Sky & Telescope - October 2023 - 53

700 km east to west. There isn't a hint
of a mountainous basin rim such as
Montes Apenninus and Montes Carpatus
circling part of Mare Imbrium.
Additionally, QuickMap's overlay of
wrinkle ridges at Tranquillitatis reveals
ridges, but they don't define inner rings
or any other basin structure. The greatest
concentration of ridges is in the west
half of the mare and traces concentric
and radial structures of Lamont, which
is thought to be either a lava-covered,
two-ring impact basin or an unusual
buried volcanic structure. Another concentration
of ridges marks a volcanic
complex east of Jansen crater.
A final morphological aspect of
Mare Tranquillitatis is its uneven
depth. Instead of a broad, symmetrical
depression, the western half (centered
on Lamont) is a 450-km-long, 275-kmwide
depression that sits 2.2 km below
the average lunar elevation. The east
side of the mare is a shallow, 400-kmwide,
roughly circular area only 0.5 km
below the average elevation. The crustal
thickness reflects these surface elevation
differences, with Lamont having a
crust only 10 to 15 km thick, whereas
the crust under the eastern half of the
mare ranges from 25 to 40 km thick.
Precise gravity measurements provided
by GRAIL also show that, unlike
many impact basins having a positive
gravity anomaly across their center,
Tranquillitatis has only an elongated
mass concentration following the
topographic trough and thin crust of its
western half. Two smaller, circular mass
concentrations occur near Jansen and
the Gardner Megadome, a volcanic
mountain and caldera that I informally
named in 2004.
A final piece of geophysical data
is provided by GRAIL-derived gravity
gradient maps. A gravity gradient
occurs where two adjacent regions have
different masses and densities. Across
the Moon, concentrations of positive
gravity gradients (shown as deep blue
on QuickMap) are found under many
basins. There are also narrow, linear
blue zones (that I call worms) under
many mare ridges, which sometimes
link volcanic structures. The worms
are interpreted as places where the
deep lunar crust was pulled apart and
filled with high-density magma. The
blue worms represent geologic dikes
that probably supplied the magma that
erupted onto the surface as lava flows
in some areas or as other volcanic features
elsewhere.
The morphological and geophysical
data for Tranquillitatis don't support
the existence of an impact basin under
its mare surface. The lack of a mountainous
rim, and the great differences
of the mare topography, its crustal
thickness, mare ridge distribution, and
gravity anomalies and gradients are all
inconsistent with large impact basins.
Furthermore, the topographically low
surface trough at the same location as
a major deep gravity worm indicates
that the trough isn't related to surface
effects such as the excavation of a
giant basin.
Instead, several characteristics of
Tranquillitatis are associated with
volcanism. First, the fact that Lamont
is located over a wide gravity worm
strongly suggests the feature is of
volcanic origin, rather than a buried
impact basin. Second, the 100-km-wide,
circular crustal thickness and positive
gravity anomaly to the east of Jansen is
consistent with a buried impact crater.
However, this area is also located on
a strong gravity worm, indicating a
connection with deep magma. The
region also has a high concentration of
linear ridges, rather than the circular
ridges that would be typical for a buried
impact structure. Additionally, the
Jansen area is a source of the sinuous
Rima Jansen and a 6-km-long collapse
feature. While Jansen crater may be of
impact origin, a third nearby volcanic
structure is the aforementioned 80-kmwide,
1.2-km-high Gardner megadome.
The most unexpected feature of Mare
Tranquillitatis is the 400-km-wide circular
rise that covers most of the mare's
eastern portion and includes a 250-kmlong,
600-m-high area that reaches its
highest elevation between Rupes Cauchy
and Rima Cauchy. The fault forms
a sharp, 300-m-high jump in elevation,
and at the rille the summit rise begins
a smooth, downward slope towards the
northeast shore of the mare.
Recently, Le Qiao of Shandong University
in China and colleagues cataloged
283 small volcanic domes, almost
all in the east portion of Tranquillitatis.
They posit that the broad volcanic rise
was built up from innumerable, smallvolume
eruptions originating with
earlier volcanic domes. They proposed
that such domes were the main sources
of Mare Tranquillitatis lavas. Like nearly
all previous researchers, Qiao's team
accepts that an ancient, now degraded
basin must have created fractures to
magma source regions and formed
a depression to collect the lavas. But
because the evidence for a basin is quite
weak, perhaps the question should be
asked if maria ever formed without the
benefit of a basin predecessor.
¢Contributing Editor CHUCK WOOD has
climbed small volcanic domes in Idaho
and Iceland but none yet on the Moon.
sk yand tele scope .o r g * OCTOBER 2023 53
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Sky & Telescope - October 2023

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