Sky & Telescope - October 2022 - 53

NASA / GSFC / ARIZONA STATE UNIVERSITY (2)
Elevation (meters)
Elevation (kilometers)
surrounding area to a mathematically
calculated topography model that uses
the fault's vertical height and length as
given values.
Nahm and Schultz find that the
best fit to the measured scarp length
and 400-m height implies that the
fault initiated about 42 km below the
surface and fractured its way upward.
They conclude that the faulting started
near the Straight Wall's highest point
(marked + in the bottom image on the
facing page). Additional fault segments
initiated north and south of the main
one and grew until they linked together,
producing the noted segment boundaries.
The estimated 42-km depth at
which the fault originated is roughly
where the crust meets the mantle, as
determined from NASA's Gravity Recovery
and Interior Laboratory (GRAIL)
orbiter lunar gravity measurements.
The crust on the east side of the fault is
35-to-40-km thick and abruptly drops
to 25-km thickness west of the fault.
Now let's take a broader look at the
region. Notice that the Straight Wall is
near the center of a half crater informally
known as Ancient Thebit, which
is defined by a curved, mountainous
rim to the east. The 57-km-wide
crater Thebit cuts across the destroyed
crater's eastern rim. What happened
to the western half of Ancient Thebit?
When the rising Sun illuminates the
area (as the Moon's phase waxes), the
buried western rim is revealed as a
set of semi-circular mare ridges that
extend the truncated northern rim
of Ancient Thebit and continue the
curved outline back towards the less
distinct southern rim. This 220-km
crater formed on the edge of Mare
Nubium. The Straight Wall marks
where the ruined crater's relatively flat
floor east of the Straight Wall suddenly
drops 400 m and then gradually
declines another 450 m westward.
Northwest of the 16-km-wide crater
Birt is Rima Birt. This rille generally
mimics the slight curve of the Straight
Wall, suggesting that the geologic stress
that caused the collapse and created the
enormous scarp also allowed magma to
fracture its way to the surface, producing
a 20-km-wide dome at Rima Birt's
north end. In addition to the curve, the
Birt rille is also offset at its midpoint,
similar to the segment boundaries of
the Wall. The rille looks like a series of
short faults but has no detectable offset.
Rather than moving up or down, it widened
and was filled by rising magma at
its north end. The 1-km-wide rille contains
roughly 30 collapse pits, though
none shows evidence of eruptions.
Grazing sunlight photos like the
one on the facing page (and confirmed
in cross-section measurements shown
below) reveal another linear feature
that isn't noted in scientific papers.
There's a crease where the gradually
sloping land 60 km west of the scarp
abruptly becomes level. It's unknown
how the change in slope occurred.
There's one more subtle linear fea2.0
1.6
East
rim
1.2
0.8
Rupes
Recta
0.4
20
40
60
80
100 120 140
160 180 200 220 240
Cross-section of Ancient Thebit (kilometers)
pThe graph above shows the southwest-to-northeast topographic profile across the center of the
ruined crater informally known as Ancient Thebit.
West rim
Flat
West floor
East floor
ture within Ancient Thebit. A shallow,
inconspicuous rille originates from the
northeast and intersects the Wall at
B. As with the crease, it's unclear how
this apparently older rille relates to the
formation of the Straight Wall.
So how did the Straight Wall form?
Ancient Thebit was originally carved
out of the western rim of the Nubium
Basin. Later, the basin was inundated
with lava, and the accumulating mass
caused the basin floor to subside, triggering
a powerful moonquake and the
sudden, catastrophic collapse of the
western half of Ancient Thebit. The
Wall is the dividing line between the
two halves.
¢Contributing Editor CHUCK WOOD
thanks Richard Schultz for an invigorating
discussion on this wonderful fault.
200
300
400
500
100
20
40
60
Distance along fault line (kilometers)
pThis graph plots the height of the Straight Wall from north to south.
sk yand tele scope .o r g * OCTOBER 2022 53
A B
C
D
80
100
120

http://skyandtelescope.org

Sky & Telescope - October 2022

Table of Contents for the Digital Edition of Sky & Telescope - October 2022