Sky & Telescope - February 2021 - 53

W Crater mor-

Lalande
24 km

Chladni
13 km

diameter, and on to Complex-terraced
craters at about 40-km diameter.
With a diameter of 231 km, Clavius
is the largest lunar crater, but the graph
also plots the much larger lunar basins.
These features are formed by projectiles
that are more massive or moving at
higher velocity than those that formed
craters. The additional impact energy
strongly transformed the resulting
formations, making them the largest
features on the Moon - 10 to 20 times
larger than complex craters.
Morphologically, basins are as different from craters as giant and supergiant
stars are from their stellar mainsequence counterparts. The energy that
formed lunar basins caused more intense
deformation of the target rocks than
what occurred in the creation of mainsequence craters. The transient cavities
of basins were deeper and wider, and the
surrounding terrain slid down deeper
and across a greater distance, creating
multiple mega-terrace rims, like Rupes
Altai and Montes Apenninus. These
large basins have three or more rings and
are called multi-ring basins. Some of their
outer rims are low and hard to trace, and

X Multi-ring basins,

Orientale
1,260 km

phology is highly
dependent on the
mass and velocity
of the impacting
object, as seen in
this comparison
among several
progressively
smaller lunar
depressions.

their inner rings appear as mostly concentric mare ridges. All circular maria
on the lunar nearside reside within
multi-ring basins.
Smaller basins have only two rings
- typically a distinctive outer rim and a
smaller, inner ring. The relatively young,
326-km-wide farside depression
Schrödinger is the best example of
a two-ring or peak-ring basin. Of the
Moon's 17 peak-ring basins, only three
are visible from Earth - Bailly, SchillerZucchius, and Grimaldi - and all have
indistinct inner rings.
A few small depressions called protobasins are caught in a transitional state
with a crater-like central peak as well as
an inner ring of small hills, like those
found on larger peak-ring basins. The
farside features Antoniadi (127 km) and
Compton (162 km) are the Moon's only
examples. They're well within the diameter range of complex-terraced craters,
but for unknown reasons have features
associated both with craters and peakring basins. Compton's ﬂoor is ﬂooded
with dark lava, as is the circular patch
within Antoniadi's peak ring.
Theoretical models and observaNectaris
860 km

tions suggest that the interior structure
differences seen in complex craters,
peak-ring basins, and multi-ring basins
are strongly inﬂuenced by the depth
and volume of the melt created below
the basin ﬂoor during an impact event.
Both melt size and depth increase as the
energy produced by the impact rises. As
the melt zone grows, it inﬂuences the
deformation of the target rocks, preventing central peaks from forming. Instead,
the rebounding material expands into
an inner ring. Impacts of even greater
energy produce even deeper basins with
more expansive melt zones. As the main
rim collapses and slides inward, it rips
interior ﬂoor material into multiple, lowangle ring faults. The inward collapse
of the rim wall releases pressure on the
surrounding crustal rocks, which break
into one or more external, concentric
faults, creating the multi-rings we see in
the largest basins. This theory is complex, but it explains many crater-to-basin
transitional features.
As you look at any lunar depression
you should be able to place it in one of
the ﬁve classiﬁcations I've described.
Note the morphology as you observe
complex craters of the main sequence
and then inspect two-ring basins (lunar
" giants " ) and then the much broader
and shallower multi-ring basins (lunar
" supergiants " ).
Unlike stars on the main sequence,
craters and basins work in reverse,
exploding into existence and then
weathering away over billions of years,
challenging the understanding of
observers and theorists alike.
 Contributing Editor CHUCK WOOD has
studied the Moon for more than 50 years.
Schiller-Zucchius
455 km

such as Orientale
(left), Nectaris (middle), and SchillerZucchius (right), are
analogous to the
supergiant stars on
the H-R diagram,
though explosive
events marked their
creation rather than
their demise.
sk yandtelescope.org * FEBRUARY 2 021

53
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