Sky & Telescope - August 2024 - 52

AUGUST 2024 OBSERVING
Exploring the Solar System by Charles A. Wood
Lunar Volcanism Seen and Unseen
Maria are a larger portion of the lunar surface than previously known.
O
bservers have long recognized
that large patches of darkness on
the Moon must be made of distinctly
different material than the surrounding
lighter regions. But it wasn't until
Apollo 11 astronauts brought home
samples from one such dark plain that
scientists confirmed the maria are
basaltic lava and therefore have volcanic
origins (S&T: July 2019, p. 14).
The Moon's volcanic history is
complex. In 1959, images from the Luna
3 spacecraft revealed that the lunar
farside largely lacks the expanses of
dark material we're so familiar with
on the nearside. This finding later
raised questions on precisely how much
volcanic material there is on the Moon,
how it's distributed, and what it tells us
about the Moon's thermal history - all
the volcanic material we see must have
started as mantle rocks deep inside that
were melted.
In a recent publication in the journal
Icarus, Adrien Broquet and Jeffrey
Andrews-Hanna (both University of
Arizona) present the first global estimates
of just how much igneous
rock there is on the Moon.
They detail both extrusive
rocks (volcanic material
erupted onto the
surface) and intrusive
rocks (magma that
solidified beneath the
surface, which we can
find through careful
study).
Measuring the surface
area of dark volcanics is relatively
easy. The vast maria make up
about 17% of the lunar surface, with
52 AUGUST 2024 * SKY & TELESCOPE
the lava-covered floors of craters such as
Plato, Billy, and Mercator contributing
a little more. There are also cryptomaria,
ancient lava flows now covered by
subsequent ejecta from the formation
of basins and craters. Cryptomaria were
originally identified by the existence of
dark-halo craters, such as the 4-kmdiameter
(2.5-mile-wide) Copernicus
H. The impactor that formed Copernicus
H excavated mare lavas that had
lain hidden beneath a covering of bright
ejecta produced by the earlier formation
of nearby Copernicus. Careful mapping
of these dark-halo craters has revealed
significant cryptomaria in the Schiller-
Schickard, Balmer-Kapteyn, and other
basins (S&T: Feb. 2023, p. 52).
But there's more igneous material
on the Moon than what we can see.
Intrusive rocks often are the subsurface
portion of a rising column of magma,
whose upper end breached the surface
and erupted as lava. On the Moon,
these intrusive rocks lie concealed
within the crust but are detectable by
their effects on the lunar surface.
An excellent example of these effects
are places where unerupted mare
basalts have seeped under the floor of
an impact crater and lifted its floor.
Gassendi, Posidonius, and Atlas are
prominent examples of such floorfractured
craters. Intrusive layers have
fractured their floors, creating concentric
rilles. In some cases, material also
was extruded to create lava-covered
floors, rilles, and small volcanic darkhalo
craters. Other inferred intrusions
manifest as ring dikes - circular or
arc-shaped deposits that invaded preexisting
fractures such as basin rings -
and under volcanic masses such as the
Marius Hills to the west of Marius.
A more recently discovered type
of intrusive activity is interpeted to
explain the widespread, linear gravity
anomalies detected in data from
Cryptomare
Mare
Im
ImSe
Se Cr
Oc.
Pr.
Cr
Oc.
Pr.
Hu
Hu
Ne
Ne
Sm
Sm
Cryptomare
Mare
A. BROQUET / J. C. ANDREWS-HANNA
Sky & Telescope - August 2024

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