Bibcode
Parviainen, H.; Muinonen, K.; Näränen, J.; Josset, J.-L.; Beauvivre, S.; Pinet, P.; Chevrel, S.; Koschny, D.; Grieger, B.; Foing, B.
Referencia bibliográfica
"EGU General Assembly 2009, held 19-24 April, 2009 in Vienna, Austria http://meetings.copernicus.org/egu2009, p.7966"
Advertised on:
4
2009
Número de citas
0
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Descripción
We analyze the single-scattering albedo and phase function, local
surface roughness and regolith porosity, and the coherent
backscattering, single scattering, and shadowing contributions to the
opposition effect for specific lunar mare regions imaged by the
SMART-1/AMIE camera. We account for shadowing due to surface roughness
and mutual shadowing among the regolith particles with ray-tracing
computations for densely-packed particulate media with a
fractional-Brownian-motion interface with free space. The shadowing
modeling allows us to derive the hundred-micron-scale volume-element
scattering phase function for the lunar mare regolith. We explain the
volume-element phase function by a coherent-backscattering model, where
the single scatterers are the submicron-to-micron-scale particle
inhomogeneities and/or the smallest particles on the lunar surface. We
express the single-scatterer phase function as a sum of three
Henyey-Greenstein terms, accounting for increased backward scattering in
both narrow and wide angular ranges. The Moon exhibits an opposition
effect, that is, a nonlinear increase of disk-integrated brightness with
decreasing solar phase angle, the angle between the Sun and the observer
as seen from the object. Recently, the coherent-backscattering mechanism
(CBM) has been introduced to explain the opposition effect. CBM is a
multiple-scattering interference mechanism, where reciprocal waves
propagating through the same scatterers in opposite directions always
interfere constructively in the backward-scattering direction but with
varying interference characteristics in other directions. In addition
to CBM, mutual shadowing among regolith particles (SMp) and
rough-surface shadowing (SMr) have their effect on the behavior of the
observed lunar surface brightness. In order to accrue knowledge on the
volume-element and, ultimately, single-scattering properties of the
lunar regolith, both SMp and SMr need to be accurately accounted for. We
included four different lunar mare regions in our study. Each of these
regions covers several hundreds of square kilometers of lunar surface.
When selecting the regions, we have required that they have been imaged
by AMIE across a wide range of phase angles, including the opposition
geometry. The phase-angle range covered is 0-109 °, with incidence
and emergence angles (ι and ε) ranging within
7-87 ° and 0-53 °, respectively. The pixel scale varies from
288m down to 29m. Biases and dark currents were subtracted from the
images in the usual way, followed by a flat-field correction. New
dark-current reduction procedures have recently been derived from
in-flight measurements to replace the ground-calibration images . The
clear filter was chosen for the present study as it provides the largest
field of view and is currently the best-calibrated channel.
Off-nadir-pointing observations allowed for the extensive phase-angle
coverage. In total, 220 images are used for the present study. The
photometric data points were extracted as follows. First, on average, 50
sample areas of 10 Ã- 10 pixels were chosen by hand from each
image. Second, the surface normal, ι, ε, °,
and α were computed for each pixel in each sample area using the
NASA/NAIF SPICE software toolkit with the latest and corrected
SMART-1/AMIE SPICE kernels. Finally, the illumination angles and the
observed intensity were averaged over each sample area. In total, the
images used in the study resulted in approximately 11000 photometric
sample points for the four mare regions. We make use of
fractional-Brownian-motion surfaces in modeling the interface between
free space and regolith and a size distribution of spherical particles
in modeling the particulate medium. We extract the effects of the
stochastic geometry from the lunar photometry and, simultaneously,
obtain the volume-element scattering phase function of the lunar
regolith locations studied. The volume-element phase function allows us
to constrain the physical properties of the regolith particles. Based
on the present theoretical modeling of the lunar photometry from
SMART-1/AMIE, we conclude that most of the lunar mare opposition effect
is caused by coherent backscattering and single scattering within volume
elements comparable to lunar particle sizes, with only a small
contribution from shadowing effects. We thus suggest that the lunar
single scatterers exhibit intensity enhancement towards the backward
scattering direction in resemblance to the scattering characteristics
experimentally measured and theoretically computed for realistic small
particles. Further interpretations of the lunar volume-element phase
function will be the subject of future research.