The sets of extraterrestrial particles collected from the air and from surface deposits (deep sea sediments and polar ice, for example) provide a rich source of data on the material properties of interplanetary dust particles in the 5micron to mm size range [1]. Most of the influx of extraterrestrial matter onto the Earth is dominated by meteoroids with diameters in the range 50 to 500 microns, (giving a mass of 10-6 g to 10-3 g, assuming a density of 2 g/cm3), with most of the mass in sizes of about 200 microns in diameter [1]. In their calculations and models, planetary scientists commonly assume a single nominal particle density for the interplanetary dust particle's density [1]. However, for real particles, there is a range of densities and even the meaning of a dust particle's "density" is complicated due to its irregular shape.
IDPs can also attach themselves to man-made debris, further complicating the identification of a particle's composition, density, and size, and charge surface effects, as seen in Figure 1.
Figure 1: Johnson Space Center catalog photo showing chondritic IDP U2015D8 rigidly attached to an aluminum oxide sphere (U2015D9). This image suggests some mechanism exists for the contamination of IDPs with man-made material found in the Earth's stratosphere. Each individual grain in this image measures 6 microns in diameter. (NASA JSC Photo S-84-41340). From Flynn, 1994 [2].
A study of the density measurement of by Love et al., (1996 [3]) found for chondritic composition particles an average at 2.0 g/cm3 (Fig. 3). Further determinations of the densities of IDPs have come from examination of the perforations of impacting meteoroids onto NASA's LDEF satellite and onto ESA's Eureca satellite [4]. The authors of this study find a slightly lower characteristic density of 1.6 g/cm3 for the meteoroid population incident on the Earth at satellite altitudes [4]. One result from particle density measurements is that common IDPs are much more porous than meteorites, but they are not as highly porous as is sometimes assumed in modeling [1].
Divine, N. et al., (1986) published a thorough analysis and model of comet Halley's dust and gas environment [5]. The parameters listed in that work are still used today in research by many dust modelers, for example, in the dust charging and dynamics papers by M. Horányi and his colleagues during the last decade. In Divine et al., (1986)'s work, the researchers surveyed the diverse evidence of particle densities from lunar microcraters, meteor stream studies, and collected Brownlee particles. The resulting densities range from 3.0 to 0.8 g/cm3, as the particle size varies from the smallest to the largest in the distributions [5]. Some sizes and densities calculated using this equation (1) can be seen in the following table (Table 1).
| radius (cm) | density (g/cm3) |
| 0. | 3. |
| 1.0e-5 | 2.9 |
| 1.44e-5 | 2.85 |
| 1.77e-5 | 2.82 |
| 2.04e-5 | 2.8 |
| 2.98e-5 | 2.72 |
| 4.51e-5 | 2.59 |
| 6.63e-5 | 2.45 |
| 1.02e-4 | 2.26 |
| 2.36e-4 | 1.8 |
| 5.57e-4 | 1.38 |
| 1.30e-3 | 1.09 |
| 2.94e-3 | .94 |
| 6.51e-3 | .86 |
| 1.42e-2 | .83 |
| 3.08e-2 | .81 |
| 6.66e-2 | .8 |
| 1.44e-1 | .8 |
| 3.1e-1 | .8 |
| 6.68e-1 | .8 |
| 1.0 | .8 |
Table 1: Some sizes and densities using the formula of Divine et al., 1986, [5] and applied in models by Horányi and his co-workers.
The elemental abundances and building blocks of both IDPs and MMs resemble those of C1 and C2 carbonaceous chondritic meteorites. The fluffy, fine-grained olivine aggregates and GEMS that are abundant in IDPs are not found so far in MMs [1].
Typical IDPs are fine-grained mixtures of thousands to millions of mineral grains and amorphous components. Most of the IDPs can have wide ranging elemental compositions at a given phase in the particle [1]. The great majority of IDPs fall into the chondritic class. The abundant elements Mg, Al, Si, S, Ca, Cr, Mn, Fe, and Ni are present within a factor of a few of CI chondritic abundance. The chondritic particles in good stratospheric collections typically comprise over 75% of the total particles that are not Al-rich space debris or clear-cut contaminants from the aircraft. Lamy, P. [7] notes other results from the laboratory analysis of IDPs. The carbon content is amorphous, and its content ranges from 5 to 25% higher than in meteorites. In addition, carbon is found in the form of chunks, but not in the particle's coating. Calcium is depleted. Volatile trace elements are enriched by factors of 2 to 5, except for sulfur and bromine. The organic material is different from that found in meteorites.
Rare gas abundances are present in both IDPs and MMs. The reason for this, is that the dust has been exposed to the solar wind and cosmic rays for a sufficiently long time so that the particle have accumulated solar noble gases and spallogenic isotopes.
GEMS are tiny submicrometer spheroids (0.1-0.5 microns in diameter) with bulk compositions that are approximately (within a factor of three) chondritic [8]. They form the building blocks of anhydrous CP IDPs in general, and cometary IDPs, in particular. Their compositions, minerology and petrography appear to have been shaped by exposure to ionizing radiation. Since the exposure occurred prior to the accretion of cometary IDPs, and therefore comets themselves, GEMS are likely either solar nebula or presolar interstellar grains. The properties of GEMS (size, shape, mineralogy) bear a strong resemblance to those of interstellar silicate grains as inferred from astronomical observations [8].
Many investigators characterize the particle's optical properties via the parameter:
Equation 1
;
where Qpr is averaged over the solar spectrum, a is a spherical
particle's radius, and 
is the
particle's density, in cgs units.
The Sun radiates nearly all of its energy in a narrow wave band around 0.6 microns
so that the transition from geometric optics to Rayleigh scattering takes place in the micrometer size range [11].
For any particle material greater than a few microns in size, geometrical
optics holds, thus Qpr is roughly constant, independent of particle size [10]. The important interactions
are for sizes comparable to the characteristic radiation wavelength divided by 2 
. The metals suffer a large radiation pressure because they backscatter radiation, whereas
the dielectrics are strong forward-scatterers. Graphite feels a relatively large radiation force because it absorbs
radiation over a wide wavelength range, very much like a metal.
The quantity is of order unity for micron-sized spherical particles [12
]. However, real dust particles are
not perfectly smooth spherical particles, and therefore, the light-scattering properties could be different.
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Dust Properties
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