Sky & Telescope - February 2023 - 52

FEBRUARY 2023 OBSERVING
Exploring the Solar System by Charles Wood
In 1971, lunar scientist William
Hidden Maria
Tiny clues that reveal the true nature of some light plains.
A
quick look at the full Moon reveals
two main types of lunar terrain:
expansive, light-hued regions and large
patches of dark material. Closer study
shows the bright material to be ancient
highlands dominated by impact craters,
while the dark regions consist of younger
basaltic lava flows (maria) at lower
elevations. Maria have far fewer impact
craters than do the highlands, implying
that by the time maria formed (mostly
after 3.8 billion years ago), the rate of
crater formation had greatly subsided.
By coincidence, the Moon's cratering
rate declined to a very low level at about
the same time as the start of lunar
volcanism, which is caused by internal
heating from radioactive decay of thorium,
potassium, and uranium.Cratering
continues today at a low level, while volcanism
peaked around 3.8 to 3.5 billion
years ago, with the last gasps apparently
occurring about a billion years ago.
Roughly 10% of the lunar surface
shares the same relatively smooth surface
of the maria but lacks the characteristic
dark colors of basaltic lava.
Known as light plains (S&T: Dec. 2022,
p. 52), these regions were originally
interpreted as volcanic material with a
different composition than basalts to
account for their lighter hue. But when
Apollo 16 landed on the light plains
near the crater Descartes, the astronauts
immediately noticed the lack of
volcanic rocks. Instead, they saw pulverized
ejecta from the formation of the
Imbrium and other giant impact basins.
K. Hartmann and I proposed ejecta
deposits as an explanation of why
older impact basins were sometimes
filled with light material while younger
basins contained mare lavas. Every
crater-forming event produces massive
amounts of pulverized rock that blankets
the surrounding terrain with layers
of light-hued debris. Some of this ejecta
falls across maria, masking the true
nature of the underlying rock.
Surefire indicators of buried mare
lavas are small dark-halo craters (DHCs),
including Copernicus H, a 4-km-wide
(2.4-mi-wide) crater just southeast of
Copernicus. In telescopes it's easy to
see at full Moon because of its broad,
dark halo. Copernicus itself formed on
dark maria but sprayed a thin deposit of
bright ejecta all around it. The subsequent
impact that formed Copernicus H
excavated beneath this coating, exposing
the dark mare material below.
Ejecta-covered dark basalt is called
Schickard
cryptomaria, or hidden maria. Many hidden
mare lavas were identified thanks
to DHCs. The other effective way to
categorize them is with spectral studies
that reveal mafic minerals (those containing
abundant magnesium and iron),
which give mare basalts their characteristic
dark tone. Impacts into cryptomare
create a regolith mixture comprising
small, fragmented ejecta particles and
underlying lava material.
In 2015, Jennifer Whitten, now at
Schiller
Tulane University in New Orleans, and
James W. Head of Brown University,
produced a list of 20 cryptomaria sites,
12 of which are visible on the lunar
nearside. These are challenging targets
for telescopic observers because cryptomaria
have no visible features except
for DHCs, which typically are small and
hard to see.
By far the largest nearside cryptoThe
Schickard/Schiller
cryptomare region
52 FEBRUARY 2023 * SKY & TELESCOPE
Zucchius
mare is the expanse coating a portion
of Schickard and beyond. Like many
cryptomaria, this one partly resides
within an impact basin, the 360-kmdiameter
Schiller-Zucchius Basin, whose
light-hued, mare-lava floor is coated
with ejecta mostly from the nearby
Orientale Basin. Schickard is famous
ALL IMAGES COURTESY OF NASA / GSFC / ARIZONA STATE UNIVERSITY
Sky & Telescope - February 2023

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