Geoscience Reference
In-Depth Information
two equal-sized boulders of radius
r
and porosity
ψ
=0
.
6, colliding at
V
rel
≤
0
.
03
r
, i.e., the bod-
ies barely attach. The porosity in the compaction zone may be reduced
substantially, but due to its small size, the overall effect is rather small.
Furthermore, one might expect that macroscopic void spaces are formed
between blocks which may compensate (or even overcompensate) the com-
pression during collisions. For this reason, it is expected that planetesimals
maintain a porosity of
ψ
10 m
/
s penetrate each other to a distance of
∼
0
.
65 until they reach the typical size of a comet,
r
= 1-10 km. Since a collision even at
V
rel
=50m
/
s only leads to a local
heating of
≥
20 K, it is expected that planetesimal growth proceeds with-
out substantial loss of volatiles.
15
∼
Applying 1
,
400
∼
ρ
comp
∼
1
,
700 kg
/
m
3
,
and a porosity
ψ
0
.
65 makes it plausible that
newly formed plan-
etesimals of cometary size in the outer Solar System had bulk densities
ρ
bulk
∼
600 kg
/
m
3
.
≥
3. Collisional and Thermal Evolution of Cometary Nuclei
Dynamical studies
17
-
21
have shown that the Jupiter Family Comets (JFCs),
i.e., comets with a Tisserand parameter 2
≤
T
J
≤
3 with respect to Jupiter,
andwithorbitalperiods
P
20 years, most likely have their origin in the
Edgeworth-Kuiper Belt (EKB). Edgeworth-Kuiper Objects (EKOs) larger
than
≤
r
−α
, where the cumulative
∼
100 km have a size distribution
N
∝
0
.
3 according to Gladman
et al.
,
22
power law index is
α
=3
.
4
±
and
α
=3
.
2
+0
.
4
−
0
.
3
according to Trujillo
et al.
23
However, for smaller objects
(40
∼
r
∼
70 km), the power law index seems to decrease abruptly
24
to
α
≈
1
.
9
±
0
.
6 (although this estimate is based on a very small sample of
objects).
For a collisionally relaxed population,
α
=1
.
9
2
.
5 is expected,
25
-
27
which seems to indicate that the smaller EKOs are severely affected by
collisions. It has been suggested
25
,
28
-
30
that 50-90% of the
r
∼
10 km EKOs
(and thereby, the JFCs) in fact are collisional fragments.
Additional support for a collisional history may be found in the size
distribution for JFCs themselves, although the estimates of
α
for this pop-
ulation vary substantially. Fernandez
et al.
31
found
α
=2
.
65
−
±
0
.
25, Lowry
and Weissman
32
0
.
03, while Meech
et al.
33
estimated
α
=1
.
59
±
report
0
.
05. However, Jewitt
et al.
34
pointed out that JFCs gener-
ally are more elongated (average photometric range
α
=1
.
45
±
∆
m
R
=0
.
54
±
0
.
07)
0
.
05), which may indi-
cate that JFCs are not a population of collision fragments after all
∆m
R
=0
.
32
±
than small main belt asteroids (
