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.
The thorium concentration across the vast South Pole – Aitken Basin on the lunar farside reveals the distribution of mantle materials violently ejected during the basin-forming impact. Here, thorium abundance is represented by a rainbow color scale, with high-thorium areas shown in red, trending to purple and grey with lower abundances. Two craters in the northwestern region of the basin exhibit especially high thorium abundance (indicated in red on the map), suggesting the presence of abundant mantle materials currently exposed on the surface. Credits: NASA/LRO/Lunar Prospector/D. Moriarty
“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.”
More about the missions and partners:
The research was funded by NASA’s Center for Research and Exploration in
Space Science and Technology II program under a cooperative agreement with the
University of Maryland. LRO is managed by NASA's Goddard Space Flight Center in
Greenbelt, Maryland, for the Science Mission Directorate at NASA Headquarters
in Washington. Launched on June 18, 2009, LRO has collected a treasure trove of
data with its seven powerful instruments, making an invaluable contribution to
our knowledge about the Moon. Launched October 22, 2008, Chandrayaan-1,
India’s first deep-space mission, played a crucial role in the discovery of
water molecules on the Moon. Among its suite of instruments, it carried NASA's
Moon Mineralogy Mapper, an imaging spectrometer that helped confirm the discovery
of water locked in minerals on the Moon. NASA's Jet Propulsion Laboratory,
Pasadena, California, designed and built the Moon Mineralogy Mapper. Lunar
Prospector, Launched January 8, 1998, was designed to collect data to compile
the first complete compositional and gravity maps of the Moon during its
planned one-year mission in lunar polar orbit. The mission was managed by
NASA’s Ames Research Center at Moffett Federal Airfield in California's Silicon
Valley.
Bill Steigerwald
William.A.Steigerwald@nasa.gov
NASA Goddard Space
Flight Center, Greenbelt, Maryland
Source: https://www.nasa.gov/feature/goddard/2021/lro-deep-secrets-of-moon
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