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


http://www.skyandtelescope.org

Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021

Contents
Sky & Telescope - February 2021 - Cover1
Sky & Telescope - February 2021 - Cover2
Sky & Telescope - February 2021 - 1
Sky & Telescope - February 2021 - Contents
Sky & Telescope - February 2021 - 3
Sky & Telescope - February 2021 - 4
Sky & Telescope - February 2021 - 5
Sky & Telescope - February 2021 - 6
Sky & Telescope - February 2021 - 7
Sky & Telescope - February 2021 - 8
Sky & Telescope - February 2021 - 9
Sky & Telescope - February 2021 - 10
Sky & Telescope - February 2021 - 11
Sky & Telescope - February 2021 - 12
Sky & Telescope - February 2021 - 13
Sky & Telescope - February 2021 - 14
Sky & Telescope - February 2021 - 15
Sky & Telescope - February 2021 - 16
Sky & Telescope - February 2021 - 17
Sky & Telescope - February 2021 - 18
Sky & Telescope - February 2021 - 19
Sky & Telescope - February 2021 - 20
Sky & Telescope - February 2021 - 21
Sky & Telescope - February 2021 - 22
Sky & Telescope - February 2021 - 23
Sky & Telescope - February 2021 - 24
Sky & Telescope - February 2021 - 25
Sky & Telescope - February 2021 - 26
Sky & Telescope - February 2021 - 27
Sky & Telescope - February 2021 - 28
Sky & Telescope - February 2021 - 29
Sky & Telescope - February 2021 - 30
Sky & Telescope - February 2021 - 31
Sky & Telescope - February 2021 - 32
Sky & Telescope - February 2021 - 33
Sky & Telescope - February 2021 - 34
Sky & Telescope - February 2021 - 35
Sky & Telescope - February 2021 - 36
Sky & Telescope - February 2021 - 37
Sky & Telescope - February 2021 - 38
Sky & Telescope - February 2021 - 39
Sky & Telescope - February 2021 - 40
Sky & Telescope - February 2021 - 41
Sky & Telescope - February 2021 - 42
Sky & Telescope - February 2021 - 43
Sky & Telescope - February 2021 - 44
Sky & Telescope - February 2021 - 45
Sky & Telescope - February 2021 - 46
Sky & Telescope - February 2021 - 47
Sky & Telescope - February 2021 - 48
Sky & Telescope - February 2021 - 49
Sky & Telescope - February 2021 - 50
Sky & Telescope - February 2021 - 51
Sky & Telescope - February 2021 - 52
Sky & Telescope - February 2021 - 53
Sky & Telescope - February 2021 - 54
Sky & Telescope - February 2021 - 55
Sky & Telescope - February 2021 - 56
Sky & Telescope - February 2021 - 57
Sky & Telescope - February 2021 - 58
Sky & Telescope - February 2021 - 59
Sky & Telescope - February 2021 - 60
Sky & Telescope - February 2021 - 61
Sky & Telescope - February 2021 - 62
Sky & Telescope - February 2021 - 63
Sky & Telescope - February 2021 - 64
Sky & Telescope - February 2021 - 65
Sky & Telescope - February 2021 - 66
Sky & Telescope - February 2021 - 67
Sky & Telescope - February 2021 - 68
Sky & Telescope - February 2021 - 69
Sky & Telescope - February 2021 - 70
Sky & Telescope - February 2021 - 71
Sky & Telescope - February 2021 - 72
Sky & Telescope - February 2021 - 73
Sky & Telescope - February 2021 - 74
Sky & Telescope - February 2021 - 75
Sky & Telescope - February 2021 - 76
Sky & Telescope - February 2021 - 77
Sky & Telescope - February 2021 - 78
Sky & Telescope - February 2021 - 79
Sky & Telescope - February 2021 - 80
Sky & Telescope - February 2021 - 81
Sky & Telescope - February 2021 - 82
Sky & Telescope - February 2021 - 83
Sky & Telescope - February 2021 - 84
Sky & Telescope - February 2021 - Cover3
Sky & Telescope - February 2021 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com