Long-term exposure to cosmic rays has created an insulating organic-rich layer on the surface of the interstellar visitor ‘Oumuamua, reports a paper published this week in Nature Astronomy. This coating could have protected an ice-rich interior from being vaporized during its recent passage close to the Sun.

‘Oumuamua is the first unambiguously detected interstellar body to visit our Solar System. Planetary scientists have long expected that the interstellar object population is dominated by ice-rich small bodies like comets; however, ‘Oumuamua didn’t exhibit any comet-like activity when it passed close to the Sun at the beginning of September 2017.

Alan Fitzsimmons and colleagues obtained a full spectrum of the object in the visible and near-infrared wavelengths (0.3-1.8 μm) in order to determine its composition. They find that there are compositional differences across its surface, and that the surface is similar to small Solar System bodies that are covered in carbon-rich ices, whose structure is modified by exposure to cosmic rays. They developed a model of the thermal properties of ‘Oumuamua and determine that a half-metre thick mantle of organic-rich material could have protected a water-ice-rich comet-like interior from vaporizing when the object was heated by the Sun.

These observations allow a better characterization of the structure and the history of this unique object, while also shedding light on the population of interstellar objects floating around our Solar System.