My first Magazine Sky & Telescope - 04.2019 | Page 54

APRIL 2019 OBSERVING
Exploring the Moon by Charles Wood
Arago
Lamont
Lamont: The Shadow Knows
Did NASA’s GRAIL mission reveal the true identity of a ghostly lunar feature?
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A PR I L 2 019 • SK Y & TELESCOPE
radial ridges, difficult to see unless the
terminator is nearby. Lamont looks like
a 75-kilometer-wide (47-mile-wide)
ghost crater, a pre-existing feature com-
pletely covered by subsequent lava flows
in Mare Tranquillitatis (S&T: June
2018, p. 53).
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 specter of a two-ring impact basin.
U.S. Geological Survey geologist David
Scott proposed this in 1974 when it was
discovered that a moderate-size mascon
was centered at Lamont. Mascons are
circular gravity highs — an excess of
mass. Mascons are centered on circular
impact basins and are believed to have
two origins: The rebound of dense lunar
p Lamont is commonly thought to be an an-
cient crater almost completely buried by lava in
Mare Tranquillitatis, visible only under low-Sun
illumination. Or is it a volcanic structure?
mantle material during the forma-
tion of impact basins, and the mass of
kilometer-thick piles of later mare fi ll.
The fact that fi ve 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 fi eld
of the entire Moon to high precision.
LUN
GSFC
amont Cranston was an identity
sometimes taken by the main
character of “The Shadow,” a serialized
radio drama that came on the air in
the 1930s. The Shadow had the abil-
ity to “cloud men’s minds,” and every
broadcast ended with an ominous voice
saying, “The Shadow knows!”
Much like the fi ctional 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, fi ne amateur images reveal that
Lamont is a complex of concentric and