Unusual Interstellar Visitor Could Be Shard of Ice

A new study of ‘Oumuamua suggests it’s water ice covered with 20 inches of dry carbon compounds

Artists rendering of ‘Oumuamua ESO/M. Kornmesser

Last month, astronomers announced that for the first time they had detected an interstellar object passing through our solar system. And it's no ordinary space rock—measuring at least the size of a football field, the object named ‘Oumuamua is ten time longer than it is wide. The object was so unusual that last week researchers decided to scan it to make sure it wasn’t an alien space probe (preliminary results say no).

Initial observations seemed to indicate the interstellar traveler was made of rock or metal, reddened by millions or billions of years absorbing cosmic rays. But as Tereza Pultarova at Space.com reports, a new study suggests ‘Oumuamua could be a large shard of ice covered in a carbon coating, conforming much more closely to what researchers expect from an interstellar visitor.

As Pultrova reports, when ‘Oumuamua passed close to the sun, researchers did not observe any gasses being released or the formation of a tail from dust, both of which are expected from an icy body like a comet. That led some astronomers to believe it was rocky object. But that, of course, isn't the end of the story. 

An international group of researchers led by Alan Fitzsimmons of Queen's University Belfast went in for a closer look, measuring the way that ‘Oumuamua reflects light. “We’ve got high signal-to-noise spectra (the 'fingerprint' of light reflected or emitted by the asteroid) both at optical wavelengths and at infrared wavelengths. Putting those together is crucial," Fitzsimmons tells Paul Rincon at the BBC. “What we do know is that the spectra don't look like something artificial.”

Instead, what they found is that the object is similar to some objects in our own neighborhood. “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,” Fitsimmon says.

As Pultarova reports, on comets and other icy objects, evaporation of surface ice over time eventually leaves reddish, pinkish crust of dry carbon compounds. About a foot and half of carbon-coating is all that is needed to keep the ice inside the object from vaporizing when it nears the sun, meaning ‘Oumuamua could be just one large shard of ice. The research appears in the journal Nature Astronomy.

The next big mystery behind ‘Oumuamua—now that we’re pretty sure it isn’t a space ship—is its unusual shape, Rincon reports. Several possibilities have been proposed, including the idea that it is two objects smooshed together, that it’s a shard of an object destroyed in a supernova or that, after millions of years speeding through space at 58,000 miles per hour, dust grains whittled it into the shape of a spear.

Though Fitzsimmons cautions that these idea are all speculation right now, but it's still exciting to think about. “I think what we’re looking at here is the initial flurry of scientists running around saying: ‘How did it get like this, where’s it come from, what’s it made of.’ It’s incredibly exciting,” he says. “I think after a few months you will see people focus down on one or two possibilities for all these things. But this just shows you: it's a symptom of what an amazing, interesting object this is... we can't wait for the next one.”

We may not have to wait too long. As Fitzsimmons tells Pultarova, the new mega-telescope, the Large Synoptic Survey Telescope in Chile, which is currently being built, will give us a better chance of catching future interstellar visitors before they depart our solar system.

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