Polarization, comet

Other events, evidence of which has not yet been detected in polar ice, may also eventually serve as useful time markers in the future. Large meteors or meteor swarms that ablated in the upper atmosphere may have left a significant chemical impact on succeeding snowfalls. The same may be true for cometary collisions or brushes with a comet s tail. As with ancient volcanic eruptions, such events need not have been historically documented. If identified and accurately dated in one ice core, these events will serve as useful time horizons in other cores. 

Stern s research has taken him to the South Pole, to a number of major astronomical observatories, and to the upper atmosphere aboard high-performance military aircraft. His areas of interest include spacecraft rendezvous theory, terrestrial polar mesospheric clouds, galactic astrophysics, and tenuous satellite atmospheres. He has been principal investigator for a number of space projects, including the European Space Agency s Rosetta/ ALICE Extreme Ultraviolet Spectrometer Experiment (a mission to study Comet 46P/Wirtanen), two Space Shuttle projects, three airborne research projects, and two research rocket projects. In 1995, Stern was selected to be a Space Shuttle Mission Specialist finalist for a forthcoming flight. 

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 2. Variation of the polarization in comet IP/Halley coma, as measured at 73 phase angle in the blue domain along the trajectory of Giotto spacecraft, for distances to the nucleus in the 1000-80000 km range. Local changes could be attributed to the crossing of swarms of particles with different scattering properties (from [11]).

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].

The polarization at a given phase angle, P ( ), usually increases with wavelength in the 30 to 90 range, at least in the optical domain. This trend, already noticed for IP/Halley [21] and later confirmed for other comets [24, 31], is illustrated for C/1995 01 in Fig. 4b. The increase appears to be hnear, and the rate of increase can thus be obtained from observations at two wavelengths and X2 from... 

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. 

In this paper, we provide both a brief historical review of the polarimetric researches of comets and analysis of the observed angular and spectral characteristics of linear and circular polarization of the scattered solar light by dust particles in order to reveal principal regularities in the polarimetric properties of cometary dust. 

The first visual polarimetrie observations of comets Arago[l] detected traces of polarization in Great comet 1819 II. The first evidence of dust in comets. 

Arago detected polarized light coming from comet IP/Halley. The existence of polarization in comets was established as faet. Cometary grains seatter the solar light. 

Secchi [8] revealed different polarization of light in the eomet s tail and nucleus. Direct evidence of polarization variations over the comet. 

Kiselev et al. [19-21] found differences in the polarization maximum for two groups of comets dust-rich and gas-rich comets. A problem of similarity and diversity of the polarization of comets was raised. 

The first successful detection of circular polarization in comet IP/Halley by Metz and Haether [5], Dollfiis and Suchail [6], Morozhenko et al. [7]. There is strong evidence that aligned non-spherical dust particles are present in eometary atmospheres. 

The first imaging polarimetry of comets by Eaton et al. [22]. Detection of morphological features, dust jets and shells in the eometary coma with a high polarization degree. 

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... 

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