NASA Astrobiology Institute (NAI)

1. Amorphization of Crystalline Water Ice in the Solar System

   Project Investigators: David Jewitt, Ralf Kaiser, Weijun Zheng

   Summary

   Our laboratory simulations show that crystalline water ice can never be completely turned into amorphous ice by cosmic rays or solar wind at temperature over 50K. Temperatures of most icy objects in the Solar System, including Jovian satellites, Saturnian satellites, and Kuiper Belt Objects, are equal to or above 50 K; this explains why water ice detected on those objects is mostly crystalline.

   Astrobiology Roadmap Objectives:

   • Objective 2.2: Outer Solar System exploration

   Project Progress

   We conducted a systematic experimental study to investigate the amorphization of crystalline water ice via ionizing radiation irradiation in the 10-50 K temperature range at doses of up to 140 ± 20 eV per molecule. We found that crystalline water ice can be converted only partially to amorphous ice by electron irradiation. The experiments showed explicitly that a fraction of the 1.65 μm band, which is characteristic for crystalline water ice, survived the irradiation, to a degree that strongly depends on the temperature.

   
   

   Recrystallization of the ice after irradiation. (a) before irradiation; (b) after 44 hours irradiation; (c) after recooling. The spectra are offset related to the bottom one. At 50 K, the recrystallization due to thermal effects is strong, and most of the crystalline ice survived the irradiation. The spectra from the recooled samples are almost identical to those before irradiation except that the recooled samples show a hydrogen peroxide feature that resulted from the irradiation.

   Quantitative kinetic fits of the temporal evolution of the 1.65 μm band clearly demonstrate for the very first time that there is a balance between thermal recrystallization and irradiation-induced amorphization, with thermal recrystallizaton dominant at higher temperatures. At 50 K, the recrystallization due to thermal effects is strong, and most of the crystalline ice survived. These results have crucial implications to the Solar System. Temperatures of most icy objects in the Solar System, including Jovian satellites, Saturnian satellites, and Kuiper Belt Objects, are equal to or above 50 K; this explains why water ice detected on those objects is mostly crystalline.

   
   

   The survival of the 1.65-μm band versus irradiation time at different temperatures. The irradiation was conducted at 10, 20, 30, 40, and 50 K. The dose from 200 hours irradiation in our experiment corresponds roughly to 6.3 billion years on the surface of a Kuiper belt object, longer than the history of our solar system. The data show that crystalline ice on Quaoar will never be completely amorphized as long as it was formed in the first place.

Publications

Zheng, W., Jewitt, D. & Kaiser, R.I.  (2007).  Amorphization of Crystalline Water Ice.  Chemical Physics Letters:submitted.

Zheng, W. & Kaiser, R.I.  (2007).  On the formation of carbonic acid (H2CO3) in solar system ices.  Chemical Physics Letters, 450:55-60  [Online].