Sky & Telescope - June 2022 - 53

COPERNICUS, CLEOMEDES, AND WALLACE: NASA / ARIZONA
STATE UNIVERSITY / APPLIED COHERENT TECHNOLOGY
CORP (3); CEPHEUS AND FRANKLIN: SEAN WALKER
Planetary Observers (ALPO) published
a Lunar Dark-Halo Crater Catalog
that gives coordinates for 103 DHCs
(https://is.gd/ALPOdhcs). But rather
than trying to observe these features by
plotting each one on a map, instead try
surveying the lunar surface when the
solar angle is high. Start with Alphonsus,
then move on to areas in and around
Theophilus, Stadius, Atlas, Franklin,
Schickard, Grimaldi, Triesnecker,
Manilius, Cleomedes, and Gauss. The
latter two are especially challenging.
As you observe, you may begin to
Copernicus
Cepheus
Franklin
Copernicus H
notice some patterns. The majority of
the DHCs around large, fresh craters
are like Copernicus H: impact craters
that excavated dark lava material from
below the bright ejecta rays deposited by
the larger, older crater.
Craters like Alphonsus are typically
located near the edges of maria and
have floors that were fractured and
uplifted by magma rising into the
impact-fractured zone beneath the crater.
Sometimes small amounts of this
magma escape through fracture rilles
and erupt onto the surface, producing
volcanic DHCs.
A few volcanic DHCs are found not
inside floor-fractured craters but rather
appear as elongated depressions along
rilles. The most unusual example is the
5.5-km × 1.7-km pit atop a 20-km-wide
dome at the north end of Rima Birt,
west of Rupes Recta (the Straight
Wall). Dark halos associated with dome
volcanoes are extremely rare.
Traditionally, the kinds of volcanic
vents that produce massive, dark pyroclastic
deposits are not designated as
DHCs, but they could be. One example
is the 7-km × 3-km vent that erupted
the roughly 100-km-diameter Bode
pyroclastic deposit abutting the east
edge of Sinus Aestuum. Similarly, the
2-km-high volcanic cone - the Cobra
Head - that was the source of the pyroclastic
deposits covering the Aristarchus
Plateau isn't considered a DHC.
Based on these examples, we can
derive a simple rule of thumb to determine
if a DHC's origin is volcanic or
impact-related. If a DHC lies within a
floor-fractured crater or is located along
pThe most prominent impact-generated
DHC is Copernicus H, located southeast of
Copernicus.
pPositioned in the northeast lunar quadrant,
the fractured floor of Franklin contains several
DHCs of volcanic origin.
Wallace
Cleomedes
pLocated just northwest of Mare Crisium,
Cleomedes contains several tiny DHCs visible
in large telescopes.
a rille, it's volcanic. All others are normal
impact craters that simply scattered
dark, underlying basalt debris on top of
bright ejecta from a nearby rayed crater
or young basin - dark ejecta atop bright
ejecta. Look at the half-dozen or so
DHCs splashing thru the rays northwest
of Copernicus, near the crater Wallace.
All of these are normal impacts with
dark halos some 2 to 3 times bigger.
Now observe Petavius and Humboldt,
large, floor-fractured craters
near the Moon's eastern limb. Both
have big patches of lava and pyroclastic
materials around their floor edges. But
there are no obvious DHCs that could
be sources of this dark material - it
may have erupted from rilles or from
DHCs hidden by thick deposits of ash.
pSeveral DHCs of impact origin are visible in
the vicinity of Wallace, which lies to the northeast
of Copernicus.
But if you greatly enlarge LRO images,
you'll see 1-km-to-2-km-wide craters
in the midst of the dark pyroclastics in
both. Are these DHCs big enough to be
the source of the 20-km-wide patch of
pyroclastics along Petavius' southern
floor? I don't know.
Finally, although there is no firm
count of the number of volcanic versus
impact DHCs, I get the impression that
the ones that formed on bright ejecta
significantly outnumber those of volcanic
origin. This makes sense since there
are far more bright crater rays than
dark, pyroclastic patches.
¢Contributing Editor CHUCK WOOD
finds plenty of interesting features to
observe when the Moon is full.
sk yand tele scope .o r g * JUNE 2022 53
https://www.is.gd/ALPOdhcs http://www.skyandtelescope.com
Sky & Telescope - June 2022

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