Australian Sky & Telescope - June 2018

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

Procellarum where no underlying

impact basins are thought to exist, and

thusthemarethicknessmightbeless

than needed to completely cover crater

rims.Thesearemostlynegativegravity

anomalies, but I would have thought

that their large diameters and shallow

Procellarumlavaswouldhavecaused

most of the impacting projectiles to

excavate into the underlying highlands

materials. Perhaps Procellarum lavas

are thicker than I imagine.

The detection of QCMAs is exactly

whatwouldbeexpected,andlikemost

good discoveries raises more questions

for investigation. Let me encourage you

to observe a sequence of craters formed

onandatdifferentdepthsbelow

the present mare surface. Start with

Copernicus, which sits atop the latest

lavas, then look at nearbyEratosthenes

andArchimedes,whoseejectaare

coveredbylavaflows.Next,searchout

LambertRwhentheSunillumination

islowtoseepeaksofarimthatbarely

rise above the mare surface. And then

lookforafewofthesubmarinecraters

shown on the map. Number 33, which

isjustsouthofAristarchus,ishintedat

bylowmareridges.Furthernorth,look

for the big QCMAs in Mare Frigoris that

seem to have no correlation with mare

ridges at all.

„Contributing EditorCHARLES WOOD

chairs the Lunar Nomenclature Task

Group of the IAU’s Working Group for

Planetary System Nomenclature.

original basin highlands floor.

Evans’s group explains QCMAs

with negative gravity anomalies as

marking craters that formed into early

mare lava flows, excavating through

thelavasintotheunderlyinglower-

densityhighlandsmaterial.Duringthe

modification stage of crater formation,

theupliftofthecentralpeakand

surroundingfloorwouldcreateacrater

infilloflow-densityhighlandsrocks

fracturedbytheimpactevent.Ifthis

low-densityfloormaterialroseto

the level of the surrounding pre-lava

terrain, the thickness of subsequent

lavafillinsidethecraterwouldbethe

sameasoutsideandtherewouldbeno

gravityanomalyor,ifthehighlands

materialrosehigher,anegativeone.

Projectilesthatimpactedintothick

mare deposits may not intersect the

low-density highlands deposits at all

and therefore not generate any gravity

anomaly — these stealth-buried craters

would be totally undetectable.

Wecanidentifyafewoftheyellow

circlesontheQCMAmapwithknown

features. Number 63, in western Mare

Tranquillitatis, isLamont,asmall

dual-ringimpactbasin.Immediately

tothesouthisthelargerQCMA#91

that underliesSinus Asperitatis,a

previously recognised buried crater

or basin. And south of the Imbrium

Basinrimaretwoothernamed

features: #50 isSinus Aestuumand

#92 isMare Vaporum,boththought

tobesmallimpactbasins.Allofthese

smallbasinshavepositivegravity

anomalies, implying that they formed

before lavas erupted in their areas.

One final identified small feature is the

aforementionedLambertR(#96),which

hasanegativeanomaly,meaningthat

it formed after lavas had erupted in its

vicinity.Thatisquitelikelyconsidering

thatbitsofitsrimemergeabovethe

surroundingMareImbriumlavas.

Notice that there is only one

QCMA within each of the impact

basins Crisium, Nectaris, Humorum,

Serenitatis and the inner part of

Imbrium. For all of these except

Nectaris, the lavas are possibly so

thickthatsubmergedcratersare

totally encased in mare lavas and

generate no anomalies.

It’s surprising that large QCMAs

occur in Mare Frigoris and Oceanus

ANOMALIES: GREGG DINDERMAN /

S&T;

LAMONT: ROBERT REEVES

Low-density bedrock

Extra-thick mare ill atop early mare low

Early pre-crater

Positive QCMA Negative QCMA mare low

W This map of circular gravitational

anomalies highlights visible craters in

pink, while those with no visible surface

features appear in yellow. The numbered

regions are known buried craters that can

be observed when sunlight strikes the

area at a low angle.

S Quasi-Circular Mass Anomalies (QCMAs) are categorised into two groups. Positive QCMAs

(left) contain higher mass than their surroundings, which is thought to be due to mare lava

looding a crater that excavated deep into the underlying terrain. Negative QCMAs have a mass

deiciency compared to their surroundings, implying a buried crater that formed in the low-mass

early lunar highlands material.

S The crater Lamont is a completely buried

impact basin that appears as a shallow

system of circular ridges.

96

50 92

63

91

33