possible chemical reactions of these materials under shock compression in planetary systems due to impact of comets or meteorites into planetary atmospheres. In addition, we were interested in learning more about the detailed kinetics of shock-induced chemistry in condensed phases, a subject in which little experimental information is available. For example, it is of some interest to examine the orientation dependence of shock impact, the spatial and temporal extent of the reaction zone, the time scale of energy thermalization, and the range of reaction products for each of the materials studied. 

Figure 4. Phase angle dependence of the whole coma polarization a) Cometary data in the red domain with (x) low maximum comets, (+) higher maximum comets and (A) C/1996 01 Hale-Bopp. Data points above 120 are from C/1999 S4 (LINEAR) at its disruption [30] b) C/1996 01 Hale-Bopp data in the green domain (lower fit) and the red domain (upper fit) [20].

Figure 5. Wavelength dependence of the polarization at a given phase angle (from [26]) a) Comet Hale-Bopp, for a equal 30, 40, 50 (fro m bottom to top) b) Asteroid Toutatis, for a equal to 20,30, 40,50 (fro m bottom to top).

The analysis of cometary observations suggests the existence of very fluffy dust aggregates. Differences are observed in the light-scattering properties, e.g. stracture of the comae, polarization phase curves maxima and minima, polarization wavelength dependence. They could be a clue to the temporal evolution of the physical properties of the dust particles, with collisional processes as well as evaporation of icy mantles and organic compoimds. Table 1 presents some polarization properties of dust particles in comets, asteroids, in the interplanetary dust cloud, and on Mars, as retrieved by remote sensing. 

For obtaining and studying the phase-angle dependence of linear polarization for comets, we used our own homogeneous data sets received with narrowband filters [31,34-40]. Only comet C/1982 Ml (Austin) was observed through the wideband V filter. These data were supplemented with data for comets C/1982 Ml (Austin), 67P/Churyumov-Gerasimenko [41], C/1989 XI (Austin) [42-45], C/1983 HI (IRAS-Araki-Alcock) [46], and IP/Halley [47]. All data for the blue and red domains of the continuum spectra are shown on the left and right panels in Fig. 1(a). Most measurements of polarization were made for the whole coma (several... 

Figure 1. The phase-angle dependence of polarization for comets in die blue (left panel) and red (right panel) continuum a all comets b dust-rich comets c gas-rich comets. Dust-rich comets -...

On the basis of wideband-filter polarimetry, Kiselev divided comets into two groups depending on the maximum degree of polarization at large phase angles [19-21]. He noted that such a distinction between comets is caused by an influence of the molecular emissions that fall within the wideband filters. The narrowband [34] and intermediateband [44] polarimetric observations of comets confirmed the existence of the two groups. Chernova et al. [34] indicated that a... 

Parameters P n and slope AP/Aa at the inversion angle ainv = 2 ° show a tendency to increase slightly in the visible domain for dust-rich comets. This result was obtained by comparison of trigonometric fits for the phase-angle dependence of polarization for dust-rich comets in the blue and red domain [40]. This tendency was recently confirmed by observations of comet C/2000 WMl (LINEAR) [77]. It should be noted that the negative polarization branch of comet... 

The spectral dependence of polarization for dust-rich comets changes with phase angle (Fig. 6b). This fact first was found for comet Halley [34,47]. A gradient (AP/AA,)/Aa is constant within the phase angle range 30 - 80° and equal to 0.025 0.001 %/1000 per 1 ° [40]. One can see in Fig. 6b that for phase angles smaller than 20°, the gradient (AP/AA,)/Aa is close to zero. 

Figure 9. Composite phase-Migle dependence of circuit- polmization for comets IP/Halley [6,7], C/1995 01 (Hale-Bopp) [26,30], and C/1999 S4 (LINEAR) [86].

Figure 1. Dependence of bolometrie albedo on phase angle. The data are from (0) [15], (1) [16], (2-7) [8], (8) [13]. The solid line represents the least-square fit to the data for comets C/1975 VI (West) and C/1980 Y1 (Bradfield) and is interpolated to eonneet smoothly to the backscattering curve from [13] normalized at a = 30°. The data from [13] are normalized to the data for comet Bradfield at a = 127°.

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