Figure 11. The unnamed crater in HiRISE image PSP_001391_2465
with rim, 1R and 2R circles (black lines) and MBARS-detected boulders as
white dots. For context, the rim is marked in Fig. 10. The rim was
selected visually, aligning the rim just interior to the areas of
increased boulder density to the South and North. The crater is
estimated to have a rim diameter of ~600 m and is close
in size to both South Ray and Camelot crater (Watkins et al., 2019)
The unnamed crater has low relief, contains abundant thermal contraction
polygons (Fig. 11), and is likely a relatively young impact into an
ice-rich substrate (Levy et al., 2018). The crater has a radius
R~300 m, as determined visually by a ring of more
abundant, large boulders and an apparent dearth of large boulders within
the ring. Given the qualitative nature of this determination, the radius
must be taken as an approximation, as no HiRISE Digital Elevation Models
are available in the area to provide stereo-derived topographic evidence
of a rim. After defining the rim, we clipped boulders from the MBARS
results out to 3R from the crater center. There are several other impact
craters of comparable size and degradation state within the HiRISE
image, so we cutoff the survey at 3R to attribute the examined boulders
more confidently to the impact event of interest. We compare results of
the unnamed martian crater to the results of Camelot (R = 303 m) and
South Ray (R = 350 m) craters (Watkins et al., 2019) as these are
closest in size to the unnamed crater. We examine the Boulder Size Range
Distributions (BSRD) and Boulder Range Frequency Distributions (BRFD),
as the size-frequency distributions are not expected to behave similarly
(Golombek et al., 2012; Watkins et al., 2019).
Comparison of results among the BSRD and BRFD of the three craters are
shown in Fig. 12, including the BRFD of only boulders >1.5
m for the unnamed crater to compensate for the higher image resolution
of HiRISE compared to the lunar images used in the original work.
Comparison of BRSDs show that much larger boulders (>7 m)
are preserved at both the lunar craters, particularly at South Ray
Crater, compared to the unnamed crater (Dmax=4.3 m). The
unnamed crater replicates the trend of decreasing maximum boulder size
at greater ejected distances seen South Ray and Camelot Craters.
However, the unnamed crater seems to show a steeper drop off in large
boulders, though this may be due to the lack of boulders ≳4.0 m.
Comparing the BRFDs, the unnamed crater has a wide area of increased
boulder abundance from ~1.5-2R that has mostly
diminished by 3 R. This broad peak is also visible when only boulders
>1.5 m are counted, showing that this is likely not an
effect of increased sensitivity to small boulders. This distribution is
unlike the sharp spike in boulder frequency at the Camelot rim, or the
more even distribution of boulders around South Ray crater.