Abstract
The occurrence of hydrogen atom-ordered form of ice Ih, ice XI, in the outer Solar System has been discussed based on laboratory experiments because its ferroelectricity influences the physical processes in the outer Solar System. However, the formation of ice XI in that region is still unknown due to a lack of formation conditions at temperatures higher than 72 K and the effect of UV-rays on the phase transition from ice I to ice XI. As a result, we observed the UV-irradiation process on ice Ih and ice Ic using a newly developed ultra-high vacuum cryogenic transmission electron microscope. We found that ice Ih transformed to ice XI at temperatures between 75 and 140 K with a relatively small UV dose. Although ice Ic partially transformed to ice XI at 83 K, the rate of transformation was slower than for ice Ih. These findings point to the formation of ice XI at temperatures greater than 72 K via UV irradiation of ice I crystals in the Solar System; icy grains and the surfaces of icy satellites in the Jovian and Saturnian regions.
Highlights
Ice Ih is a thermodynamically stable phase of water at temperatures higher than 72 K and pressures lower than 200 MPa
After 30 min of UV-irradiation at 75 K (UV fluence ∼4 × 1016 photons cm−2), the transmission electron microscope (TEM) image shows that the area where ice is present has increased, implying that the heights of islands have decreased while their area has increased (Figure 6B)
We confirmed that the formation of ice XI does not occur without UV-irradiation, indicating that the irradiation of 80–120 keV electrons for TEM does not affect the formation of ice XI and that UV-irradiation is essential
Summary
Ice Ih is a thermodynamically stable phase of water at temperatures higher than 72 K and pressures lower than 200 MPa. The characteristics of the crystal structure of ice Ih is an ordered arrangement of oxygen atoms (wurtzite structure) but the disordered arrangement of hydrogen atoms. At temperatures lower than 72 K, the hydrogen atom-ordered form of ice Ih, ice XI, becomes a thermodynamically stable phase (Tajima et al, 1984). As a metastable variant of ice Ih, ice Ic exists at lower temperatures and has an ordered arrangement of oxygen atoms (diamond structure) but a disordered arrangement of hydrogen atoms. Amorphous ice (a-H2O) is formed at temperatures lower than ∼130 K by various methods: vapor deposition, quenching of liquid water, pressurizing of ice I crystals, UVirradiation onto ice I, and so on (e.g., Petrenko and Whitworth, 1999). When amorphous ice is heated, the irreversible transition from a-H2O through ice Ic to ice Ih occurs
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