Optical properties of dust

http://arxiv.org/abs/0808.4123Except in a few cases cosmic dust can be studied in situ or in terrestrial laboratories, essentially all of our information concerning the nature of cosmic dust depends upon its interaction with electromagnetic radiation. This chapter presents the theoretical basis for describing the optical properties of dust -- how it absorbs and scatters starlight and reradiates the absorbed energy at longer wavelengths.Partial support by a Chandra Theory program and HST Theory Programs is gratefully acknowledged

Similarities between in situ measurements of local dust light scattering and dust flux impact data within the coma of 1P/Halley

In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density are only available for one comet, 1P/Halley. These data, returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DIDSY) aboard the European Space Agency spacecraft Giotto, are re-assessed in light of the recent improvements in the analysis and calibration of the OPE data after the encounter of the spacecraft with the comet 26P/Grigg-Skjellerup. We find that the local brightness and dust flux are remarkably consistent (even though the DIDSY data samples a limited range of particle mass) for distances from the nucleus in the 10(3) to 10(5) km range, and broadly speaking both data sets exhibit a -2 radial gradient (albeit with localised features). An interesting deviation from the -2 radial gradient is seen for the local brightness as Giotto approaches within 2000 km of the nucleus (which corresponds to Giotto crossing from the anti-sunward side of the terminator plane to the sunward side). These two in situ data sets, and their similarities and correlation, now offer an excellent diagnostic test of the effectiveness and validity of cometary coma models

In situ dust measurements from within the coma of 1P/Halley: First-order approximation with a dust dynamical model

In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density, are only available for one comet, 1P/KalIey. These data were returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DID) aboard the European Space Agency spacecraft Giotto. Due to a detailed calibration of OPE at the time of Giotto's encounter with comet 26P/Grigg-Skellerup, as well as improved analysis techniques, the similarities and correlation between the OPE and DID data sets can be reassessed. In this paper, we utilize this opportunity to compare these unique observations with a cometary coma dynamical model. Our results indicate that, to first order, the data can be fitted by a coma model that incorporates a grain size distribution index (at the nucleus), which need not be time dependent. Further, we find that the general shape of both the OPE and DID data can largely be explained by Keplerian dynamics alone, without recourse to fragmentation processes. The model is used to constrain the cometary dust bulk density, and a likely range of 50 < p < 500 kg m(-3) is found, although a value of rho = 100 kg m(-3) is favored. In addition, the corresponding favored solution of the geometric albedo A(p)(alpha = 0 degrees) is found to be 0.04. Within the quoted density range, the ratio between density and albedo remains constant at rho/A(p)(alpha = 0 degrees) approximate to 2500 kg m(-3) The modeling also indicates that the grain size distribution power index at the nucleus is constant (in the range 10(-12) < in < 10(-3) kg) and has a likely value k = -2.6 +/- 0.2 (i.e., cumulative mass distribution index b = -0.53 +/- 0.1)

Experimental scattering matrices of clouds and randomly oriented particles

In the atmospheres of planets and satellites, liquid particles may occur in the form of clouds, hazes, fog, and rain. The liquid can be water as is the case in the atmosphere of the Earth but also other materials, like sulfuric acid that occurs in the atmosphere of Venus. These liquid particles can be safely assumed to be homogeneous spherical particles. There is, however, also a large variety of non-spherical particles in very different astronomical environments ranging from the Earth’s atmosphere to other planetary and cometary atmospheres in the solar system, the interplanetary medium, reflection nebulae, atmospheres of brown dwarfs, etc. In these cases, the assumption of spherical particles is highly unrealistic. This is well known for the Earth’s atmosphere where mineral dust is one of the most prominent aerosol classes. The main sources of mineral dust are the big deserts and their margins (Mishchenko et al. 2002; Nousiainen 2009). In addition, volcanic eruptions inject gas and volcanic ash into the Earth’s atmosphere affecting its radiative balance. Moreover, a large number of cloud particles consist of non-spherical ice crystals (see, e.g., Yang and Liou 2006; Baran 2009). There are other examples in the solar system where we can find small irregular (mineral) particles. This is the case for the atmosphere of Mars, where dust particles from its surface are regularly swept up into its atmosphere by winds (e.g. Wolff and Clancy 2003; and Chapter 17). The atmospheres of the giant planets such as Jupiter and Saturn, and satellites like Titan have solid particles of condensed material in clouds and hazes (e.g. West and Smith 1991; and Chapter 19). In studies of solar system objects, light scattered by those particles is usually the only tool we have for determining their physical characteristics, such as size, shape, and refractive index. By analyzing the spectral dependence of the observed intensity and polarization of the scattered light we can retrieve information on the physical characteristics and location in the atmosphere of the scattering particles

The inner dust coma of Comet 26P Grigg-Skjellerup: multiple jets and nucleus fragments?

On 1992 July 10 the European Space Agency's spaceprobe Giotto passed the nucleus of the comparatively inactive comet 26P/Grigg-Skjellerup at a relative velocity of 14 km s(-1). This Giotto Extended Mission (GEM) followed a highly successful encounter in 1986 with Comet 1P/Halley. We present results returned from the Optical Probe Experiment (OPE) and in particular consider data gathered by the channels sensitive to the scattering of solar light by cometary dust grains, in emission-free continuum bands. Owing to the demise of the Halley Multicolour Camera (HMC) during the Halley encounter, and the low number of impacts registered by the Dust Impact Detection System (DIDSY), OPE data offer the best indication of the actual encounter geometry. We find that it is likely that Giotto was on the sunward side of the shadow terminator plane at closest approach, with our modelling results suggesting that Giotto passed similar to 100 km from the nucleus (although distances of up to 300 km cannot be ruled out). We investigate possible causes of the striking 'spike' features, or 'events', in the OPE data. While scattering of sunlight from ejecta particles as a result of dust impacts on the spacecraft body cannot be ruled out, considerations of the hypervelocity impact mechanisms and impact geometry show that this explanation is not without problems, and more investigation is needed before it can be conclusively accepted. As an alternative solution, we find that the complex data profiles can be fitted by jet activity in the innermost coma (which was not resolvable by ground-based observers). One particular event occurring at least 1000 km from the nucleus can be fitted if the OPE line of sight passes close to a nucleus fragment of radius 10-100 m which is situated around 50 km from the spacecraft and which is producing a small dust coma

Physics and chemistry of icy particles in the universe: answers from microgravity

During the last century, the presence of icy particles throughout the universe has been con:rmed by numerous ground and space based observations. Ultrathin icy layers are known to cover dust particles within the cold regions of the interstellar medium, and drive a rich chemistry in energetic star-forming regions. The polar caps of terrestrial planets, as well as most of the outer-solar-system satellites, are covered withan icy surface. Smaller solar system bodies, suchas comets and Kuiper Belt Objects (KBOs), contain a signi:cant fraction of icy materials. Icy particles are also present in planetary atmospheres and play an important role in determining the climate and the environmental conditions on our host planet, Earth. Water ice seems universal in space and is by far the most abundant condensed-phase species in our universe. Many research groups have focused their eAorts on understanding the physical and chemical nature of water ice. However, open questions remain as to whether ices produced in Earth's laboratories are indeed good analogs for ices observed in space environments. Although temperature and pressure conditions can be very well controlled in the laboratory, it is very diDcult to simulate the time-scales and gravity conditions of space environments. The bulk structure of ice, and the catalytic properties of the surface, could be rather diAerent when formed in zero gravity in space. The author list comprises the members of the ESA Topical Team: Physico-chemistry of ices in space. In this paper we present recent results including ground-based experiments on ice and dust, models as well as related space experiments performed under microgravity conditions. We also investigate the possibilities of designing a new infrastructure, and /or making improvements to the existing hardware in order to study ices on the International Space Station (ISS). The type of multidisciplinary facility that we describe will support research in crystal growth of ices and other solid refractory materials, aerosol microphysics, light scattering properties of solid particles, the physics of icy particle aggregates, and radiation processing of molecular ices. Studying ices in microgravity conditions will provide us with fundamental data on the nature of extraterrestrial ices and allow us to enhance our knowledge on the physical and chemical processes prevailing in diAerent space environments