Interstellar Visitor's Icy Core May Be Coated by Organic Crust From Cosmic Rays

Astronomers studying 1I/2017 U1 ’Oumuamua, the first known interstellar object to visit our solar system, suggest that a crust protects its frozen insides.

The first known interstellar object to visit from the deep beyond entered our solar system at a speed of about 15.8 miles a second. It doesn’t have an icy comet’s telltale halo of gas and dust, and it’s apparently not your typical rock-and-metal asteroid. Astronomers who have been observing it since its identification in October suggest that it’s actually covered in a peculiar organic coating that disguises a frozen, comet-like interior.

The object, known as 1I/2017 U1 ’Oumuamua, was first spotted because its trajectory was wildly different from those of the comets that pass through the inner solar system. Announcing the discovery in October, astronomers noted that it was 1,300 feet in diameter and that it appeared to have come from the constellation Lyra.

The object survived a close brush with our sun at 23 million miles (37 million kilometers), less than half the equivalent distance between the sun and Mercury, and then swung by Earth at a distance of about 15 million miles, which is roughly 60 times the distance between Earth and the moon

According to a new paper in Nature Astronomy authored by researchers who have analyzed the sunlight reflected off of ’Oumuamua, the object likely survived its solar encounter because it was covered with a crust that protected its icy insides.

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The idea is that this coating was created over millions or billions of years (’Oumuamua is estimated to be up to 10 billion years old) as the object was hit by cosmic rays — highly energetic particles that zip through space at close to the speed of light. Over the course of eons, the rays desiccated ices and left the remaining organics to cohere into a crust.

“We have discovered that the surface of ’Oumuamua is similar to small solar system bodies that are covered in carbon-rich ices, whose structure is modified by exposure to cosmic rays,” said lead author Alan Fitzsimmons, an astronomer at the Queen's University Belfast Astrophysics Research Centre in Ireland, in a statement.

The researchers analyzed ’Oumuamua in both visible and infrared wavelengths to better understand its composition, and found that there were different compositions across its surface. After modelling the thermal properties of ’Oumuamua, they estimated that the object is covered with a half-meter (1.6-foot) covering of materials rich in organics.

“We have also found that a half-meter thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporizing when the object was heated by the sun,” Fitzsimmons said, “even though it was heated to over 300 degrees centigrade [572 degrees Fahrenheit].”

The interstellar interloper has been the subject of much intrigue since its discovery, with some of it focused on the hope of finding life beyond Earth. Breakthrough Listen, an initiative supported by the Russian billionaire Yuri Milner that bills itself as “the largest ever scientific research program aimed at finding evidence of civilizations beyond Earth,” suggested at one point that it might be an interstellar craft based on its oblong shape.

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Previous research had posited that interstellar spacecraft could be cigar- or needle-shaped because this would make it easier for a spacecraft to pass through dust or gas between galaxies, Breakthrough said in a statement. “While a natural origin is more likely,” it added, “there is currently no consensus on what that origin might have been, and Breakthrough Listen is well positioned to explore the possibility that ’Oumuamua could be an artifact.”

Last week, Breakthrough said it would listen to `Oumuamua for 10 hours across four radio bands to see if it was transmitting anything interesting. But these early observations turned up empty, with Breakthrough announcing this week that there were no continuous signals from the frequencies it observed.

Meanwhile, a separate study from Queen's researcher Michele Banister looked at the color of ’Oumuamua. Her team wrote in Astrophysical Journal Letters that ’Oumuamua is the same shade as some of the small, icy objects at the edge of our solar system. This means that planetary systems outside of our own solar system likely have small, icy objects as well.

“We’ve discovered that this is a planetesimal with a well-baked crust that looks a lot like the tiniest worlds in the outer regions of our solar system, has a greyish/red surface and is highly elongated,” Banister said in a statement. “It's fascinating that the first interstellar object discovered looks so much like a tiny world from our own home system. This suggests that the way our planets and asteroids formed has a lot of kinship to the systems around other stars.

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