Lunar and Planetary Science XLVIII (2017)
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APPLICATION OF A REVISED HAPKE MODEL FOR CHARACTERIZING THE PHYSICAL
PROPERTIES OF ICY REGOLITHS. V. Singh1, A. Rhoden1, H. Sato2, 1School of Earth and Space Exploration,
Arizona State University, ISTB4, 781 Terrace Road, Tempe, AZ 85287-6004 ([email protected]), 2LROC
Science Operation Team, Arizona State University, Tempe, AZ.
Introduction: Europa displays unusual photometric
properties that are likely related to the detailed structure
of its regolith, but our ability to interpret our
observations is limited by the lack of “ground-truth” and
a dearth of models linking optical and physical
properties. Our current understanding of the bulk
physical properties of ices on the surfaces of outer solar
system satellites is primarily confined to [a] photometric
model predictions for bidirectional reflection [1–4] and
[b] experimental studies of terrestrial ices over a range
of phase angles, including opposition geometry [5–7].
Laboratory studies for characterization of different icy
planetary analog surfaces with independent parameter
variation are rare [6–11], and research on subsequent
application of photometry to determine physical
properties of an icy satellite has yet to be established.
Such studies are particularly relevant for future mission
preparation to Galilean satellites and Saturn’s rings.
The Europa Imaging System (EIS), with a suite of
narrow and wide angle cameras will acquire images at a
resolution of 50m/pixel with near global coverage
(>95%) at incidence 20-85°, emission 0-75° and phase
angles 0-75° during 42 flybys [12]. This wealth of data
can provide great insight into the physical, thermal, and
chemical properties of Europa’s ice shell if we are
prepared to interpret Europa’s observed optical
properties. The detailed characteristics of Europa’s
regolith are also critically relevant for development of a
Europa lander, which is currently under study. Here, we
present a pathway towards detailed simultaneous
characterization of the bidirectional reflectance function
and physical state for icy satellites, with a focus on
Europa and interpreting the EIS measurements, using a
combination of [a] photometric modeling, [b] field work
and [c] lab experiments.
Lunar Photometry: [a] A main goal of our
approach is to refine several key parameter assumptions
and the associated mathematical coupling, within the
Hapke [4] and radiative transfer models [3], for
application to icy satellites. We are particularly
interested in the treatment of CBOE in opposition surge
[6], neglecting porosity, and the assumption of constant
roughness. As a starting point, we are analyzing the
abundance of datasets available on the Earth’s moon to
test model assumptions and correlate photometric
properties with physical properties of the regolith. The
Lunar Reconnaissance Orbiter Camera (LROC)
multispectral observations have a comparable range of
viewing geometry and illumination angles, lending to its
analog photometric model application. 66,00 LROC
Wide Angle Camera (WAC) images were used for the
initial analyses [13], to establish near-global Hapke
parameter maps of the Moon. For this study on the
correlation of parameter variations to physical
properties, we select regions of fresh, immature
highland material near young craters owing to the
comparable higher albedo (geometric albedo of 0.6-0.9)
and slightly higher values of opposition surge which
may be related to a variation in grain size distribution of
the regolith. We use multiple LROC Narrow Angle
Camera (NAC) images for each region, with varying
illumination angles (incidence, emission, and phase
angles), which are radiometrically calibrated and
photometrically normalized to a common viewing
geometry and illumination angle. This is achieved by
using the tile-by-tile method [18] with repeat NAC
image coverage of each region allowing the
determination of a median radiance factor (or I/F).
The Hapke simplified bidirectional reflectance
function is used for fitting to NAC reflectance data by
comparing the calculated radiances of these sites to
measured radiances, with variation of free parameters
informing physical and chemical properties of the
surface. The opposition effect observed on both icy
satellites and our Moon, is an anomalous increase in
brightness as the faces become fully-illuminated; it is
described by four parameters representing shadow
hiding effect (SHOE) and coherent backscatter (CBOE).
The existing models are hindered by their inability to
fully incorporate multiple scattering in these
parameters, which is considered unimportant for the
Moon and generally ignored. However, any application
to icy satellites cannot be achieved successfully without
a thorough consideration of multiple-scattering effects,
owing to their prominent role in opposition surge.
We further intend to achieve a successful fit of
model-determined physical properties for a region to [a]
observations using pre-existing lunar datasets and [b]
measured properties of lunar return samples. This will
help us refine existing assumptions and establish a more
physically acceptable range of parameters for
compaction, porosity, roughness, particle size
distribution and composition [17].
Laboratory and Field Analogs: [b] Work by
Kassalainen et al., 2001, 2003; Jost et al., 2013 [5, 6, 14]
established terrestrial ice sheets as planetary regolith
Lunar and Planetary Science XLVIII (2017)
analogs (without considerations for radiation,
atmosphere, and temperature conditions) by replicating
opposition surge in measured phase curves from field
and lab experiments. We intend to extend the
measurements of an analog surface’s bidirectional
reflectance over a large range of incidence, emission
and phase angles to ground test our modified
photometric model. Multiple field measurements in the
stable ice sheets of Antarctica (instrumentation listed in
[6]) and concurrent lab experiments on acquired
samples, with a radio-goniometer operating in the VIS–
NIR spectral range [14], will enable us to interpret the
optical properties in terms of composition and physical
state of the surface.
[c] A comprehensive study of Hapke parameters
using various samples in laboratory experiments, with
the
application
of
near-infrared
reflectance
spectroscopy and imaging with cold-compliant
instrumentation, will help identify well-characterized
and stable analog materials. We intend to create a 3D
analog surface for Europa, using pure ice as the initial
material, prior to weathering and chemical analog
studies undertaken by [16]. After establishing a
correlation using backscattering peaks observed in
phase curves, the reflectance data can be fit with our
modified Hapke model to reproduce data and compare
them to field measurements and Voyager imagery in our
regions of interest.
Conclusions: Collectively, this research will: [a]
enable development of refined photometry models for
determining the global and regional-scale physical
properties of outer solar system icy satellites; [b] inform
approaches to overcoming the limitations in phase angle
observations; [c] improve terrestrial analog sample
selection by utilizing field and lab experimental studies,
and [d] enable assessment of the temporal evolution of
photometric properties of ice samples. [e] It will also
offer a unique opportunity for predictive analysis by
application to EIS datasets, and [f] can inform regolith
particle studies and landing site assessment for a future
lander, in conjugation with E-THEMIS. We will discuss
results of our lunar analog study as well as the details of
the laboratory and field approaches we are developing
for analysis of icy satellite regolith properties.
References: [1] Minnaert, M. (1941) Astrophysical
Journal, 93, 403–410. [2] Shkuratov, Y. et al. (1999)
Icarus, 141, 132–155. [3] Buratti, B. et al. (1985)
Icarus, 61, 208–217. [4] Hapke, B. et al. (1981) JGR,
86, 3055–3060. [5] Kaasalainen, S. et al. (2001) JQSRT,
70, 529–543. [6] Kaasalainen, S. et al. (2003)
Astronomy and Astrophysics, 409, 765–769. [7] Jost, B.
et al. (2016) Icarus, 264, 109–131. [8] Shepard, M. and
Helfenstein, (2007) JGR, 112, E3. [9] Shkuratov, Y. et
al. (2007) JQSRT, 106, 487–508. [10] Pommerol, A.
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and Schmitt, B. (2008) JGR, 113, E10. [11] Pommerol,
A. and Schmitt, B. (2008) JGR, 113, E12. [12] Turtle,
Z. et al. (2016) LPI Contribution No. 1903, 1626. [13]
Sato, H. et al. (2014) JGR- Planets, 119, 1775–1805.
[14] Jost, B. et al. (2013) Icarus, 225, 352–366. [15]
Pommerol, A. et al. (2011) PSS, 59, 1601–1612. [16]
Hand, K. et al. (2016) The Astronomical Journal,
145:110. [17] Goguen, J. et al. (2012) 43rd LPSC,
Abstract #2568. [18] Sato, H. et al. (2011) 42nd LPSC,
Abstract #1974.