Searching for water ice in the coma of interstellar object 2I/Borisov

TL;DR

This study searched for water ice in the interstellar object 2I/Borisov's coma using near-infrared spectroscopy, finding no detectable water ice features and suggesting water grains are a minor component.

Contribution

First near-infrared spectral analysis of 2I/Borisov aimed at detecting water ice features, providing constraints on its ice composition compared to Solar system objects.

Findings

No water ice absorption features detected.

Water grains, if present, are less than 10% of the coma cross-section.

Spectral slope similar to Solar system comets and 'Oumuamua.

Abstract

Interstellar Objects (ISO) passing through our Solar System offer a rare opportunity to probe the physical and chemical processes involved in solid body and planet formation in extrasolar systems. The main objective of our study is to search for diagnostic absorption features of water ice in the near infrared (NIR) spectrum of the second interstellar object 2I/2019 Q4 (Borisov) and compare its ice features to those of the Solar system icy objects. We observed 2I in the NIR on three separate occasions. The first observation was made on 2019 September 19 UT using the SpeX spectrograph at the 3-m IRTF and again on September 24 UT with the GNIRS spectrograph at the 8-m GEMINI telescope and the last observation was made on October 09 UT with IRTF. The spectra obtained from all three nights appear featureless. No absorption features associated with water ice are detected. Spectral modeling suggests that water grains, if present, comprise no more than 10% of the coma cross-section. The comet consistently exhibits a red D-type like spectrum with a spectral slope of about 6% per 100nm, which is similar to that of 1I/'Oumuamua and is comparable to Solar system comets.