M. Staid

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.

Compositional analyses of lunar pyroclastic deposits

Abstract The 5-band Clementine UVVIS data at ∼100 m/pixel were used to examine the compositions of 75 large and small lunar pyroclastic deposits (LPDs), and these were compared to representative lunar maria and highlands deposits. Results show that the albedo, spectral color, and inferred composition of most LPDs are similar to those of low-titanium, mature lunar maria. These LPDs may have consisted largely of fragmented basalt, with substantial components of iron-bearing mafic minerals (pyroxenes, olivine) and smaller amounts (if any) of volcanic glass. Several smaller LPDs also show substantial highland components. Three classes of very large deposits can be distinguished from most LPDs and from each other on the basis of crystallinity and possible titanium content of their pyroclastic components. One class has spectral properties that are dominated by high-titanium, crystallized “black beads” (e.g., Taurus–Littrow), a second consists of a mixture of high-titanium glasses and beads with a higher glass/bead ratio (Sulpicius Gallus) than that of Taurus–Littrow, and a third has a significant component of quenched iron-bearing volcanic glasses (Aristarchus) with possible moderate titanium contents. Although areally extensive, these three classes of very large pyroclastic deposits compose only 20 of the 75 deposits studied (∼27%), and eruption of such materials was thus likely to have been less frequent on the Moon.

A Sharper View of Impact Craters from Clementine Data

The ultraviolet-visible camera on the Clementine spacecraft obtained high-spatial resolution images of the moon in five spectral channels. Impact craters mapped with these multispectral images show a scale of lithologic diversity that varies with crater size and target stratigraphy. Prominent lithologic variations (feldspathic versus basaltic) occur within the south wall of Copernicus (93 kilometers in diameter) on the scale of 1 to 2 kilometers. Lithologic diversity at Tycho (85 kilometers in diameter) is less apparent at this scale, although the impact melt of these two large craters is remarkably similar in this spectral range. The lunar surface within and around the smaller crater Giordano Bruno (22 kilometers in diameter) is largely dominated by the mixing of freshly excavated material with surrounding older soils derived from a generally similar feldspathic lithology.

Mineralogy of the last lunar basalts: Results from Clementine

The last major phase of lunar volcanism produced extensive high-titanium mare deposits on the western nearside which remain unsampled by landing missions. The visible and near-infrared reflectance properties of these basalts are examined using Clementine multispectral images to better constrain their mineralogy. A much stronger 1 μm ferrous absorption was observed for the western high-titanium basalts than within earlier maria, suggesting that these last major mare eruptions also may have been the most iron-rich. These western basalts also have a distinctly long-wavelength, 1 μm ferrous absorption which was found to be similar for both surface soils and materials excavated from depth, supporting the interpretation of abundant olivine within these deposits. Spectral variation along flows within the Imbrium basin also suggests variations in ilmenite content along previously mapped lava flows as well as increasing olivine content within subsequent eruptions.

Mare Tranquillitatis: Basalt emplacement history and relation to lunar samples

Galileo and Clementine multispectral data of the Mare Tranquillitatis region have been analyzed to investigate the stratigraphy of basaltic units and the effects of lateral and vertical mixing processes within the mare. The distribution of compositionally distinct mare units is observed to be correlated with previous UV/VIS ratio images, although estimates of soil titanium contents are low in some areas as a result of mixing of local basalts with nonmare feldspathic materials. Basalt units identified by their spectral properties and spectral mixture analysis are compared with groups of Apollo 11 samples defined by previous workers on the basis of age and chemistry. Spectral studies presented here indicate that the Apollo 11 site lies at the edge of a localized western mare unit which includes the youngest and most titanium-rich basalts in Tranquillitatis (Apollo 11 high-K, high-Ti samples). In southern Tranquillitatis, these basalts have been contaminated by a large degree of mixing with nonmare feldspathic materials. Nonmare materials near the Apollo 11 site are attributed largely to crater rays from Theophilus (100 km in diameter), which is located approximately 300 km to the south. A more extensive and stratigraphically older unit exposed near Apollo 11 is related to the low-K, high-Ti Apollo 11 samples and appears to extend as a coherent surface unit as far north as the Apollo 17 site in southern Serenitatis. The distribution of this spectrally identified basalt unit supports petrologic and geochemical evidence for the grouping of the high-Ti, low-K Apollo 11 and 17 basalt samples into the same regional volcanic events. Multispectral analysis of Tranquillitatis deposits also identify low-titanium basalts in the northeastern and southeastern portions of the basin that are older than the high-Ti basalts and are believed to be unsampled by Apollo 11. Several lines of evidence suggest that the Cayley Formation along the western Tranquillitatis margin may indeed lie on top of an ancient mare deposit buried by Imbrium basin ejecta (e.g., a cryptomare deposit). The distribution of vertically excavated feldspathic premare material within the mare provides information on the depth of the mare units and the proximity of the underlying basin topography. Compositional stratigraphy observed in both sets of multispectral data supports an asymmetric premare-fill basin topography containing thicker basalts in the northwestern portion of the basin than previously predicted by crater flooding data.

Thermal infrared spectroscopy and modeling of experimentally shocked plagioclase feldspars

Abstract Thermal infrared emission and reflectance spectra (250-1400 cm-1; ~7-40 mm) of experimentally shocked albite- and anorthite-rich rocks (17-56 GPa) demonstrate that plagioclase feldspars exhibit characteristic degradations in spectral features with increasing pressure. New measurements of albite (Ab98) presented here display major spectral absorptions between 1000-1250 cm-1 (8-10 mm) (due to Si-O antisymmetric stretch motions of the silica tetrahedra) and weaker absorptions between 350-700 cm-1 (14-29 mm) (due to Si-O-Si octahedral bending vibrations). Many of these features persist to higher pressures compared to similar features in measurements of shocked anorthite, consistent with previous thermal infrared absorption studies of shocked feldspars. A transparency feature at 855 cm-1 (11.7 μm) observed in powdered albite spectra also degrades with increasing pressure, similar to the 830 cm-1 (12.0 μm) transparency feature in spectra of powders of shocked anorthite. Linear deconvolution models demonstrate that combinations of common mineral and glass spectra can replicate the spectra of shocked anorthite relatively well until shock pressures of 20-25 GPa, above which model errors increase substantially, coincident with the onset of diaplectic glass formation. Albite deconvolutions exhibit higher errors overall but do not change significantly with pressure, likely because certain clay minerals selected by the model exhibit absorption features similar to those in highly shocked albite. The implication for deconvolution of thermal infrared spectra of planetary surfaces (or laboratory spectra of samples) is that the use of highly shocked anorthite spectra in end-member libraries could be helpful in identifying highly shocked calcic plagioclase feldspars.

A semianalytical approach to the calibration of AVIRIS data to reflectance over water application in a temperate estuary

Abstract Calibration of at-sensor radiance to reflectance is a critical step for the use of remotely sensed hyperspectral data of water to determine the concentrations of optically active components. Because water has such a low reflectance, sources of radiance other than the water-leaving radiance contribute significantly to the at-sensor radiance and vary with wavelength and spatially. While radiative transfer models have improved significantly, there are still large uncertainties in prescribing the spatial and spectral properties of aerosol scattering. In addition, errors in the sensor calibration of radiance can lead to additional uncertainty. An atmospheric correction method based entirely on scene information, but not in situ data, is described. This approach accounts for nonuniform aerosol scattering, glint from the water surface, and reflected skylight. Normalization by an estimate of the direct solar irradiance reaching the surface, also derived from scene information, then provides an estimate of reflectance. This method is applied to AVIRIS data acquired over a temperate estuary. The resultant reflectance estimates are shown to be consistent with field observations. While the method is empirical, it is based on physical principles that allow it to be applied under a wide range of conditions.

Marking Time:The Epic Quest to Invent the Perfect Calendar

The arrival of the new millennium has renewed interest in the evolution of our modern calendar. Unraveling the story of this evolution requires extensive knowledge of science, history, and religion. Most of us have little understanding of how the date of Easter is determined, or why we celebrate the New Year on January 1st. In Marking Time: The Epic Quest to Invent the Perfect Calendar, Duncan Steel takes readers on a revealing, meandering trip from several millennia B.C. to the present. As the author states, “By the time you finish reading this tome…it will be clear that the calendar has had much greater effect upon human destiny than you may have fondly imagined. Its not just about numbering the days: its about the rises and falls of empires and religions…about controlling the masses and subverting your rivals. The calendar story is one of human strife and aspirations.” And so Steel reveals how our calendar is based as much on religion and politics as it is on physical science.