Sky & Telescope - April 2019 - 53

t NASA'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.
Gardner
Megadome
Jansen

Lamont

t The 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.
Gardner
Jansen

CREDIT

Gardner
Megadome

Mascons and many other previously
unknown gravitational features were
mapped out in great detail. By looking not just at the gravity values but
their rate of change, GRAIL scientists
discovered entirely new features. Most
are enigmatically long, narrow lines
of excess gravity that GRAIL scientists

propose represent subsurface dikes, vertical sheets
of magma that fractured
the crust and rose from
deeper sources toward
the surface.
Maurice Collins,
coauthor of our book
21st Century Atlas of the
Moon, has combined
the GRAIL gravity data
and Lunar Reconnaissance Orbiter (LRO) low-illumination
topography for western Mare Tranquillitatis. The blue excess gravity "worms,"
as I call them, coincide almost exactly
with Lamont and the major mare ridges
to the north and south. An impact
basin wouldn't normally be associated
with radial ridges that we see on the

surface and their presumed underlying
magma-ﬁlled dikes. But it's common on
Earth for large volcanic centers 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 ﬁ lled
with dark lava. It is part of an irregular
plateau that's 200 to 300 meters 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 is an impact
structure. 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.
s k y asnkdyt e
a lnedstceol e
p se c.coopm
e . c*o mA *P A
RPI RLI L2 20011 9

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Sky & Telescope - April 2019

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