Laboratory simulations of ice growth in space: An expected nonuniform ice mantle composition

¹ INAF – Osservatorio Astronomico di Palermo, P.za Parlamento 1, 90134 Palermo, Italy
e-mail: antonio.jimenez@inaf.it
² Department of Physics, National Central University, Zhongli Dist., Taoyuan City 320317, Taiwan
e-mail: asperchen@phy.ncu.edu.tw
³ Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan
⁴ Centro de Astrobiología (INTA-CSIC), Ctra. de Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 4 October 2023
Accepted: 6 March 2024

Abstract

Context. In dense, cold molecular regions, gas-phase chemical species freeze out onto grain surfaces. These icy condensates become an important reservoir of volatile elements and feedstock for molecular diversity.

Aims. While there is a fairly general agreement on the chemical composition of icy mantles, there are differences in how the various molecular components are perceived to be present. Should the materials composing the ice be mixed or are they segregated into distinct chemical zones?

Methods. To answer such a question, we performed a few exploratory experiments that allowed the adsorbing surface (mimic dust grains) to slowly relax to very low temperatures while gas-phase mixtures of H₂O, NH₃, and CO embed onto it.

Results. We find that mantles are far from being uniform, and they could evolve into completely mixed ices only if the ambient temperature undergoes a catastrophic collapse.

Conclusions. Under the typical conditions of an interstellar dense cloud, ices present a high degree of molecular segregation, with possible consequences on the ice chemistry and the desorption mechanisms.