Geoscience Reference
In-Depth Information
Fig. 1. Dust agglomerate of r =0 . 95 µ mSiO 2 spheres formed by differential sedimen-
tation. Image reproduced from Ref. 10 with permission from the author.
Increasing impact velocities leads to a certain degree of compaction,
and at some point the clusters no longer grow as fractals (although the
porosity remains high). Wurm and Blum 7 showed experimentally that this
restructuring limit is reached at V rel
0 . 2m / s. At even higher velocities,
the sticking eciency decreases and collisions no longer lead to growth.
Blum and Wurm 9 showed that this fragmentation limit occurs abruptly at
V rel
2m / s for spherical SiO 2 grains. At that point, 10
the clusters have
0 . 1m at r h =1AUand10 4 mat r h = 100 AU (where r h
is the heliocentric distance).
The fragmentation limit may therefore be a serious problem, since it is
dicult to explain how larger bodies could have formed, especially since the
largest relative velocities expected 10 , 11
only grown to
at r h 30 AU during planetesimal
50 m / s. However, Poppe et al. 12 showed that grains with
irregular surfaces have a strikingly different behavior compared to spherical
grains — even at V rel
formation is V rel
50 m / s, 30-80% of the grains stick upon impact.
Furthermore, Bridges et al. 13 have demonstrated experimentally that the
presence of frost on the grain surfaces may facilitate sticking, especially if
the water ice is mixed with methanol.
What is the expected porosity for bodies built by particle-by-particle
agglomeration at relative velocities V rel 50 m / s? Blum and Schrapler 10 , 14
have investigated experimentally how the porosity of so-called “dust cakes”
(i.e., large grain aggregates with an initial porosity of ψ
0 . 85) is affected
Search WWH ::




Custom Search