bluuebunny/crossref_metadata_2025 huggingface dataset:
Primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of Mars
Authors: Anderson Robert C. , Dohm James M. , Golombek Matthew P. , Haldemann Albert F. C. , Franklin Brenda J. , Tanaka Kenneth L. , Lias Juan , Peer Brian
Published: February 2004

Five main stages of radial and concentric structures formed around Tharsis from the Noachian through the Amazonian as determined by geologic mapping of 24,452 structures within the stratigraphic framework of Mars and by testing their radial and concentric orientations. Tectonic activity peaked in the Noachian (stage 1) around the largest center, Claritas, an elongate center extending more than 20° in latitude and defined by about half of the total grabens which are concentrated in the Syria Planum, Thaumasia, and Tempe Terra regions. During the Late Noachian and Early Hesperian (stage 2), extensional structures formed along the length of present‐day Valles Marineris and in Thaumasia (with a secondary concentration near Warrego Vallis) radial to a region just to the south of the central margin of Valles Marineris. Early Hesperian (stage 3) radial grabens in Pavonis, Syria, Ulysses, and Tempe Terra and somewhat concentric wrinkle ridges in Lunae and Solis Plana and in Thaumasia, Sirenum, Memnonia, and Amazonis are centered northwest of Syria with secondary centers at Thaumasia, Tempe Terra, Ulysses Fossae, and western Valles Marineris. Late Hesperian/Early Amazonian (stage 4) structures around Alba Patera, the northeast trending alignment of Tharsis Montes, and Olympus Mons appears centered on Alba Patera. Stage 5 structures (Middle‐Late Amazonian) represent the last pulse of Tharsis‐related activity and are found around the large shield volcanoes and are centered near Pavonis Mons. Tectonic activity around Tharsis began in the Noachian and generally decreased through geologic time to the Amazonian. Statistically significant radial distributions of structures formed during each stage, centered at different locations within the higher elevations of Tharsis. Secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout Tharsis.
Porosities of lunar meteorites: Strength, porosity, and petrologic screening during the meteorite delivery process
Authors: Warren Paul H.
Published: February 2004

Porosity has been directly measured for eight lunar meteorite breccias and calculated for two more on the basis of literature density measurements. Lunar meteorite regolith breccias display systematically low porosity in comparison to otherwise analogous Apollo regolith breccias. Among seven meteoritic regolith breccias, porosity ranges from 1 to 11% and averages 7.5 ± (1‐σ) 3.2%, whereas for 44 analogous Apollo samples (porosities mostly calculated from literature density data) the average is 25 ± (1‐σ) 7%. The origin of this disparity is enigmatic, but the trend probably reflects mainly a bias in favor of strong, compact breccias among fragments that manage to survive the violent process of launch to lunar escape velocity (2.38 km/s). In addition, compaction during launch may play an important role. The population of lunar meteorites is clearly not a random, unmodified sample of lithic materials near the surface of the parent body.
Occultation of stars in the UV: Study of the atmosphere of Mars
Authors: Korablev O. I. , Bertaux J.‐L. , Dubois J.‐P.
Published: February 2004

The stellar occultation technique permits effective sounding of a tenuous planetary atmosphere, such as the atmosphere of Mars. On the basis of measuring characteristics of the SPICAM Light UV spectrometer for the spectral range of 118–320 nm, resolution 0.9 nm (to be flown at Mars‐Express ESA mission), we have made some model simulations in order to estimate how the vertical profiles of CO2, temperature, O3, aerosols, and clouds can be determined by the stellar occultation method. From the atmospheric absorption imprinted progressively on the star spectrum, the line densities of CO2, O3, and dust are retrieved as a function of altitude. An inversion yields local densities of CO2 (and therefore the temperature), ozone, and dust particle concentration. Several (3–5) occultations per orbit, preferably on the night side of the orbit, are foreseen, allowing good geographical coverage independently on the orbit. Observing a bright star (one out of ∼30 in the sky), atmospheric density will be determined in the altitude range of 10–170 km with an accuracy of 0.5–4%; temperature will be determined at 20–140 km with an accuracy of ∼3 K, and ozone can be retrieved with an accuracy of a few percent in a layer between 25 and 40 km or below, depending on its vertical distribution. Optically thin cloud structures can be studied, including night clouds, which cannot be observed in scattered light. Molecular oxygen can be measured with an accuracy of 10%, and a search of SO2 will be performed. The results of this study can easily be applied to other instrumental characteristics.
Interior trough deposits on Mars: Subice volcanoes?
Authors: Chapman Mary G. , Tanaka Kenneth L.
Published: February 2004

Widespread, several‐kilometer‐thick successions of layered deposits occur as mounds that partly fill the troughs or chasmata that compose the Valles Marineris on Mars. Like terrestrial subice volcanoes, the layered deposits occur in a volcano‐tectonic setting within basins that may have held ponded water or ice. On the basis of their dimensions, morphologies, and associated catastrophic floods and other geologic events as shown in Viking and new Mars Global Surveyor (MGS) data sets, we suggest that the interior deposits are volcanic in origin and possibly generated by subice eruptions. A tuya origin for the mounds can explain the lack of external sediment, mound heights that can rival the plateau, local flat‐topped mesas, morphologically distinct mounds of different ages, horizontal to steep dips, fine‐grained materials, indications of rare volcanic vents and lava flows, and spectral composition. The extremely diverse layering of west Candor Chasma and possible volcanic cones in Melas may have formed by related subaerial eruptions. Consistent with the suggestion that interior deposits are eroding out of the wall rock, some deposits could have been erupted from sites along the walls.
Martian wrinkle ridge topography: Evidence for subsurface faults from MOLA
Authors: Golombek M. P. , Anderson F. S. , Zuber M. T.
Published: February 2004

Mars Orbiter Laser Altimeter (MOLA) topography shows distinct elevation offsets across wrinkle ridges that can be explained simply by subsurface thrust faults. MOLA has provided the first reliable topographic data of wrinkle ridges to test models for their origin; although previous work has shown that wrinkle ridges result from compressional folding and faulting of near‐surface units, the role of thrust faulting and its depth penetration have been argued. Topographic profiles across wrinkle ridges in Solis Planum, Lunae Planum, Tempe Terra, Xanthe Terra, Arcadia, Terra Sirenum, Thaumasia Planum, Arabia Terra, Syrtis Major, and Hesperia Planum show characteristic features such as superposed hills, crenulations, and elevation offsets between the plains on either side of the ridge. The characteristic elevation offsets between plains surfaces on either side of the ridges shown by MOLA are easily explained by subsurface thrust faults that underlie the ridges and produce the offset. In Solis Dorsa, wrinkle ridges are 10–20 km wide (well resolved by the 300 m spaced MOLA elevations), have a total relief of 80–250 m, and have elevation offsets of 50–180 m (well above the uncertainty in the MOLA elevations). MOLA topography shows that the plains decrease in elevation toward the southeast and are deformed into folds or arches not visible in images that are parallel to and in between the ridges. The elevations of the plains on either side of the ridges in Solis and Lunae Plana are characteristically down to the southeast and east, respectively, suggesting that faults beneath the wrinkle ridges offset the plains. This offset indicates underlying thrust faults that dip to the northwest and west, respectively, and systematically lower the southeast side of the plains. The observation that the elevation offsets across the ridges are maintained to the next ridge implies that the thrust fault penetrates to depths of tens of kilometers and thus well into the mechanical lithosphere. Small foreland basement uplifts on Earth, in which slip on shallowly dipping thrust faults in the crystalline basement is accommodated by folding in the overlying sedimentary rocks, appear to be broadly analogous to these groups of wrinkle ridges and suggest that they are examples of thick‐skinned compressional deformation on Mars. These observations are not consistent with models for the formation of wrinkle ridges that involve folding of near‐surface layers only or subsurface thrust faults that flatten into decollements at shallow depths.
Mars Orbiter Camera observations of the Martian south polar cap in 1999–2000
Authors: James Philip B. , Cantor Bruce A. , Davis Scott
Published: February 2004

The spring‐summer recession of the south polar cap of Mars in 1999–2000 has been investigated using the wide‐angle cameras of the Mars Orbiter Camera experiment on Mars Global Surveyor. The 1999–2000 regression closely resembles the recession observed by Viking orbiters in 1977. Although the rates of recession are quite similar, the current recession was slightly ahead of that in 1977 throughout spring. The rapid development of dark terrain, dubbed “cryptic terrain” by Hugh Kieffer, in one sector of the south cap between LS = 198° and 223° is documented. The emergence and separation of the Mountains of Mitchel has also been recorded in detail, culminating with its complete disappearance at solstice. The 2000 residual cap is almost identical to that in 1977; this observation does not resolve the mystery of the very different appearance of the residual cap in 1972. The albedos in the residual cap region and in bright, seasonal frost increase rapidly to high values at around solstice and then level out until the seasonal frost disappears. Observations of more Martian years will be needed to constrain the amount of interannual variability and its relation to other phenomena such as dust storms.
Extension and uplift at Alba Patera, Mars: Insights from MOLA observations and loading models
Authors: McGovern Patrick J. , Solomon Sean C. , Head James W. , Smith David E. , Zuber Maria T. , Neumann Gregory A.
Published: February 2004

High‐resolution Mars Orbiter Laser Altimeter (MOLA) profiles of the large shield volcano Alba Patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice. An extensive system of graben cuts across Alba Patera, extending from Ceraunius Fossae to the south to Tantalus Fossae to the northeast. In the vicinity of the edifice, the otherwise generally north‐south orientations of these graben are deflected to circumferential directions. MOLA topography reveals that the circumferential graben are located well up the flanks of Alba Patera. Both the type (extensional) and location (midflank) of faulting on Alba Patera are inconsistent with the state of stress predicted for purely surface‐loaded flexure models. To constrain the conditions governing the evolution of Alba Patera, we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms. Buoyant sublithospheric loads (representing underplated magma, low‐density mantle residuum, or dynamic support from mantle convection) and intralithospheric displacements (representing sills “trapped” by horizontal compressive stresses in the upper lithosphere) can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface load's characteristic radial extent. Both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent. However, bottom loading requires a disk‐like load geometry and an elastic lithosphere thickness Te sufficiently small to allow the load's shape to be apparent in the surface deformation (Te ≤ 32 km). Such low Te is inconsistent with Mars Global Surveyor (MGS) gravity and topography data. In contrast, intralithospheric sill complexes can produce the observed topography and faulting at Te values consistent with those inferred from gravity and topography (50 ≤ Te ≤ 100 km). For these conditions, principal stress orientations on the lower flanks are also consistent with the orientations of lower‐flank graben, provided that faulting was induced by dike intrusion or tensile failure rather than shear failure.
Evidence for a weakly stratified Europan ocean sustained by seafloor heat flux
Authors: Thomson Richard E. , Delaney John R.
Published: February 2004

We interpret chaos‐type features on the surface of Europa as melt‐through structures formed by rotationally confined, steady and/or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor. Smaller lenticular features in the vicinity of chaos‐type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites. The ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above. Seafloor heating, maintained by tidal dissipation in the rocky interior, generates an estimated global heat flux of 8.7×1012 W and limits the mean ice thickness to 2–5 km. For seafloor heat sources with radii r that are less than the ocean's deformation radius, rD = ND/|ƒ| (N is the Brunt‐Väisälä frequency, D is the water depth, and ƒ is the Coriolis parameter), the diameters of chaos‐type regions are expected to diminish from O (100 km) within equatorial regions to O (10 km) at high latitudes (assuming spatially uniform water depth and density structure). Where r > rD, the scale of the source region determines the scale of the melt‐through features. Provided there is sufficient time before refreezing, ice rafts in large melt‐through regions are imbedded in episodes of preferentially anticyclonic circulation, corresponding to clockwise (counterclockwise) motions in the northern (southern) hemisphere. We calculate that 1021 J were required to melt the ice in the ∼100 km diameter Conamara Chaos region and that for a steady, localized heat flux F ≈ 1011 W (∼1% of the global heat flux) it took ∼1000 years for the initial melt‐through to occur. Assuming that ice raft displacements in Conamara Chaos occurred during a major melt‐through event, maximum current speeds in the region were O (10 cm s−1), and refreezing occurred within ∼20 hours. A lack of well‐defined ice drift in other major melt‐through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion.
A new look at the Apollo 11 regolith and KREEP
Authors: Korotev Randy L. , Gillis Jeffrey J.
Published: February 2004

Although the Apollo 11 mission landed in Mare Tranquillitatis, ∼50 km from the nearest exposure of highlands, small nonmare particles are conspicuous in the regolith. The nonmare portion of the Apollo 11 regolith is compositionally similar to the Apollo 16 regolith. At both sites most of the nonmare material is from the feldspathic highlands, but some is mafic impact‐melt breccia with the chemical signature known as KREEP for its high concentrations of K, rare earth elements, and P. The composition of the Apollo 11 regolith corresponds best to a mixture of 66% crystalline mare basalt, 5% orange volcanic glass, 20% material of the feldspathic highlands, 8% KREEP‐bearing impact‐melt breccia, and 1% meteoritic material. The volcanic‐glass and KREEP‐bearing melt‐breccia components account for the high concentrations of Mg and Cr in the regolith. The most KREEP‐rich sample known from Apollo 11, 10085,1187, is an impact‐melt breccia that bears a strong textural and compositional similarity to a unique Apollo 16 melt breccia, 64815. Although not of the maria, such breccias are also not of the highlands provenance. Global data obtained from orbit show that KREEP‐bearing materials are most common at low elevations in the Imbrium‐Procellarum region and are rare at high elevations. Thus, as at Apollo 16, the KREEP‐bearing breccias of Apollo 11 are probably ejecta from the Imbrium impact into the low‐lying, Procellarum KREEP Terrane. On the basis of these observation and others, we suggest that the general acceptance of KREEP as a material of the highlands is not supported by the data and results largely from historical accident.
Evidence for sulfur dioxide, sulfur monoxide, and hydrogen sulfide in the Io exosphere
Authors: Russell C. T. , Kivelson M. G.
Published: February 2004

Molecules in the Io exosphere are ionized and accelerated in the Jovian magnetospheric electric field creating a distribution in velocity space that is unstable to the generation of magnetic fluctuations at the frequency of the gyrating ion. On the inbound portion of first pass by Io, Galileo detected strong waves centered on the SO2+ gyrofrequency, but on the outbound portion, waves near the SO+ gyrofrequency were seen, as well as a weak burst near the H2S+ gyrofrequency. On passes I24 and I25, waves at both the SO+ and SO2+ gyrofrequencies were present. On the I27 pass, additional components were also observed, close to mass 34, 35, and 37 amu, as would be expected if H2S+, 35Cl+, and 37Cl+ ions were present. These disparate observations on geometrically similar passes suggest that the atmosphere of Io is quite variable, possibly changing with solar zenith angle and perhaps changing in concert with the relative activity of different volcanic sources.
Electron‐ and photon‐stimulated desorption of K from ice surfaces
Authors: Yakshinskiy B. V. , Madey T. E.
Published: February 2004

Motivated by controversy concerning the origin of alkali metal vapor in the atmosphere of the Jovian satellite Europa, we present laboratory studies of the electron‐stimulated desorption (ESD) and photon‐stimulated desorption (PSD) of atomic K deposited on the surface of thin water ice films. These ice films of both crystalline and amorphous modifications simulate the surface of icy satellites. A pulsed low‐energy electron gun and a mercury arc lamp serve as sources of electron and UV irradiation. X‐ray photoelectron spectroscopy (XPS) is used for surface chemical state control. Both ESD and PSD of potassium atoms from ice exhibit appearance thresholds at ∼4 eV, which is more pronounced for the case of crystalline ice than for amorphous ice. The velocity distribution of desorbed K atoms, measured at 100 K, is peaked at ∼500 m/s (by comparison, the peak for Na is at ∼800 m/s), with the high‐energy portion extended up to ∼3000 m/s. The mechanism of desorption is identified as an electronically excited charge transfer from ice to alkali ion, followed by desorption. We conclude that along with magnetospheric energetic ion sputtering processes, UV solar photons and electron fluxes with E > 4 eV may cause alkali metal desorption from Europa's surface.
Tempe Fossae, Mars: A planetary analogon to a terrestrial continental rift?
Authors: Hauber Ernst , Kronberg Peter
Published: February 2004

Tempe Terra, the northeastern part of the Tharsis Region on Mars, is characterized by several extensional structures differing in style and age. The Tempe Fossae, a particularly well‐developed system of graben striking N45°E, have been studied for the first time in detail on the basis of Viking Orbiter imagery and Mars Observer Laser Altimeter (MOLA) topographic data. The graben system appears to be unique in the whole region because of its extent, the remarkable width and depth of the graben, the varying graben pattern, and the associated volcanism. Single graben can be up to 35 km wide and 120 km long, while their depth can reach nearly 3000 m. They typically show an asymmetric architecture, often in halfgraben or step fault style. The graben system widens and changes its style along strike from NE to SW from a single, very deep, and narrow graben to a complex set of several shallower, sinuous graben and halfgraben. Crustal extension was measured in the northeastern part from the observable throw and amounts to 2.5–3.1 km. Volcanic structures can be found at several locations along the graben. In the SW the graben system seems to be connected with the Tempe Terra volcanic province marked by flood basalts and plains volcanism. Craters were counted on volcanic units unaffected by extensional tectonics. Crater statistics indicate an absolute crater model age for the upper end of the extensional deformation of ∼3.5 Ga. The dimensions and the characteristics of the mapped graben system resemble those of terrestrial continental rifts, and a comparison between them and the Kenya Rift reveals striking similarities. Therefore the Tempe Fossae are interpreted as a Martian analog to a continental rift associated with an underlying mantle plume. This hypothesis seems to be supported by recent geophysical models based on topography and gravity data from Mars Global Surveyor which indicate regional uplift for Tempe Terra.
Radio science observations with Mars Global Surveyor: Orbit insertion through one Mars year in mapping orbit
Authors: Tyler G. Leonard , Balmino Georges , Hinson David P. , Sjogren William L. , Smith David E. , Simpson Richard A. , Asmar Sami W. , Priest Patricia , Twicken Joseph D.
Published: February 2004

Mars Global Surveyor (MGS) radio science comprises studies of the atmosphere and gravity of the planet. Perturbations of the 3.6‐cm λ radio path by the atmosphere during periods of atmospheric occultation provide the vertical temperature‐pressure structureT[p(r)] to accuracies at the surface of ΔT≈ 0.4 K and Δp≈ 2 Pa, and ∼10 K and ∼0.6 Pa at altitudes of 40–50 km; the error in radius is Δr≈ 1 m at all levels. Accurate knowledge of the radius permits fixing of theT‐pstructure to the geopotential and use of the gradient wind equation to calculate components of the wind. Systematic sampling of the atmosphere in combination with the accuracy of the MGS radio system supports studies of the large‐scale dynamics of the atmosphere, including seasonal variations of the atmospheric fields and embedded waves such as Kelvin and Rossby waves. Terminator region ionospheric electron density profiles are obtained successfully much of the time but on occasion are undetectable with the MGS system. Two‐way radio tracking of MGS with uncertainties in the line‐of‐sight velocity of several to tens of μm s−1and less supports solution for spherical harmonic models of the gravity field of order and degree in the range of 75×75, although the degree and order of meaningful terms is limited by the ∼400 km spacecraft altitude to ∼62×62, corresponding to a resolution of a few degrees of arc on the surface. This resolution of gravity is sufficient to support geophysical studies of the planet's interior structure and history. Additional radio science investigations include the search for gravitational radiation and observation of very low grazing angle forward scattering by the surface of Mars.
Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 2. Application to Martian surface spectra from the Mars Global Surveyor Thermal Emission Spectrometer
Authors: Hamilton Victoria E. , Wyatt Michael B. , McSween Harry Y. , Christensen Philip R.
Published: February 2004

Atmospherically corrected thermal infrared spectra of large regions of the Martian surface from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES) previously have been interpreted to represent two general spectral classes. One class represents a basalt to basaltic andesite composition, and the other class represents a basaltic andesite to andesite composition. We have performed new linear deconvolutions of the two Martian surface type spectra with an end‐member set tailored to represent volcanic rock types. Our preparatory study of laboratory spectra of terrestrial volcanic rocks (acquired at 2 cm−1 sampling), convolved to TES spectral sampling (10 cm−1), shows little degradation in deconvolution results when compared to results acquired using the higher spectral resolution data, indicating that the deconvolution technique is valid for analyzing data at TES resolution. Our spectral fits to the Martian data agree well with previous models and do not exhibit any notable deviations from the Martian spectra that would indicate the absence of any significant end‐members in our model. Modal mineralogies obtained with these new spectral fits also compare favorably (within the previously stated uncertainties) to prior results. The newly derived modal mineralogies are used with new and traditional classification schemes for volcanic igneous rocks (introduced in a companion paper [Wyatt et al., this issue]) to classify the Martian compositions. Our results substantiate the previously proposed hypothesis that these two spectral classes on the Martian surface represent volcanic compositions with distinguishable differences in silica content ranging from basalt to andesite.
Thermal signature, eruption style, and eruption evolution at Pele and Pillan on Io
Authors: Davies Ashley Gerard , Keszthelyi Laszlo P. , Williams David A. , Phillips Cynthia B. , McEwen Alfred S. , Lopes Rosaly M. C. , Smythe William D. , Kamp Lucas W. , Soderblom Laurence A. , Carlson Robert W.
Published: February 2004

The Galileo spacecraft has been periodically monitoring volcanic activity on Io since June 1996, making it possible to chart the evolution of individual eruptions. We present results of coanalysis of Near‐Infrared Mapping Spectrometer (NIMS) and solid‐state imaging (SSI) data of eruptions at Pele and Pillan, especially from a particularly illuminating data set consisting of mutually constraining, near‐simultaneous NIMS and SSI observations obtained during orbit C9 in June 1997. The observed thermal signature from each hot spot, and the way in which the thermal signature changes with time, tightly constrains the possible styles of eruption. Pele and Pillan have very different eruption styles. From September 1996 through May 1999, Pele demonstrates an almost constant total thermal output, with thermal emission spectra indicative of a long‐lived, active lava lake. The NIMS Pillan data exhibit the thermal signature of a “Pillanian” eruption style, a large, vigorous eruption with associated open channel, or sheet flows, producing an extensive flow field by orbit C10 in September 1997. The high mass eruption rate, high liquidus temperature (at least 1870 K) eruption at Pillan is the best candidate so far for an active ultramafic (magnesium‐rich, “komatiitic”) flow on Io, a style of eruption never before witnessed. The thermal output per unit area from Pillan is, however, consistent with the emplacement of large, open‐channel flows. Magma temperature at Pele is ≥1600 K. If the magma temperature is 1600 K, it suggests a komatiitic‐basalt composition. The power output from Pele is indicative of a magma volumetric eruption rate of ∼250 to 340 m3 s−1. Although the Pele lava lake is considerably larger than its terrestrial counterparts, the power and mass fluxes per unit area are similar to active terrestrial lava lakes.
Effect of porosity on the adsorption, desorption, trapping, and release of volatile gases by amorphous solid water
Authors: Ayotte Patrick , Smith R. Scott , Stevenson K. P. , Dohnálek Z. , Kimmel Greg A. , Kay Bruce D.
Published: February 2004

We compare the adsorption, desorption, trapping, and release of Ar, N2, O2, CO, and CH4 by dense (nonporous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques are used to control the porosity of the vapor‐deposited ASW thin films. Experiments where the gas species is deposited on top of and underneath dense and porous ASW are conducted. For the film thickness used in this study, the porous films are found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas is also dependent on the porosity of the ASW film. Differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results are largely independent of the gas studied, confirming that the adsorption and trapping of gases are dominated by the ASW porosity. These findings show that laboratory studies must account for the growth conditions and their effects on ASW morphology in order to accurately predict the properties of astrophysical ices.
Presence and detection of carbonates on the Martian surface
Authors: Fonti S. , Jurewicz A. , Blanco A. , Blecka M. I. , Orofino V.
Published: February 2004

The messages we receive from Mars about the presence of carbonates are quite contradictory. On the one hand, images of the planet clearly show the signatures of past bodies of standing water, where the accumulation of sedimentary deposits should have occurred. On the other hand, the apparent absence of carbonates in the homogeneous fines covering the Martian surface raises the question: where could carbonates hide now? In this paper we discuss the subject and demonstrate that a proposed destruction mechanism, the photodecomposition caused by UV radiation, cannot account alone for the “missing” carbonates. By means of simulations we show that the difficulties in detecting the carbonates can be ascribed to the low temperatures of the emitting zones, observed up to now. Finally, we derive the minimum amount of such minerals that could possibly be detected in future observations of Mars, and we compare the results of simulations with observed spectra.
Io's gravity field and interior structure
Authors: Anderson John D. , Jacobson Robert A. , Lau Eunice L. , Moore William B. , Schubert Gerald
Published: February 2004

Radio Doppler data generated by the Deep Space Network (DSN) from four encounters of the Galileo spacecraft with Io, Jupiter's innermost Galilean satellite, are used to infer Io's gravitational quadrupole moments. By combining the four flybys into a single solution for the gravity field, the response of Io to the second degree tidal and rotational potentials is accurately determined. This is characterized by the value of the second degree potential Love number k2 = 1.2924 ± 0.0027. We construct interior models that satisfy constraints imposed by the mean radius R = 1821.6 ± 0.5 km, the mean density
, and the normalized axial moment of inertia C/MR2 = 0.37685 ± 0.00035. The gravitationally derived figure of Io has principal axes (c < b < a) a = 1830.0 ± 0.5 km, b = 1819.2 ± 0.5 km, and c = 1815.6 ± 0.5 km, consistent with the shape determined by imaging. Gravitational and other data strongly suggest that Io is in hydrostatic equilibrium. In this case, models of Io's interior density show that Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe‐FeS core or between 350 and 650 km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the gravitational data.
Mapping of crustal magnetic anomalies on the lunar near side by the Lunar Prospector electron reflectometer
Authors: Halekas J. S. , Mitchell D. L. , Lin R. P. , Frey S. , Hood L. L. , Acuña M. H. , Binder A. B.
Published: February 2004

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley‐type light plains, terra materials, and pre‐Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin‐forming impacts were involved in their formation. The magnetization of pre‐Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small‐scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.
Ground‐based observations of volcanism on Io in 1999 and early 2000
Authors: Howell Robert R. , Spencer John R. , Goguen Jay D. , Marchis Franck , Prangé Renée , Fusco Thierry , Blaney Diana L. , Veeder Glenn J. , Rathbun Julie A. , Orton Glenn S. , Grocholski Aaron J. , Stansberry John A. , Kanner Gary S. , Hege E. Keith
Published: February 2004

Ground‐based observations of volcanism on Io during the period of the 1999 and early 2000 Galileo close flybys have detected several types of activity, providing information which complements the spacecraft observations. At Loki a brightening began between August 25 and September 9 and continued through February. On August 2 a major outburst was observed near (14°N, 74°W) whose brightness corresponds to an area of approximately 350 km2 at a temperature of 1100 K. Observations of eruptions in late June (9906A) and in late November (9911A, at Tvashtar) provide temporal and photometric constraints on activity also seen by Galileo. High‐resolution adaptive optics images provide further information on the fainter sources distributed across the surface.
An experimental estimate of Europa's “ocean” composition independent of Galileo orbital remote sensing
Authors: Fanale F. P. , Li Y.‐H. , De Carlo E. , Farley C. , Sharma S. K. , Horton K. , Granahan J. C.
Published: February 2004

We have conducted a series of experiments designed to simulate, in the laboratory, the development of any subsurface aqueous phase on Europa. In our theoretical‐experimental approach we select a single natural sample (a CM meteorite) that based on cosmochemical considerations, we consider to approximately represent the bulk material that accreted to form Europa. We then subject the sample to a hot water leaching procedure designed to simulate low‐ to moderate‐temperature aqueous alteration. The resulting leach solution was then subjected to a series of sequential fractional crystallization steps producing a series of ices and residual brines. Then all this brines and ices are multiply analyzed for Na, Ca, Mg, Sr, Ba, Fe, Mn, K, Cl, and SO4. Results were found to be remarkably consistent between brines and ices in the same stages of crystallization and also between stages. We found that any putative aqueous phase below Europa's ice crust is probably a brine with cations: Na ∼ Mg > Ca, K > Fe and anions: SO4 ≫ Cl. Our results are in harmony with inferences drawn from one of the two main current interpretations of the orbital spectral data but cannot definitively rule out inferences drawn from the alternative interpretation. This is so because the mineralogy of the top 200 μm may not reflect the chemical composition of bodies of brine below the solid surface owing to extensive alteration caused by magnetospheric bombardment.
Interior of the Moon: The presence of garnet in the primitive deep lunar mantle
Authors: Neal Clive R.
Published: February 2004

New trace element data have been generated by inductively coupled plasma‐mass spectrometry (ICP‐MS) for 65 high‐ and low‐Ti mare basalts from the Apollo 11, 12, 14, 15, and 17 landing sites. These data demonstrate that ratios of the high field strength elements and yttrium are generally chondritic in the mare source regions for the low‐Ti basalts but nonchondritic in the high‐Ti basalts and potassium, rare earth elements, and phosphorus (KREEP); such differences can be explained by addition and subtraction (respectively) of ilmenite. This interpretation is consistent with derivation of the basalts from lunar magma ocean (LMO) cumulates through which late stage ilmenite has sunk and mixed with earlier magma ocean products. Compositional differences between the mare basalts and volcanic glass beads suggest that at least some of the latter were derived from a garnet‐bearing source region. Siderophile and chalcophile element data suggest that the garnet‐bearing source escaped the LMO melting event and thus represents primitive Moon material. The remaining glasses are generally very low Ti (VLT) and appear to have been derived from early LMO cumulates (i.e., olivine and orthopyroxene, similar to the VLT basalts). The remaining glasses were derived from below the LMO cumulates, and as the magmas rose they underwent sequential assimilation (to varying degrees) initially of an ilmenite‐rich cumulate followed by a KREEP component. This study presents geochemical evidence for the existence of garnet in a mare source and demonstrates the likelihood that the LMO was not “whole Moon,” thus preserving primitive lunar material, which has acted as the source region for at least some of the volcanic glasses.
Composition and stability of salts on the surface of Europa and their oceanic origin
Authors: Zolotov Mikhail Y. , Shock Everett L.
Published: February 2004

We present theoretical models of the composition, the relative abundances, and the stability of hydrated salts on the surface and in the icy shell of Jupiter's satellite Europa and discuss whether those salts have an oceanic origin. The evaluations were done with thermodynamic calculations of (1) salt dehydration equilibria at the conditions of the surface of Europa and its icy shell, (2) chemical equilibria involving solids and water vapor in the Na‐K‐Mg‐Ca‐S‐Cl‐H2O system at surface temperatures and variable partial pressures of water vapor, and (3) changes in aquatic chemistry and sequences of salt precipitation from freezing oceanic water, using cosmochemical, mass balance, and physical‐chemical constraints on the elemental and ionic composition of the ocean. Mass balance calculations of total or partial extraction of elements into an ocean from a carbonaceous chondrite type mantle show that magnesium and sulfate rather than chloride and sodium could be the most abundant solutes in the ocean. Freezing oceanic water of this composition leads to brines that differ in composition from the original water and to deposition of ice and highly hydrated sulfates of Mg, Na, and Ca as well as alkali chlorides. After freezing is complete, highly hydrated salts remain stable in ice‐bearing surface materials and throughout the icy shell. For hypothetical surface salt lag deposits, formed through sublimation/sputtering of ice and dehydration of salts, we predict hydration stratification with depth, approaching the highest hydration states in ice‐bearing materials in the lowest parts of the deposits. We discuss the effects of fast disequilibrium freezing and variable dehydration rates of salts on the predicted mineral assemblages at the surface. All of our models, which are independent of observations, predict the predominance of Mg and Na sulfates in surface salts, in agreement with spectroscopic models for the nonicy surface material in the near infrared spectral region.
Rock types of South Pole‐Aitken basin and extent of basaltic volcanism
Authors: Pieters C. M. , Head J. W. , Gaddis L. , Jolliff B. , Duke M.
Published: February 2004

The enormous pre‐Nectarian South Pole‐Aitken (SPA) basin represents a geophysically and compositionally unique region on the Moon. We present and analyze the mineralogical diversity across this basin and discuss the implications for basin evolution. Rock types are derived from Clementine multispectral data based on diagnostic characteristics of ferrous absorptions in fresh materials. Individual areas are characterized as noritic (dominated by low‐Ca pyroxene), gabbroic/basaltic (dominated by high‐Ca pyroxene), feldspathic (<3–6% FeO), and olivine‐gabbro (dominated by high‐Ca pyroxene and olivine). The anorthositic crust has effectively been removed from the interior of the basin. The style of volcanism within the basin extends over several 100 Myr and includes mare basalt and pyroclastic deposits. Several areas of ancient (pre‐Orientale) volcanism, or cryptomaria, have also been identified. The nonmare mafic lithology that occurs across the basin is shown to be noritic in composition and is pervasive laterally and vertically. We interpret this to represent impact melt/breccia deposits derived from the lower crust. A few localized areas are identified within the basin that contain more diverse lithologies (gabbro, olivine‐gabbro), some of which may represent material from the deepest part of the lower crust and perhaps uppermost mantle involved in the SPA event.
Probing Mars' crustal magnetic field and ionosphere with the MGS Electron Reflectometer
Authors: Mitchell D. L. , Lin R. P. , Mazelle C. , Rème H. , Cloutier P. A. , Connerney J. E. P. , Acuña M. H. , Ness N. F.
Published: February 2004

The Electron Reflectometer (ER) on board Mars Global Surveyor measures the energy and angular distributions of solar wind electrons and ionospheric photoelectrons. These data can be used in conjunction with magnetometer data to probe Mars' crustal magnetic field and to study Mars' ionosphere and solar wind interaction. During aerobraking, ionospheric measurements were obtained in the northern hemisphere at high solar zenith angles (SZAs, typically ∼78°). The ionopause was crossed at altitudes ranging from 180 km to over 800 km, with a median of 380 km. The 400‐km‐altitude polar mapping orbit allows observations at SZAs from 25° to 155° in both the northern and southern hemispheres. The near‐planet ionosphere and magnetotail structure of the night hemisphere is dominated by the presence of intense crustal magnetic fields, which can exceed 200 nT at the spacecraft altitude. Closed field lines anchored to highly elongated crustal sources form “magnetic cylinders,” which exclude solar wind plasma traveling up the magnetotail. When the spacecraft passes through one of these structures, the ER count rate falls to the instrumental background, representing an electron flux drop of at least two orders of magnitude. A map of these flux dropouts in longitude and latitude closely resembles a map of the crustal magnetic sources. When the crustal magnetic cylinders rotate into sunlight, they fill with ionospheric plasma. Since many of these crustal fields are locally strong enough to stand off the solar wind to altitudes well above 400 km, the ionosphere can extend much higher than would otherwise be possible in the absence of crustal fields. Even weak crustal fields may locally bias the median ionopause altitude, which provides an indirect method of detecting crustal fields using ER observations.
Io in the near infrared: Near‐Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000
Authors: Lopes Rosaly M. C. , Kamp L. W. , Douté S. , Smythe W. D. , Carlson R. W. , McEwen A. S. , Geissler P. E. , Kieffer S. W. , Leader F. E. , Davies A. G. , Barbinis E. , Mehlman R. , Segura M. , Shirley J. , Soderblom L. A.
Published: February 2004

Galileo's Near‐Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ∼1 μm. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High‐resolution spectral observations of Emakong caldera show thermal emission and SO2 within the same pixels, implying that patches of SO2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO2 distribution maps show that the highest concentration of SO2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO2. The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.
Ancient drainage basin of the Tharsis region, Mars: Potential source for outflow channel systems and putative oceans or paleolakes
Authors: Dohm J. M. , Ferris J. C. , Baker V. R. , Anderson R. C. , Hare T. M. , Strom R. G. , Barlow N. G. , Tanaka K. L. , Klemaszewski J. E. , Scott D. H.
Published: February 2004

Paleotopographic reconstructions based on a synthesis of published geologic information and high‐resolution topography, including topographic profiles, reveal the potential existence of an enormous drainage basin/aquifer system in the eastern part of the Tharsis region during the Noachian Period. Large topographic highs formed the margin of the gigantic drainage basin. Subsequently, lavas, sediments, and volatiles partly infilled the basin, resulting in an enormous and productive regional aquifer. The stacked sequences of water‐bearing strata were then deformed locally and, in places, exposed by magmatic‐driven uplifts, tectonic deformation, and erosion. This basin model provides a potential source of water necessary to carve the large outflow channel systems of the Tharsis and surrounding regions and to contribute to the formation of putative northern‐plains ocean(s) and/or paleolakes.