Sky and Telescope - February 2017 - 52

FEBRUARY 2017 OBSERVING
Exploring the Moon by Charles A. Wood

How Are Crater Rims Made?
The circular rim crests that surround big lunar impacts tell interesting stories.
ithout rims, craters would just
be holes in the ground. Rims
provide visual drama. As the first rays
of sunlight hit a majestic crater such as
Copernicus, its elevated circumference
brightly reflects the morning light -
while the interior floor remains in inky
blackness. A little later, as the Sun rises
higher, we delight in long shadows dramatically cast across the floor by individual peaks along the enclosing rim.
Craters form when mountain-size
fragments of cosmic debris slam into
the Moon's surface. Their high velocity
and great mass excavate huge volumes

of crustal rock. This material is forcefully ejected, spreading debris near and
far. In fact, fragments of lunar rock
from the Copernicus impact probably
lie buried in billion-year-old sediments
here on Earth.
Some ejecta, shot out of the forming crater nearly vertically, land in and
around the excavated hole. This fallback
builds up at least part of a crater's rim.
Another component is the uplift of the
preexisting rock layers. The impact's
powerful shock wave compresses the
surrounding rock, pushing it sideways
and upward out of the crater. This uplift
tilts the target rock to a higher elevation
than it had originally.
For a simple lunar crater, typically
less than 10 to 15 kilometers across,
the preexisting rock layers are strong
enough to resist collapsing into the relaW Big craters are more complex than their
simple, bowl-shaped siblings. The initial (transient) cavity slumps inward, forming terraces
and enlarging the crater's width. This inward
slumping and upward rebound of deeper rocks
form central peaks in craters larger than 15 km.
T A recent analysis shows that the rims of
big, complex craters mostly involve uplift of
the original surface, with ejected debris adding
only about 30% to the total height.

tively small excavated hole. At impact
sites that puncture mare plains, these
craters display upwarped exposures of
the previously flat-lying lava flows on
their inner walls, capped by fallback
ejecta. Most of these are too small to
clearly see their interiors telescopically,
but try examining Chladni (13 km) and
Pickering (15 km).
Early studies showed that, for simple,
bowl-shaped craters, structural uplift
and the overlying blanket of fallback
debris each account for about 50% of
the rim height. But recent investigations
by Tim Krüger and Thomas Kenkmann
(Albert-Ludwigs-University, Frieburg,
Germany) show that this is not the case
for larger, complex excavations.

Slope-Shifting in Big Craters
During especially powerful impacts, the
outward-spreading shock wave excavates a large transient cavity with steep
slopes and a high rim. But near the cavity's edge, the preexisting rock layers are
strongly shaken and fractured, and they
collapse into the hole along a series of
concentric faults, creating ring-shaped
terraces. You can easily see this geologic
staircasing in large, fresh craters such
as Copernicus and Tycho.
The terraces don't result from a

Ejecta thickness
Crater
center

Original surface

Uplift thickness

E je c
ted debr
is

Layered deposits

Fractures

52

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

IMPACT SEQ UENCE: DON DAVIS; CR ATER PROFILE: S&T: LE A H TISCIONE, SOURCE: TIM K RÜG ER & THO M AS K ENK M A NN

W



Sky and Telescope - February 2017

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