Scientists could soon get a precise age for the giant impact basins on the moon, and a better understanding of the impact history of the young Earth, thanks to a new geological map of the moon's youngest large impact site, the Mare Orientale basin.
Mare Orientale sits on the edge of the face of the moon that we can see from Earth, on the boundary between the lunar near and far side. A mare is a low-lying expanse on the moon that is flooded with basaltic lava and appears darker than the surrounding highlands (creating the visage of the "man in the moon"). Before the telescopic era resolved the maria for what they really are, they were thought to be seas. ("Mare" is the Latin word for sea.)
Many of the lunar maria are the sites of enormous ancient impacts, which gouged out huge basins in the lunar surface. The Orientale basin is considered the youngest of these, but how young is uncertain since no samples have ever been recovered from its location. Estimates place it at 3.8 billion years old, compared to the oldest impact basin, the South Pole-Aitken basin, which has been calculated to be over 4.3 billion years old.
Orientale's structure features a stunning double ring, with the outermost ring having a diameter of 580 miles (930 kilometers). Within its rings is a crumbling surface originally formed when the energy of the impact rendered the lunar surface white-hot, melting it. It's this original impact melt, now hardened into basaltic rock on the floor of the basin, that can tell scientists how long ago it solidified, and hence how long ago the basin formed.
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The problem is that, after nearly four billion years, the surface of the Orientale basin has become covered with younger lava flows, fresh impact craters and assorted debris that has churned up the original impact melt. A new study, led by Kirby Runyon of the Planetary Science Institute in Tucson, Arizona, has produced a map that will help scientists identify the original impact melt amongst all the breccia.
In particular, the map recognizes two types of geological structure. One is material from the smooth but cracked basin floor, designated "BFsc." Some of this can be seen in the map as being buried underneath later lava flows, which are highlighted in red. Meanwhile, the stars on the map highlight young, smaller impact craters on top of Mare Orientale and their ejecta debris that has been scattered across the landscape.
The concern has been that these more recent impact sites could pollute the measurements of the age of the basin. However, it is possible that the debris from those smaller, younger craters also results from the original impact melt, rather than rocks rendered molten by the later impacts.
However, "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 in a statement.
A future lunar sample-return mission could put this to the test. If successful, it would provide a way to date other impact basins on the moon that have become much more degraded than the younger Orientale.
"We chose to map Orientale basin because it's simultaneously old and young," said Runyan. "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."
This loose estimate of its age as 3.8 billion years comes in part from crater counts. The more craters a surface has, the older that surface must be.
By discovering the precise ages of Orientale and the other basins, planetary scientists will be able to determine the rate of giant impacts in the young solar system. Earth, being a larger target with stronger gravity, would have been hit more often than the moon, but the evidence of those impacts on Earth has all but disappeared via erosion and plate tectonics. However, the airless moon preserves its impact record and can act as a guide to Earth's impact history.
"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," said Runyon.
Related: Scientists have dated the moon's oldest, and largest, impact site
These giant impacts could have had a huge effect on the early Earth, vaporizing entire oceans and perhaps delaying the origin of life, or even wiping life out completely, only for it to begin again and get wiped out again, repeatedly.
"Some recent modeling has shown that we probably never totally sterilized Earth during these big impacts, but we don't know for sure," said Runyon. "At some point our oceans could have been vaporized from impacts, then re-condensed 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."
Had Earth experienced one less, or one more, giant impact, then life could potentially have developed quite differently on our planet.
The findings were published on Nov. 18 in the Planetary Science Journal.
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