Sky & Telescope - August 2024 - 53

A. BROQUET / J. C. ANDREWS-HANNA (2)
NASA's Gravity Recovery and Interior
Laboratory (GRAIL) spacecraft. Gravity
anomalies are created by variations
in rock density. In a previous publication,
Andrews-Hanna and colleagues
proposed that the 100- to 1,000-kmlong
linear anomalies in GRAIL data
were ancient magma intrusions or dikes
that I like to call worms. Based on the
number, lengths, and likely depths of
worms, these features are the secondlargest
intrusive contributor to the total
lunar igneous inventory. Their existence
also indicates that the Moon's diameter
slightly expanded early in its history.
Identifying all the types of intrusive
and extrusive lunar volcanism is much
easier than determining their depths
and extents in order to calculate their
volumes. Broquet and Andrews-Hanna
combine GRAIL gravity data with
Lunar Reconnaissance Orbiter (LRO)
altimetry to do the latter. The mathematical
models are complex, but the
basic idea is that they estimate the
igneous body's volume by determining
the extent of its gravity anomaly and
the subsidence of the crust and upper
mantle due to a mare deposit, intruded
slab, or dike of solidified magma surrounded
by crustal rocks. The models
rely on best estimates of several uncertain
quantities, such as the density
difference between the solid magma
and the adjacent non-intrusive crust,
as well as the lunar crust's thickness
and elasticity.
Using various
methods, Broquet
and AndrewsHanna
calculated
a total
volume of
more than 29
million cubic
kilometers
of igneous
deposits on
the Moon.
The estimated
18.2 cubic kilometers
of maria are the
single biggest contributor
of lunar igneous materials. The next
largest are basin-ring dikes at 4.6 km3,
linear intrusions and worms with 3.9
km3, cryptomaria with 2.2 km3, and
floor-fractured craters contributing 0.4
km3. They also derived the thicknesses
of maria to be about 8 km in the major
basins like Imbrium and Serenitatis.
Away from circular basin centers, mare
thickness averages about 1.6 km, which
accounts for the appearance of many
semi-buried and ghost craters in southern
Oceanus Procellarum.
As the diagram on the facing page
shows, the occurrence of various rock
types strongly differs on the Moon's
two hemispheres, with the nearside
dominated by extrusive rocks and the
farside by intrusive rocks. The farside's
total volume of igneous rocks is also
only one-third that of the nearside.
 This pie chart
shows the global
volumetric fractions
of extrusive (gray)
and various intrusive
igneous materials.
62% Maria
1% FFC intrusions
16% Ring dikes
13% Linear intrusions
8% Cryptomaria
Nearside volcanism is concentrated in
the Moon's northwest quadrant, right
where the crust is thinnest and the
abundance of heat-producing radioactive
elements is highest.
The maria are the most conspicuous
lunar features, and many observers who
are familiar with floor-fractured craters
and volcanic shields are now becoming
aware of cryptomaria. These are among
the most intriguing lunar features to
explore on our satellite. Yet this new
inventory shows that the volcanism that
makes the Moon more than a graveyard
of endless impact craters constitutes
just 1.5% of the Moon's 40-km-thick
crust. We get so much from so little.
¢Contributing Editor CHUCK WOOD
continually marvels at the complex story
the lunar surface tells.
This Mollweide projection shows the estimated nominal global mare thickness model with
an elastic-crustal thickness of 40 kilometers (25 miles). The color scale shows the estimated
area-weighted thickness, with mean thicknesses for the plains, impact basins, and on a global
scale. Labels correspond to Oceanus Procellarum (Oc. Pr.), Mare Humorum (Hu), Mare Imbrium
(Im), Mare Serenitatis (Se), Mare Nectaris (Ne), Mare Crisium (Cr), and Mare Smythii (Sm).
Plains:
1.6 km
Global:
2.8 km
Impact basins:
7.9 km
Mare Thickness
Mare
Impact
Basin
Imbrium
Serenitatis
Crisium
Nectaris
0.0
2.5
5.0
7.5
10.0
12.5
Mare thickness (km)
15.0
17.5
20.0
Humorum
Smythii
Volume
(million
km3)
2.5
2.1
1
0.7
0.6
0.5
Maximum
thickness &
standard
deviation (km)
8.7 ± 0.7
8.8 ± 0.6
8 ± 0.4
7.4 ± 0.3
7.3 ± 0.3
6.2 ± 0.3
sk yand tele scope .o r g * AUGUST 2024 53

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