Our Solar System’s recent interstellar visitor — a space rock named `Oumuamua — may be covered in a thick coat of organic material, an outer layer that’s protecting a cache of ice within. This veneer may have acted like a shield, preventing the ice inside `Oumuamua from getting overheated and evaporating when the rock passed close by our Sun. That means this interstellar object may be more like an icy comet and less like an asteroid.
The main distinction between asteroids and comets is their composition: comets are made up of a mixture of ice and rocks, whereas asteroids are mostly rock and metal, without much water. Astronomers think that most of the objects passing in between stars — called interstellar space — are more comet-like than asteroid-like. When our planetary system formed, the larger planets likely flung trillions of small objects at the system’s edge out into interstellar space. Most of the distant objects in our Solar System are icy bodies, so astronomers assume other planetary systems ejected mostly icy rocks, too.
That’s why astronomers initially thought `Oumuamua was a comet when they realized it wasn’t from our cosmic neighborhood. But further observations of `Oumuamua showed it wasn’t acting much like a comet. Comets are typically surrounded by tails of gas and dust that are caused when the Sun vaporizes the icy material within the rock. Astronomers couldn’t find any traces of a comet tail around `Oumuamua, though, so the rock was reclassified as an asteroid. Now, a new analysis of `Oumuamua, published in Nature, explains how the object may be like a comet without behaving like one. “We found that even though this object passed really quite close to our Sun — closer than Mercury — a crust like that could have insulated the interior,” lead author Alan Fitzsimmons, an astronomer at Queen's University Belfast, tells The Verge.
Fitzsimmons and his team think `Oumuamua may have been covered in ice long ago, but has been reshaped after hundreds of millions of years of traveling through deep space. During its cosmic wanderings, the rock was constantly bombarded by deep-space cosmic rays — highly energized particles that get ejected into space when stars explode. These rays may have worn down that outer ice layer, eventually creating a layer of organic material that’s over a foot and a half thick. “Our data is consistent with this object being out there a long, long time,” says Fitzimmons.
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To find this coat, the researchers looked over data collected by two telescopes — one on the Canary Islands and one in Chile — on October 25th and 26th. These observatories measured how the sunlight reflected off of `Oumuamua as it traveled through the Solar System, and broke that light apart into its component colors. Scientists didn’t find any signatures of minerals that would make up rocky objects. And there were no signatures of ice either.
Instead, they found that the composition of the rock’s surface differs from place to place, but ultimately, the rock showed signatures of carbon-rich compounds on its surface — materials that are often associated with life here on Earth. That doesn’t mean `Oumuamua hosts life, but it does look similar to small comet-like objects in our own Solar System that are also covered in icy carbon. Lab studies also show that icy objects that travel a long time through deep space would look a bit like this space rock. “That’s what we expect to get from taking something that used to have an icy surface and then exposing it to high-energy radiation for a long time,” says Fitzsimmons.
There’s a lot more to learn about `Oumuamua, of course. But follow-up observations are no longer an option: the object is tiny and it’s currently getting farther away from Earth, making it incredibly difficult to see. Astronomers, however, can continue poring over data that’s already been collected on the space rock to see if there are any more details to unearth.
One thing most astronomers can agree on is that this object is definitely not some artificially made spacecraft from a star system far away. Last week, the research program Breakthrough Listen hosted a campaign to see if any artificial communication signals were coming from `Oumuamua. The first results confirmed that this visitor is most likely just a rock — but now we know it might also have a thick carbon coat, too.