2019-07-01_Australian_Sky_&_Telescope

50 AUSTRALIAN SKY & TELESCOPE July 2019

EXPLORING THE MOON by Charles Wood

CREDIT

L

amontCranstonwasanidentity

sometimestakenbythemain

characterof‘TheShadow,’ a

serialisedradiodramathatcameon

theairinthe1930s.TheShadowhad

the ability to “cloud men’s minds,” and

every broadcast ended with an ominous

voice saying, “The Shadow knows!”

Much like the fictional character, the

lunar feature Lamont has clouded the

minds of many observers and scientists,

for it is unlike any other landform on

the Moon. Barely noticed by early lunar

mappers, the feature wasn’t named

until 1898 by the young Bavarian

observer Johann Nepomuk Krieger, who

added visually observed details to Lick

and Paris Observatory photographs.

Today, fine amateur images reveal that

Lamont is a complex of concentric and

radialridges,difficulttoseeunlessthe

terminatoris nearby.Lamontlooks

likea 75-kilometre-wideghostcrater,a

pre-existingfeaturecompletelycovered

bysubsequentlavaflowsinMare

Tranquillitatis.

But Lamont is more than a ghostly

ring. It’s partially surrounded by a

discontinuous outer ring about 135 km

in diameter, making it appear like the

spectre of a two-ring impact basin.

US Geological Survey geologist David

Scott proposed this in 1974 when it was

discovered that a moderate-size mascon

was centred at Lamont. Mascons are

circular gravity highs — an excess of

mass. Mascons are centred on circular

impact basins and are believed to have

two origins: The rebound of dense lunar

mantle material during the formation LAMONT: WESLEY HIGGINS; GRAVITY MAP: NASA / LUNAR RECONNAISSANCE ORBITER / JPL-CALTECH / MIT / GSFC / M. COLLINS; GARDNER MEGADOME: JOCELYN SEROT

of impact basins, and the mass of

kilometre-thick piles of later mare fill.

The fact that five large mare ridges

radiate from Lamont is strange. Impact

basins don’t have such ridges, so perhaps

the impactor that created Lamont just

happened to land at an intersection of

mare ridges. My mind feels clouded...

Today, new information provides

answers — or an expanded array of

questions. NASA’s GRAIL (Gravity

Recovery and Interior Laboratory)

spacecraft measured the gravity field

of the entire Moon to high precision.

Mascons and many other previously

pLamont is commonly thought to be an

ancient crater almost completely buried by lava

in Mare Tranquillitatis, visible only under low-

Sun illumination. Or is it a volcanic structure?

Lamont: The shadow knows

Did NASA’s GRAIL mission reveal the true identity of a ghostly lunar feature?

Arago

Lamont