Sky & Telescope - December 2021 - 53

NASA / GSFC / ARIZONA STATE UNIVERSITY (3)
Alphonsus
Rima Marius
Marius B
Pyroclastic
deposits
Milichius A
Milichius
Dome
Milichius
pSinuous rilles (left) are almost exclusively found in the KREEP terrane, whereas pyroclastic deposits (center) and volcanic domes (right) occur in
most nearside maria.
and dating of samples collected during
the Apollo missions provided ground
truth to accurately calibrate the lunar
crater counts used to estimate the ages
of maria.
One of the major discoveries from
these multitudes of data was of the PKT
in 2000 by Brad Jolliff and colleagues
at Washington University in St. Louis,
Missouri. Scientists had previously discovered
basalts, anorthosite-rich highlands
materials, and other, less-common
rock types from Apollo samples.
But they also found an unexpected suite
of rocks (called KREEP) rich in certain
elements, especially radioactive potassium
(atomic symbol K), phosphorous
(P), and rare-earth elements (REE) -
the so-called incompatible elements
that don't bond readily with those in
most rocks and so are the last to form
when magma solidifies.
KREEP was found in many Apollo
samples as brecciated (fragmented)
rocks, but also in non-brecciated volcanic
rocks in Apollo 15 samples from the
Hadley Rille area of Mare Imbrium.
This finding suggests that the source
of all KREEP materials is buried in the
Hadley Rille region. Presumably, the
formation of the Imbrium impact basin
brecciated and ejected bits of KREEP
basalt, scattering debris all over the
Moon. But data from the NASA Lunar
Prospector's gamma-ray spectrometer
shows that high-radiation levels (as
identified by thorium measurements)
were almost entirely concentrated in the
area covered by Oceanus Procellarum,
Mare Imbrium, the western half of Mare
Serenitatis, and the Imbrium ejecta
northwest of the crater Ptolemaeus.
Jolliff and his colleagues recognized that
this area is unique and called it the PKT.
So have all lunar scientists ever since.
The thorium map shows that the
PKT includes most of the maria on the
Moon. Only the eastern maria, including
Crisium, Tranquilitatis, Nectaris,
and Fecunditatis are excluded. On the
farside there are fewer maria, and only
the ones inside the South Pole-Aitken
basin have any significant levels of
thorium. Another surprising characteristic
of the PKT is that, although most of
the lunar maria formed between about
3.8 and 3.2 billion years ago, only the
basalts in the PKT have crater-count
ages as young as 2 to 1 billion years. The
PKT is the only area of the Moon that
had voluminous quantities of magma
for a prolonged period. The older PKT
mare rocks appear chemically similar
to other mare rocks found over much
of the Moon, but the younger lavas that
originate in the PKT have nearly twice
as much titanium as older rocks outside
of the PKT. This means that the source
region or processing of the younger
magmas had changed since the time of
earlier eruptions.
PKT magma also favored the formation
of sinuous rilles carved by flowing
lava - nearly all of which occur in the
PKT. However, domes (another volcanic
landform) are common in the PKT
as well as elsewhere, such as in the
eastern maria, especially Tranquillitatis.
Similarly, pyroclastic deposits, such as
volcanic ash, are also widespread both
in and beyond the PKT, especially in
floor-fractured craters.
Models of the various styles of lunar
volcanic eruptions by Lionel Wilson at
Lancaster University, Lancaster, UK,
and James Head, at Brown University,
Providence, Rhode Island, help explain
these differences. They found that
sinuous rilles form when eruption rates
and volumes are large, whereas domes
(small shield volcanoes) form from
small-volume activity. The sinuous-rille
distribution may imply that PKT eruptions
were generally larger than those in
other regions. And the more widespread
occurrence of pyroclastic deposits and
domes may be explained by noting that
the beginning of all eruptions is driven
by escaping gases, while the end phases
produce smaller eruptions with lavas
that flow only short distances and sometimes
build up small domes. Pyroclastics
are deposited at beginnings of eruptions,
and domes form during declining periods,
with sinuous rilles occurring any
time a lot of magma erupts quickly.
Next time you observe the Moon
with your telescope, try to appreciate
the impact of the PKT on all aspects of
lunar volcanism - each rille, dome, and
ash deposit you detect tells you something
about local eruption conditions
billions of years ago. To fully embrace
your role as lunar volcanism explorer,
you may want to wear a hard hat and
leather boots while observing.
¢Contributing Editor CHUCK WOOD
sees hints of lunar history every time he
views the Moon.
sk yand tele scope .o r g * DECEMBER 2021 53

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