http://www.skyandtelescope.com.au 57Their flat floors are larger and better-
defined, and large central mountains are
more prevalent. Think of them as the
‘young adults’ of the crater population.CHARLES WOOD has been taking a
special interest of late in the craters of
Saturn’s moon Titan.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
terraces around its circumference.
You might also observe 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 differentlydependingontheirlocationandthe
vicissitudesoftime.Impactcraterson
maria often have lava-flooded floors
thatburycentralpeaksanddebris
mounds;MariusandKopffare two
good examples.
Overthecourseofbillionsofyears,
all craters’ walls get smoothed and lose
detail;theirfloorsbecomeshallowand
widenthankstodownwardcascades
fromthewallsandbyejectadroppedin
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 characterise the full ‘Tycho’
morphology — terraced walls, large
central mountains and a broad flat floor
— become more commonplace. Three
60-km-wide craters worth examining
are Erathosthenes, Bullialdus and
Zucchius. Each has incipient terraces
giving way to full terraces, with mounds
NASA / LUNAR RECONNAISSANCE ORBITER of wall-hugging debris on their floors.
SThe morphology(shape and characteristics)
oflunarcratersbecomesmorecomplex
oncetheirdiametersreach25to40km.Note
how the inner walls have slumped in these
examples.25 km
(all 3 photos)MöstingHerschelBürg