Sky & Telescope - February 2022 - 53

CRATERS: NASA / ARIZONA STATE UNIVERSITY / APPLIED
COHERENT TECHNOLOGY CORP; GRAPH: H. HIESINGER
Cumulative number of craters (>1 km/km2)
research was new in the 1960s, so it's
not surprising that the age estimates for
lunar maria differed widely. Shoemaker
and Baldwin predicted young ages for
the lavas around the Apollo 11 landing
site, as little as 0.6 billion years.
Four years earlier, Hartmann derived
an age of 3.6 billion years, exactly what
the samples returned by the Apollo 11
mission turned out to be. Hartmann's
success must have been a combination
of expertise and luck.
Samples brought back from the
You can observe the key sites I've
2
3
1
4
5
Apollo 11 landing site on Mare Tranquillitatis
provided the first calibration
point for crater counting. Four more
Apollo missions landed on maria and
yielded additional calibrations for crater
counts. But the mare ages of Apollo
samples spanned only the period before
3.2 billion years ago. As a result, there
were no calibration points for lunar
crater counts covering 3.2 billion years
ago to today - the most recent 60% of
lunar history. This gap was creatively
filled by radiometric dating of certain
Apollo 17 samples thought to be Tycho
ejecta, which provided an age of 0.11
billion years for Tycho and its crater
counts. Similarly, the age of 0.8 billion
years for Copernicus stems from dating
an Apollo 12 soil sample inferred to be
ray material from that giant crater. All
the sparse and uncertain crater count/
absolute age data are memorialized on a
famous graph that remains the basis of
lunar chronology, nearly 50 years after
the end of the Apollo program.
The situation will drastically
improve now that samples returned
in December 2020 from northern
Oceanus Procellarum by China's
Chang'e 5 mission have been precisely
dated. China selected the landing site
(recently named Statio Tianchuan,
after a Chinese constellation) because
crater counts indicate that the mare
lavas there, originating from the nearby
sinuous rille Rima Sharp, are among
the youngest on the Moon, forming
sometime between 1.5 and 2 billion
years ago. The laboratory-dated Chang'e
5 samples are 1.96 billion years old,
which is consistent with the age derived
through crater counting.
pThis image plots the order of major events
in Mare Imbrium. Montes Apenninus (1) formed
first, followed by Archimedes (2). Imbrium lavas
then filled the basin (3). Later, impacts formed
Eratosthenes (4) and finally Copernicus (5).
Adding a precise age and crater count
number for such a young lava implies
that the standard crater-counting/age
models need only small corrections to
the accepted lunar chronology (and
all other chronologies across the solar
system that are based on lunar results).
The scientists who reported the new age
also state that the samples are low in
radioactive elements whose decay could
provide heat for creating magma more
than a billion years after most lunar
volcanism ended. What this means is
that scientists need to create new models
of lunar thermal history. In science,
data drive theories.
This graph updates the
absolute lunar chronology,
which plots the
ages of measured lunar
samples on the x axis
against the cumulative
number of craters larger
than 1 km diameter per
each square kilometer
of lunar surface along
the y axis. The ages of
Copernicus, Tycho, and
Nectaris are estimates,
while samples returned
by the Apollo missions
(Ap), the Soviet Luna
missions (L), and China's
Chang'e 5 mission (Ch5)
were directly measured.
Gray bars represent the
margin of error in the
crater counts.
1.0
mentioned. The southeast quadrant of
the Imbrium Basin where Shoemaker
and Hackman established the first lunar
stratigraphy is a familiar region, with
prominent craters and mountains. Two
details to notice are rays and secondary
craters from Copernicus crossing
Imbrium east of Pythias, and the
lack of rays and secondaries radiating
from Archimedes. The landing site of
Chang'e 5 is not conspicuous but look
east of Mons Rümker for the isolated
peak Mons Heng, newly named after
a Chinese mountain (see facing page).
Northeast of Mons Heng is Statio Tianchuan
and Rima Sharp. The rille is quite
indistinct here but is more conspicuous
120 km (75 miles) further northeast
near the 11-km-wide crater Louville DA.
The 1.96-billion-year age at Statio
Tianchuan is considered young because
the majority of the lunar surface is
more than 3 billion years old, whereas
the average age of Earth's surface is
about 0.5 billion years. We live on
a dynamic world that tends to erase
ancient features. Fortunately, there's a
museum of solar system antiquity orbiting
just 226,000 miles away.
¢Contributing Editor CHUCK WOOD
can't wait for more dated samples to fill in
the crater curve gaps.
0.1
0.01
Ap11
Ap16
10-3
Nectaris
Ap14
Ap17
Ap15
L16
L24
Ap12
Ch'5
Copernicus
10-4
Terrestrial craters
Tycho
North Ray
Cone
10-5
5
4
3
2
1
Measured age of returned samples (in billions of years)
sk yand tele scope .o r g * FEBRUARY 2022 53
Sky & Telescope - February 2022

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