Chapter 4
Dust Emission from the Surface
It is dust that makes bright comets visible to the naked eye through scattering of
sunlight. The source of the dust is obviously the nucleus. But the details of the
ejection mechanism are not well known. The simplest concept is that the dust is
embedded in the ice and that when the ice sublimes, the dust is ejected by the
subliming ice that surrounds it. While there are issues with this concept, it is useful as
a starting point.
Some of the energy acquired by the gas through the sublimation process is
transferred to the dust via gas drag. Hence, as the gas expands to form the inner
coma, dust is accelerated away from the nucleus surface. Eventually, the gas density
becomes sufficiently low that drag on the dust becomes insignificant, the acceleration drops to near zero and the dust reaches a terminal velocity with respect to the
nucleus. Even for relatively active comets, this gas drag acceleration region is quite
small in radial extent being typically <<50 km. However, as the distance from the
nucleus increases, the influence of more subtle forces begins to become apparent
resulting in, for example, the production of the cometary dust tail. By following the
forces acting, we are able to describe several phenomena of interest.
4.1 The Point Source Approximation
As with the gas, a point source approximation can be used for the dust in the first
instance. In analogy with Eq. (3.1), the dust density can be written as
n d r
ð Þ ¼
Q d
4πr 2 v d
ð4:1Þ
where v d is a bulk velocity for the dust. This approximation has many of the
deficiencies already discussed for the gas including the inadequate assumption of
isotropic emission. Furthermore, the gas flow does tend to homogenize more readily
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_4
281
Dust Emission from the Surface
It is dust that makes bright comets visible to the naked eye through scattering of
sunlight. The source of the dust is obviously the nucleus. But the details of the
ejection mechanism are not well known. The simplest concept is that the dust is
embedded in the ice and that when the ice sublimes, the dust is ejected by the
subliming ice that surrounds it. While there are issues with this concept, it is useful as
a starting point.
Some of the energy acquired by the gas through the sublimation process is
transferred to the dust via gas drag. Hence, as the gas expands to form the inner
coma, dust is accelerated away from the nucleus surface. Eventually, the gas density
becomes sufficiently low that drag on the dust becomes insignificant, the acceleration drops to near zero and the dust reaches a terminal velocity with respect to the
nucleus. Even for relatively active comets, this gas drag acceleration region is quite
small in radial extent being typically <<50 km. However, as the distance from the
nucleus increases, the influence of more subtle forces begins to become apparent
resulting in, for example, the production of the cometary dust tail. By following the
forces acting, we are able to describe several phenomena of interest.
4.1 The Point Source Approximation
As with the gas, a point source approximation can be used for the dust in the first
instance. In analogy with Eq. (3.1), the dust density can be written as
n d r
ð Þ ¼
Q d
4πr 2 v d
ð4:1Þ
where v d is a bulk velocity for the dust. This approximation has many of the
deficiencies already discussed for the gas including the inadequate assumption of
isotropic emission. Furthermore, the gas flow does tend to homogenize more readily
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_4
281