Sky & Telescope - August 2021 - 52

AUGUST 2021 OBSERVING
Exploring the Solar System by Charles A. Wood
Triesnecker (25 km)
Euler (26 km)
Hills in
the Middle
The central peaks in larger
lunar craters testify to the
power of gravity in the
moments after impact.
I
mpact craters are by far the most
abundant landforms on the Moon.
Thousands of them having diameters
larger than about 5 kilometers are visible
in most any backyard telescope. All
can be classifi ed into three fundamental
types by inspecting their interior zones
- the walls, fl oors, and central peak
elevations (S&T: Feb. 2021, p. 52).
Most lunar craters are bowl-shaped
simple craters with diameters up to
about 15 km (9 mi). They have featureless
walls, rounded or fl at fl oors, and
no central hills. Piazzi Smyth, for
example, is a 13-km simple crater in
southern Mare Imbrium.
Complex craters are larger than simple
craters, and you need to observe their
interiors closely to separate them into
their two subgroups.
First are complex-slump craters typically
15 to 35 km wide. Their interior
walls start with a steep scarp and
then descend to the fl oor as chaotic
52 AUGUST 2021 * SKY & TELESCOPE
slumps of material. Their fl oors are
often largely fi lled with mounds of this
slumped material, sometimes overwhelming
any central hills.
Then there are complex-terraced
craters larger than about 35 km. These
complex craters also have a shear scarp
at their tops, with a cascade of terraces
resembling concentric stair steps. Floors
of these big craters are usually relatively
fl at and smooth, often peppered
with small hills. The central peaks of
complex-terraced craters are often massive
mountains. Near the centers of the
fl oors are clumps of hills or mountains
- intriguing features that are the focus
of this month's column.
Let's examine the proportions of
these features using the 55-km-wide
complex crater Aristillus as a typical
example. Aristillus's wall zone is about
45% of the crater's diameter and its
fl oor roughly 55%; the central peaks
encompass 45% of the fl oor. Largerdiameter
craters have wider fl oors,
making the walls and central peaks a
smaller proportion of the whole diameter.
Despite their small relative size,
the central peaks are in many ways the
most varied and interesting interior
landforms within large craters.
To understand the formation of
crater interiors, lunar geologists study
only fresh impacts that preserve the
morphology at the end of the craterformation
process. They recognize these
craters by their sharp rim crests, few
These four examples illustrate the increased
complexity of inner walls, fl oors, and central
peaks that craters exhibit at ever-larger sizes.
superposed craters piercing their ejecta
fi elds and interiors, and bright rays that
many (but not all) possess. Try viewing
them under oblique lighting, which
makes the subtle topography stand out.
Above is a sequence of complex
craters, arranged by increasing diameter
to aid in comparing their walls, fl oors,
and central mountains. First is the
25-km-wide Triesnecker crater, familiar
to many observers because it shares its
name with a complex of nearby rilles to
its east. Notice that the crater outline
isn't round, especially on the west (left);
there a bulge or scallop marks where a
large part of the western wall slid down
and scooted halfway across the crater
fl oor as a jumble of ridges and mounds.
Smaller collapses along its opposite wall
created a rounded mound of debris along
the eastern side of the fl oor, with the
lowest part covered by smooth impact
melt. Barely noticeable low hills in the
center, rising just 300 meters above the
melt pond, are nearly overwhelmed by
piles of slumped wall material.
Although only slightly larger than
Triesnecker, Euler in western Mare
Imbrium has a 2-km-high wall scarp
that marks the path of material that
crashed down like a landslide and ended
on the fl oor as ridges and crumpled
mounds. A 7-km-wide collection of
rounded hills rise 500 meters above EulCRATER
IMAGES: NASA / ARIZONA STATE UNIVERSITY (4)
Sky & Telescope - August 2021

Table of Contents for the Digital Edition of Sky & Telescope - August 2021
