Sky & Telescope - February 2021 - 52

FEBRUARY 2021 OBSERVING
Exploring the Solar System by Charles Wood

Antoniadi
127 km

Copernicus
96 km

Aristarchus
40 km

A Lunar Main Sequence
Understanding lunar crater formation based on appearances.

I

H-R diagram. For example, one can
consider the main sequence of impact
craters as the systematic morphological
difference from small conical pits such
as the 13-km-diameter (8-mile) crater
Chladni, to mid-sized craters with
partial wall collapses as seen in 24-km
Lalande, and on to large craters with
terraced walls and central peaks, like
96-km Copernicus. Crater diameters
increase and morphology changes as
the size and/or velocity of the impac-

n astronomy, the main sequence is the
continuous band of stars spanning
the Hertzsprung-Russell diagram, which
plots stellar color and intrinsic brightness. When a large main-sequence star
runs out of hydrogen fuel, it becomes
cooler, redder, and radically expands in
size as it moves off of the main sequence.
Unlike stars, impact craters do not
dynamically evolve. But, based on size
and morphology, they often have parallels to the distribution of stars on the

2,400

S
Imbrium up
er
Basin
-g
ia
Nectaris
nt
s
Basin

1,000
600

Gi

an

Bailly
Proto-basin

Peak-ring basins

ts

Clavius

M

1E+24

230

Antoniadi
Copernicus

1E+22

ai

n-

se

1E+20

Complex-terraced craters

Aristarchus

qu

en

FEBRUARY 2 021 * SK Y & TELESCOPE

40

Lalande Complex-slump craters

ce

cr

at

15

Chladni

er

1E+18

Impact energy (kilojoules)

52

100

10

s

1E+16

Petit
Simple craters

1E+14

1E+12

Crater diameter (kilometers)

Grimaldi

Multi-ring basins

tors increase, delivering more energy
for fracturing, melting, excavation, and
ejecting lunar crustal rocks.
Crater-wall morphology stems from
how the impacted target rocks deform
as the initially excavated hole - the
transient cavity - becomes deeper and
wider. For small transient cavities, the
impacted rocks are strong enough to
withstand collapse into the resulting
depression, and a simple, conical crater
forms. With medium-sized transient
cavities, sections of the surrounding
walls collapse, creating complex-slump
craters. And for large cavities, the entire
surrounding wall slides into the hole,
creating complex-terraced craters. Rim
collapses signiﬁcantly increase the crater's overall diameter. Central peak sizes
also change across the main sequence.
Simple craters don't have central peaks
at all because the pressure of the impact
didn't compress the subsurface rocks
sufﬁciently to cause a rebound. As the
impact energy increases, the compressed
target rocks rebound, transporting
deeper rocks to the surface to form a
central peak.
Inspired by the H-R diagram, I've
created a graph that tracks diameter and
morphology differences arising from
increasing impact energy. The horizontal axis represents the kinetic energy
produced by the impacting body, and the
vertical axis plots the resulting crater
diameter. I've also included approximate
transitional stages, from Simple to
Complex-slump craters at around 15-km
W The main sequence of lunar impact features
showing the relationship between morphology
and the energy required to create each

A LL PHOTOS: N ASA / GSFC / A RIZON A STATE
UNIV ERSIT Y; G R A PH: LE A H TISCIONE / S&T

Schrödinger
325 km
Sky & Telescope - February 2021

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