The phase of water ice which forms in cold clouds in the mesospheres of
Mars, Venus and Earth
Abstract
Water ice clouds form in the mesospheres of terrestrial planets in the
solar system (and most likely elsewhere) by vapor deposition at low
pressures and temperatures. Under these conditions a range of
crystalline and amorphous phases of ice might form. The phase is
important because it influences nucleation kinetics, density, vapor
pressure over the solid, growth rates and particle shape. In the past
the temperature range over which these different phases exist has been
defined on the basis of depositing ice at low temperature and warming it
while observing phase changes. However, the direct deposition of ice at
a range of temperatures relevant for the terrestrial planets has not
been systematically investigated. Here we present X-ray Diffraction
(XRD) measurements of water ice deposited at temperature intervals
between 88 and 145 K in a vacuum chamber. XRD patterns showed that low
density amorphous ice was formed at ≤ 120 K, stacking disordered ice I
formed from 121 – 135 K and hexagonal ice I formed at 140 and 145 K.
Direct deposition results in the stable hexagonal phase at much lower
temperatures than when warming stacking disordered ice. All three phases
of water ice observed here are possible in clouds in the mesospheres of
Earth and Mars, while on Venus only amorphous ice is likely to form.