Temperature and grain size dependence of near-IR spectral signature of crystalline water ice: From lab experiments to Enceladus' south pole
- Research areas:
- Year:
- 2012
- Authors:
- Journal:
- PLANETARY AND SPACE SCIENCE
- Volume:
- 61
- Number:
- 1, SI
- Pages:
- 124-134
- Month:
- February
- ISSN:
- 0032-0633
- Abstract:
- An experimental facility has been set up to acquire infrared spectra of
pure water ices with controlled grain sizes ranging from 80 to 700 mu m
and temperatures ranging from 80 to 140 K at 10(-2) mbar. Forty-seven
near IR spectra (1.0-5.0 mu m), selected among the total acquired
spectra, permit us to determine how spectral characteristics between 1.0
and 1.8 mu m depend on both grain size and temperature. It will be shown
that the conventional spectral characteristics derived from Gaussian
fitting cannot be used as good temperature or grains size indicators. On
the contrary, spectral band areas allow for a proper determination. In
this work, we confirm the strong dependence of the 1.05 mu m, 1.30 mu m
and 1.50 mu m band areas on grain size. A new linear scaling law,
established from our experimental data, is proposed for grain sizes'
estimations. It is also demonstrated that the temperature effect on band
areas is much weaker than the grain size effect, because temperature has
opposite effects on both sides of two inflection points. However, the
``reduced{''} area, computed between the two inflection points, turns
out to be a very good indicator of the ice temperature, especially for
the 1.30 pm and 1.50 pm bands. A second linear scaling law, established
from the experimental data, is then computed to describe the temperature
dependence of these reduced areas. These near-IR spectral properties of
crystalline water ice have been established from laboratory experiments.
It will be shown that the scaling laws proposed in this work are indeed
suitable for retrieving temperature and grain size of laboratory icy
samples. But they may also be a good tool for characterizing the icy
surfaces of the outer solar system. As an example, the VIMS spectral
images of Enceladus' south pole have been used to retrieve both the
grain size distribution around the tiger stripes area and the
distribution of temperature anomalies along the ridges. The results are
in very good agreement with previous studies. (C) 2011 Elsevier Ltd. All
rights
