In a paper in Nature Astronomy today, Fitzsimmons et. al. report spectroscopic
analysis shows that an icy interior with a mantle crust for the object -
consistent with the types of insulating mantle produced by
long-term cosmic ray exposure for objects in our own outer Solar System -
cannot be ruled out.
So if it needs to be neatly placed in a category box, it may be classified
as a comet despite the lack of outburst as it rounded the Sun.
Quote:
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Originally Posted by Fitzsimmons, Nature
Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua
ABSTRACT
During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space.
It is reasonable to expect that the same happened for planetary systems other than our own.
Detection of such interstellar objects would allow us to probe the planetesimal formation processes around other stars, possibly together with the effects of long-term exposure to the interstellar medium.
1I/2017 U1 ‘Oumuamua is the first known interstellar object, discovered by the Pan-STARRS1 telescope in October 2017.
The discovery epoch photometry implies a highly elongated body with radii of ~ 200 × 20 m when a comet-like geometric albedo of 0.04 is assumed.
The observable interstellar object population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity of 'Oumuamua implies a lack of surface ice.
Here, we report spectroscopic characterization of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System.
We show that this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure4.
An internal icy composition cannot therefore be ruled out by the lack of activity, even though ‘Oumuamua passed within 0.25 au of the Sun.
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Paper here (requires subscription) :-
http://www.nature.com/articles/s41550-017-0361-4
Story by Tereza Pultarova at space.com :-
https://www.space.com/39129-oumuamua...-disguise.html