Sky and Telescope - October 2017 - 53

N ASA / LUN A R RECONN AISSA NCE ORBITER

sits near the Moon's western limb and
is difﬁcult for telescopic observers to
see into. But Lunar Reconnaissance
Orbiter images show that this young
rayed crater displays two alcoves along
its northeastern side, marking where
the rim gave way and material slid
downslope and partway across the ﬂoor.
In fact, Glushko's small central peak
is totally surrounded by this cascade
of debris from the slumped crater wall.
Impact melt coats some of these ﬂows,
demonstrating that the wall collapsed
as the crater formed, not years later.
A somewhat older crater, Herschel,
is in the middle of the nearside, just
north of Ptolemaeus, and thus much
easier to see. Myriads of small cratering
events have smoothed the impact melt
that accumulated on its ﬂoor and erased
its rays, but the basic structure is still
prominent. The inner wall likewise has
slid downslope - not as giant mounds
of debris but apparently more continuously, resulting in a smooth contour.
Perhaps the inward cascades were a
long-term process rather than a jumble
of events in rapid succession. More
massive ﬂows from the inner wall, probably dating from the crater's formation,
reached the central peak. Look about
3 km inward from the rim crest for a
crisp ridge that parallels the rim all the
way around; this an incipient terrace.
Bürg represents another step forward in the transition to a complex,
Tycho-like crater. Its central mountain
is bigger and comprises two massive
peaks. Wreaths of mounded wall material step down from the rim scarp to the
ﬂoor - not quite in terraces, yet more
coherent than the simple slump blocks
prevalent in smaller craters. Smooth
material broadly covers much of the
ﬂoor, which is well deﬁned as a circle.
Aristarchus is the most famous
40-km-wide crater on the Moon. It is
very young and has a bright ray system
that can be traced from its ﬂoor, up
its walls (seen as distinct light-hued
bands), and across the surrounding
mare. A pool of impact-generated melt
deﬁnes a broad, relatively ﬂat ﬂoor, with
only a small central peak. Aristarchus
displays the beginnings of three or four

Bürg

25 km
(all 3 photos)

p The morphology (shape and characteristics)
of lunar craters becomes more complex once
their diameters reach 25 to 40 km. Note how the
inner walls have slumped in these examples.

terraces around its circumference.
You might also observe the 40-kmwide craters Harpalus, Lansberg, and
Cepheus, all marked on the image on
the facing page, to see how they compare to Herschel, Aristarchus, and Bürg.
Although the examples I've cited are
typical, some craters evolved differently depending on their location and
the vicissitudes of time. Impact craters
on maria often have lava-ﬂooded ﬂoors
that bury central peaks and debris
mounds; Marius and Kopff are two
good examples.
Over the course of billions of years,
all craters' walls get smoothed and lose
detail; their ﬂoors become shallow and
widen thanks to downward cascades
from the walls and by ejecta dropped in
from newly formed basins and nearby
craters. Alpetragius is a peakless,
smooth-walled, 40-km-wide crater that
might once have looked like Aristarchus.
More heavily modiﬁed is Polybius,
only 2 km deep with a wide ﬂat ﬂoor.
Compare its appearance to that of Aristarchus, which is about 3.2 km deep and
has a smaller ﬂoor. Like many humans,
the bottoms of craters widen with age.
As you examine craters larger than
these, notice how features that characterize the full "Tycho" morphology -
terraced walls, large central mountains,
and a broad ﬂat ﬂoor - become more
commonplace. Three 60-km-wide craters

Mösting

Herschel

worth examining are Erathosthenes,
Bullialdus, and Zucchius. Each has
incipient terraces giving way to full
terraces, with mounds of wall-hugging
debris on their ﬂoors. Their ﬂat ﬂoors
are larger and better-deﬁned, and large
central mountains are more prevalent.
Think of them as the "young adults" of
the Moon's crater population.
¢ Contributing Editor CHARLES WOOD
has been taking a special interest of late
in the craters of Saturn's moon Titan.

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Sky and Telescope - October 2017

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