On the State of Water Ice on Saturn's Moon Titan and Implications to Icy Bodies in the Outer Solar System

TL;DR

This study investigates how water ice on icy bodies like Titan changes between amorphous and crystalline states due to temperature and radiation, revealing a balance influenced by thermal recrystallization and irradiation effects.

Contribution

It provides systematic experimental data on the effects of ionizing radiation and temperature on water ice's crystalline state relevant to outer Solar System bodies.

Findings

Crystalline water ice can only be partially amorphized by electron irradiation.

Thermal recrystallization dominates at temperatures above 50 K.

Most icy bodies in the Solar System have crystalline water ice due to their temperature conditions.

Abstract

The crystalline state of water ice in the Solar System depends on the temperature history of the ice and the influence of energetic particles to which it has been exposed. We measured the infrared absorption spectra of amorphous and crystalline water ice in the 10-50 K and 10-140 K temperature range, respectively, and conducted a systematic experimental study to investigate the amorphization of crystalline water ice via ionizing radiation irradiation at doses of up to 160 \pm 30 eV per molecule. We found that crystalline water ice can be converted only partially to amorphous ice by electron irradiation. The experiments showed that a fraction of the 1.65 \mum band, which is characteristic for crystalline water ice, survived the irradiation, to a degree that strongly depends on the temperature. Quantitative kinetic fits of the temporal evolution of the 1.65 \mum band clearly demonstrate that there is a balance between thermal recrystallization and irradiation-induced amorphization, with thermal recrystallizaton dominant at higher temperatures. Our experiments show the amorphization at 40K was incomplete, in contradiction to Mastrapa and Brown's conclusion (Icarus 2006, 183, 207.). At 50 K, the recrystallization due to thermal effects is strong, and most of the crystalline ice survived. Temperatures of most icy objects in the Solar System, including Jovian satellites, Saturnian satellites (including Titan), and Kuiper Belt Objects, are equal to or above 50 K; this explains why water ice detected on those objects is mostly crystalline.