Roger N. Clark

Spectroscopic Determination of Leaf Biochemistry Using Band-Depth Analysis of Absorption Features and Stepwise Multiple Linear Regression

Abstract We develop a new method for estimating the biochemistry of plant material using spectroscopy. Normalized band depths calculated from the continuum-removed reflectance spectra of dried and ground leaves were used to estimate their concentrations of nitrogen, lignin, and cellulose. Stepwise multiple linear regression was used to select wavelengths in the broad absorption features centered at 1.73 μm, 2.10 μm, and 2.30 μm that were highly correlated with the chemistry of samples from eastern U.S. forests. Band depths of absorption features at these wavelengths were found to also be highly correlated with the chemistry of four other sites. A subset of data from the eastern U.S. forest sites was used to derive linear equations that were applied to the remaining data to successfully estimate their nitrogen, lignin, and cellulose concentrations. Correlations were highest for nitrogen (R2 from 0.75 to 0.94). The consistent results indicate the possibility of establishing a single equation capable of estimating the chemical concentrations in a wide variety of species from the reflectance spectra of dried leaves. The extension of this method to remote sensing was investigated. The effects of leaf water content, sensor signal-to-noise and bandpass, atmospheric effects, and background soil exposure were examined. Leaf water was found to be the greatest challenge to extending this empirical method to the analysis of fresh whole leaves and complete vegetation canopies. The influence of leaf water on reflectance spectra must be removed to within 10%. Other effects were reduced by continuum removal and normalization of band depths. If the effects of leaf water can be compensated for, it might be possible to extend this method to remote sensing data acquired by imaging spectrometers to give estimates of nitrogen, lignin, and cellulose concentrations over large areas for use in ecosystem studies.

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L’Eau, les Glaces et l’Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars’s history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

Detection of Crystalline Hematite Mineralization on Mars by the Thermal Emission Spectrometer: Evidence for Near-surface Water

The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor (MGS) mission has discovered a remarkable accumulation of crystalline hematite (a-Fe2O3) that covers an area with very sharp boundaries approximately 350 by 350 -750 km in size centered near 28S latitude between 08 and 58W longitude (Sinus Meridiani). Crystalline hematite is uniquely identified by the presence of fundamental vibrational absorption features centered near 300, 450, and .525 cm21 and by the absence of silicate fundamentals in the 1000 cm 21 region. Spectral features resulting from atmospheric CO 2, dust, and water ice were removed using a radiative transfer model. The spectral properties unique to Sinus Meridiani were emphasized by removing the average spectrum of the surrounding region. The depth and shape of the hematite fundamental bands show that the hematite is crystalline and relatively coarse grained (.5-10 mm). Diameters up to and greater than hundreds of micrometers are permitted within the instrumental noise and natural variability of hematite spectra. Hematite particles ,5-10 mm in diameter (as either unpacked or hard-packed powders) fail to match the TES spectra. The spectrally derived areal abundance of hematite varies with particle size from ;10% (.30 mm diameter) to 40 - 60% (10 mm diameter). The hematite in Sinus Meridiani is thus distinct from the fine-grained (diameter ,5-10 mm), red, crystalline hematite considered, on the basis of visible, near-IR data, to be a minor spectral component in Martian bright regions like Olympus-Amazonis. Sinus Meridiani hematite is closely associated with a smooth, layered, friable surface that is interpreted to be sedimentary in origin. This material may be the uppermost surface in the region, indicating that it might be a late stage sedimentary unit or a layered portion of the heavily cratered plains units. We consider five possible mechanisms for the formation of coarse- grained, crystalline hematite. These processes fall into two classes depending on whether they require a significant amount of near-surface water: the first is chemical precipitation that includes origin by (1) precipitation from standing, oxygenated, Fe-rich water (oxide iron formations), (2) precipitation from Fe-rich hydrothermal fluids, (3) low-temperature dissolution and precipitation through mobile ground water leaching, and (4) formation of surface coatings, and the second is thermal oxidation of magnetite-rich lavas. Weathering and alteration processes, which produce nanophase and red hematite, are not consistent with the coarse, crystalline hematite observed in Sinus Meridiani. We prefer chemical precipitation models and favor precipitation from Fe-rich water on the basis of the probable association with sedimentary materials, large geographic size, distance from a regional heat source, and lack of evidence for extensive groundwater processes elsewhere on Mars. The TES results thus provide mineralogic evidence for probable large-scale water interactions. The Sinus Meridiani region may be an ideal candidate for future landed missions searching for biotic and prebiotic environments, and the physical characteristics of this site satisfy all of the engineering requirements for the missions currently planned.

Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1

Lunar Water The Moon has been thought to be primarily anhydrous, although there has been some evidence for accumulated ice in permanently shadowed craters near its poles (see the Perspective by Lucey, published online 24 September). By analyzing recent infrared mapping by Chandrayaan-1 and Deep Impact, and reexamining Cassini data obtained during its early flyby of the Moon, Pieters et al. (p. 568, published online 24 September), Sunshine et al. (p. 565, published online 24 September), and Clark et al. (p. 562, published online 24 September) reveal a noticeable absorption signal for H2O and OH across much of the surface. Some variability in water abundance is seen over the course of the lunar day. The data imply that solar wind is depositing and/or somehow forming water and OH in minerals near the lunar surface, and that this trapped water is dynamic. Space-based spectroscopic measurements provide evidence for water or hydroxyl (OH) on the surface of the Moon The search for water on the surface of the anhydrous Moon had remained an unfulfilled quest for 40 years. However, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 has recently detected absorption features near 2.8 to 3.0 micrometers on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer hydrogen abundance data suggests that the formation and retention of hydroxyl and water are ongoing surficial processes. Hydroxyl/water production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.

Thermal emission spectrometer experiment: Mars Observer mission

Thermal infrared spectral measurements will be made of the surface and atmosphere of Mars by the thermal emission spectrometer (TES) on board Mars Observer. By using these observations the composition of the surface rocks, minerals, and condensates will be determined and mapped. In addition, the composition and distribution of atmospheric dust and condensate clouds, together with temperature profiles of the CO2 atmosphere, will be determined. Broadband solar reflectance and thermal emittance measurements will also be made to determine the energy balance in the polar regions and to map the thermophysical properties of the surface. The specific science objectives of this investigation are to determine (1) the composition and distribution of surface materials, (2) the composition, particle size, and spatial and temporal distribution of suspended dust, (3) the location, temperature, height, and water abundance of H2O clouds, (4) the composition, seasonal behavior, total energy balance, and physical properties of the polar caps, and (5) the particle size distribution of rocks and fines on the surface. The instrument consists of three subsections: a Michelson interferometer, a solar reflectance sensor, and a broadband radiance sensor. The spectrometer covers the wavelength range from 6 to 50 μm (∼1600–200 cm−1) with nominal 5 and 10 cm−1 spectral resolution. The solar reflectance band extends from 0.3 to 2.7 μm; the broadband radiance channel extends from 5.5 to 100 μm. There are six 8.3-mrad fields of view for each sensor arranged in a 3 × 2 array, each with 3-km resolution at the nadir. Uncooled deuterated triglycine sulphate (DTGS) pyroelectic detectors provide a signal-to-noise ratio (SNR) of over 500 at 10 μm for daytime spectral observations at a surface temperature of 270 K. The SNR of the albedo and thermal bolometers will be approximately 2000 at the peak signal levels expected. The instrument is 23.6 × 35.5 × 40.0 cm, with a mass of 14.4 kg and an average power consumption of 14.5 W. The approach will be to measure the spectral properties of thermal energy emitted from the surface and atmosphere. Emission phase angle studies and day-night observations will be used to separate the spectral character of the surface and atmosphere. The distinctive thermal infrared spectral features present in minerals, rocks, and condensates will be used to determine the mineralogic and petrologic character of the surface and to identify and study aerosols and volatiles in the atmosphere.

Opaline silica in young deposits on Mars

High spatial and spectral resolution reflectance data acquired by the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument reveal the presence of H_2O- and SiOH-bearing phases on the Martian surface. The spectra are most consistent with opaline silica and glass altered to various degrees, confirming predictions based on geochernicall experiments and models that amorphous silica should be a common weathering product of the basaltic Martian crust. These materials are associated with hydrated Fe sulfates, including H_3O-bearing jarosite, and are found in finely stratified deposits exposed on the floor of and on the plains surrounding the Valles Marineris canyon system. Stratigraphic relationships place the formation age of these deposits in the late Hesperian or possibly the Amazonian, implying that aqueous alteration continued to be an important and regionally extensive process on Mars during that time.

Identification of a basaltic component on the Martian surface from Thermal Emission Spectrometer data

The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument collected 4.8×106 spectra of Mars during the initial aerobraking and science-phasing periods of the mission (September 14, 1997, through April 29, 1998). Two previously developed atmosphere-removal models were applied to data from Cimmeria Terra (25°S, 213°W). The surface spectra derived for these two models agree well, indicating that the surface and atmosphere emission can be separated and that the exact atmosphere-removal model used has little effect on the derived surface composition. The Cimmeria spectra do not match terrestrial high-silica igneous rocks (granite and rhyolite), ultramafic igneous rocks, limestone, or quartz- and clay-rich sandstone and siltstone. A particulate (sand-sized) sample of terrestrial flood basalt does provide an excellent match in both spectral shape and band depth to the Cimmeria spectrum over the entire TES spectral range. No unusual particle size effects are required to account for the observed spectral shape and depth. The implied grain size is consistent with the thermal inertia and albedo of this region, which indicate a sand-sized surface with little dust. The identification of basalt is consistent with previous indications of pyroxene and basalt-like compositions from visible/near-infrared and thermal-infrared spectral measurements. A linear spectral deconvolution model was applied to both surface-only Cimmeria spectra using a library of 60 minerals to determine the composition and abundance of the component minerals. Plagioclase feldspar (45%; 53%) and clinopyroxene (26%; 19%) were positively identified above an estimated detection threshold of 10–15% for these minerals. The TES observations provide the first identification of feldspars on Mars. The best fit to the Mars data includes only clinopyroxene compositions; no orthopyroxene compositions are required to match the Cimmeria spectra. Olivine (12%; 12%) and sheet silicate (15%; 11%) were identified with lower confidence. Carbonates, quartz, and sulfates were not identified in Cimmeria at detection limits of ∼5, 5, and 10%, respectively. Their presence elsewhere, however, remains open. The Cimmeria spectra are not well matched by any one SNC meteorite spectrum, indicating that this region is not characterized by a single SNC lithology. The occurrence of unweathered feldspar and pyroxene in Cimmeria, together with the inferred presence of pyroxene and unweathered basalts in other dark regions and at the Viking and Pathfinder landing sites, provides evidence that extensive global chemical weathering of materials currently exposed on the Martian surface has not occurred.

Non‐water‐ice constituents in the surface material of the icy Galilean satellites from the Galileo near‐infrared mapping spectrometer investigation

We present evidence for several non-ice constituents in the surface material of the icy Galilean satellites, using the reflectance spectra returned by the Galileo near infrared mapping spectrometer (NIMS) experiment. Five new absorption features are described at 3.4, 3.88, 4.05, 4.25, and 4.57 μm for Callisto and Ganymede, and some seem to exist for Europa as well. The four absorption bands strong enough to be mapped on Callisto and Ganymede are each spatially distributed in different ways, indicating different materials are responsible for each absorption. The spatial distributions are correlated at the local level in complex ways with surface features and in some cases show global patterns. Suggested candidate spectrally active groups, perhaps within larger molecules, producing the five absorptions include C-H, S-H, SO2, CO2, and C≡N. Organic material like tholins are candidates for the 4.57- and 3.4-μm features. We suggest, based on spectroscopic evidence, that CO2 is present as a form which does not allow rotational modes and that SO2 is present neither as a frost nor a free gas. The CO2, SO2, and perhaps cyanogen (4.57 μm) may be present as very small collections of molecules within the crystal structure, perhaps following models for radiation damage and/or for comet and interstellar grain formation at low temperatures. Some of the dark material on these surfaces may be created by radiation damage of the CO2 and other carbon-bearing species and the formation of graphite. These spectra suggest a complex chemistry within the surface materials and an important role for non-ice materials in the evolution of the satellite surfaces.

Detection of Adsorbed Water and Hydroxyl on the Moon

Lunar Water The Moon has been thought to be primarily anhydrous, although there has been some evidence for accumulated ice in permanently shadowed craters near its poles (see the Perspective by Lucey, published online 24 September). By analyzing recent infrared mapping by Chandrayaan-1 and Deep Impact, and reexamining Cassini data obtained during its early flyby of the Moon, Pieters et al. (p. 568, published online 24 September), Sunshine et al. (p. 565, published online 24 September), and Clark et al. (p. 562, published online 24 September) reveal a noticeable absorption signal for H2O and OH across much of the surface. Some variability in water abundance is seen over the course of the lunar day. The data imply that solar wind is depositing and/or somehow forming water and OH in minerals near the lunar surface, and that this trapped water is dynamic. Space-based spectroscopic measurements provide evidence for water or hydroxyl (OH) on the surface of the Moon. Data from the Visual and Infrared Mapping Spectrometer (VIMS) on Cassini during its flyby of the Moon in 1999 show a broad absorption at 3 micrometers due to adsorbed water and near 2.8 micrometers attributed to hydroxyl in the sunlit surface on the Moon. The amounts of water indicated in the spectra depend on the type of mixing and the grain sizes in the rocks and soils but could be 10 to 1000 parts per million and locally higher. Water in the polar regions may be water that has migrated to the colder environments there. Trace hydroxyl is observed in the anorthositic highlands at lower latitudes.

The identification of liquid ethane in Titan's Ontario Lacus

Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan’s dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan’s Ontario Lacus.