At least one interstellar object (ISO) is likely to have crashed onto Earth’s moon over the eons, a new paper suggests.
That moonfilled with thousands of craters is therefore a good hunting ground for objects originating from interstellar space, the authors suggest, based on statistics and simulations.
“Given the number of ISOs we expect to see in the solar system, there are likely a few craters formed by very fast ISOs throughout the solar system, and there are likely one or two on the moon,” lead author Sam Cabot, a Ph. D Candidate at Yale University, Space.com said.
The challenge, however, is to find such a crater in the first place.
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ISOs are comets or asteroids that formed beyond the confines of our solar system. Only two have been confirmed so far: ‘Oumuamua and Borisov. The new study suggests that if we could detect a crater on the moon caused by an interstellar impact, we could learn more about the composition of these enigmatic objects.
Decades of observations of our nearest large neighbor, particularly with NASA’s Lunar Reconnaissance Orbiter, have provided high-resolution maps used for NASA’s Artemis program. The Artemis program aims to land humans on the moon later in the 2020s if all goes according to plan.
However, maps can only provide limited information about the craters depicted, the authors say. The challenge is that maps provide little spectroscopic information about the composition of the craters. While some analysis can be performed from orbit, the authors say it will likely require “ground truth” to determine if a crater was actually formed by an ISO, Cabot said.
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It will be difficult to predict what astronauts will find, Cabot added, because the two ISOs already discovered are quite different. Borisov is most similar to other solar system objects that we know; However, astronomers were surprised at how much carbon monoxide was in its composition, he said. ‘Oumuamua is even more enigmatic, he said, because there is no “satisfactory theory” that fully explains its composition.
Something outgassed from the surface of ‘Oumuamua, causing acceleration as it exited our solar system into interstellar space, Cabot explained.
“The dilemma,” he continued, “is that we in the community have been observing with space telescopes and looking for the typical gases that you would expect to evaporate from the surface that are found in astronomical objects. None of these have been reliably proven.”
Because astronomers couldn’t find typical outgassing products like water, they instead think the object has unique types of volatiles on its surface, Cabot said. (Fugitives are chemical elements and compounds that vaporize with relative ease.) To better understand what ISOs are made of in general, the moon could provide a place to gather concrete evidence, he added.
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Better still, there could be boots on the lunar surface relatively soon. Provided the funding and technological development of the Artemis program goes according to plan, humans can seek the sources of craters in their landing area.
The challenge, however, is that there is no way to predict exactly where an ISO might have ended up. In addition, human excursions will be limited to the south pole of the moon for now; That’s where NASA and other space agencies hope to deploy their astronauts in the near future.
Nonetheless, Cabot said, human missions give us “many opportunities to characterize regoliths, [meaning] find out the composition of the lunar soil and try to answer questions about the early solar system.
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The little we know about ISOs suggests a strong hypothesis of how these craters might be different, he said. ISOs tend to travel at faster speeds compared to other objects in our solar system. That’s because objects locked to the sun have a kind of “speed limit” as they are limited by the sun’s gravity.
“ISOs flowing freely through the galaxy can enter the solar system at much faster speeds,” Cabot said. “So that was the premise of our work: to study telltale signatures from extremely high-velocity impacts.”
The astronomers chose the limit speed of 225,000 mph (360,000 km/h) because it is extremely rare for objects in the solar system to reach anything close to that speed. The authors suggest that signs of melt at the impact site may be higher at this increased velocity, although the composition of the melt would depend on the rock composition in the area.
What’s needed next, Cabot said, is “widespread characterization of lunar regolith, which we hope to see with Artemis.” The challenge, he continued, is that astronauts and their equipment would need to figure out how large amounts of regolith could be processed by the moon to make a meaningful comparison with what an ISO might contain.
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Some of NASA’s future robotic landing missions could serve as test drives for large-scale regolith processing. NASA has a program called Commercial Lunar Payload Services (CLPS) that aims to bring private landers and payloads to the moon in support of the Artemis missions. A selection of these payloads may be able to process regolith as a secondary target for other scientific exploration, Cabot said.
In the meantime, the authors, along with the rest of the astronomical community, are still searching for other ISOs through powerful wide-field telescopes. And that search will get a boost in the near future as instruments like the Vera C. Rubin Observatory come online.
An article based on the research was published in the Planetary Science Journal (opens in new tab). A preprint version is available on Arxiv. (opens in new tab)
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