Quadrant 48:
— Images and text provided by Howard Eskildsen
Craters dotted with saucers, scars
Wow, saucers on the Moon! But theses
saucers do not fly or carry little green
creatures from outer space. Rather, they
are depressions in the floor of the 153 km
crater Ptolemaeus that reveal secrets about
the early history of the moon. They can be
seen only at very low sun angles and appear
almost soft and velvety. One prominent
saucer can be seen just above Ammonius
on the quadrant image and others can be
detected in the Ptolemaeus image.
What are they and what happened?
Ptolemaeus is an ancient crater, thought to
be over 3.9 billion years old, that formed
during an era of intense cratering. It has
been battered and scarred by later impacts
and by distant basin-forming collisions.
Early in its lifetime many craters pocked its
original floor leaving depressions that were
later coated by “fluidized ejecta,” the gritty
slurry of rocks and dust violently expelled
in the formation of a gigantic impact basin,
perhaps 3.85 billion years ago. Albategnius,
Klein, Hipparchus and other craters on the quadrant
image have similar ejecta fill and some have saucer-like
depressions as well. Ammoinus, the only crater formed
in Ptolemaeus after the basin-forming catastrophe, was
spared since it arrived nearly 3 billion years later!
Scars between Herschel and Gylden point accusingly
towards Mare Imbrium, hundreds of miles to the
northwest, as the source of ejecta sculpturing. Similar
scars can be seen on the southeast rim of the badly
battered Hipparchus. Its southwestern wall has been
obliterated by younger local impacts, and it is so worn
that it is difficult to detect unless the sun is at a low
angle.
The larger craters on the image are all scarred and
much older than the smaller craters. Early in the history
of the moon large space rocks battered the surface of the
moon, but the cratering rate fell off as the objects were
depleted. After the Imbrium impact about 3.85 billion
years ago cratering events became less common and the
impacting objects smaller. Hence most of the younger
craters are smaller and widely scattered over the ancient
moonscape.
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Well-preserved
Herschel and
Horrocks, 41
km and 30
km diameter
respectively,
are considered
“Eratosthenian”
in age and may
range in age
The ancient Ptolemaeus crater is
dotted with depressions.
from 3.2 to 1.1
billion years
old. The youngest craters, Ammonius, Pickering,
and a few other unnamed craters scattered about
the image are “Copernican” in age; less than 1.1
billion years old.
Other interesting objects include Rima
Flammarion and Rima Oppolzer, which are
surface fractures with the dropped floors
between the fracture walls and are known as
grabens. Near Muller, a crater chain crosses diagonally
for a short distance. It was formed by an object —
possibly a comet — that fragmented prior to impact
Sky ’ s
Up
creating a series of craters.
Like the Rosetta Stone, the craters and scars across
this tortured landscape reveal an ancient past, filling
in details that have been erased on Earth by erosion
Sky ’ s
Up
and crustal motion. Our solar system endured a violent
distant past before sweeping most of the space debris
from the surroundings of the planets and settling into the
relative quiescence that we enjoy today.
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