doi:10.1016/j.icarus.2005.10.018
Copyright © 2005 Elsevier Inc. All rights reserved.
The contribution of icy grains to the activity of comets: I. Grain lifetime and distribution
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E.H. Beer
, M. Podolak
, and D. Prialnik
Department of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
Received 13 January 2005;
revised 26 September 2005.
Available online 15 December 2005.
Abstract
We have developed a computer code (GEM—grain evolution model) to simulate the behavior of ice grains in a comet coma. The grains are assumed to be composed of water–ice with an admixture of dark material (“dirt”). An initial size distribution of grains is assumed to be ejected from the nucleus. The ejected mass is taken to be proportional to the rate of gas production by the nucleus. The efficiency for absorption and re-radiation of sunlight is computed from Mie scattering theory. The grain temperature and sublimation rate at a given heliocentric distance is then derived from energy balance considerations. The evolution of the grain size distribution is followed as a function of distance from the nucleus.
Keywords: Comets; Ices; Photometry
Fig. 1. Grain heating and cooling as a function of heliocentric distance. The grains presented are for dirty ice grains (X1=0.9). Squares represent radiative cooling; circles represent sublimation; pluses represent solar heating.
Fig. 3. The grain surface temperature as a function of heliocentric distance for various grain radii, for pure ice grains (X1=1).
Fig. 6. Pvap at different heliocentric distances as a function of grain radius. For pure ice (X1=1) and dirty ice (X1=0.9) grains, the distance and composition associate with each symbol is shown in the inset.
Fig. 7. The contribution of grains to the production rate (
) divided by the contribution of the nucleus to the production rate (
) for pure ice (circles) and dirty ice (squares) grains as a function of heliocentric distance.
Fig. 8. Lifetime of dirty ice (X1=0.9) grain distribution for different heliocentric distances. The distribution associated with each symbol is given in the inset.
Fig. 9. Same as Fig. 8 for pure ice grains.
Fig. 10. Amount of grains vs time since ejection for dirty ice grains (X1=0.9) at 3.68 AU. The different symbols represent the different groups of grain radii as shown in the inset.
Fig. 11. Same as Fig. 10 for dirty ice grains at 2.69 AU.
Fig. 12. Same as Fig. 10 for dirty ice grains at 0.48 AU.
Fig. 13. Number of grains vs distance traveled inside the coma at 1.1 AU. The different symbols represent the different groups, which are the same as Fig. 12.
Fig. 14. Number of grains as a function of grain radius, for different distances from the nucleus or dirty ice grains (X1=0.9). The nucleus is 1.1 AU from the Sun.
Fig. 15. Number of grains as a function of grain radius (both on a log scale), for a heliocentric distance of 1.1 AU at an inner distance inside the coma of 3.9 km. The grains are dirty ice grains (X1=0.5). The black bold line represents the grain distribution. The dotted lines represent the different fits according to their symbols: dotted line plus squares represent N(a)
a−5.8; dotted line plus triangles represent N(a)a−2.0; dotted line plus circles represent N(a)a−3.2.
Fig. 16. Number of grains as a function of grain radius (both on a log scale), for a heliocentric distance of 1.1 AU at an inner distance inside the coma of 237 km. The grains are dirty ice grains (X1=0.5). The black bold line represents the grain distribution. The dotted lines represent the different fits according to their symbols: dotted line plus squares represent N(a)a−1.3; dotted line plus triangles represent N(a)a−1.7; dotted line plus circles represent N(a)a−2.0.
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