Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 'Oumuamua

TL;DR

This study provides spectroscopic analysis and thermal modeling of 'Oumuamua, revealing surface properties similar to outer Solar System objects and suggesting an insulating mantle that explains its inactivity despite potential internal ice.

Contribution

It presents the first spectroscopic characterization of 'Oumuamua, combining observations with thermal modeling to infer surface composition and internal structure.

Findings

'Oumuamua has surface features similar to outer Solar System objects.

The object shows no signs of surface ice despite its close solar approach.

An insulating mantle likely explains the lack of activity.

Abstract

During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space$^{1,2}$. It can be reasonably expected that the same happened for planetary systems other than our own. Detection of such Inter- stellar Objects (ISOs) 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 ISO, discovered by the Pan-STARRS1 telescope in October 2017$^3$.The discovery epoch photometry implies a highly elongated body with radii of $\sim 200 \times 20$ m when a comet-like geometric albedo of 0.04 is assumed. Here we report spectroscopic characterisation of 'Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. The observable ISO population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity implies a lack of surface ice. We show this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. 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.