Quadrant 40:
— Images and text provided by Howard Eskildsen
Ruined craters, impressive rilles
The top of the image speaks of devastation as
if a slurry of mud had been smeared from the
top right to the central part of the image. Of
course, it was fluidized ejecta from Imbrium,
not mud, that did the dirty deed, but the effect
is the same. Ruined craters Julius Caesar and
Boscovich fared better than the craters above
them that are now discernible only as wretched
rectangles or as the depressions at the top of
the image that are now filled with lava. Some
of the basalt-flooded depressions include Lacus
Lenitatis, Lacus Hiemalis, and Sinus Honoris.
Boscovich and Julius Caesar also have lava-
coated floors that are mostly smooth except for
the rill seen crossing Boscovich.
A large crack known as Ariadaeus Rille can be
seen angling across the center of the image. Its
steeply sloping walls and flat floor show it to be
a type of fault common on Earth that is known
as a graben. Forces from below spilt the outer
surface apart to form parallel cracks that spread
apart while the central part sank downward. The
rille is about 220 km long, 4-5 km wide and about 0.8 km
deep. Based on the geometry of the parallel faults, they are
believed to meet at a depth of 2-3 km where there is likely
a more coherent layer of rock beneath the heavily fractured
outer layer of ejecta known as the megaregolith (regolith is
the term for the lunar surface soil).
Note how the ridge just north of Silberschlag also
drops downward across the rill and hence predates the
formation of the rille. Also, a branch on the western end
intersects with The Hyginus rille. The opposite end of the
rille disappears near the crater Ariadaeus, which abuts a
smaller crater known as Ariadaeus A. They are separated
by a straight thin wall indicating that they formed from
two objects that impacted simultaneously, suggesting that
either they had been traveling as a gravitationally bound
pair prior to impact, or had been split by tidal forces just
prior to impact. On the lower right of the image the craters
Ritter and Sabine may have formed in a comparable
manner, but far enough apart that their rims did not
overlap.
Ritter and Sabine rest on the margin of Mare
Tranquillitatis, and rilles can be seen near them that follow
the margin of the mare. More rilles are seen north of Ritter,
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near Ariadiaeus and Manners and near Sosigenes.
These rilles likely relieved pressure at the margin of
the mare as the solidified lava sank near the middle
of the mare and formed arcs around the margin.
Also, the settling of the center Tranquillitatis caused
compression at the surface resulting in overthrust
faults known as wrinkle ridges or dorsa. Some of
these ridges are spectacularly illuminated just to the
right of Arago.
Two volcanic domes, Arago Alpha and Arago
Beta, are visible as well. They are about 15-20
km wide, rise to a few hundred meters high, and
resemble Earthly shield volcanoes. These represent
some of the final volcanic activity in that region of
the Moon. Whereas the basaltic lava that filled in
the basin was quite fluid to form the flat floor, the
lava forming the domes was thicker, or more viscus
to be able to pile up as they did.
The lower part of the image shows more ruined
craters, although not to the extent of the upper
part of the image. Dembowski, Temple and D’Arrest
suffered severely from the Imbrium impact though
more than 650 kilometers away from the Apennine rim.
Agrippa and Godin arrived after the disaster, however,
Sky ’ s
Up
and so were spared.
Finally, the diminutive crater Cayley deserves mention as
the namesake for the Cayley Formation. The flat areas of
the formation extend southward and includes an area near
Sky ’ s
Up
Descartes, a very forgettable crater had it not been for the
landing of Apollo 16 near there. What they were seeking
was not found, and their unexpected discoveries rewrote
our knowledge of the lunar geology.
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