Australian Sky Telescope MayJune 2017

http://www.skyandtelescope.com.au 55

since the outer rim always looks like a

perfectlynormalimpactcrater.

An answer arising

In my now-ancient paper, I speculated

that CCs were impact craters whose

excavation near the edges of maria

allowed magma to rise to the surface

along pre-existing fractures in the

floorsofthelargebasinsthatunderlie

the maria. Exactly why the lavas would

make a ring, rather than flow smoothly

across the floor — as occurred inPlato

andArchimedes—wasunclear.

Perhaps these flows were moreviscous

(sluggish) and piled up in a ring.

David Trang, Jeffrey Gillis-Davis and

the late B. Ray Hawke (University of

Hawai‘i) now offer a better explanation.

As they detail in the November 2016

issue ofIcarus,concentriccratersare

apparently simple bowl craters that have

been modified by goings-on below them.

In this new model, the deep fracture

systeminagivenbasin’sfloorprovided

conduits for magma to well upward.

This encroachment led to an increase

in volume that pushed the crater floor

upward to form a concentric ridge.

Key to their new interpretation is

the much higher-resolution imagery

available from Lunar Reconnaissance

Orbiter and Kaguya. The researchers

used these and other data to catalogue

144concentriccraters,75%ofwhich

are on the lunar nearside (where

volcanism had been much more

common). Their mean crater diameter

and ring-to-rim diameter ratio match

my older and more limited data.

Importantly, they found that the

composition of a given CC’s inner ring is

very similar to that of its surroundings

— it is not some unique material.

Additionally, the ages of these craters

coincide with the timing of most mare

eruptions (3.8 to 2.8 billion years ago).

The Trang trio also realised that CCs

occur in the same mare-edge locations

as do much largerfloor-fractured craters

(FFCs) such asAlphonsus,Posidonius,

Taruntius,Petavius,Atlasand

Gassendi. These are among the most

observationally interesting craters on

the Moon because their floors are cut

by concentric fractures and ridges, and

theyoftencontainrilles,darkhalo

craters and ponds of lava.

Interestingly, CCs have diameters

that range from 3 or 4 km to about 15

km, whereas FFC diameters typically

extend from 15 km to more than 100

km.Asitturnsout,15kmisalsothe

dividing diameter between simple

and complex craters. Simple craters

have steep walls and small, flat floors,

looking like they were turned out by a

lathe. Craters are considered complex

if they have central peaks and if parts

of their rims have collapsed onto their

floors.

Trang, Gillis-Davis and Hawke draw

upon a model first proposed in 1976 by

Peter Schultz (Brown University). They

think floor-fractured craters formed

when magma rose along basin fractures

and lifted their entire floors — central

peaks and all. Concentric cracks and

ridges resulted as the floors’ edges frac-

tured, and magma ‘leaks’ produced

rilles, dark halos and mare ponds.

Notice that this explanation neatly

handles both simple and complex cases.

The three researchers suggest that CCs

form when magma rising along basin

margins invades simple craters, and

that FFCs form when larger amounts of

rising magma colonise complex craters.

So their differences are due to the size

and the nature of the host crater.

This is a very satisfying solution,

one that unites two types of craters

previously considered as distinct. We

still lack some specifics of how simple

craters end up with doughnuts inside

them — but we’ve got a better sense of

the process.

„ Contributing Editor CHARLES WOOD

used a typewriter to pound out his 1978

article on concentric craters. You can

read it here: https://is.gd/wood_cc.

Fractures

l, fresh

ater

Upwelling

magma

Magma-filled

fractures

ifted

ter

or

œConcentric craters might have formed when deep-seated lava pushed

upward through underlying fractures. The swelling volume then caused the

craters’ floors to bulge upward, forming distinctive rings inside the outer rim.

CONCENTRIC CRATERS: A PUSH FROM BELOW?

Concentric craters

(yellow symbols)

aren’t randomly

distributed

around the Moon

but instead are

typically found

on the nearside

around the edges

of maria. Floor-

fractured craters

(not shown here)

have a similar

distribution.

MAP: DAVID TRANG ET AL. /

ICARUS

2016; DIAGRAM:

S&T

/ LEAH TISCIONE (SOURCE: BARRY BAYS / DAVID TRANG)