Geology Reference
In-Depth Information
Figure 7.8. Lowell crater, west of the Argyre basin, is 201 km in
diameter and exhibits the multi-ring structure typical of larger
craters on Mars (NASA PIA02836).
on a mesa-like platform that corresponds roughly to their
ejecta deposits. This effect was
first seen on Mariner 9
images, and it was suggested that impacts occurred on
materials that could be de
ated but that the mesas were
rendered more resistant to erosion by the presence of the
ejecta that armored the surface. Although this idea still has
merit, it is dif
cult to imagine how
fine-grained ejecta at the
distances seen in
Fig. 7.9
would be effective in resisting
de
ation.
Impact crater size
Figure 7.9.
craters, such as this structure 5 km in diameter
in the northern hemisphere, are typi
“
Pedestal
”
ed by a surrounding mesa-like
platform (the pedestal), which corresponds approximately to the
extent of ejecta deposits (NASA THEMIS V02215005).
Third, in the absence of radiogenically dated surfaces on
Mars, there is no direct calibration for the crater counts, and
the data from the Moon
(Fig. 2.10)
must be extrapolated to
Mars. This requires making adjustments to account for the
difference in factors such as gravity (higher g results in
smaller craters on Mars than on the Moon for the same
impact event) and Mars
frequency distributions (i.e., crater
counts) provide a means for assessing surface ages in the
absence of radiogenic age determinations. As discussed in
Chapter 2
, crater count ages can be derived assuming that
(1) only primary impact craters are counted (or that secon-
dary craters are taken into account), (2) there has been no
“
-
proximity to the asteroid belt,
which could lead to more impacts as a function of time in
comparison with the Moon.
The issues surrounding crater counts for Mars have
been partly addressed, employing certain assumptions
and models. For example, volcanic craters are assumed
to be recognizable from their morphologies (e.g., they
tend to be non-circular), while the issue of secondary
craters was analyzed statistically and shown to be resolv-
able (Werner et al.,
2009
). The physics of gravity scaling
of impacts is relatively well understood, and crater sizes
can be adjusted accordingly. Extrapolating the crater
count curve from the Moon to Mars, however, is problem-
atic, keeping in mind that the curve for the Moon itself has
'
of craters, as from erosion, and (3) a valid method
for calibrating the data exists. Mars is particularly challeng-
ing on all three issues. First, non-impact processes, such as
volcanism, can produce circular depressions, and much of
Mars involves volcanic surfaces, as described below.
Moreover, planetary geologist Alfred McEwen and col-
leagues have noted the abundance of secondary craters on
Mars, which could strongly in
uence the distributions of
craters, especially in the smaller sizes. Second, much of the
surface has been highly modi
ed by wind, water, and (in
places) glaciation, leading to erosion of impact craters.
erasure
”
