Sky & Telescope - February 2022 - 52

FEBRUARY 2022 OBSERVING
Exploring the Solar System by Charles A. Wood
Louville DA
Statio Tianchuan
Rima Sharp
Mons Heng
Mons Rümker
Mairan
Chang'e 5 and the Age of Lunar Lavas
U
Does China's sample return mission change our understanding of lunar chronology?
ntil recently, scientists since
Galileo's time who studied the
Moon were astronomers, not geologists.
It wasn't until the late 1950s and
early 1960s that Eugene Shoemaker and
Robert Hackman of the U.S. Geological
Survey (USGS) developed a lunar stratigraphic
sequence based on telescopic
and photographic studies of the spatial
relations among lunar landforms. They
chose the southeast quadrant of Mare
Imbrium to determine sequences of
events by mapping the overlap of deposits
created by each event.
The most recent Imbrium episode
that Shoemaker and Hackman recognized
was the formation of the crater
Copernicus, whose radiating bright
rays and secondary craters adorn nearby
Mare Imbrium, Montes Apenninus
(Apennine mountains), and more distant
terrains. Copernicus's formation
must be the youngest major event in
this area of the Moon, they concluded.
The fact that rays and secondaries are
on Mare Imbrium means that those
mare lavas are older, and since the arc
of the Apennine mountains borders the
52 FEBRUARY 2022 * SKY & TELESCOPE
mare, the range must be older than the
lavas. The USGS scientists considered
the tall Apennines to be the rim of a
giant crater or basin that the mare lavas
erupted within. And the Apennines
must have formed on some pre-existing
terrain, which would be the oldest landform
in the region.
Going to a deeper level of stratigraphy,
the pair noted that the crater
Eratosthenes lacks rays, but its small
secondary craters are visible on Mare
Imbrium. Eratosthenes is also overlapped
by debris from Copernicus, so
it must be older than Copernicus, and
younger than the mare lavas. Going one
step further, they saw that Archimedes
has no rays or secondary craters
and is almost completely surrounded by
Imbrium lavas. Therefore, Archimedes
must have formed on the giant basin
floor or early lavas before Imbrium's
later lavas flooded the basin and buried
Archimedes' rays and secondary craters.
Over the next decade, USGS scientists
mapped the entire Moon, producing
a stratigraphy that delineated
various young and old landforms.
pSeen above are the newly named lunar
feature Mons Heng and Statio Tianchuan,
where China's Chang'e 5 mission landed and
collected samples.
Unfortunately, they didn't know the
absolute age for any of the features.
The solution at the time was to
estimate absolute age by counting the
number of craters on the maria. The
idea was that impact craters continually
accumulate over time. Thus, the heavily
cratered, bright lunar highlands must
be older than the sparsely cratered mare
lavas. But to convert crater counts to
absolute ages requires knowing the rate
at which impact craters form - and that
rate apparently changed from being very
high just after the Moon's formation to
the much lower rate we see today.
Before the Apollo program, Shoemaker,
astronomer Ralph Baldwin,
and graduate student Bill Hartmann
independently estimated crater formation
rates based on the number of
impact craters of known ages on Earth,
and from the predicted number of lunar
impact craters formed by the collision
of asteroids and comets. This kind of
NASA / ARIZONA STATE UNIVERSITY / APPLIED
COHERENT TECHNOLOGY CORP
Sky & Telescope - February 2022

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