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https://arstechnica.com/science/2021/10/lunar-samples-returned-by-change-5-tell-of-recent-volcanism/

Lunar samples returned by Chang’e-5 tell of recent volcanism

"Recent" is relative—the samples are roughly two billion years old.

JOHN TIMMER - 10/7/2021, 12:42 PM

Greyscale image showing a slice through a rock with a complex structure.
Enlarge / An electron micrograph of some of the volcanic material
returned by the Chang'e 5 mission.
Beijing SHRIMP Center, Institute of Geology, CAGS

Chang'e-5 represented a major step forward for China's lunar program, as
it was the first of the country's missions to return samples to Earth.
Now the results of dating the samples are in, and it's clear that while
the deposits are old, they're young enough to be a bit of an oddity.

To the Moon and back
China has now successfully landed several probes and rovers on the lunar
surface as part of a larger exploration program. Chang'e 5 represented
the next step, as it had a drill to obtain subsurface samples and a
return vehicle that could take them back to Earth. The spacecraft
successfully landed almost two years ago, and a few weeks later, it
returned roughly 2 kilograms of rock to Earth from the Moon.

China has the benefit of decades of knowledge gained since the Apollo
missions, along with a greater theoretical understanding of the Moon's
formation. And it carefully chose Chang'e 5's landing site, a large
volcanic deposit called the Oceanus Procellarum, thought to be one of
the younger areas of the Moon's surface. But "younger" covers a lot of
ground, as estimates based on its crater count ranged from 3.2 billion
years old down to just 1.2 billion.

Getting a precise age on the deposit would provide a number of benefits.
To begin with, the Moon presents a "clock" for the cratering of our
entire Solar System. It's the only place where we can match crater
counts to ages obtained from rock samples; we then use those numbers to
put ages on other bodies based on their crater counts. The date range
for the Chang'e 5's landing site includes periods for which we don't
have dates from radioactive decay.

In addition, understanding when the Moon was volcanically active to
produce large deposits like the Oceanus Procellarum can help us craft
more accurate models of the Moon's formation and evolution. Volcanic
activity requires heat, and that heat comes from a combination of the
body's formation and its composition, which will include radioactive
isotopes that produce additional heat.

Marking time
The samples used for the new study were remarkably small: two cubes,
each about three to four millimeters per side. Yet that tiny volume held
a remarkable mix of minerals (clinopyroxene, plagioclase, olivine,
quartz, cristobalite, and ilmenite all make appearances). The bulk
composition is consistent with other volcanic deposits on the Moon,
however, and most of the differences between the two samples can be
ascribed to the rate at which they cooled.

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The research team, representing a large international collaboration,
used lead isotopes to estimate dates for multiple sites within each
sample. (Volcanic rocks often contain materials that solidified at
different time points and therefore can produce a range of ages.) The
results indicate that one of the samples was 1.893 ± 0.280 billion years
old and 1.966 ± 0.059 billion years old. Combining all the data from
both samples produces an age of 1.963 ± 0.059 billion years.

In good news, all of the uncertainty ranges overlap. And while
radioactive dating can be skewed by some factors, there's no indication
that those factors are in play here. There's no sign of contamination by
other material, either present at the eruption site or emplaced there by
nearby impacts.

The overall composition matched well with remote sensing done both from
orbit and by the hardware that China landed. So overall, the evidence
argues that the entire Oceanus Procellarum was likely to be deposited
about two billion years ago.

Implications
When it comes to the Solar System's crater clock, the new date for the
Oceanus Procellarum rules a few proposed chronologies out, as it
indicates fewer impacts since its formation than those chronologies
would have predicted. Other models are consistent with the date,
however, so the data doesn't call for major revisions.

The bigger surprise is in explaining the evolution of the Moon. The age
"implies that nearly 2,000 cubic kilometers of basaltic magma erupted
near the landing site almost 1 billion years later than the emplacement
of any previously measured lunar basalts." If these dates represent the
deposit as a whole, the Moon had a more active volcanism than we might
otherwise expect.

In other areas of the Moon, younger volcanic deposits are associated
with elements like potassium and thorium, which provide heat through
radioactive decay. The presence of these elements seemed suggestive, as
it could indicate that higher radioactivity provided the heat needed to
keep things molten later. The samples obtained by Chang'e 5 also have
higher levels of these radioactive elements but not higher than other
volcanic deposits, so the idea that there's a linear relationship
between radioactivity and age of volcanism doesn't hold up.

We're therefore left with a fair bit of uncertainty about how the Moon
managed to support such large-scale eruptions so late in its history.
Smaller-scale eruptions clearly went on later, caused by local
variations in areas where hot material was trapped. But it's not clear
whether these processes could support something like the Oceanus
Procellarum eruptions.

Science, 2021. DOI: 10.1126/science.abl7957 (About DOIs).

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