Geoscience Reference
In-Depth Information
Table 4. Near-surface bulk densities (kg
/
m
3
) derived from radar cross sections observed
by Harmon and colleagues,
59
,
60
assuming two different compositions — water snow and
silicate powder. The average bulk density may represent ice/dust mixtures.
Density
Density
Density
Comet
(snow)
(Si powder)
(average)
C/1983 H1 IRAS-Araki-Alcock
500
900
700
2P/Encke
500
900
700
C/1983 J1 Sugano-Saigusa-Fujikawa
700
1,300
1,000
26P/Grigg-Skjellerup
700
1,300
1,000
C/1996 B2 Hyakutake
200
400
300
4.5.
Nucleus fragmentation and disruption
About 30 comet nuclei have been observed to split or partially fragment (see
the excellent review by Boehnhardt
61
), often for no apparent reason (i.e.,
non-tidally). Desvoivres
et al.
62
have developed a model of the dynam-
ics of fragments, based on the nucleus gravity and sublimation-induced
non-gravitational forces acting on them, which can be used to estimate
the mass of such fragments. The authors applied their model to Comet
C/1996 B2 Hyakutake, which expelled a number of fragments in March
1996. The estimated bulk density of the fragments depend on their assumed
sizes. Taking the sizes as 20, 10 and 7 m, yields corresponding bulk den-
sities of 300, 600, and 800 kg
/
m
3
, respectively, showing that their densi-
ties hardly could have been as high as
ρ
comp
unless they were extremely
small.
4.6.
Binary Edgeworth-Kuiper objects
Given the probable relation between small EKOs and JFCs, it is of
interest to study the bulk densities of the former objects, at least for
bodies small enough (
r
∼
100 km) to be unaffected by gravitational
compression.
10
A number of binary EKOs have already been discovered,
63
which allows system mass estimates to be made if the orbital periods and
separation distances are determined. In case optical and thermal infrared
observations are performed simultaneously, estimates of the object albe-
dos and sizes are obtained, which would yield the sought-for bulk den-
sity of the pair. Although the primaries discovered so far typically have
sizes above the gravitational compression limit, this technique may pro-
vide important information on bulk densities of small EKOs in the near
future.
