Thermal Emission Spectrometer Spectra Research Articles

Seeking phyllosilicates in thermal infrared data: A laboratory and Martian data case study

Previous analyses of Thermal Emission Spectrometer (TES) data produce results that could suggest a widespread distribution of phyllosilicate minerals over the surface of Mars, whereas studies of visible to near‐infrared (VNIR) data indicate a more limited distribution. We use VNIR detections of phyllosilicates in the vicinity of the Nili Fossae to determine the spectral characteristics of phyllosilicate‐bearing material in the thermal infrared (TIR). By investigating areas of VNIR phyllosilicate detection in more detail, we find that the phyllosilicate‐bearing material corresponds to spectral variation in Thermal Emission Imaging System decorrelation‐stretched TIR images and differences in infrared spectral shape that are consistent with, but not uniquely attributable to, mixtures of phyllosilicates and basalt. Phyllosilicate phases are modeled from TES data at abundances that average 5% over the region and at abundances near the 10–15% detection limit in our specific regions of interest. Deconvolution of numerical mixtures of phyllosilicate and basalt spectra indicates that these low abundances of phyllosilicates likely are not influenced by uncertainties greater than the 10–15% uncertainty of the method. TES spectra and modeled abundances vary between the phyllosilicate‐bearing material and the surrounding region, but this difference in composition cannot be attributed solely to the presence of phyllosilicates. We believe the inconsistencies in phyllosilicate occurrence between TES and VNIR analyses may be explained by the inclusion of phyllosilicates in the models of TES data as substitutes for poorly crystalline phases (e.g., allophane) not currently available in public infrared spectral libraries.

Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of variations in atmospheric dust optical depth over cold surfaces

The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument has returned over 200 million thermal infrared spectra of Mars taken between March 1999 and August 2004. This represents one of the most complete records of spatial and temporal changes of the martian atmosphere ever recorded by an orbiting spacecraft. Previous reports of the standard TES retrieval of aerosol optical depth have been limited to those observations taken over surfaces with temperatures above 210 K, limiting the spatiotemporal coverage of Polar Regions with TES. Here, we present an extension to the standard TES retrieval that better models the effects of cold surfaces below 200 K. This modification allows aerosol optical depth to be retrieved from TES spectra over a greater spatiotemporal range than was previously possible, specifically in Polar Regions. This new algorithm is applied to the Polar Regions to show the seasonal variability in dust and ice optical depth for the complete temporal range of the TES database (Mars Year 24, L s = 104 ° , 1 March 1999 to Mars Year 24, L s = 82 ° , 31 August 2004).

Thermal Emission Spectrometer hyperspectral analyses of proposed paleolake basins on Mars: No evidence for in‐place carbonates

Several studies have described photogeologic evidence for paleolacustrine basins on Mars, mostly within impact craters. If these basins contained persistent standing water in the past, they could still contain deposits of evaporite minerals (e.g., carbonates, sulfates). Many such deposits, if exposed at the surface to a sufficient extent, would be detectable in thermal infrared spectra taken from orbit. Using data from the Mars Global Surveyor Thermal Emission Spectrometer (TES), we have conducted a hyperspectral investigation of 43 putative paleolake basins to search for the spectral signatures of evaporite minerals exposed at a scale comparable to the spatial resolution of a single TES pixel (∼3 × 5 km). Seven basins displaying sufficient surface‐related spectral variation were identified using a principal component analysis on the TES spectral image cube covering the basin and its surroundings and spectral regions of interest (ROIs) were defined. Averaged spectra from ROIs were evaluated using previously developed dust cover index. Those spectra determined to be “dust‐free” were analyzed for composition using linear spectral deconvolution. The same spectra were also analyzed using a spectral ratio and a set of carbonate indices developed in the present work. Most TES spectra in this study were well‐modeled using only previously defined TES spectral end‐members. In addition, the spectral ratios and the carbonate index analyses of these basins indicated that carbonates are not present in abundances greater than the detection limits of these methods. Therefore this study did not find any spectral evidence for evaporite deposits in the basins studied.

Thermal infrared spectral band detection limits for unidentified surface materials

Infrared emission spectra recorded by airborne or satellite spectrometers can be searched for spectral features to determine the composition of rocks on planetary surfaces. Surface materials are identified by detections of characteristic spectral bands. We show how to define whether to accept an observed spectral feature as a detection when the target material is unknown. We also use remotely sensed spectra measured by the Thermal Emission Spectrometer (TES) and the Spatially Enhanced Broadband Array Spectrograph System to illustrate the importance of instrument parameters and surface properties on band detection limits and how the variation in signal-to-noise ratio with wavelength affects the bands that are most detectable for a given instrument. The spectrometer's sampling interval, spectral resolution, signal-to-noise ratio as a function of wavelength, and the sample's surface properties influence whether the instrument can detect a spectral feature exhibited by a material. As an example, in the 6-13-mum wavelength region, massive carbonates exhibit two bands: a very strong, broad feature at ~6.5 mum and a less intense, sharper band at ~11.25 mum. Although the 6.5-mum band is stronger and broader in laboratory-measured spectra, the 11.25-mum band will cause a more detectable feature in TES spectra.

Mars south polar spring and summer behavior observed by TES: Seasonal cap evolution controlled by frost grain size

Thermal Emission Spectrometer (TES) observations of the recession phase of Mars' south polar cap are used to quantitatively map this recession in both thermal and visual appearance. Geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior. For most of the cap, while temperatures remain near the CO2 frost point, albedos slowly increase with the seasonal rise of the Sun, then drop rapidly as frost patches disappear over a period of ∼20 days. A “Cryptic” region remains dark and mottled throughout its cold period. TES observations are compared with first‐order theoretical spectra of solid CO2 frost with admixtures of dust and H2O. The TES spectra indicate that the Cryptic region has much larger grained solid CO2 than the rest of the cap and that the solid CO2 here may be in the form of a slab. The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small size of the CO2 frost grains; we found little evidence for a significant presence of H2O. Although CO2 grain size may be the major difference between these regions, incorporated dust is also required to match the observations; a self‐cleaning process carries away the smaller dust grains. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO2 sublimation budgets of up to 1250 kg m−2. Regional atmospheric dust is common; localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.