Geology Reference
In-Depth Information
Figure 6.10. A Magellan image of the 150 km in diameter Meitner
crater, a typical multi-ring impact structure on Venus (NASA
Magellan C1 MIDRP60S319).
hemisphere
(Fig. 6.10)
. Radar-dark
flat surfaces are seen
on the
floor and between the inner ring and the main rim,
while the continuous ejecta deposits extend only about
50 km beyond the rim, evidently a result of the high sur-
face gravity on Venus and dense atmosphere which would
retard the trajectories of the ejecta.
Even before radar data were available from spacecraft,
there was speculation that small craters would not be
found on Venus. The reasoning was that the dense atmo-
sphere would fracture small incoming objects into small
pieces, or perhaps even consume them. The smallest
obvious impact crater seen in Magellan data is about
1.5 km across; and, as speculated, it appears that incoming
objects do break apart into smaller pieces that reach the
surface to form a cluster of craters, much like the blast
from a shotgun
(Fig. 6.11)
. Thus, craters smaller than 10
-
20 km in diameter have irregular outlines and
oors,
unlike the simple bowl-shaped craters of this size seen
on other terrestrial planets.
For some inferred impact features, there are no obvious
topographic depressions, but only a radar-dark zone on
the surface
(Fig. 6.12)
. It is thought that as smaller bolides
pass through the dense atmosphere they are so fragmented
that only the shock wave and, perhaps, fine material
reaches
Figure 6.9. Impact crater Adivar is 30 km in diameter and has typical
features for complex craters on Venus, including a scalloped outline,
terraced walls, and a central peak. The continuous ejecta is radar-
bright, indicative of a rough surface, while the radar-bright
horseshoe-shaped zone beyond the continuous ejecta and the
“
tail
”
of bright ejecta extending to the left both indicate the in
uence of
the prevailing winds toward the west (left) at the time of the impact
(NASA Magellan P-38387).
(Fig. 6.9)
, which has a scalloped rim, central peak, and
at
floor. Except for the
floor deposits, Adivar
s rim and inner
wall are radar-bright, a consequence of the rugged, irreg-
ular topography. The continuous ejecta deposits are
sharply de
ned by a distinct outer boundary and are
radar-bright, which is indicative of rocky debris. The
“
'
and the horseshoe bright zone extending toward
the west are ejecta deposits that were caught by prevailing
winds from the east at the time of the impact and carried
westward for deposition.
Craters larger than 50
-
60 km in diameter transition
from complex features to multi-ring structures, such as
the 150 km in diameter Meitner
tail
”
the surface,
leaving the blast-scars visible
in the southern
on images.
