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Figure 2.12. An experiment similar to that shown in Fig. 2.11 to
illustrate stages in crater modi cation by impact degradation. The
arrows in the first and last images point to a crater that is subjected
to small impacts, which gradually wear down the crater rim and ll in
the crater
floor until it is nearly
erased
from the surface (NASA-
Figure 2.13. A diagram of idealized crater size
frequency
distributions (number of craters per unit area versus crater diameter)
for three surfaces; surface 1 is the oldest, re
-
Ames photograph AAA481
-
8, courtesy Don Gault).
ected by its having the
greatest number of craters and craters of largest sizes. The steep
parts of all three curves represent crater production, while the less
steep parts represent crater equilibrium. Relative age-dates can be
determined either by the
number of secondary craters. Other considerations include
target properties that could cause variations in crater sizes.
For example, experiments show that craters formed in
targets containing fluids are larger than craters formed
in dry targets. This difference could cause the size
between production and
equilibrium (in which progressively older surfaces have their break
point at larger crater size), or by the position of the production curve,
which shifts to the right with increasing age.
break point
-
frequency distribution for the volatile-containing target
to be interpreted as representing an older surface.
Despite these dif culties and uncertainties, impact crater
statistics are commonly used as a means for obtaining rela-
tive ages for different planetary surfaces. Ages derived from
crater counts have been compared with (and calibrated
against) radiometric dates obtained from lunar samples and
demonstrate the validity of the technique, at least on the
Moon, where surface-modifying processes are minimal.
This result suggests that crater counts can be used to obtain
dates for surfaces on other airless bodies, such as Mercury.
Great caution must be exercised, however, in using crater
counts on planets where differences in erosion might occur
as a function of location, or where signi cant differences in
target properties may alter the crater morphology.
In principle, crater counts can also be used to derive
absolute ages for planetary surfaces, as discussed by
Michael and Neukum ( 2010 ). This has been done with
some con dence on the Moon where cratered surfaces
have been sampled and radiometric ages determined
(Neukum et al., 2001 ); extrapolation of the calibrated crater
curve ( Fig. 2.10 ) to surfaces that have not been sampled
enables estimates of their ages. The same can be done for
surfaces on other planets by extrapolation of the calibrated
lunar crater counts. This requires, however, that correct
adjustments can be made for gravity (which in uences the
sizes of craters), impact flux as a function of location in the
Solar System (proximity to the asteroid belt, as with Mars,
which experiences a higher impact rate than that on the
Moon), and the potential for degradation in the presence of
an atmosphere, as on Venus. These and other factors result
in complex algorithms for extrapolation, and potentially
large error bars on the results, depending on the assump-
tions and uncertainties in the age calculations.
2.5 Remote sensing data
Most of our knowledge of the geomorphology of Solar
System objects is derived from remote sensing, de ned as
the collection of information without coming into physical
contact with the object of study. This is accomplished by
designing instruments that can be carried on some plat-
form
to collect useful information. Typically, platforms
on Earth include airplanes and spacecraft, but can also
include balloons, helicopters, or robots operating on the
surface. To the extent possible, similar platforms are used
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