Geology Reference
In-Depth Information
Figure 7.5. An oblique view of the 210m in diameter Bonneville
crater and the path of the Spirit rover; A, B, and C show the locations
of various ripples and dunes, some of which
fill many of the small
craters on Mars, partly accounting for the shallow depths of the
craters (NASA HiRISE frame PSP_00151_1655, part).
Figure 7.7. An oblique view of a complex impact crater 20 km in
diameter in the northern hemisphere of Mars, showing terraced
inner walls, a small central peak, and
flow-like ejecta deposits; also
shown are
uvial channels on which the impact was superposed (ESA
HRSC image #435).
sites, many of which are thought to be secondary craters
highly modified by erosion and in fill by windblown sedi-
ments
(Fig. 7.5)
. In general, primary martian craters show
a progression in morphology with size from simple bowl-
shaped features
(Fig. 7.6)
, through complex craters with
terraced walls and central peaks
(Fig. 7.7)
, to central
peak
ring structures, such as Lowell crater
(Fig. 7.8)
.
This progression is similar to that on the Moon, but the
transitions in morphology occur at smaller sizes on Mars,
due in part to the higher gravity.
Ejecta deposits of many martian impact craters exhibit
flow-like patterns
(Fig. 7.7)
distinct from those seen on the
Moon and Mercury and suggest that the impacts occurred
in terrain that was wet or contained ice that was melted by
the impact. The mixture of rock, soil, and water is thought
to have formed a slurry-like mass that was ballistically
emplaced but then slid outward across the surface from the
crater. If this idea is correct, then the distribution and
timing of these impacts could provide insight into the
nature of the surface through Mars
'
history, and numerous
groups have been mapping the craters showing the
ow-
like patterns for this purpose (Barlow et al.,
2000
). An
alternative suggestion by Pete Schultz of Brown
University is that
flow-like ejecta could result from inter-
action of the ejecta with the atmosphere.
Craters in some regions show extensive modi
cation,
mostly by processes of gradation. Many craters in the higher
latitudes are described as
“
pedestal
”
craters because they are
-
Figure 7.6. Small fresh impact craters on Mars are typically bowl-
shaped, exempli
ed by this crater 2.6 km in diameter on northern
Elysium Planitia (NASA PIA02084).
expand and shrink with the martian seasons and are under-
lain by more permanent water-ice, all interlayered with
windblown dust and other deposits.
7.5.2 Impact craters
Martian impact craters range in size from the 1,800 km
Hellas basin down to meter-size depressions seen at lander
