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In-Depth Information
THE BULK DENSITY OF COMETARY NUCLEI
BJ
¨
ORNJ.R.DAVIDSSON
Department of Astronomy and Space Physics, Uppsala University
Box 515, SE-751 20 Uppsala, Sweden
bjorn.davidsson@astro.uu.se
This paper reviews the current knowledge about bulk densities and porosities of
cometary nuclei. Grain agglomeration and formation of small planetesimals in
the Solar Nebula, as well as the subsequent collisional and thermal evolution
of cometary nuclei are discussed, in order to provide a theoretical expecta-
tion on the range of bulk densities for the present comet population. Various
methods for estimating cometary bulk densities, and corresponding results are
reviewed, showing a reasonable agreement with the previously mentioned theo-
retical range. Current empirical estimates indicate that comets are bodies with
rather low density and high porosity (typically
ρ
bulk
= 600
±
400 kg/m
3
and
ψ
=0
.
6
±
0
.
3, respectively).
1. Introduction
Comets are the most primitive and least evolved bodies in the Solar Sys-
tem. As such, studies of their chemical composition and physical prop-
erties will yield invaluable information about the conditions prevailing in
the Solar Nebula more than 4 billion years ago. For example, their bulk
densities and porosities may place important constraints on planetesimal
formation scenarios. However, it is not clear to what extent the present pop-
ulation of comets differ from the small planetesimals which built the Jovian
planets and their moons — a substantial collisional and thermal evolution
has possibly taken place, and it is necessary to investigate how this may
have affected the structure of these bodies. Furthermore, bulk densities are
dicult to measure and increased efforts are needed to improve current
estimates.
It is first necessary to define a plausible upper limit on the nucleus
density. Greenberg and Hage
1
and later Greenberg
2
tried to define
the composition of a “typical” comet nucleus by considering the ele-
mental abundances in the interstellar medium, the Solar System, the
dust of Comet 1P/Halley, and the gas comae of several comets.
They obtained mass fractions of silicates, organic refractories, volatiles
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