Geoscience Reference
In-Depth Information
(although the discrepancy could be explained by sublimation-induced shape
modification).
The presence of active and inactive areas on comets (the former typi-
cally covering less than 20% of the surface of JFCs, according to Fernandez
et al.
35
), could in principle be yet another sign of cometary collisional evo-
lution. One can imagine that water ice was a globally distributed surface
species for newly formed comets, given the previously mentioned small
degree of heating during the formation processes. However, impact veloc-
ities in the present-day EKB are substantially higher
29
500 m
/
s
at 42 AU) than in the Solar Nebula, due to the absence of gas. The energy
delivered by an impact could then deprive the surface material of ice in
the immediate vicinity of the crater, thereby forming a “dead spot” on the
surface (i.e., little or no gas would subsequently be produced there, even
when solar illumination is strong). Durda and Stern
30
estimated that a 1 km
body in the EKB would experience 90-300 impacts by bodies larger than
4 m during 3
.
5 Gyr, and that 20-224% of its surface would be cratered (i.e.,
the same place might be hit more than once) — numbers that are compa-
rable to the cometary surface fraction of inactive areas. The reason that
substantial fractions of JFC surfaces are inactive could therefore be that
surface volatiles have been removed long ago in the EKB due to collision-
induced melting. Consequently, “active areas” on cometary surfaces may
therefore represent old pristine material which has not experienced impacts
(i.e., surface ice is still present). It is noted that the Oort cloud comets
may also have been collisionally evolved
36
(
V
rel
≈
before leaving the planetary
region.
Due to the potential importance of cometary collisional evolution, it is
not obvious that the density
ρ
bulk
∼
600 kg
/
m
3
, characterizing the earliest
planetesimals, also is representative for the current population of comets.
Whether the collisions predominantly have led to compaction (increase of
ρ
bulk
) or crack formation (decrease in
ρ
bulk
), is hard to say. However, impor-
tant lessons can be learned by considering the rapidly growing body of
information available for asteroids (see, e.g., the review by Britt
et al.
37
).
With the exception of a few of the largest asteroids, which appear to be
compact and coherent objects, most investigated asteroids seem to have
porosities in the range 0
.
3
0
.
7, with a tendency for clustering
at the lowest value. The dominant mechanism for producing a poros-
ity in excess of
≤
ψ
≤
∼
30% appears to be impact-induced shattering, followed
by dispersion of the fragments and, later, gravitational reaccumulation
