Quadrant 18:
— Images and text provided by Howard Eskildsen
Aristarchus Plateau: The fire within
If any area of the Moon could
be considered unique, this is
the place, a strange, crustal
block thrust upward by some
catastrophic event, possibly the
Imbrium impact. Measuring
170 by 200 kilometers, it rises
as much as 2,000 meters above
the surrounding mare basalts
of Oceanus Procellarum. It is
bounded by the Aristarchus
Crater on the south and by the
Montes Agricola to the north
and is covered with low hills. It
is darker than the surrounding
maria in visible light as well as
in ultraviolet light, and radar
reflection studies show its surface
to be smoother than other areas
of the Moon, implying that it
is nearly devoid of rocks and
boulders. Multiple rilles that
radiate in various directions once
belched lava from reservoirs deep
below the surface.
Vallis Schröteri (Schröter’s
Valley) is the most notable rille.
It winds for 160 kilometers from
the origin, known as the Cobra
Head, to the plains of Procellarum,
and is by far the largest rille on
the Moon. It is nearly 10 km wide
near the Cobra Head and, at its
peak activity, may have gushed
forth between 10 million and
10 billion kilograms of molten
basalt every second. If so, it could
have covered the entire Oceanus
Procellarum to a depth of 200
feet. Later lava flows from a different source cover
significant portions of lava from Schröteri, so its actual
extent is not known.
These massive flows were accompanied by fire-
fountains spewing glass beads that spread over the
plateau, coating it with tens of meters of volcanic ash
and producing the unusual surface smoothness. It also
may account for the unusual color noted at various
illuminations. Near full moon it has been described
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as mustard-colored, ruddy or even olive in color.
Other curious sightings in recent times have raised
the question of further eruptions or haze emanating
from the valley. While any active volcanism has been
essentially ruled out by thermal studies, Apollo 15 did
detect radon gas in the region, presumably produced
from the decay of uranium and thorium, which may
have escaped through fissures from beneath the
surface.
Other rilles show that Schröter’s Valley was not
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the only source of flows from the
plateau. These various smaller rilles
are collectively known as Rimae
Aristarchus and are marked with
Roman numerals on the image. All
are much smaller than the valley, but
note the apparent fossil flow-channel
remnant that is unofficially named
Fossa Caruso near the end of rima
VIII. This appears to have been an
outflow channel that was eventually
blocked and partly flooded by
subsequent flows. Its source has been
obscured, but it makes one wonder
what, if any, connection it might have
had with Vallis Schröteri.
Long after the volcanic fires died
out, a huge space rock crashed on
the southern margin of the plateau
and created the name-sake crater
Aristarchus. This 40-kilometer crater
is thought to be around 500 million
years in age and is the brightest area
on the moon. It is readily visible to
the naked eye and even notable during
lunar eclipses or in the Earthshine on
the dark side of the terminator during
the crescent phases of the Moon.
Ironically it is located next to one of
the darkest areas of the moon. Its rays
scatter about but are hard to see on the
plateau. On its interior, two distinct
types of dark basalt are visible as well
as bright anorthosite in the central
peak. Radial bands can be seen within
the crater at high sun angles, and
an especially bright ray plume from
Aristarchus drapes across the floor of Herodotus.
In contrast to Aristarchus, Herodotus looks tired and worn
with its central peak buried in basalt that partly fills the crater.
Faring even worse, the crater Prinz has much of its rim buried
as well. Had volcanic activity continued much longer Prinz
might have been totally obliterated.
North of the plateau the strange Montes Agricola forms a
thin linear ridge unlike most other ranges on the Moon, which
are usually arcuate rather than linear. I know of no official
explanation for its morphology, but it does remind me of
buckling seen on Earth where one plate pivots against another
like a hinge.
The whole area is a place of wonder with its complex
structure, history and appearance. Its startling landforms, and
changing appearance as the sun-elevation changes, entices the
observer to return again and again to this wonderfully unique
plateau.
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