Geoscience Reference
In-Depth Information
81P/Wild 2 by the same authors
49
yielded
ρ
bulk
∼
600-800 kg
/
m
3
(depend-
ing on the applied nucleus volume).
4.2.
Tidal disruption
A handful of comets have been observed to split due to tidal forces in
the vicinity of Jupiter or the Sun.
50
The most well studied of these
objects is Comet D/1993 F1 Shoemaker-Levy 9, which disrupted in 1992
and later impacted Jupiter in 1994. By modeling the morphology of the
post-disruption nucleus chain, Asphaug and Benz
51
±
100 kg
/
m
3
if the parent body did not rotate, and
ρ
bulk
∼
1
,
000 kg
/
m
3
if
a prograde 9 h period was assumed. A similar study by Solem
52
yielded
ρ
bulk
= 500-600 kg
/
m
3
. Furthermore, a study of the impact features on
Jupiter made by Crawford
53
found
ρ
bulk
= 600
suggested that the impactors had densities
around
ρ
bulk
≈
250 kg
/
m
3
.
4.3.
Rotational stability
In order for a comet nucleus to remain intact, self-gravity and material
forces must provide a sucient centripetal force to avoid breakup due to
nucleus rotation. Formulae derived by Davidsson
54
,
55
show how the critical
breakup rotational period depends on body properties such as size, shape,
bulk density, tensile, and shear strength. When the size, shape, and spin
period of a comet are estimated empirically, these formulae may also be used
to place a
lower limit
on the bulk density, although an assumption must be
made regarding the material strength. None of the 14 objects studied by
Davidsson
55
600 kg
/
m
3
to
stay intact, when assuming zero strength (see Table 3), although it cannot
be excluded that the true densities are higher. The same conclusion was
reached by Lowry and Weissman,
32
when studying a sample of 13 objects.
Although this technique has limited importance for individual comets
or small samples of objects, a larger sample may reveal a true physical cut-
off in the parameter space, i.e., non-existence of rotational periods below
a certain value for comets with a particular size, which in turn places a
constraint on the bulk density and material strength of the objects. The
investigation by Toth and Lisse,
56
in which 29 objects were studied, is an
important step in this direction — the cut-off appear consistent with a
bulk density of
therefore
required
a bulk density higher than
∼
300 kg
/
m
3
250 Pa. A tendency
of finding shorter rotational periods for smaller objects may indicate that
this in fact represents the
physical
cut-off limit.
∼
and tensile strength of
∼
