Thomas B. McCord

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.

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.

The geologically recent giant impact basins at Vesta's south pole.

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. Dawn’s global mapping of Vesta reveals that its observed south polar depression is composed of two overlapping giant impact features. These large basins provide exceptional windows into impact processes at planetary scales. The youngest, Rheasilvia, is 500 kilometers wide and 19 kilometers deep and finds its nearest morphologic analog among large basins on low-gravity icy satellites. Extensive ejecta deposits occur, but impact melt volume is low, exposing an unusual spiral fracture pattern that is likely related to faulting during uplift and convergence of the basin floor. Rheasilvia obliterated half of another 400-kilometer-wide impact basin, Veneneia. Both basins are unexpectedly young, roughly 1 to 2 billion years, and their formation substantially reset Vestan geology and excavated sufficient volumes of older compositionally heterogeneous crustal material to have created the Vestoids and howardite–eucrite–diogenite meteorites.

Delivery of dark material to Vesta via carbonaceous chondritic impacts

NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 lm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6 vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the � 400 km Veneneia basin by a low-velocity (<2 km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.

Galileo infrared imaging spectroscopy measurements at Venus

During the 1990 Galileo Venus flyby, the Near Infaied Mapping Spectrometer investigated the night-side atmosphere of Venus in the spectral range 0.7 to 5.2 micrometers. Multispectral images at high spatial resolution indicate substanmial cloud opacity variations in the lower cloud levels, centered at 50 kilometers altitude. Zonal and meridional winds were derived for this level and are consistent with motion of the upper branch of a Hadley cell. Northern and southern hemisphere clouds appear to be markedly different. Spectral profiles were used to derive lower atmosphere abundances of water vapor and other species.

Thermal and radiation stability of the hydrated salt minerals epsomite, mirabilite, and natron under Europa environmental conditions

We report studies on the thermal and radiolytic stability of the hydrated salt minerals epsomite (MgSO4·7H2O), mirabilite (Na2SO4·10H2O), and natron (Na2CO3·10H2O) under the low-temperature and ultrahigh vacuum conditions characteristic of the surface of the Galilean satellite Europa. We prepared samples, ran temperature-programmed dehydration (TPD) profiles and irradiated the samples with electrons. The TPD profiles are fit using Arrhenius-type first-order desorption kinetics. This analysis yields activation energies of 0.90±0.10, 0.70±0.07, and 0.45±0.05 eV for removal of the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (<130 K), epsomite should remain hydrated over geologic timescales (∼1011–1014 years), whereas natron and mirabilite may dehydrate appreciably in approximately 108 and 103 years, respectively. A small amount of SO2 was detected during and after 100 eV electron-beam irradiation of dehydrated epsomite and mirabilite samples, whereas products such as O2 remained below detection limits. The upper limit for the 100 eV electron-induced damage cross section of mirabilite and epsomite is ∼10−19 cm2. The overall radiolytic stability of these minerals is partially due to (1) the multiply charged nature of the sulfate anion, (2) the low probability of reversing the attractive Madelung (mostly the attractive electrostatic) potential via Auger decay, and (3) solid-state caging effects. Our laboratory results on the thermal and radiolytic stabilities of these salt minerals indicate that hydrated magnesium sulfate and perhaps other salts could exist for geologic timescales on the surface of Europa.

Detection of local H2O exposed at the surface of Ceres

INTRODUCTION Dwarf planet Ceres’ low average-density (2162 ± 3 kg m−3) indicates that it must contain considerable water. Water is likely a key component in the chemical evolution and internal activity of Ceres, possibly resulting in a layer of ice-rich material and perhaps liquid in the mantle. Mineral hydroxides (OH-bearing) and hydrates (H2O-bearing), such as clays, carbonates, and various salts, would be created. These hypotheses were supported by the detection of hydroxyl (OH)–rich materials, OH-bearing molecule releases, H2O vapor molecules, and haze. However, the presence of H2O on the surface has not previously been confirmed. The detection and mapping of H2O on Ceres is one objective of the Dawn spacecraft, in orbit around Ceres since March 2015. RATIONALE The purpose of the Dawn space mission at Ceres is to study the geology, geophysics, and composition remotely by means of high-resolution imagery and spectrometry. Dawn’s Visible and InfraRed Mapping Spectrometer (VIR) measures the sunlight scattered by the surface of Ceres in a range of wavelengths between 0.25 and 5.1 μm. The position and shape of absorption features in VIR reflectance spectra are sensitive to the surface mineral and molecular composition. In spectroscopy, absorption bands at 2.0, 1.65, and 1.28 μm are characteristic of vibration overtones in the H2O molecule. RESULTS Dawn has detected water-rich surface materials in a 10-km-diameter crater named Oxo, which exhibit all absorption bands that are diagnostic of the H2O molecule (see the figure). These spectra are most similar to those of H2O ice, but they could also be attributable to hydrated minerals. Oxo crater appears to be geologically very young (~1 million to 10 million years); it has sharp rims and its floor is almost devoid of impacts, suggesting a recent exposure of surface H2O. The high latitude and morphology of the Oxo crater protects much of the surface area from direct solar illumination for most of the cerean day, presenting favorable conditions for the stability of water ice or heavily hydrated salts. CONCLUSION Four ways to create or transport H2O on Ceres are considered: (i) Exposure of near-surface H2O-rich materials by a recent impact or an active landslide seems most consistent with the presence of both mineral hydrates and water ice. (ii) Release of subsurface H2O may occur on Ceres, similar to release on comet nuclei, but may never recondense on the surface. (iii) Infall of ice-bearing objects is not likely to deposit water on Ceres, because the H2O molecule likely would dissociate upon impact. (iv) Implantation of protons from the solar wind on the surface is not a probable origin of OH on Ceres because of the low flux of solar wind charged particles. We therefore conclude that surface H2O or hydrated minerals are the most plausible explanation. Dawn VIR infrared observations of Oxo crater on Ceres demonstrate the detection of H2O at the surface. (A) Reflectance spectrum collected where absorption bands of H2O at 1.28, 1.65, and 2 μm are the strongest (in blue) compared with a laboratory spectrum of H2O ice (black). The lab spectrum is scaled and vertically shifted for clarity. (B) Perspective view of Oxo crater observed by the Dawn Framing Camera (FC), where the two high-albedo areas right next to the scarps contain H2O-rich materials. The surface of dwarf planet Ceres contains hydroxyl-rich materials. Theories predict a water ice-rich mantle, and water vapor emissions have been observed, yet no water (H2O) has been previously identified. The Visible and InfraRed (VIR) mapping spectrometer onboard the Dawn spacecraft has now detected water absorption features within a low-illumination, highly reflective zone in Oxo, a 10-kilometer, geologically fresh crater, on five occasions over a period of 1 month. Candidate materials are H2O ice and mineral hydrates. Exposed H2O ice would become optically undetectable within tens of years under current Ceres temperatures; consequently, only a relatively recent exposure or formation of H2O would explain Dawn’s findings. Some mineral hydrates are stable on geological time scales, but their formation would imply extended contact with ice or liquid H2O.

Mineralogical-petrological characterization of near-earth asteroids

Studies of near-Earth asteroids are aimed at determining their dynamical and structural history. The mineralogy and petrology of 17 near-Earth asteroids are characterized using reflectance spectroscopy with ground-based telescopes as one method to address their major issues. Implications for the origin and evolution are discussed in a separate paper. Assuming the surfaces are composed of cosmically abundant materials, the presence of certain mineralogical species can be determined from diagnostic absorption features and spectral characteristics which have been studied under known laboratory conditions and understood in terms of crystal field theory. With one possible exception, the surface composition of near-Earth asteroids consists of common rock-forming minerals such as olivine, pyroxene, and phyllosilicates. Opaque components are present but cannot be mineralogically identified with existing experimental data. The spectrum of 2201 Oljato cannot be interpreted in terms of common rock-forming minerals. This spectrum was examined for cometary features because its high orbital eccentricity suggests a possible relation to comets. No common cometary features are identified in its spectrum. The predominance of mafic silicate absorption features in spectra of near-Earth asteroids compared to the majority of main-belt asteroids may be a primary compositional feature or may be the signature of relatively fresher asteroid material.

Saturn's rings: Spectral reflectivity and compositional implications

Abstract A photometric study of the rings of Saturn was carried out during the 1969 apparition. A reflection spectrum of the A and B rings was obtained for the spectral region 0.3–1.05 μ. The reflectivity for both rings decreases sharply toward blue and ultraviolet wavelengths. A comparison of the ring reflection spectrum with spectra for other solar system objects shows that the ring curve most closely resembles the curve for the Galilean satellite J1. The ring spectrum lacks distinctive absorption features found in lunar, Martian, and Vesta spectra in the 0.3–1.1 μ region. Absorption features characteristic of water frost have been found recently in the 1.25–2.5 μ region, but the decrease in reflectivity of the rings toward shorter wavelengths indicates that material other than pure water frost also must be present. A physical mixture of frost and silicates seems to be unlikely as ring material. Frost-covered silicates and mixtures of frosts with other compounds, perhaps modified by ultraviolet or high-energy particle radiation, remain possible ring constituents.

Imbrian-Age Highland Volcanism on the Moon: The Gruithuisen and Mairan Domes

The Gruithuisen and Mairan domes on the moon represent morphologically and spectrally distinct nonmare extrusive volcanic features of Imbrian age. The composition, morphology, and age relationships of the domes indicate that nonmare extrusive volcanism in the northern Procellarum region of the moon continued until about 3.3 x 109 to 3.6 x 109 years ago and was partially contemporaneous with the emplacement of the main sequence of mare deposits.