Early molten moon's deep secrets

Date:
August 3, 2021
Source:
NASA/Goddard Space Flight Center
Summary:
Recently, a pair of NASA studies identified the most likely
locations to find pieces of the Moon's mantle on the surface,
providing a map for future lunar sample return missions such as
those under NASA's Artemis program. If collected and analyzed,
these fragments from deep within the Moon can provide a better
understanding of how the Moon, the Earth, and many other solar
system worlds evolved.



FULL STORY ========================================================================== Shortly after it formed, the Moon was covered in a global ocean of molten
rock (magma). As the magma ocean cooled and solidified, dense minerals
sank to form the mantle layer, while less-dense minerals floated to form
the surface crust.

Later intense bombardment by massive asteroids and comets punched through
the crust, blasting out pieces of mantle and scattering them across the
lunar surface.


========================================================================== Recently, a pair of NASA studies identified the most likely locations to
find pieces of mantle on the surface, providing a map for future lunar
sample return missions such as those under NASA's Artemis program. If
collected and analyzed, these fragments from deep within the Moon can
provide a better understanding of how the Moon, the Earth, and many
other solar system worlds evolved.

"This is the most up-to-date evaluation of the evolution of the lunar
interior, synthesizing numerous recent developments to paint a new picture
of the history of the mantle and how and where it may have been exposed
on the lunar surface," said Daniel Moriarty of NASA's Goddard Space Flight Center, Greenbelt, Maryland and the University of Maryland, College Park.

Magma oceans evolve as they cool down and dense materials sink while light materials rise. The formation of magma oceans and their evolution are
thought to be common processes among rocky planets and moons throughout
our solar system and beyond. Earth's Moon is the most accessible and well-preserved body to study these fundamental processes.

"Understanding these processes in more detail will have implications
for important follow-up questions: How does this early heating affect
the distribution of water and atmospheric gases of a planet? Does water
stick around, or is it all boiled away? What are the implications for
early habitability and the genesis of life?" adds Moriarty, lead author
of the papers, published August 3 in Nature Communications and January
2021 in the Journal of Geophysical Research.

Large rocky objects such as planets, moons, and large asteroids can
form magma oceans with the heat generated as they grow. Our solar
system formed from a cloud of gas and dust that collapsed under its
own gravity. As this happened, dust grains smacked into each other and
stuck together, and over time this process snowballed into larger and
larger conglomerations, eventually forming asteroid and planet-sized
bodies. These collisions generated a tremendous amount of heat. Also, the building blocks of our solar system contained a variety of radioactive elements, which released heat as they decayed. In larger objects, both processes can release enough heat to form magma oceans.

However, the details of how magma oceans evolve as they cool and how
the various minerals in them crystalize are uncertain, which affects
what scientists think mantle rocks may look like and where they could
be found on the surface.

"The bottom line is that the evolution of the lunar mantle is more
complicated than originally thought," said Moriarty. "Some minerals that crystallize and sink early are less dense than minerals that crystallize
and sink later. This leads to an unstable situation with light material
near the bottom of the mantle trying to rise while heavier material closer
to the top descends. This process, called 'gravitational overturn',
does not proceed in a neat and orderly fashion, but becomes messy,
with lots of mixing and unexpected stragglers left behind." The team
reviewed the most recent laboratory experiments, lunar sample analysis,
and geophysical and geochemical models to develop their new understanding
of how the lunar mantle evolved as it cooled and solidified. They used
this new understanding as a lens to interpret recent observations of
the lunar surface from NASA's Lunar Prospector and Lunar Reconnaissance
Orbiter spacecraft, and NASA's Moon Mineralogy Mapper instrument on
board India's Chandrayaan-I spacecraft. The team generated a map of
likely mantle locations using Moon Mineralogy Mapper data to assess
mineral composition and abundance, integrated with Lunar Prospector observations of elemental abundances, including markers of the last
remaining liquid at the end of lunar magma ocean crystallization, and
imagery and topography data from Lunar Reconnaissance Orbiter.

At around 1,600 miles (about 2,600 kilometers) across, the South Pole
-- Aitken basin is the largest confirmed impact structure on the Moon,
and therefore is associated with the deepest depth of excavation of all
lunar basins, so it's the most likely place to find pieces of mantle,
according to the team.

For years, scientists have been puzzled by a radioactive anomaly in
the northwest quadrant of the South Pole -- Aitken Basin on the lunar
farside. The team's analysis demonstrates that the composition of this
anomaly is consistent with the "sludge" that forms in the uppermost
mantle at the very end of magma ocean crystallization. Because this
sludge is very dense, scientists have previously assumed that it should completely sink into the lower mantle early in lunar history.

"However, our more nuanced understanding from recent models and
experiments indicates that some of this sludge gets trapped in
the upper mantle, and later excavated by this vast impact basin,"
said Moriarty. "Therefore, this northwest region of the South
Pole -- Aitken Basin is the best location to access excavated
mantle materials currently on the lunar surface. Interestingly,
some of these materials may also be present around the proposed
Artemis and VIPER landing sites around the lunar South Pole." ========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Original written by Bill
Steigerwald. Note: Content may be edited for style and length.


========================================================================== Journal References:
1. Daniel P. Moriarty, Nick Dygert, Sarah N. Valencia, Ryan N. Watkins,
Noah
E. Petro. The search for lunar mantle rocks exposed on the
surface of the Moon. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021-24626-3
2. D. P. Moriarty, R. N. Watkins, S. N. Valencia, J. D. Kendall, A. J.

Evans, N. Dygert, N. E. Petro. Evidence for a Stratified
Upper Mantle Preserved Within the South Pole‐Aitken
Basin. Journal of Geophysical Research: Planets, 2021; 126 (1)
DOI: 10.1029/2020JE006589 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210803175259.htm

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