Light scattering by irregular dust particles in the solar system: observations and interpretation by laboratory measurements

Light scattering by irregular dust particles in the solar system: observations and interpretation by laboratory measurements

Journal of Quantitative Spectroscopy & Radiative Transfer 79–80 (2003) 903 – 910 www.elsevier.com/locate/jqsrt Light scattering by irregular dust pa...

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Journal of Quantitative Spectroscopy & Radiative Transfer 79–80 (2003) 903 – 910

www.elsevier.com/locate/jqsrt

Light scattering by irregular dust particles in the solar system: observations and interpretation by laboratory measurements A. Chantal Levasseur-Regourd∗ , Edith Hadamcik Aeronomie CNRS-IPSL, Universite Paris VI, BP 3, 91371 Verrieres, France Received 5 June 2002; accepted 18 September 2002

Abstract Results on the polarimetric properties of cometary dust are updated, with emphasis on the phase angle and wavelength dependences. They are compared to results obtained for asteroids and interplanetary dust. Noticeable di6erences are found between the cometary dust and the S-type asteroids polarization color. Interpretation through laboratory measurements performed under reduced gravity conditions suggests that dust in cometary comae consists of highly porous aggregates. Numerous measurements are expected to be performed with ICAPS on board the ISS, to monitor the evolution of the light scattering properties of the dust particles while they aggregate and, possibly, while ices condense on or evaporate from them under di6erent physical conditions. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Light scattering; Polarization; Dust; Comets; Asteroids; Interplanetary dust; Laboratory experiments

1. Introduction Cosmic dust, built up of irregular particles with sizes in a 0:1 m to 1 mm range, is found in a large variety of locations in the solar system. Information about these dust particles is mainly provided through in situ studies by space probes and through collection of IDPs (i.e. interplanetary dust particles in the near earth environment). For regions where in situ studies are not feasible, remote observations provide unique information on the composition of the dust particles through infrared spectroscopy, and on their physical properties through polarimetry of the scattered light. Extensive reviews on such observations and their analysis are available in [1]. In a ?rst part, this paper updates the main results on the observations of light scattered by cometary dust, as previously summarized in [2,3], and compares their properties to those of light scattered ∗

Corresponding author. Tel.: +33-1-64-47-42-93; fax: +33-1-69-20-29-99. E-mail address: [email protected] (A.C. Levasseur-Regourd).

0022-4073/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0022-4073(02)00327-8

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by asteroids and interplanetary dust. In a second part, interpretations provided by laboratory measurements on realistic clouds under microgravity conditions are discussed, and future measurements allowing further interpretation of light scattering observations in the solar system are presented. 2. Light scattering observations in the solar system 2.1. Light scattering and linear polarization Solar light scattered by optically thin media (such as cometary comae and the interplanetary dust cloud) is essentially partially linearly polarized. Observations of the polarization do not require any normalization and enhance changes in the physical properties of the dust, as demonstrated on polarization images of inner cometary comae (Fig. 1). For a given type of particles, the degree of linear polarization (hereafter called polarization) mainly depends upon the phase angle. The changing geometry of the scattering body and of the observer with respect to the Sun can be used to de?ne a polarization phase curve. All the phase curves retrieved for dust in the solar system are quite similar, with a slightly negative branch for small phase angles (electric ?eld vector parallel to the scattering plane), and a wide positive branch (electric ?eld vector perpendicular to the scattering plane) with a 90 –100◦ maximum for larger phase angles. Such smooth curves can be ?tted by polynomial or trigonometric functions, which actually lead to similar ?ts within phase angle ranges where enough well distributed data points are available. For phase angles above 30 –40◦ , where the polarization is high enough, a variation of the polarization with the wavelength of the scattered light is usually noticed, at least in the visible domain. 2.2. Light scattered by cometary dust The average linear polarization has been determined for numerous dust comae, mainly between 2◦ and 120◦ phase angle. As can be noticed in Fig. 2, updated from [4,5], cometary phase curves

Fig. 1. Brightness (left) and polarization (right) image of the coma of comet C/1995 O1 Hale-Bopp, as observed from Haute-Provence Observatory on 9 April 1997 (?eld of view of about 80; 000 km × 80; 000 km).

A.C. Levasseur-Regourd, E. Hadamcik / J. Quan. Spectro. & Radiative Transfer 79–80 (2003) 903 – 910

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Fig. 2. Polarization of light scattered by cometary dust as a function of the phase angle, for all observations performed through narrow-band green ?lters (left, a) and red ?lters (right, b). (x) Comets with low maximum in polarization; (+) Comets with higher maximum; (o) Comet Hale-Bopp.

allow to point out some di6erences between comets. Three classes may be de?ned: (i) comets with a low maximum, of about 0.10 – 0.15, (ii) comets with a higher maximum, of about 0.25 – 0.30, and (iii) comet C/1995 O1 Hale-Bopp, the polarization of which always seems to be the highest. The brightness of the latter was high enough to allow a determination of the circular polarization, actually found to be lower than 0.25% [6]. A comparison between observations performed in the green and red domains (Fig. 2) immediately indicates that the (positive) polarization increases with the wavelength. It could nevertheless follow an opposite trend in the vicinity of the nucleus (for distances approximately below 2000 km), as revealed by Giotto in situ observations of 1P/Halley and suspected in remote observations of C/1995 O1 Hale-Bopp [7,8]. The numerous observations available for Comet Hale-Bopp (e.g. [5,6,8,9]) allow us to derive ?ts in di6erent wavelengths. The resulting polarization values in the visible domain, for phase angles equal to 30◦ , 40◦ and 50◦ , are presented as a function of the wavelength in Fig. 3, which clearly points out the so-called red polarization color. In the visible domain, a linear increase in polarization, slighter greater for higher phase angles, is noticed. For a phase angle equal to 50◦ , the slope of the line ?tting the data is of about 14% m−1 for Hale-Bopp. It should nevertheless be pointed out that, taking into account the data obtained at larger wavelengths [10,11], the rate of increase of the polarization with the wavelength is not constant but seems to decrease in the near infrared [12]. 2.3. Comparison with asteroidal and interplanetary dust Numerous polarization measurements have been performed on asteroids by various authors (as reviewed in [13]), mainly at small phase angles because of the relative geometry of the Earth and of the main belt asteroids. Although multiple scattering is to be considered for asteroidal surfaces, it is possible to compare the characteristics of the linear polarization of asteroids and comets. The

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Fig. 3. Wavelength dependence of the polarization in the dust coma of comet C/1995 O1 Hale-Bopp (red polarization color). The data points are retrieved from trigonometric ?ts in four di6erent wavelength ranges. The lines correspond, from bottom to top, to phase angles equal to 30◦ , 40◦ and 50◦ .

Fig. 4. Wavelength dependence of the polarization on the surface of asteroid 4179 Toutatis (blue polarization color). The data points are retrieved from trigonometric ?ts in ?ve di6erent wavelength ranges. The lines correspond, from bottom to top, to phase angles equal to 20◦ , 30◦ , 40◦ and 50◦ .

shapes of the polarization phase curves are similar, and some of their characteristics may be used to identify the taxonomic types [14]. However, the S-type asteroids polarization color, again for phase angle greater than 30◦ , is opposite. Numerous observations are available for 4179 Toutatis, which is (up to now) the only NEA (near Earth asteroid) observed on a wide range of both phase angles and wavelengths (see e.g. [15,16]). From the derived trigonometric ?ts for 0.42, 0.46, 0.53, 0.65 and 0:70 m, the inversion angle is (within the error bars) the same for all the ?ts, with phase curves seeming to focus at the same inversion point. Fig. 4 presents the spectral dependence of the polarization at 20◦ , 30◦ , 40◦ and 50◦ phase angle, and points out a blue polarization color. The decrease in polarization is more important for higher phase angles. For 50◦ , the slope of the line ?tting the data points is of about −4% m−1 . It is much more diJcult to compare the previous results to those obtained from zodiacal light observations, which actually correspond to solar light scattered by interplanetary dust all along the line-of-sight, from the observer to the outer fringe of the asteroid belt. An inversion is thus required to retrieve local information and derive polarization phase curves. Results obtained in the vicinity of the ecliptic plane by two di6erent approaches, the nodes of lesser uncertainty method [17,18] and the kernel of Volterra integral method [19], agree fairly well, as summarized in [18]. The local polarization at 90◦ phase angle in the ecliptic plane had been noticed to increase with increasing solar distance in the symmetry plane of the interplanetary dust cloud (see [2], Fig. 2). This trend remains true for large enough phase angles, at least between 0.3 and 1:5 AU. Such a result suggests a temporal evolution of the particles, which su6er some weathering (collisions, evaporation,

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sputtering) while they spiral towards the Sun under Poynting–Robertson e6ect. The wavelength dependence is only estimated from line-of-sight integrated data. A trend to a blue polarization color is found (see [20], Eq. (24)), with a slope tentatively estimated to be of about −6% m−1 at 90◦ elongation, corresponding to phase angles decreasing from 90◦ to 0◦ along the line-of-sight. To summarize, signi?cant discrepancies, in term of the values of the slope at inversion and of the maximum in polarization, have been noticed between di6erent classes of comets and between di6erent types of asteroids. Even greater discrepancies, in terms of polarization color and of dependence of this color e6ect on the phase angle, are found between cometary dust and asteroidal regolith. These di6erent scattering properties indicate signi?cant di6erences in the morphological and physical properties of the dust particles that scatter the solar light. They need to be compared with the scattering properties of well-documented particles. 3. Interpretation through laboratory measurements EJcient codes now exist for light scattering computations by low density clouds of irregular particles with relevant size parameters (see e.g. [21]). They lead to smooth polarization phase curves, and the results obtained for such “virtual” particles are of major interest. However, questions remain about the uniqueness of the solutions, and about the estimation of the complex refractive indices (which strongly depend upon the wavelength). A complementary approach is thus provided by laboratory measurements with “real” particles. Multiple scattering on gravity packed layers is avoided through elaborate techniques, such as microwave analogue (see e.g. [22]) and levitation. We have actually performed, under reduced gravity conditions, light scattering measurements on low-density clouds of natural particles with the PROGRA2 and CODAG-LSU experiments, which rely on nephelometer type instruments working at two wavelengths. 3.1. Key results from PROGRA2 and CODAG-LSU PROGRA2 has validated the concept of light scattering measurements during parabolic Pights campaigns [23]. CODAG-LSU has validated the concept of dust particles aggregation during a rocket Pight [24]. Besides, numerous phase curves have been retrieved for various types of particles and aggregates [25,26], specially with the upgraded version of the PROGRA2 experiment, which provides polarization images at two wavelengths. Mixtures of Pu6y aggregates of submicron-sized grains of silica and of carbon, which agglomerate in highly porous structures, are found to reproduce the cometary phase curves; they present a maximum in polarization that decreases with increasing size of the submicronic grains, and a red polarization color [7,26]. It may thus be proposed that dust in cometary comae mainly consists of aggregates, with an average dust grain size smaller for Hale-Bopp than for other comets. Such a result agrees with the observation of a strong silicate emission feature (see e.g. [27]). On the opposite, gray compact particles (with sizes greater than the wavelength) present a blue polarization color, and a maximum in polarization that increases with the size. The asteroidal regoliths and the interplanetary dust would thus mainly consist of such compact particles, the size of which decreases while they spiral towards the Sun and su6er weathering e6ects. The blue polarization

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color suspected in the vicinity of some cometary nuclei could be a clue to the presence of compact (possibly icy) particles, which would rapidly evaporate and allow the formation of Pu6y aggregates as they move away from the nucleus. 3.2. Future measurements Such a scenario for the evolution of cometary and interplanetary dust does need laboratory measurements to be validated. Both PROGRA2 and CODAG experiments have actually contributed to the proposal to ESA of multi-user facility for research on Interactions in Cosmic and Atmospheric Particles Systems on board the International Space Station [28]. The phase A study of the so-called ICAPS experiment is now completed, and a precursor experiment, CODAG-2, is to take place in 2004 on a rocket Pight. Some of the scienti?c objectives of this dual low-pressure chamber are related to the investigation of formation and evolution of small dust grains and aggregates under a variety of physical conditions representative of the protoplanetary nebula, to the monitoring of the scattering properties of the dust (particles, aggregates, regoliths), and to the validation of light scattering codes. It is anticipated to perform light scattering measurements in three colors, to study (from about 5 –175◦ ) the evolution of the brightness and polarization phase curves while aggregation processes take place. Questions related to the formation and evolution of ices in interstellar dust clouds, on cometary dust particles, and on regoliths, are still open. Particles with water ice could have formed at very low temperatures in the interstellar medium and have later evolved from an amorphous structure in the outer solar system to a crystalline (cubic or hexagonal) structure. The light scattered by such particles carries signi?cant information about their physical properties, and thus about their origin. In a ?rst step, it will be of major interest, in order to interpret the available observations, to study the evolution of the dust light scattering properties during the condensation or sublimation of ices on submicron sized dust grains (cosmic grains), micron sized grains (cometary dust, atmospheric solid aerosols), aggregates with di6erent packing densities and sizes (cometary dust aggregates), and regoliths of high porosity (cometary nuclei, asteroids). It is already anticipated to perform measurements on icy particles with temperatures reaching −20◦ C with ICAPS experiment. Later, measurements at lower temperatures will hopefully be performed on board the ISS with a modi?ed ICAPS experiment. A new Topical Team, devoted to the physics and chemistry of ices in space [29], has precisely been selected by ESA, in relation with such topics. 4. Conclusion Polarimetric properties of dust, such as the values of the maximum in polarization and of the slope at inversion, are keys to the understanding of the physical properties of dust clouds or regoliths. The wavelength dependence of the polarization, both in the linear region (corresponding to phase angles in the 25 –50◦ range) and in the inversion region (near 20◦ ), is also of major importance. Laboratory measurements on levitating dust particles already provide clues to the physical properties of cometary and interplanetary dust particles, as well as to the di6erences existing between di6erent comets and di6erent cometary regions. Extended comparisons with microwave analogue measurements and with results of elaborate computations should, together with future measurements (with

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