Sky and Telescope - February 2017 - 53

N ASA / A RIZON A STATE UNIV.

20 km

Elevation (m)

strictly vertical drop; instead, structural
failure occurs along curved faults that
slide previously ﬂat-lying rocks downward into the hole. In the process, the
slabs rotate so that their surfaces end
up tilting downward away from the
crater's center and toward its rim. These
outward-facing tilts are easy to recognize in high-resolution Lunar Reconnaissance Orbiter (LRO) images, which
show where impact-produced molten
rock fell as a liquid onto a terrace,
ﬂowed downhill, and collected in ponds
against the walls of adjacent terraces.
The inward slumping of terraces
makes the ﬁnal diameters of big craters
30% to 40% larger than their transient
cavities were initially. And because the
thickness of ejected debris tapers outward from its maximum at the transient cavity's rim, it's not as thick at the
ﬁnal, terraced-enlarged rim crest. This
means that the contribution of fallback
ejecta to the rim's height should be less
than 50% for large craters.
To test this assumption, Krüger and
Kenkmann used high-resolution images
from LRO and the Japanese Kaguya
lunar orbiter to look in detail in crater
walls. They calculated the amount of
structural uplift of preexisting rocks and
measured the thickness of ejecta at the
rim for 11 craters, ranging in size from
Bessel (16 km) to Pythagoras (130 km).
As an example, they found that the
ﬁnal rim of 41-km-wide Bürg shows
about 850 meters of structural uplift
and 300 m of overlying ejecta. This
ratio ﬁt the pattern seen in all of the
craters studied: ejecta constitutes about
30% of a rim's height, and uplift makes
up the other 70%.
So now that you know how crater
rims are made, can you observe any
of these details? Start by looking at
Copernicus under a high Sun, when just
a narrow line of shadow hugs the rim.
Just below the rim crest is a steep, bright
scarp, 1½ km high, and below it are the
terraces that widened the original diameter of the crater. This scarp includes
both uplifted preexisting surface and
the ejecta that rained down on top of it.
Interestingly, the bottom of the scarp is
the same elevation as the surrounding,

Rim crest
(+269 m)

Scarp

-1,000
-2,000

Floor
(-3,560 m)

Elevation
of original
terrain

Terraces

-3,000
A

100

Distance from center (km)

S This topographic cross-section shows that Copernicus is nearly 4 km deep from its floor to the
crest of its rim. The dramatic scarp just inside its rim crest (bright in the image) is 1½ km high.

pre-impact terrain well beyond the rim.
For a real challenge, observe Bürg.
It's on the lava-covered ﬂoor of Lacus
Mortis and less than half the diameter
of 93-km-wide Copernicus. With high
power you just might notice that the
uppermost part of Bürg's rim scarp
looks brighter than the bottom twothirds. The bright material is ejecta, and
below it is the structural uplift of the
mare lavas. These brightness differences
make sense, because lavas are dark, but
pulverized ejecta (which makes rays) is
highly reﬂective. High-quality amateur
images show the same brightening atop
the north rim scarp of Plato - though I
have never observed it visually.
To see these rims more in proﬁle,
observe large, fresh craters near the

limb. You can gaze across the near rims
and ﬂoors of the limb-hugging craters
Moretus, Pythagoras, Langrenus,
and Zucchius to the bright rim-crest
scarps and the jumble of terraces below
on their far walls. Compare the narrow
extent of the scarps - typically they're
1 to 1½ km high - and the greater
heights of the terraces. This shows how
much farther craters excavate below the
original terrain than they pile up ejecta
on their rims. And it reminds us of the
powerful forces that shaped the Moon's
heavily battered face.
¢ CHUCK WOOD would prefer to study
the rim scarps of lunar craters by repelling down one.
For more information: www2.lpod.org.

s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

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Sky and Telescope - February 2017

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