Sky & Telescope - December 2024 - 52

DECEMBER 2024 OBSERVING
Exploring the Solar System by Charles A. Wood
The Werner-Airy Enigma
Does this lunar region contain a forgotten basin?
A
ll parts of the lunar surface have a
history, but if there are no grand
craters, towering mountains, or curving
rilles, that history is more challenging to
discern. One such place stretches about
500 kilometers west from the northwestern
extent of Rupes Altai scarp
and nearly to Mare Nubium. This area
I'm writing about is made up of neither
smooth maria nor heavily cratered highlands
but instead contains about a dozen
large, flat-floored craters and smaller,
partially rimmed, and degraded craters
with light-hued, smooth material within
and between the craters. What is the
history of this unusual area?
More than 50 years ago, pioneer
lunar astronomer Ralph Baldwin
noticed that portions of this area are
relatively bland and suggested that it
could be the remains of an ancient,
highly degraded impact basin. As
the image below shows, a circle that
encloses much of the area is roughly
traceable clockwise from Almanon
through Fermat and Wilkins to
Gemma Frisius at its southern extent.
It then continues northwest through
Aliacensis and Werner on the west,
past Airy to the northeast, and back
to Almanon. Although this putative
Werner-Airy Basin had been included in
various lists of possible impact basins,
most scientists discount it, for several
reasons. It isn't topographically lower
than its environs, it has few (if any)
remnants of a rim, and NASA's GRAIL
mission failed to detect any gravitational
anomalies within its proposed
borders - all typical basin features.
However, China's recent Geologic Map
of the Moon (S&T: Aug. 2023, p. 52)
accepts the Werner-Airy feature as a
basin, with rings bearing diameters of
350, 515, and 765 km. These rings are
difficult to identify visually.
The most common characteristic of
the Werner-Airy area is its widespread
patches of smooth plains. During the
1960s, such plains were commonly
interpreted as old volcanic materials
different from the dark, basaltic lavas
making up the maria. Then in 1972
the Apollo 16 spacecraft landed just
north of the Werner-Airy Basin and
collected samples. But as astronaut Ken
Mattingly immediately recognized, the
rocks they found were not volcanic but
comprised of fractured and pulverized
material. Mattingly's comment was:
" Well, it's back to the drawing boards,
or wherever geologists go " to come up
with a better theory.
Due to the Apollo 16 results, all
Almanon
Airy
Fermat
Pontanus E
Werner
Wilkins
Aliacensis
Gemma Frisius
bright, smooth lunar plains are considered
fluidized ejecta from the formation
of basins and large craters. And probably
almost all are, but two features in
the Werner-Airy Basin create a bit of
uncertainty. Within the inner ring of
the putative basin is Pontanus E, a
13-km-wide, concentric crater. Slightly
more than 100 concentric craters are
found on the Moon, and the majority
are on or very near maria. The innerdonut-ridge
of a concentric crater is
thought to form by the intrusion of
magma through faults beneath the
crater, uplifting a ring around the floor.
The existence of Pontanus E implies
that magma was once under the middle
of this region.
Some 50 km southeast of Pontanus
pThe area above is sometimes designated as the Werner-Airy Basin. The location of the area's
three concentric rings comes from the Chinese Academy of Sciences Geologic Map of the Moon.
52 DECEMBER 2024 * SKY & TELESCOPE
E are 8 small hills about 2-3 km wide in
a 12-km straight line. They are smaller
and steeper-sloped than volcanic mare
domes. Any one of the hills could be
ALL IMAGES COURTESY OF NASA / GSFC / ARIZONA STATE UNIVERSITY

Sky & Telescope - December 2024

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