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(i.e., rubble-pile formation). Crack formation, on the other hand, is only
considered to yield very modest increases in porosity. If this also is true for
comets, substantial increases of their porosity during collisions should not
be expected.
The perhaps most likely outcome of an impact on a porous target like
a comet, is therefore compaction. However, the degree of porosity decrease
may not be severe, as illustrated by the oddly shaped asteroid 253 Mathilde,
which presently has a porosity of about
ψ
0
.
4. It has endured at least
five giant collisions which, according to Housen
et al.
,
38
only has increased
its bulk density by
≈
20%. If similar numbers apply for the collisional evo-
lution that JFCs may have experienced in the EKB, the expected cometary
bulk density limit would just be adjusted to
ρ
bulk
∼
700 kg
/
m
3
. But even
if collisions had a more severe effect on the porosity, it is not likely to
decrease below
ψ
∼
≈
0
.
36, the random close packing limit (as long as the
0
.
1
µ
m grains are not forced to merge, or are ground down
to even smaller pieces). The reason is that it becomes increasingly di
cult
to crush particles with decreasing size
l
(for silicate material, compressive
strength roughly scales as
l
−
1
/
2
), and to compact silicate soils significantly
beyond the close packing limit requires much higher pressures than can be
delivered in collisions at a few kilometers per second.
37
This may in fact
be the reason why most non-monolithic asteroids do not have a porosity
below
ψ
∼
constituent
0
.
3. If the close packing limit is applied to comet material, this
would suggest a bulk density limit of
ρ
bulk
∼
1
,
000 kg
/
m
3
.
Another mechanism that could modify the density, is radiogenic core
melting. Prialnik
et al.
39
showed that if the planetesimal abundance of
26
Al
was as high as in the Allende meteorite, icy bodies with
r
∼
6km would
melt. Liquid water could then fill microcavities before freezing, potentially
creating a core region in larger bodies characterized by
ρ
comp
. If such a body
was disrupted in a collision in the EKB, the fragments (of which some may
be seen as JFCs today) could potentially be relatively solid bodies. However,
the authors pointed out that presence of amorphous water ice (which may
be responsible for the frequently observed cometary outbursts), rules out
such radiogenic core melting.
To conclude, it is expected that truly pristine comets have bulk densi-
ties
ρ
bulk
∼
600 kg
/
m
3
. In case collisions have compacted the bodies, it is
still not likely that the bulk density exceeds
≈
1
,
000 kg
/
m
3
, since collisions
hardly could have compressed the bodies beyond the close packing limit.
However, if radiogenic core melting indeed took place, at least a fraction of
∼
