Spatial structure in the color of the dust coma of comet P/Halley

Physical Properties of Cometary Dust

“…So far, there have been few interpretations limited to one particular observational result. Jewitt and Meech (1986a) found that size sorting by radiation pressure is able to explain the color gradient in the solar direction, an interpretation disputed by Hoban et al (1989), who claim that all the points measured were outside the dust paraboloid. However, they find this mechanism adequate to explain their own observation of redness of the envelopes and jets, as it leads to an apparent excess of intermediate-size particles (typically 0.5 to 0.9 \im).…”

“…Hoban et al (1989) found dust envelopes redder than the surrounding dust coma with a strong jet even redder. Lamy et al (1989a) observed a strong asymmetry between the solar and anti-solar hemisphere, with the former being redder than the latter; a similar trend, in which the coma became redder with increasing distance in the sunward direction, was observed in April 1986 by Jewitt and Meech (1986b) Figure 15.…”

“…The observations yield the color of the volume scattering function \|/(0, X), i.e., the phase function of a unit volume of the dust cloud (that is integrated over the size distribution), which involves differential cross-sections. Model calculations such as those performed by Jewitt and Meech (1986) or Hoban et al (1989) are based on the total scattering cross-sections (also averaged over the size distribution). There is strictly no relationship between differential and total cross-sections, but, furthermore, experimental studies have shown that the shape of the phase function or \(i(0) varies with wavelength (Killinger, 1987).…”

Properties of the dust in the coma of 67P/Churyumov-Gerasimenko observed with VIRTIS- M

“…Unfortunately, these measurements show a large amount of scatter because most of them were taken with filter photometry, which can be perturbed by unknown cometary gas emissions, and so we presently do not review all of these reports. Probably, the most reliable ones are those for comet 1P/Halley by Meech & Jewitt (1987), who reported a colour of 9 ± 2 per cent per 100 nm for the wavelength range of 440-689 nm and those of Hoban et al (1989), who obtained 7 per cent per 100nm using two continuum filters at 484 and 684 nm. The reflectance colours for 1P/Halley can also be seen in eight spectra from 520 to 1040 nm in Fink (1994).…”

Coma morphology of Jupiter-family comets

“…This suggests that either the grains from active areas have an inherently different size distribution than from the ambient coma, or else the collimation mechanism (e.g., dust flowing through a vent or tube) batters the fragile grains, breaking them up before or shortly after they leave the nucleus. However, there are comets that have features that are redder than the surrounding coma (Hoban et al, 1989;Jockers and Bonev, 1997) and others where the features have the same color (e.g., Schulz and Stüwe, 2000). Radial color variations may also indicate an evolution of the dust as it expands away from the nucleus.…”