Charles Arthur Hibbitts

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.

Hydrated salt minerals on Europa's Surface from the Galileo near-infrared mapping spectrometer (NIMS) investigation

We reported evidence of heavily hydrated salt minerals present over large areas of Europas surface from analysis of reflectance spectra returned by the Galileo mission near infrared mapping spectrometer (NIMS) [McCord et al., 1997a, b, 1998a, b]. Here we elaborate on this earlier evidence, present spatial distributions of these minerals, examine alternate water-ice interpretations, expand on our hydrated-salts interpretation, consider salt mineral stability on Europa, and discuss the implications. Extensive well-defined areas on Europa show distinct, asymmetric water-related absorption bands in the 1 to 2.5-μm region. Radiative transfer modeling of water ice involving different particle sizes and layers at Europa temperatures does not reproduce the distinctive Europa water bands. However, ice near its melting temperature, such as in terrestrial environments, does have some characteristics of the Europa spectrum. Alternatively, some classes of heavily hydrated minerals do exhibit such water bands. Among plausible materials, heavily hydrated salt minerals, such as magnesium and sodium sulfates, sodium carbonate and their mixtures, are preferred. All Europa spectral features are present in some salt minerals and a very good match to the Europa spectrum can be achieved by mixing several salt spectra. However, no single or mix of salt mineral spectra from the limited library available has so far been found to perfectly match the Europa spectrum in every detail. The material is concentrated at the lineaments and in chaotic terrain, which are technically disrupted areas on the trailing side. Since the spectrum of the material on Europa is nearly the same everywhere so-far studied, the salt or salt-mixture composition may be nearly uniform. This suggests similar sources and processes over at least a near-hemispheric scale. This would suggest that an extensive subsurface ocean containing dissolved salts is the source, and several possible mechanisms for deposit emplacement are considered. The hydrogen bonds associated with hydration of these salts are similar or greater in strength and energy to those in pure water ice. Thus, once on the surface, the salt minerals should be as stable to disruption as water ice at the Europa temperatures, and mechanisms are suggested to enhance the stability of both materials. Spectra obtained of MgSO4·6H2O at 77 K show only small differences from room temperature spectra. The main difference is the appearance of the individual absorptions composing the broad, composite water features and associated with the several different H2O sites in the salt hydrate molecule. This suggests that the Europa absorption bands are also composites. Thus higher spectral resolution may reveal these diagnostic features in Europas spectrum. The specific salts present and their relative abundances would be indicators of the chemistry and conditions of an ocean environment, and areas of fresh, heavy concentration of these minerals should make ideal lander mission sampling sites.

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.

Near-Infrared Spectroscopy and Spectral Mapping of Jupiter and the Galilean Satellites: Results from Galileo's Initial Orbit

The Near Infrared Mapping Spectrometer performed spectral studies of Jupiter and the Galilean satellites during the June 1996 perijove pass of the Galileo spacecraft. Spectra for a 5-micrometer hot spot on Jupiter are consistent with the absence of a significant water cloud above 8 bars and with a depletion of water compared to that predicted for solar composition, corroborating results from the Galileo probe. Great Red Spot (GRS) spectral images show that parts of this feature extend upward to 240 millibars, although considerable altitude-dependent structure is found within it. A ring of dense clouds surrounds the GRS and is lower than it by 3 to 7 kilometers. Spectra of Callisto and Ganymede reveal a feature at 4.25 micrometers, attributed to the presence of hydrated minerals or possibly carbon dioxide on their surfaces. Spectra of Europas high latitudes imply that fine-grained water frost overlies larger grains. Several active volcanic regions were found on Io, with temperatures of 420 to 620 kelvin and projected areas of 5 to 70 square kilometers.

Impact crater lakes on Mars

The robotic search for life on Mars centers on identifying accessible environments where the biological catalyst, water, has existed. The formation of large impact craters on Mars (>65 km diameter) may have resulted in the creation of ice-covered impact crater lakes, which would not freeze for thousands of years, even under present climatic conditions. Water could be supplied from deep confined aquifers penetrated by the impact craters, without the need for surface melt water. Freezing of the lakes is postponed owing to heat from impact generated melt-bearing deposits, from impact-related uplift of hotter rocks from depth, and from the latent heat of freezing of a deep crater lake. Abundant morphologic evidence for ancient crater lakes has not been found in Viking images, except for craters associated with outflow channels. However ice-covered crater lakes could have formed, and further searches for evidence of these lakes are warranted. The lake deposits from dissected impact craters may represent one of the best targets for future surface exobiology investigations or sample return missions from Mars.

Distributions of CO2 and SO2 on the surface of Callisto

Absorption bands in the infrared reflectance spectra from the Galileo Near-Infrared Mapping Spectrometer (NIMS) which are attributed to the presence of CO2 and SO2 on the surface of Callisto have been analyzed and mapped in detail. CO2 of varying concentrations appears to exist everywhere on Callisto, except at higher latitudes, where it may be masked by frost. The CO2 concentration on the trailing hemisphere has a longitudinal distribution largely consistent with a sinusoid centered on the equator near 270° longitude. The approximately sinusoidal pattern suggests that exogenic effects related to Jupiters corotating magnetic field are involved. Closer inspection of both hemispheres reveals that in many cases, visibly bright and ice-rich impact craters have high CO2 concentrations within or near them. The CO2 sometimes appears to be associated more with dark material near the craters than with the water ice. These correlations suggest impact processes may also affect the distribution of CO2 on the surface of Callisto. The center of the absorption band has been refined to be at 4.258±0.004 μm. The presence of a single band shape and band minimum wavelength position in all data sets for the CO2 absorption implies the physical state of CO2 is similar over the surface of Callisto. The distribution of SO2 on the surface is less well defined owing to characteristically shallower band depths, but it appears generally mottled, with some areas of high concentrations correlated with ice-rich impact craters. Large-scale patterns include the depletion of SO2 in the polar regions and a depletion of SO2 on the trailing side relative to the leading side. There is no sinusoidal pattern to this depletion. The center of the SO2 band is determined to be between 4.01 and 4.02 μm.

Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus: Detection of CO2

The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus in which new absorption bands are seen in the spectra of both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice slightly darkened with tholin. The detection of CO2 in the low-albedo material on the leading hemisphere supports the contention that it is carbon-bearing material from an external source that has been swept up by the satellites orbital motion.

Science measurements and instruments for a planetary science stratospheric balloon platform

Balloon platforms operating in Earths upper stratosphere offer a unique platform to conduct new, high value planetary science observations of our solar system and exoplanets. There are compelling science drivers for conducting observations from such a balloon platform, with several potential high value science measurements that can be accomplished with one of several instrument concepts. Observations from 100,000 to 120,000 feet, which can last from hours to months, night and day, offer significant advantages over observations from ground and aircraft platforms. The stability of the airmass at float altitude is indistinguishable from space so that diffraction-limited performance can be obtained without adaptive optics, resulting in performance at visible wavelengths better than many ground based assets with larger apertures. With >99% of the atmosphere, and almost all the telluric water and CO2, beneath the platform, previously obscured spectral windows are also now open (e.g. water, CO2, and the organic fingerprint region of 5-8 μm), others are now fully free from telluric contributions, and observations in the mid through thermal infrared (IR), as well as shortward into the near ultraviolet (NUV), experience more than an order of magnitude less downwelling radiance than do ground based measurements enabling longer integration times and higher contrast observations. Instrument types that would support high value science include broadband and multispectral high spatial resolution NUV-NIR imagers, multispectral and hyper spectral imagers in the 2.5-5 μm range, as well as in the 5-8 μm range.

OPTICAL SIGNATURES OF BURIED MINES

Soils can be disturbed by the burial of mines. When disturbed, the soils have a smaller effective size distribution than when undisturbed and this difference can be detected optically. Apparent emissivity differences in the thermal infrared is an effective passive optical technique developed by others for discriminating disturbed silicate soils from undisturbed soils and appears particularly sensitive to the presence of grains less than about 60 μm in diameter. It is not, however, an effective means for detecting grain size variations in carbonate sands. Optical techniques that may prove less sensitive to composition include 1) Changes in the reflectance in the shortwave infrared outside of absorption features nominally near 2.4 μm, and 2) Reflectance changes due to photometric backscattering by particulate surfaces. The reflectance of particulate surfaces increases with decreasing phase angle, peaking sharply at opposition when the illumination and detector are collocated (0o phase angle). For a given high-albedo material, finer-grains are brighter than larger-grained powders when observed at

Characterizing optical properties of disturbed surface signatures

The burial of objects disturbs the ground surface in visually perceptible ways. This project investigated how such information can inform detection via imaging from visible through mid-infrared wavelengths. Images of the ground surface where objects were buried were collected at multiple visible through mid-infrared wavelengths prior to burial and afterward at intervals spanning approximately two weeks. Signs of soil disturbed by emplacement change over time and exposure in the natural environment and vary in salience across wavelengths for different time periods. Transient cues related to soil moisture or illumination angle can make signatures extraordinarily salient under certain conditions. Longpass shortwave infrared and multi-band mid-infrared imaging can enhance the signature of disturbed soils over visible imaging. These findings add knowledge and understanding of how soil disturbances phenomena can be exploited to aid detection.