Acquiring samples of such impact melt is of great value to scientists, who can analyze them in laboratories to determine the exact timing of the melt's solidification and, by extension, the age of the impact event. However, post-impact geological activities like lava flows and smaller meteor strikes have obscured and mingled much of the original impact material.
Identifying the source craters of lunar rocks is critical for understanding the evolution of the impact rate across the Solar System, as well as the formative events shaping the Moon, Earth, and the emergence of early life.
Research Scientist Kirby Runyon from the Planetary Science Institute is a principal author of a study published in the 'Planetary Science Journal' presenting a high-resolution geological map of Orientale basin. This detailed map aims to assist future research efforts, particularly those focused on sample return missions, by pinpointing original impact melt locations.
"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 features BFsc (smooth, cracked basin floor material) to delineate unaltered impact melt from the original formation of Orientale basin. These regions hold rocks that record the age of the basin and may be buried under other geological features such as lava flows. Marked stars indicate smaller impact craters whose debris may have uncovered previously buried melt. If rocks from these starred sites match the age of those in BFsc areas, it supports the reliability of using similar smaller craters to date other lunar basins.
Earth's attic
The Earth's early impact history, covering approximately 4.5 to 3.5 billion years ago, has largely been erased due to processes like tectonics, erosion, and biological activity. Although Earth is about 4.5 billion years old, most of its existing surface rocks are under half a billion years in age. The Moon, on the other hand, has rocks predominantly older than 2.5 billion years, serving as a more stable archive of early Solar System events.
Because of its proximity, scientists infer that the impact rates observed on the Moon reflect similar rates on Earth, adjusted for Earth's greater size and 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."
These insights are essential for understanding Earth's early conditions and the emergence of life. "Giant impacts - like the one that formed Orientale - can vaporize an ocean and kill any life that had already started," Runyon explained. "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 anticipate that their mapping methodology will be useful for studying other impact basins across the Moon. This could aid in planning sample collection missions that would further validate the approach.
"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.
Related Links
Planetary Science Institute
Mars News and Information at MarsDaily.com
Lunar Dreams and more
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