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Adrian J. Brown

Publications and source records attributed to Adrian J. Brown.

31 records · Page 2Linked to original sources
arXiv2014

Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) north polar springtime recession mapping: First Three Mars years of observations

We report on mapping of the north polar region of Mars using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument. We have observed three Mars Years (28-30) of late-winter and spring recessions (Ls=304°-92°). Our investigations have led to the following observations: 1. We classify the retreat of the north polar seasonal cap into 'pre-sublimation', 'early spring', 'asymmetric' and 'stable' periods according to the prevalent H2O ice grain size distributions. 2. During the early spring, the signatures of CO2 ice at the edge of the cap are obscured by H2O ice, which increases the apparent size of the H2O ice annulus around the seasonal CO2 cap at this time. At around Ls=25°, this process changes into an asymmetrical distribution of H2O deposition, covering CO2 signatures more rapidly in the longitude range from 90-210°E. 3. We detect signatures of 'pure' CO2 ice in extremely limited locations (in Lomonosov Crater) even in mid winter. H2O ice signatures appear everywhere in the retreating CO2 seasonal cap, in contrast with the south polar seasonal cap. 4. We find that average H2O ice grain sizes continuously increase from northern mid-winter to the end of springtime - this is the inverse of the behavior of CO2 ice grain sizes in the southern springtime.

astro-ph.EP↗
arXiv2014

Hydrothermal formation of Clay-Carbonate alteration assemblages in the Nili Fossae region of Mars

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has returned observations of the Nili Fossae region indicating the presence of Mg- carbonate in small (<10km sq2), relatively bright rock units that are commonly fractured (Ehlmann et al., 2008b). We have analyzed spectra from CRISM images and used co-located HiRISE images in order to further characterize these carbonate-bearing units. We applied absorption band mapping techniques to investigate a range of possible phyllosilicate and carbonate minerals that could be present in the Nili Fossae region. We also describe a clay-carbonate hydrothermal alteration mineral assemblage in the Archean Warrawoona Group of Western Australia that is a potential Earth analog to the Nili Fossae carbonate-bearing rock units. We discuss the geological and biological implications for hydrothermal processes on Noachian Mars.

astro-ph.EP↗
arXiv2014

CRISM south polar mapping: First Mars year of observations

We report on mapping of the south polar region of Mars using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument. Our observations have led to the following discoveries: 1. Water ice is present in the form of pole-circling clouds originating from the circum-Hellas region, beginning prior to Ls=162 and diminishing markedly at Ls=200-204. 2. It has previously been inferred by temperature measurements(Titus et al., 2003) and CO2-H2O mixture spectral models (Langevin et al., 2007) that surface water ice was present in the Cryptic Region in the final stages of sublimation. The high resolution of CRISM has revealed regions where only water ice is present (not a CO2-H2O ice mixture). This water ice disappears completely by Ls=252 and may be the source of water vapor observed by CRISM in southern latitudes between Ls=240-260 (Smith, et al., this issue). 3. We have estimated surface CO2 ice grain size distributions for the South Pole Residual Cap (SPRC) and the seasonal CO2 ice cap that covers it throughout summer spring and summer. Our analysis suggests that grain sizes peak at Ls=191-199 with an apparent grain size of ~7 +/-1 cm. By the end of the summer period our analysis demonstrates minimum apparent grain sizes of ~5 +/-1 mm predominate in the SPRC. 4. Fine grained CO2 ice condenses from Ls=0-40, and extends symmetrically away from the geographic pole, extending beyond 80 deg S by Ls=4-10. No evidence for unusual CO2 depositional processes in the Cryptic Region is observed up to Ls=16.

astro-ph.EP↗
arXiv2014

Louth Crater: Evolution of a layered water ice mound

We report on observations made of the ~36km diameter crater, Louth, in the north polar region of Mars (at 70°N, 103.2°E). High-resolution imagery from the instruments on the Mars Reconnaissance Orbiter (MRO) spacecraft has been used to map a 15km diameter water ice deposit in the center of the crater. The water ice mound has surface features that include roughened ice textures and layering similar to that found in the North Polar Layered Deposits. Features we interpret as sastrugi and sand dunes show consistent wind patterns within Louth over recent time. CRISM spectra of the ice mound were modeled to derive quantitative estimates of water ice and contaminant abundance, and associated ice grain size information. These morphologic and spectral results are used to propose a stratigraphy for this deposit and adjoining sand dunes. Our results suggest the edge of the water ice mound is currently in retreat.

astro-ph.EP↗
arXiv2014

The MARTE VNIR Imaging Spectrometer Experiment: Design and Analysis

We report on the design, operation, and data analysis methods employed on the VNIR imaging spectrometer instrument that was part of the Mars Astrobiology Research and Technology Experiment (MARTE). The imaging spectrometer is a hyperspectral scanning pushbroom device sensitive to VNIR wavelengths from 400-1000 nm. During the MARTE project, the spectrometer was deployed to the Rio Tinto region of Spain. We analyzed subsets of 3 cores from Rio Tinto using a new band modeling technique. We found most of the MARTE drill cores to contain predominantly goethite, though spatially coherent areas of hematite were identified in Core 23. We also distinguished non Fe-bearing minerals that were subsequently analyzed by X-ray diffraction (XRD) and found to be primarily muscovite. We present drill core maps that include spectra of goethite, hematite, and non Fe-bearing minerals.

astro-ph.IM↗
arXiv2014

MR PRISM - Spectral Analysis Tool for the CRISM

We describe a computer application designed to analyze hyperspectral data collected by the Compact Infrared Spectrometer for Mars (CRISM). The application links the spectral, imaging and mapping perspectives on the eventual CRISM dataset by presenting the user with three different ways to analyze the data. One of the goals when developing this instrument is to build in the latest algorithms for detection of spectrally compelling targets on the surface of the Red Planet, so they may be available to the Planetary Science community without cost and with a minimal learning barrier to cross. This will allow the Astrobiology community to look for targets of interest such as hydrothermal minerals, sulfate minerals and hydrous minerals and be able to map the extent of these minerals using the most up-to-date and effective algorithms. The application is programmed in Java and will be made available for Windows, Mac and Linux platforms. Users will be able to embed Groovy scripts into the program in order to extend its functionality. The first collection of CRISM data will occur in September of 2006 and this data will be made publicly available six months later via the Planetary Datasystem (PDS). Potential users in the community should therefore look forward to a release date mid-2007. Although exploration of the CRISM data set is the motivating force for developing these software tools, the ease of writing additional Groovy scripts to access other data sets makes the tools useful for mineral exploration, crop management, and characterization of extreme environments here on Earth or other terrestrial planets. The system can be easily implemented for use by high school, college, and graduate level students.

astro-ph.IM↗
arXiv2014

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.

astro-ph.EP↗
arXiv2014

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.

astro-ph.IM↗
arXiv2014

Hyperspectral imaging spectroscopy of a Mars analogue environment at the North Pole Dome, Pilbara Craton, Western Australia

A visible and near infrared (VNIR) to shortwave infrared (SWIR) hyperspectral dataset of the Early Archaean North Pole Dome, Pilbara Craton, Western Australia, has been analysed for indications of hydrothermal alteration. Occurrence maps of hydrothermal alteration minerals were produced. It was found that using a spatial resolution on the ground of approximately 5 m and spectral coverage from 0.4 to 2.5 mm was sufficient to delineate several hydrothermal alteration zones and associated veins, including phyllic, serpentinitic and chloritic alteration. These results suggest this level of spectral and spatial resolution would be ideal for localising shallow epithermal activity, should such activity have existed, on the surface of Mars.

astro-ph.EP↗
arXiv2014

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.

astro-ph.EP↗
arXiv2013

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.

physics.optics↗
arXiv2009

An improvement to the volcano-scan algorithm for atmospheric correction of CRISM and OMEGA spectral data

The observations of Mars by the CRISM and OMEGA hyperspectral imaging spectrometers require correction for photometric, atmospheric and thermal effects prior to the interpretation of possible mineralogical features in the spectra. Here, we report on a simple, yet non-trivial, adaptation to the commonly-used volcano-scan correction technique for atmospheric CO_2, which allows for the improved detection of minerals with intrinsic absorption bands at wavelengths between 1.9-2.1 $μ$m. This volcano-scan technique removes the absorption bands of CO_2 by ensuring that the Lambert albedo is the same at two wavelengths: 1.890 $μ$m and 2.011 $μ$m, with the first wavelength outside the CO_2 gas bands and the second wavelength deep inside the CO_2 gas bands. Our adaptation to the volcano-scan technique moves the first wavelength from 1.890 $μ$m to be instead within the gas bands at 1.980 $μ$m, and for CRISM data, our adaptation shifts the second wavelength slightly, to 2.007 $μ$m. We also report on our efforts to account for a slight ~0.001 $μ$m shift in wavelengths due to thermal effects in the CRISM instrument.

astro-ph.EP↗