http://www.skyandtelescope.com.au 51verticalsheetsofmagma
thatfracturedthecrust
androsefromdeeper
sourcestowardthe
surface.
MauriceCollins,
coauthorofourbook
21stCenturyAtlasof the
Moon, hascombinedthe
GRAILgravitydataand
LunarReconnaissance
Orbiter(LRO)low-
illuminationtopographyforwestern
MareTranquillitatis.Theblueexcess
gravity‘worms,’asI callthem,coincide
almostexactlywithLamontandthe
majormareridgestothenorthand
south.Animpactbasinwouldn’t
normallybeassociatedwithradial
CREDIT ridgesthatweseeonthesurfaceand
unknowngravitationalfeatureswere
mappedoutingreatdetail.Bylooking
notjustat thegravityvaluesbut
theirrateofchange,GRAILscientists
discoveredentirelynewfeatures.Most
areenigmaticallylong,narrowlines
ofexcessgravitythatGRAILscientists
proposerepresentsubsurfacedikes,tNASA’s LRO topographical data combined
with the GRAIL gravity map reveals excess
gravity signal coinciding with Lamont and
mare ridges that run to the north and south.their presumed underlying magma-filled
dikes. But it’s common on Earth for
large volcanic centres such as caldera
magma chambers to have fractured the
surrounding crust and to have forced
magma radially outward as dikes. Is
Lamont a buried caldera rather than a
buried impact basin?
Such an outrageous idea is consistent
with two other likely volcanic
landforms in this region that also
have significant excess mass. The first
is the Jansen complex about 330 km
to the northeast of Lamont. Jansen is
a 24-km-wide circular crater nearly
filled with dark lava. It is part of an
irregular plateau that’s 200 to 300
metres high and 85 km wide in Mare
Tranquillitatis. A small rille and
a few rimless collapse depressions
demonstrate that volcanism occurred
near Jansen as well as inside it.
The second likely volcanic landform
is a mountain about 1,000 to 1,200 m
high and 70 km wide with a caldera-like
central depression, 150 km northeast of
the Jansen complex. When I discovered
this feature 19 years ago I called it
the Gardner Megadome, after nearby
Gardner crater, and also because it
looks like many smaller volcanic domes
that occur in maria.
Neither the Jansen area nor the
Gardner Megadome are impact
structures. Both are clearly volcanic
landforms, and like Lamont, both
have strong mass excesses. Nearly two
decades ago I proposed that all three
are large volcanic complexes and are
aligned along some tectonic feature.
With the GRAIL gravity worms we now
see that all appear to have dikes that
brought up magma to feed them. So
perhaps the Shadow isn’t the only one
to know!¢Contributing Editor CHUCK WOOD
has studied the Moon for more than five
decades as both an amateur astronomer
and professional scientist.tpThe Gardner Megadome
is another volcanic landform
located just south of the
crater Gardner. To the west
the lava-floored crater
Jansen sits on a rille-crossed
volcanic plateau.JansenLamontGardnerGardner
MegadomeGardner
MegadomeJansen