IEEE - Aerospace and Electronic Systems - March 2020 - 47
$0.1 up to $10 wt% in the south polar PSRs, with a
patchy distribution [11]-[13]. However, the distribution of
apparent water frost at the surface does not match the subsurface distribution, and neither is its occurrence proven
everywhere temperatures are cold enough to permit trapping of water molecules [14]. Current data are not yet sufficient to conclude that the form, quantity, or distribution
of lunar H2O is present at concentrations sufficient for
IRSU, or to predict the distribution of ice at scales of a
rover or human landed mission. To be "operationally
useful" for such missions, H2O concentrations of greater
than $0.5 wt% are required [15].
Because of this uncertainty, and the potential utility
of water as a resource to human exploration, NASA identified a SKG for Human Exploration to understand the
composition, quantity, distribution, and form of water and
other volatiles associated with PSRs [16]. The Lunar Flashlight mission will use infrared reflectance spectroscopy to
make definitive detections of surficial water frost within
PSRs if it is present in quantities above $2 wt% in areas
measured by the mission. Reflectance spectroscopy is a surface measurement technique; the depth of penetration is the
same scale as the wavelength being employed (i.e., IR
spectroscopy probes the top 1-2 mm of the surface). Dry
lunar regolith and water ice have very different infrared
spectra (see Figure 1): water ice has two prominent molecular absorption features at $1.5 and 2 mm, whereas dry
lunar regolith has a featureless spectrum and an increasing
albedo with wavelength (red slope). When observing mixtures of regolith and ice (either areal or volumetric
Figure 1.
Lunar Flashlight uses four-point spectroscopy to distinguish dry regolith from water ice frost. Two of the four wavelengths ($1.495 and
$1.99 mm) correspond to overtone vibrational absorption features for water ice, while the two other wavelengths ($1.064 and $1.85 mm)
correspond to nearby continua. The measurement will ratio the reflectance value in the absorption features to the continuum to determine
water ice abundance.
MARCH 2020
IEEE A&E SYSTEMS MAGAZINE
47
IEEE - Aerospace and Electronic Systems - March 2020
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