Sky & Telescope - February 2022 - 9

CHANG'E 5 LANDING: CNSA / CLEP / DON DAVIS; BINARY
ASTEROID FORMATION: NASA / JHAPL / SWRI
MOON
Lunar Samples Confirm Late Volcanism on the Moon
ANALYSIS OF LUNAR MATERIAL,
delivered to Earth from the lunar nearside
by China's Chang'e 5 mission, confirms
that the samples are the youngest
collected to date. But their composition
surprised scientists.
A team led by Xiaochao Che (Chinese
Academy of Geological Sciences)
analyzed the content in two 3-4 millimeter-size
basalt fragments, determining
that they are 1.96 billion years old
- a billion years younger than material
returned by the Apollo and Soviet-era
Luna missions. The team published the
results in the October 8th Science.
Collecting young lunar fragments
was one of the main objectives for
the Chang'e-5 mission, which sent
a lander to the Moon in December
2020. The craft grabbed 1.7 kilograms
(3.7 pounds) of lunar regolith from the
vast volcanic plain of Oceanus Procellarum
and flew back to Earth within the
month. With the help of observations
SOLAR SYSTEM
New Horizons Finds
" Twins " in the Kuiper Belt
NASA'S NEW HORIZONS spacecraft
has spotted two asteroid pairs in the
outer solar system, reported Hal Weaver
(Johns Hopkins University Applied
Physics Laboratory) at the annual Division
for Planetary Sciences meeting.
The probe's Long Range Reconnaissance
Imager took pictures of 2011
JY31 and 2014 OS393 from a distance of
0.15 a.u. and 0.09 a.u., respectively.
The images of both targets appeared
slightly elongated compared to a nearby
star, so the team fit each shape with
a two-body model. Even though the
images couldn't resolve the individual
objects, the models showed that two
bodies better explained the observed
elongation and brightness. For 2011
JY31, the two objects orbit each other
about 200 km (120 miles) apart, while
for 2014 OS393, the model indicates
slightly smaller bodies 150 km apart.
the magma flowing longer. Some of
these elements would have been radioactive
(namely uranium, thorium, and
potassium). Yet Che's team finds only
usual amounts of these radioactive elements;
the concentrations are similar to
those in the basalts returned by Apollo
and Luna missions.
Scientists also expected the rocks
from this site to contain lots of heatproducing
materials known as KREEP
(short for potassium, rare Earth elements,
and phosphorus). Instead, the
pThis is a rectified version of the landing site
panorama taken by cameras aboard China's
Chang'e 5 lander in Oceanus Procellarum.
from lunar orbit, scientists had determined
this mare was relatively young;
its paucity of craters suggested recent
lava flows. The samples' ages confirm
that volcanism occurred later there
than in other areas of the Moon.
The samples' composition poses
new questions, however. Scientists had
suspected that an abundance of heatproducing
elements in the lunar mantle
underneath this mare might have kept
Pathways to binary asteroid formation
Arrokoth
Protosolar disk
team found only small amounts.
" The unexpected composition challenges
our traditional thinking, " says
Long Xiao (China University of Geosciences),
who was not involved in the
study. If not heat-producing elements,
though, what kept the material under
Oceanus Procellarum hot?
Besides raising new questions,
the radiometric dating also provides
" ground truth " for comparison with
crater counts, the method used to date
other inner solar system surfaces.
¢ANDREW JONES
Slow speeds within collapsing pebble clouds allow
accretion, enabling binaries to form naturally.
The pairs were two of four " cold classical "
Kuiper Belt targets, Weaver says.
This family of objects hasn't changed
much since it formed early on in solar
system history. A third target wasn't a
binary; a fourth remains undetermined.
New Horizons imaged one other
object that's part of the " scattered disk "
family, whose members have undergone
an interaction that altered their original
orbit. That one doesn't appear to be a
binary, Weaver says.
The twin asteroids - like doublelobed
Arrokoth, which New Horizons
imaged in early 2019 - support a for2011
JY31
2014 OS393
mation model in which gentle collisions
among small " pebbles " make dense
pebble-filled clouds, which then collapse
into larger planetesimals.
Another flyby target isn't out of the
question for New Horizons, but Kuiper
Belt objects are few and far between.
The probe's position in star-rich Sagittarius
further complicates the hunt.
" It's a long-shot, " Weaver says, " but
this is humanity's only mission to the
Kuiper Belt for the next couple decades,
so we're trying to squeeze every last bit
we can out of it. "
¢DAVID DICKINSON
sk yand tele scope .o r g * FEBRUARY 2022
9
Sky & Telescope - February 2022

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