Australian Sky Telescope MayJune 2017

54 AUSTRALIAN SKY & TELESCOPE May | June 2017

The curious case

of concentric craters

Why do some small lunar craters display a remarkable ring-and-rim structure?

Researchers might finally have the answer.

N

early all lunar craters formed

by impact, but some have

peculiarities that can’t be

explained as simply punching a hole

in the Moon’s surface. For example,

concentric craters (CCs) contain

circular rings or ridges, like a doughnut

snuggled onto their inner walls and

floors. The most famous of these,

and one of the biggest, is 15-km-wide

Hesiodus A, on the southern shore of

Mare Nubium. You can spot its inner

ring even through small telescopes

using medium- to high-magnification.

A few CCs have been known since at

least the 1950s, but not until 1978 was

a catalogue published, by me, listing

51 of them. I found that these craters

average 8 km in diameter and that the

inner ring is about half as wide. Only a

handful could be detected by telescopic

observations or in photographs with

the 1- to 2-km resolution of the day.

Instead, I was able to find so many

only by searching through photographs

taken during NASA’s Lunar Orbiter

and Apollo programs, which typically

show features as small as a few hundred

metres across.

My most surprising discovery

— and one that remains a key to

understanding their origin — is that

70% of them lie near the edges of maria

and 20% more occur on the lava-

flooded floors of remarkable craters

such as Humboldt (on the Moon’s

southeastern limb). Almost none occur

in the highlands.

Their remarkable ring-within-

rim structure led to some odd

interpretations. One was that CCs

resulted from two impacts at the same

location. This unlikely occurrence could

happen if an incoming asteroid had

broken into two pieces, one travelling

behind the other, before striking the

Moon. But there is no logical reason

why such double projectiles would only

hit near the edges of maria.

A second hypothesis held that a

single projectile, smashing into the edge

of a mare plain (where presumably the

lava is thin), would gouge out one crater

in the lava and a second, smaller one in

the underlying highlands material. In

fact, that does happen for excavations a

few hundred metres across, which yield

small craters in a lava flow on a mare

surface and in a separate flow just below.

But Crozier H and some other CCs

occur in highlands terrain adjacent to

maria, where the two-layer case does

not exist. Another counterexample is an

unnamed, 7-km-wide concentric crater

on the lava-covered floor of Humboldt.

Immediately next to it are two similar-

size craters, one slightly larger and the

other somewhat smaller — and neither

exhibits an inner ring.

What about volcanism? Might these

odd little craters be volcanic calderas of

some sort? It’s highly unlikely, especially

SThe easiest concentric crater to spot

telescopically is 15-km-wide Hesiodus A.

WThe floor-fractured crater Pitatus displays

many arc-shaped fractures just inside its rim.

Nearby is Hesiodus A, which has a doughnut-

shaped ridge concentric to its rim. DAMIAN PEACH; CLOSEUP: NASA / LUNAR RECONNAISSANCE ORBITER

20 km

MARE NUBIUM

Hesiodus

A

Pitatus

EXPLORING THE MOON by Charles Wood