Australian Sky & Telescope — November-December 2017

56 AUSTRALIAN SKY & TELESCOPE November | December 2017

EXPLORING THE MOON by Charles Wood

The vagaries of crater ‘tweens’

Some lunar impacts are neither ‘simple’ nor ‘complex,’ but somewhere in between.

A BIOLOGIST ONCE SAID THAT God

must have an inordinate fondness for

beetles because he made so many of

them. The Almighty must also have

loved impact craters, for millions of

them adorn our Moon, other moons,

asteroids, solid planets, and probably

the surfaces of rocky exoplanets. I like

impact craters too, and the Moon’s

eons-old landscape is my favourite

venue for admiring their variations.

Over the last 60 to 70 years, the

basic facts of impact crater formation

have become well established. On the

Moon, crater diameters range from

micron-size zap pits to 1,000-km-wide

basins, and their morphologies (shapes

and characteristics) systematically

change as their sizes climb. Lunar

craters smaller than about 15 km

across — such as 13-km Chladni — are

commonly flat-floored bowls with

smooth walls, looking like they’d been

turned on a potter’s wheel.

For diameters larger than 15 km,

these simple characteristics start to

break down, literally. Craters’ outer

walls begin to slump, and clumps

of hills rise in their centres. Look at

25-km-wide Mösting, an extreme

example of a small-yet-complex crater.

It has no flat floor, for the crater walls

have collapsed nearly everywhere and

sent piles of debris cascading toward the

middle, inundating all but the top of

its central peak. Some areas of collapse

have undercut parts of the rim, creating

alcoves that distort its circularity.

The walls and floors of ever-larger

craters slowly become more orderly,

as individual alcoves of collapses and

slumps give way to stately terraces that

sometimes circle the entire wall. Central

hills become towering mountains. At

their peak of perfection, as at Tycho,

craters are magnificent sights.

As target rocks respond to increasing

impact energy and the downward

pull of lunar gravity, the resulting

progression of crater morphology

toward complexity is not smooth and

uniform. Like children struggling to

find their identities once they become

‘tweens,’ and then transition from

the teen years to adulthood, these

depressions exhibit many variations due

to local circumstances.

Nowhere are these transitional

characteristicsfromdisorderly(Mösting)

to orderly (Tycho) morphology more

evident—andvaried—thantheyarefor

freshcraterswithdiametersofabout40

km.Followingaresomeexamplesthat

reveal key details when viewed through

amateur telescopes.

Glushko,onthehighlandsjustwest

of southern Oceanus Procellarum,

sits near the Moon’s western limb and

isdifficultfortelescopicobserversto

seeinto.ButLunarReconnaissance

Orbiterimagesshowthatthisyoung

rayedcraterdisplaystwoalcovesalong

itsnortheasternside,markingwhere

the rim gave way and material slid

downslopeandpartwayacrossthefloor.

In fact, Glushko’s small central peak

WThis 30-panel mosaic, captured in red light,

reveals a wealth of bright and dark markings

on the full Moon of July 20, 2016. Labels

identify craters mentioned in the text.

Harpalus

Aristarchus

Marius

Glushko

Eratosthenes

Cepheus

Bürg

Chladni

Mösting

Herschel

Alpetragius

Polybius

Bullialdus

Lansberg

Kopff

Tycho

Zucchius

GIUSEPPE PETRICCA