Researchers at the Planetary Science Institute have compiled a 1:200,000-scale geological map of the lunar Orientale basin, focusing on identifying the most widespread and accessible occurrences of impact melt deposits from the basin-forming impact to help guide sample-return missions.
1:3M mapping (originally mapped at 1:200,000) of a portion of the Orientale basin. The small craters near ‘Pa’ (red and yellow stars) excavate Orientale melt from beneath the mare and serve as a useful type example for similar small craters in similar basins. Maunder, the large crater in the NW part of the map, is Eratosthenian in age. Image credit: Runyon et al., doi: 10.3847/PSJ/ad862f.
Billions of years ago, a giant asteroid struck the Moon with so much energy that it melted rock until it was super-heated and white-hot, or what scientists call impact melt.
This eventually cooled and hardened, creating a multi-ringed impact crater that is known today as Orientale basin.
Having samples of impact melt is valuable because scientists can use laboratory techniques to determine the exact time of the melt’s solidification, and therefore the age of the impact.
The problem is, geologic processes after impact — such as lava flows and smaller impacts — have buried and mixed up much of the original impact melt.
But parsing out the impact crater from which a rock originated is worth the effort because that knowledge can help scientists understand how the impact rate changed throughout the Solar System’s history, as well as how impacts shaped the Moon, the Earth and early life on our planet.
To identify original basin impact melt, Planetary Science Institute researcher Kirby Runyon and colleagues created a new high-resolution geologic map of the Orientale basin.
“We chose to map Oriental basin because it’s simultaneously old and young,” Dr. Runyon said.
“We think it’s about 3.8 billion years old, which is young enough to still have its impact melt freshly exposed at the surface, yet old enough to have accumulated large impact craters on top of it as well, complicating the picture.”
“We chose to map Orientale to test melt-identification strategies for older, more degraded impact basins whose ages we’d like to know.”
The scientists used BFsc — shorthand for smooth, cracked basin floor material — to map original, unpolluted impact melt from the Orientale basin’s formation.
Those rocks record the age of the basin, and parts of this melt deposit would have been buried beneath other geologic units, such as the lava flows mapped in red.
Stars mark rims and debris from smaller impact craters that have unearthed previously buried Orientale melt.
So, if rocks from starred locations turn out to be the same age as rocks from the BFsc areas, geologists know they can rely on rocks from similar small craters on other, more degraded basins to record the ages of those basins’ formation.
“We hope that our mapping method can be used in other impact basins across the Moon so that future sample return missions could test this approach by sampling rocks from areas similar to those highlighted on the map,” the authors said.
“If samples collected from any of the starred areas on our map are the same age as samples collected from the BFsc areas that denote original impact melt, then we have confidence that we can apply the impact melt sampling technique to other basins.”
The team’s map appears in a paper published in the Planetary Science Journal.
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Kirby Runyon et al. 2024. Orientale Basin as a Guide for Identifying Lunar Basin Datable Impact Melt and Assessing Impact Melt Differentiation. Planet. Sci. J 5 (11): 249; doi: 10.3847/PSJ/ad862f
This article is a version of a press-release provided by the Planetary Science Institute.
