New lunar map can help guide future sample return missions

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, Earth, and early life on our planet.

Planetary Science Institute Research Scientist Kirby Runyon is a lead author on a paper published in The Planetary Science Journal and containing a new high-resolution geologic map of Orientale basin that attempts to identify the original basin impact melt. The hope is that future researchers will use this map to target sample return missions and pin down impact dates for this and other impact basins.

“We chose to map Orientale basin because it’s simultaneously old and young,” 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 map uses BFsc—shorthand for smooth, cracked basin floor material—to map original, unpolluted impact melt from Orientale basin’s formation. Those rocks record the age of Orientale 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 in other, more degraded basins to record the ages of those basins’ formation.

‘Earth’s attic’

Earth’s early impact record from its first billion years—about 4.5 to 3.5 billion years ago—has been erased by shifting continents, water, weather and the general disturbance by living things. In fact, Earth itself is 4.5 billion years old, while most of its surface rocks are less than about a half-billion years old. In contrast, most of the moon’s surface rocks are older than about 2.5 billion years.

Luckily, the moon is close enough in the solar system that scientists can infer that the rate of impacts that happened there was likely the same for Earth, scaled for Earth’s larger size and higher gravity.

“The moon is sort of like Earth’s attic in terms of preserving the impact records; it’s the only place where we can get Earth’s baby photos,” Runyon said. “The moon is so nearby to us that its impact record is a reliable proxy record for early Earth’s, and we can scale impact statistics to have some reasonable approximation for what Earth’s first billion years were like, impact-wise. Earth has more gravity and we’re bigger, so we would have gotten hit a little more often and harder than the moon.”

Understanding Earth’s early environment is key to understanding when and even how life first arose on Earth.

“Giant impacts—like the one that formed Orientale—can vaporize an ocean and kill any life that had already started,” Runyon said. “Some recent modeling has shown that we probably never totally sterilized Earth during these big impacts, but we don’t know for sure. At some point, our oceans could have been vaporized from impacts, then recondensed and rained out repeatedly. If that happened a number of times, it’s only after the last time that life could have gotten a foothold.”

Runyon and his co-authors hope that their 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 their map.

“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,” Runyon said.