3,344 research outputs found
Equivalence relations for Mueller matrix symmetries of laboratory, LIDAR and planetary scattering geometries
Symmetry relationships for optical observations of matter generally fall into
several common scattering geometries. The 'planetary' configuration is
preferred among a group of observers of extraterrestrial planets, 'laboratory'
observations are performed in the biomedical research field and the LIDAR
configuration is preferred among those using lasers to probe optical properties
of horizontal surfaces with mirror or axial symmetry. This paper starts with
the Stokes matrix formalism and uses symmetries of Mueller matrix scattering to
establishes links between the mathematical symmetries of each geometric
configuration.Comment: 13 pages, 3 figure
Observations of a new stabilizing effect for polar water ice on Mars
Using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we
map the temporal variability of water ice absorption bands over the near-polar
ice mound in Louth crater, Mars. The absorption band depth of water ice at 1.5
microns can be used as a proxy for ice grain size and so sudden reductions can
time any switches from ablation to condensation. A short period of deposition
on the outer edge of the ice mound during late spring coincides with the
disappearance of seasonal water frost from the surrounding regolith suggesting
that this deposition is locally sourced. The outer unit at Louth ice mound
differs from its central regions by being rough, finely layered, and lacking
wind-blown sastrugi. This suggests we are observing a new stabilizing effect
wherein the outer unit is being seasonally replenished with water ice from the
surrounding regolith during spring. We observe the transport distance for water
migration at Louth crater to be ~4km, and we use this new finding to address
why no water ice mounds are observed in craters <9km in diameter.Comment: 26 pages, 8 figure
SWIR Investigation of sites of astrobiological interest
Rover missions to the rocky bodies of the Solar System and especially to Mars
require light- weight, portable instruments that use minimal power, require no
sample preparation, and provide suitably diagnostic mineralogical information
to an Earth-based exploration team. Short-wave infrared (SWIR) spectroscopic
instruments such as the Portable Infrared Mineral Analyser (PIMA, Integrated
Spectronics Pty Ltd., Baulkham Hills, NSW, Australia) fulfill all these
requirements. We describe an investigation of a possible Mars analogue site
using a PIMA instrument. A survey was carried out on the Strelley Pool Chert,
an outcrop of stro- matolitic, silicified Archean carbonate and clastic
succession in the Pilbara Craton, interpreted as being modified by hydrothermal
processes. The results of this study demonstrate the ca- pability of SWIR
techniques to add significantly to the geological interpretation of such hy-
drothermally altered outcrops. Minerals identified include dolomite, white
micas such as il- lite-muscovite, and chlorite. In addition, the detection of
pyrophyllite in a bleached and altered unit directly beneath the succession
suggests acidic, sulfur-rich hydrothermal activity may have interacted with the
silicified sediments of the Strelley Pool Chert.Comment: Key Words: Pilbara, Short-wave infrared, spectroscopy, Mars,
Stromatolites, Archea
Hydrothermal alteration at the Panorama Formation, North Pole Dome, Pilbara Craton, Western Australia
An airborne hyperspectral remote sensing dataset was obtained of the North
Pole Dome region of the Pilbara Craton in October 2002. It has been analyzed
for indications of hydrothermal minerals. Here we report on the identification
and mapping of hydrothermal minerals in the 3.459 Ga Panorama Formation and
surrounding strata. The spatial distribution of a pattern of subvertical
pyrophyllite rich veins connected to a pyrophyllite rich palaeohorizontal layer
is interpreted to represent the base of an acid-sulfate epithermal system that
is unconformably overlain by the stromatolitic 3.42 Ga Strelley Pool Chert.Comment: 29 pages, 9 figures, 2 table
Correlations of atmospheric water ice and dust in the Martian Polar regions
We report on the interannual variability of the atmospheric ice/dust cycle in
the Martian polar regions for Mars Years 28-30. We used CRISM emission phase
function measurements to derive atmospheric dust optical depths and data from
the MARCI instrument to derive atmospheric water ice optical depths. We have
used autocorrelation and cross correlation functions in order to quantify the
degree to which dust and ice are correlated throughout both polar regions
during Mars Years 28-29. We find that in the south polar region, dust has the
tendency to "self clear", demonstrated by negative autocorrelation around the
central peak. This does not occur in the north polar region. In the south polar
region, dust and ice are temporally and spatially anti correlated. In the north
polar region, this relationship is reversed, however temporal correlation of
northern dust and ice clouds is weak - 6 times weaker than the anticorrelation
in the south polar region. Our latitudinal autocorrelation functions allow us
to put average spatial sizes of event cores and halos. Dust events in the south
are largest, affecting almost the entire pole, whereas dust storms are smaller
in the north. Ice clouds in north are similar in latitudinal extent to those in
the south (both have halos < 10{\deg}). Using cross-correlation functions of
water ice and dust, we find that dust events temporally lag ice events by 35-80
degrees of solar longitude in the north and south poles, which is likely due to
seasonality of dust and ice events.Comment: 29 pages, 7 figure
Interannual observations and quantification of summertime H2O ice deposition on the Martian CO2 ice south polar cap
The spectral signature of water ice was observed on Martian south polar cap
in 2004 by the Observatoire pour l'Mineralogie, l'Eau les Glaces et l'Activite
(OMEGA) (Bibring et al., 2004). Three years later, the OMEGA instrument was
used to discover water ice deposited during southern summer on the polar cap
(Langevin et al., 2007). However, temporal and spatial variations of these
water ice signatures have remained unexplored, and the origins of these water
deposits remains an important scientific question. To investigate this
question, we have used observations from the Compact Reconnaissance Imaging
Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter
(MRO) spacecraft of the southern cap during austral summer over four Martian
years to search for variations in the amount of water ice.
We report below that for each year we have observed the cap, the magnitude of
the H2O ice signature on the southern cap has risen steadily throughout summer,
particularly on the west end of the cap. The spatial extent of deposition is in
disagreement with the current best simulations of deposition of water ice on
the south polar cap (Montmessin et al., 2007).
This increase in water ice signatures is most likely caused by deposition of
atmospheric H2O ice and a set of unusual conditions makes the quantification of
this transport flux using CRISM close to ideal. We calculate a 'minimum
apparent' amount of deposition corresponding to a thin H2O ice layer of 0.2mm
(with 70 percent porosity). This amount of H2O ice deposition is 0.6-6 percent
of the total Martian atmospheric water budget. We compare our 'minimal
apparent' quantification with previous estimates.
This deposition process may also have implications for the formation and
stability of the southern CO2 ice cap, and therefore play a significant role in
the climate budget of modern day Mars.Comment: 35 pages, 5 figures, 2 tables and supplementary information of 2
table
Ultramafic talc-carbonate unit -- The North Pole Dome and Dresser Formation
Here we describe the ultramafic talc-carbonate unit of the North Pole Dome.
The North Pole Dome (NPD) is located in the centre of the East Pilbara Terrane
(Van Kranendonk et al., 2007). The NPD is a structural dome of bedded,
dominantly mafic volcanic rocks of the Warrawoona and Kelly Groups that dip
gently away from the North Pole Monzogranite exposed in the core of the dome
(Figure 1) (Van Kranendonk, 1999, 2000). Average dips vary from 30 to 60
degrees in the inner part of the dome to about 60 to 80 degrees in the outer
part of the dome (Van Kranendonk, 2000). The North Pole Monzogranite is
interpreted to represent a syn-volcanic laccolith to the Panorama Formation
(Thorpe et al., 1992) and has been estimated to extend approximately 1.5km
below the surface, based on gravity surveys (Blewett et al., 2004). Felsic
volcanic formations are interbedded with the greenstones (Hickman, 1983), and
these are capped by cherts that indicate hiatuses in volcanism (Barley, 1993;
Van Kranendonk, 2006). An overall arc-related model for hydrothermal activity
is favored by Barley (1993), whereas more recent studies have indicated a
mantle-plume model for igneous and hydrothermal activity at the North Pole Dome
(Van Kranendonk et al., 2002, 2007; Smithies et al., 2003; Van Kranendonk and
Pirajno, 2004).Comment: 8 pages, 4 Figures, chapter 10 of The North Pole Dome and Dresser
Formation Field Guide by M. van Kranendon
On the effects of size factor on albedo versus wavelength for light scattered by small particles under Mie and Rayleigh regimes
Scattering by particles significantly smaller than the wavelength is an
important physical process in the rocky bodies in our solar system and beyond.
A number of observations of spectral bluing (referred to in those papers as
"Rayleigh scattering") on planetary surfaces have been recently reported,
however, the necessary mathematical modeling of this phenomenon has not yet
achieved maturity. This paper is a first step to this effect, by examining the
effect of grain size and optical index on the albedo of small conservative and
absorbing particles as a function of wavelength. The basic conditions necessary
for spectral bluing or reddening to be observed in real-world situations are
identified. We find that any sufficiently monomodal size distribution of
scattering particles will cause spectral bluing in some part of the EM spectrum
regardless of its optical index.Comment: 28 pages, 6 figure
Circular Polarization and Coherent Backscattering
We extend the work of Mishchenko et al. (2000) regarding the exact results of
the polarization effect, using the theory of Amic et al. (1997) to derive a
model prediction for the polarization coherent opposition effect at small
angles. Our extension is to assess the effect of circular polarized light, thus
completing exact derivation of the full M\"uller matrix for the semi infinite
slab of Rayleigh sized particles. We find the circular polarization peak is
narrower than the coherent backscattering intensity peak, and weaker in
intensity.Comment: 13 pages, 5 figure
Spectral Curve Fitting for Automatic Hyperspectral Data Analysis
Automatic discovery and curve fitting of absorption bands in hyperspectral
data can enable the analyst to identify materials present in a scene by
comparison with library spectra. This procedure is common in laboratory
spectra, but is challenging for sparse hyperspectral data. A procedure for
robust discovery of overlapping bands in hyperspectral data is described in
this paper. The method is capable of automatically discovering and fitting
symmetric absorption bands, can separate overlapping absorption bands in a
stable manner, and has relatively low sensitivity to noise. A comparison with
techniques already available in the literature is presented using simulated
spectra. An application is demonstrated utilizing the shortwave infrared
(2.0-2.5 micron or 5000-4000 cm-1) region. A small hyperspectral scene is
processed to demonstrate the ability of the method to detect small shifts in
absorption wavelength caused by varying white mica chemistry in a natural
setting.Comment: 18 pages, 2 figures, 6 table
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