Geoscience Reference
In-Depth Information
Table 3. Lower limits on
ρ
bulk
obtained by Davidsson,
55
using empir-
ical data on the nucleus size, shape, and rotational period. Note that
the material strength was assumed negligible and that only 7 out of 14
objects required
ρ
bulk
in excess of 200 kg
/
m
3
(due to long rotational
periods in comparison with the critical breakup periods).
Lower limit on
ρ
bulk
(kg
/
m
3
)
Comet
31P/Schwassmann-Wachmann 2
530
6P/d'Arrest
440
95P/Chiron
370
46P/Wirtanen
350
107P/Wilson-Harrington
340
10P/Tempel 2
250
C/1991 L3 Levy
200
It is noted that the unusual fast-spinning Comet 133P/Elst-Pizarro,
with a rotational period
P
=3
.
471 h and nucleus semi-axes
a
=3
.
0km,
b
=
c
=2
.
1 km (according to observations by Hsieh
et al.
57
), requires a
bulk density of at least
ρ
bulk
=1
,
300 kg
/
m
3
to remain intact, in case it is
strengthless. However, if the comet material possesses some tensile strength
(which does not even have to be as large as the strength predicted by
Greenberg
et al.
,
58
for porous core-mantle grain material), the bulk density
could easily be as low as 100 kg
/
m
3
, without risking rotational breakup (all
according to a formula valid for prolate ellipsoids, derived by Davidsson
55
).
In particular, the largest tensile strength required in the interval 100
≤
1
,
300 kg
/
m
3
ρ
bulk
≤
amounts to
∼
200 Pa (needed if the bulk density is
600 kg
/
m
3
).
ρ
bulk
≈
4.4.
Radar observations
Harmon
et al.
59
,
60
have detected radar echoes from a number of comet
nuclei. The ratio between the radar and geometric cross sections (i.e., the
radar albedo) can be used to estimate the dielectric constant in the surface
material. This in turn yields estimates of the bulk density in the uppermost
layer of the nucleus, for various assumptions regarding the composition.
Table 4 shows such estimates for a number of comets, assuming either pure
water snow or a silicate powder to cover the nucleus surface. Note that
these values may not be representative for the entire nucleus (e.g., presence
of macroscopic void spaces in the deep comet interior may reduce
ρ
bulk
further).
