The particles in the dust trails are suggested to have diameters of the order of
1 mm although there are numerous assumptions in this value such that it should be
taken as indicating an order of magnitude. Trails represent the principal means by
which comets contribute dust to the zodiacal light and their total contribution is of
the order of 10
11 kg/year (Sykes et al. 2004) but with large uncertainties.
4.12 Radiometric Properties of Dust
4.12.1 The Colour of Dust
Studies of the colour of dust emitted from nuclei have been made for many years.
Lamy et al. (1989), for example, summarized measurements of 1P/Halley made
through ground-based and spacecraft observations. Dust colours using the spectral
gradient definition from Eq. (2.85) appear to cover a range of values. For 1P/Halley,
values of the spectral gradient (Sect. 2.7) of between 6 and 9% (100 nm)
À1 were
obtained. Kolokolova et al. (2004) give a general range of 5–18% (100 nm)
À1 for
wavelengths between 350 and 650 nm.
It is widely assumed that the dust colour should be the same as that of the
non-volatile material at the source. However, because light is being scattered
preferentially by particles close to the wavelength, there may be subtleties to account
for. One of the interesting but not yet fully explained results from the observations of
1P/Halley by the Halley Multicolour Camera (HMC) onboard Giotto spacecraft
concerns the relative colour difference between the dust and the nucleus. Thomas
and Keller (1989) showed difference images that revealed that the nucleus was less
red than dust in the coma at a scattering angle of 73
. Although the absolute colour of
the dust and the nucleus were the same within error, the relative difference was
clearly visible and equal to ~3–4% (100 nm)
À1 for the range 440 nm to 650 nm.
Given that the visible nucleus surface is the source of the dust, the surface might
contain an additional component that, when combined with the dust, produces a
more blue surface colour. A mixture with water ice might produce such an effect.
Before major activity starts, the surface may be more dusty and redder. But when
erosion is rapid, more ice becomes evident and one no longer see the pure, redder
colour of the dust. Such a model was suggested by Fornasier et al. (2016) for
observed colour changes on 67P. Given that water ice must be present close to the
surface in order to drive dust emission and given its relatively blue colour in pure
form (Pommerol et al. 2015), this is clearly plausible although it requires a mixture
of the components that provides an almost constant colour over much of the
illuminated and visible surface of 1P/Halley as seen by HMC. In other words, it
requires a large degree of homogeneity over the surface.
An alternative in the case of 1P/Halley is that dust in the coma between the Sun
and the nucleus preferentially scatters blue light in the forward direction so that the
nucleus is illuminated by a slightly bluer light distribution than a solar spectrum
(Fig. 4.5). This effect was seen at Mars where relative variations in the colour of
surfaces with time of day could be used to demonstrate clearly the effectiveness of
4.12 Radiometric Properties of Dust
375
1 mm although there are numerous assumptions in this value such that it should be
taken as indicating an order of magnitude. Trails represent the principal means by
which comets contribute dust to the zodiacal light and their total contribution is of
the order of 10
11 kg/year (Sykes et al. 2004) but with large uncertainties.
4.12 Radiometric Properties of Dust
4.12.1 The Colour of Dust
Studies of the colour of dust emitted from nuclei have been made for many years.
Lamy et al. (1989), for example, summarized measurements of 1P/Halley made
through ground-based and spacecraft observations. Dust colours using the spectral
gradient definition from Eq. (2.85) appear to cover a range of values. For 1P/Halley,
values of the spectral gradient (Sect. 2.7) of between 6 and 9% (100 nm)
À1 were
obtained. Kolokolova et al. (2004) give a general range of 5–18% (100 nm)
À1 for
wavelengths between 350 and 650 nm.
It is widely assumed that the dust colour should be the same as that of the
non-volatile material at the source. However, because light is being scattered
preferentially by particles close to the wavelength, there may be subtleties to account
for. One of the interesting but not yet fully explained results from the observations of
1P/Halley by the Halley Multicolour Camera (HMC) onboard Giotto spacecraft
concerns the relative colour difference between the dust and the nucleus. Thomas
and Keller (1989) showed difference images that revealed that the nucleus was less
red than dust in the coma at a scattering angle of 73
. Although the absolute colour of
the dust and the nucleus were the same within error, the relative difference was
clearly visible and equal to ~3–4% (100 nm)
À1 for the range 440 nm to 650 nm.
Given that the visible nucleus surface is the source of the dust, the surface might
contain an additional component that, when combined with the dust, produces a
more blue surface colour. A mixture with water ice might produce such an effect.
Before major activity starts, the surface may be more dusty and redder. But when
erosion is rapid, more ice becomes evident and one no longer see the pure, redder
colour of the dust. Such a model was suggested by Fornasier et al. (2016) for
observed colour changes on 67P. Given that water ice must be present close to the
surface in order to drive dust emission and given its relatively blue colour in pure
form (Pommerol et al. 2015), this is clearly plausible although it requires a mixture
of the components that provides an almost constant colour over much of the
illuminated and visible surface of 1P/Halley as seen by HMC. In other words, it
requires a large degree of homogeneity over the surface.
An alternative in the case of 1P/Halley is that dust in the coma between the Sun
and the nucleus preferentially scatters blue light in the forward direction so that the
nucleus is illuminated by a slightly bluer light distribution than a solar spectrum
(Fig. 4.5). This effect was seen at Mars where relative variations in the colour of
surfaces with time of day could be used to demonstrate clearly the effectiveness of
4.12 Radiometric Properties of Dust
375