Sky & Telescope - October 2023 - 52
OCTOBER 2023 OBSERVING
Exploring the Solar System by Charles A. Wood
Gardner
Gardner
Megadome
Jansen
Mare Tranquillitatis
Lamont
No Tranquil
Solution
Mare Tranquillitatis doesn't
appear to fill an impact basin.
Why not?
T
he major lunar maria tend to fill
large impact basins. Imbrium,
Serenitatis, Humorum, and Crisium all
fit this pattern. So why doesn't Mare
Tranquillitatis? It's a vast lava plain
seemingly without an underlying basin.
Or is there one?
Major impact basins on the Moon
span from about 300 to some 2,000
kilometers (185 to 1,250 miles) wide.
These are mega-craters that excavated
broad, deep cavities and deeply fractured
the underlying crust, creating
conduits that allowed magma from the
lunar mantle to erupt onto the surface.
Today they appear as circular lava plains
some 3 to 5 km deep, with concentric
inner rings and partially surrounded by
arcuate mountain ranges.
52 OCTOBER 2023 * SKY & TELESCOPE
Until recently, the traditional morphological
characteristics of basins were
identified visually. Now, with instruments
aboard NASA's Lunar Reconnaissance
Orbiter (LRO) and data provided
by the Gravity Recovery and Interior
Laboratory (GRAIL), we can image
below the surface to understand the
underlying structure of basins. These
data reveal that the basins have thin
crusts (due to the excavation during
formation) and circular gravity anomalies
characterized by excess mass in
their centers surrounded by collars with
lower mass. This central mass concentration
is caused by a rebound of dense
mantle rocks beneath the thinned
crust with some contribution from the
kilometers-thick piles of lava on top.
The surrounding collars are thought
to be due to the pile-up of fragmented
ejecta.
The co-registered collection of
data at the QuickMap Moon website
(https://is.gd/quickmap) permits an
easy means to investigate if Tranquillitatis
has a hidden underlying basin.
Looking first at the LRO Wide-Angle
Camera image of Tranquillitatis, it's
obvious that the mare isn't circular,
having a north-south dimension of
about 525 km and extending roughly
These five images show different aspects of Mare Tranquillitatis ordered left to right. The first
picture displays laser altimeter elevation measurements, with blue representing the lowest area. The
next highlights mare ridges. On the facing page, the left image displays the crustal thickness, with
thickest areas as orange. In the middle is a Bouguer gravity map with positive values assigned red
and negative values blue. The final map shows the gravity gradients that cross the maria.
ALL IMAGES COURTESY OF NASA / GSFC / ARIZONA STATE UNIVERSITY
Rima Jansen
Rima Cauchy
Rupes Cauchy
https://www.is.gd/quickmap
Sky & Telescope - October 2023
Table of Contents for the Digital Edition of Sky & Telescope - October 2023
Contents
Sky & Telescope - October 2023 - Cover1
Sky & Telescope - October 2023 - Cover2
Sky & Telescope - October 2023 - 1
Sky & Telescope - October 2023 - Contents
Sky & Telescope - October 2023 - 3
Sky & Telescope - October 2023 - 4
Sky & Telescope - October 2023 - 5
Sky & Telescope - October 2023 - 6
Sky & Telescope - October 2023 - 7
Sky & Telescope - October 2023 - 8
Sky & Telescope - October 2023 - 9
Sky & Telescope - October 2023 - 10
Sky & Telescope - October 2023 - 11
Sky & Telescope - October 2023 - 12
Sky & Telescope - October 2023 - 13
Sky & Telescope - October 2023 - 14
Sky & Telescope - October 2023 - 15
Sky & Telescope - October 2023 - 16
Sky & Telescope - October 2023 - 17
Sky & Telescope - October 2023 - 18
Sky & Telescope - October 2023 - 19
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Sky & Telescope - October 2023 - 84
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Sky & Telescope - October 2023 - Cover4
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