Moon's early volcanic activity reshaped by internal movements

Moon's early volcanic activity reshaped by internal movements
by Staff and Agency Writers
Los Angeles CA (SPX) Apr 09, 2024

Approximately 4.5 billion years ago, a collision between a small planet and the young Earth dispersed molten debris into space, which eventually coalesced to form the moon. This widely accepted theory of lunar formation provides a backdrop to recent discoveries by the University of Arizona Lunar and Planetary Laboratory, as detailed in a new study in Nature Geoscience. The research underscores significant insights into the moon's internal evolution, with implications for terrestrial planets like Earth and Mars.

Historical analyses of lunar rock samples, retrieved by Apollo astronauts and supplemented by theoretical models, reveal the moon was once engulfed in a global magma ocean. These studies show that titanium-rich basaltic rocks, prevalent on the moon's nearside, emerged from this molten state. The mystery of their distribution, however, remained unsolved until recent findings.

The moon's rapid formation meant it was covered by molten rock, which solidified into its mantle and the bright crust visible from Earth. Below this crust, the last remnants of the magma ocean crystallized into dense minerals such as ilmenite, containing titanium and iron. "Because these heavy minerals are denser than the mantle underneath, it creates a gravitational instability, and you would expect this layer to sink deeper into the moon's interior," explained Weigang Liang, the study's lead researcher.

Over millennia, this dense material did sink, mix with the mantle, melt, and resurface as the titanium-rich lava flows observable today. "Our moon literally turned itself inside out," noted Jeff Andrews-Hanna, associate professor at LPL. The exact sequence of these events remains debated, with various models proposing different timelines and mechanisms for the sinking and resurfacing processes.

Recent simulations, aligned with data from NASA's GRAIL mission, which measured the moon's gravitational variations, support a model where the dense ilmenite layer migrated to the nearside, influenced perhaps by an ancient impact on the farside, then sank in sheetlike formations. "The models and data tell a remarkably consistent story of ilmenite materials migrating, sinking, and leaving behind a vestige that alters the moon's gravity field," said Liang.

The timing of these events is traced back to before 4.22 billion years ago, predating the moon's oldest and largest impact basins. This timeline is critical for understanding the subsequent volcanic activity on the lunar surface. "Analyzing these variations in the moon's gravity field allowed us to peek under the moon's surface and see what lies beneath," added Adrien Broquet, co-lead author.

The unique characteristics of the nearside, particularly the Oceanus Procellarum region, with its lower elevation, thinner crust, and higher concentrations of elements like titanium and thorium, contrast sharply with the moon's farside. This asymmetry is likely linked to the historical mantle overturn, a process integral to the moon's unique structural and historical identity.

"Our work connects the dots between the geophysical evidence for the interior structure of the moon and computer models of its evolution," Liang concluded, highlighting the significance of this evidence in understanding lunar history.

Research Report:Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data