bluuebunny/crossref_metadata_2025 huggingface dataset:
Distant lunar sodium tail observed in the Japanese local‐time sector during the Leonid meteor shower of 1998
Authors: Shiokawa K. , Ejiri M. K. , Ogawa T. , Nakamura T.
Published: February 2004

The distant lunar sodium tail was discovered by Smith et al. [1999] from all‐sky observations of sodium emissions made at the McDonald Observatory, Texas (31°N, 256°E), during the Leonid meteor shower of November 1998. The optical emission from the lunar sodium tail was enhanced after the meteor shower and appeared as a bright spot in the all‐sky images. We conducted similar all‐sky sodium observations during the same interval at Shigaraki (35°N. 136°E), Japan, and observed the sodium spot of the lunar tail on November 18–20. The changes in the spot shape during these days are quite similar to those reported by Smith et al. [1999], although the intensities are much weaker. The local time difference between Texas and Shigaraki (∼10 hours) offers a unique opportunity to study the time history of the sodium emission enhancement after the Leonid meteor shower. Combining these two measurements, we conclude that the duration of the most enhanced sodium emission was <1 day, which is consistent with the duration of the meteor shower.
Galilean satellite obliquities
Authors: Bills Bruce G. , Ray Richard D.
Published: February 2004

The obliquities, or angular separations between spin pole and orbit pole, for the Galilean satellites of Jupiter are all small but nonzero. We present calculations of the expected obliquity values based on the well known orbital parameters and recent estimates of the moments of inertia of the satellites, under the assumption that all are in Cassini states. The nonzero obliquities of these bodies are dynamically significant because they imply the existence of components of tidal and librational stress and dissipation which have been previously ignored. In addition, accurate observational determination of the obliquities will provide more direct estimates of the moments of inertia of these bodies than are currently available.
Kilometer‐scale roughness of Mars: Results from MOLA data analysis
Authors: Kreslavsky Mikhail A. , Head James W.
Published: February 2004

The Mars Orbiter Laser Altimeter (MOLA) data are used to characterize the kilometer‐scale surface roughness of Mars. The median absolute value of the differential slope at a given baseline is proposed as a data‐derived measure of the surface roughness at this scale. Study of the scale dependence of roughness for the smoothest terrains gives an independent estimate of 20 cm for MOLA ranging accuracy. The baseline lengths from 0.6 to ∼20 km are used, and kilometer‐scale roughness is mapped for the entire surface. The maps show that different geological units have distinctive roughness characteristics. Scale dependence of roughness for a number of geological units is presented and discussed. The southern polar cap is rougher than the northern at kilometer and subkilometer scale, which suggests differences in the sublimation/condensation balance. The Vastitas Borealis Formation has a distinctive 3‐km‐scale background surface topography, which suggests a nonvolcanic origin for its upper layer. Young volcanic plains in Amazonis Planitia and the eastern part of Elysium Planitia are very similar to each other in their roughness characteristics and differ from other volcanic plains on Mars, which suggests a distinctive eruption style. There are systematic latitudinal variations of roughness in both the southern highlands and the northern lowlands: terrains at high latitude are smoother at short baselines; the characteristic vertical scale related to this difference is several meters. Processes that could be responsible for formation of this trend include creep of ice‐rich near‐surface material at high latitudes, treatment of the surface with repetitive deposition and sublimation of seasonal frost, climate‐controlled deposition and/or cementation of dust at high latitudes, and repetitive sublimation and accumulation of subsurface ice at low latitudes with climate variations. All mechanisms of origin could operate more effectively under different climate conditions. Relevant morphological observations favor mechanisms involving deposition of smooth blankets at high latitudes.
Shock pressure attenuation in water ice at a pressure below 1 GPa
Authors: Kato Manabu , Higa Michiya , Shirai Kei , Iijima Yu‐ichi , Kiyono Tatsuya , Nakazawa Satoru , Arakawa Masahiko
Published: February 2004

Shock pressure attenuation in water ice was studied at an impact pressure below 1 GPa and a temperature of 255 K. The observed shock wave showed a multiple shock wave structure: A precursor wave was followed by a main wave, which had a longer rise time and higher amplitude. The Hugoniot elastic limit (HEL) of water ice was measured to be in the range from 0.1 to 0.3 GPa when associated with precursor waves traveling at 3.86 km/s. The peak amplitude of the main wave Pm was observed to decrease with its propagation x from 3 to 60 mm (from 0.4 to 8 times as large as a projectile radius) in two series of experiments in which initial shock pressures Pi at the impact point were 0.60 and 0.87 GPa. The Pm was described as the power law relation Pm/Pi = (x/2.6 mm)−89. The precursor wave disappears as the Pm attenuated to a pressure <0.1 GPa. The measured wave profiles were used to calculate the loading path of water ice in shock compression between the HEL and 0.6 GPa. The loading path obtained by Lagrangian analysis was closely consistent with previous Hugoniot data regarding water ice.
A model of Europa's crustal structure: Recent Galileo results and implications for an ocean
Authors: Spaun Nicole A. , Head James W.
Published: February 2004

Recent results from the Galileo Near‐Infrared Mapping Spectrometer (NIMS) experiment suggest that hydrated magnesium sulfate salts may be present on the surface of Europa. We use this interpretation and a model for the chemical budget of Europa's outer water‐rich layer to determine (1) how Europa's profile atop the mantle may have evolved by cooling and fractional crystallization from an initial liquid state and (2) the potential effects of this evolution on the surface geology. This model leads to predictions that the lower part is composed of layers of heavily hydrated magnesium sulfates, which effectively isolate any ocean from direct physical contact with the silicate mantle. These lower layers are overlain by an ocean which has an icy solid upper layer present at the surface; this ocean may have persisted through time. Eutectic conditions are predicted to lead to thickening of the overlying ice shell and freezing out of the ocean, although tidal heating may maintain the liquid layer. Mechanisms such as solid‐state convection can further serve to thicken, thin, and/or mix the ice shell. Thermal diapirism could be induced by convection, and compositional diapirism could be driven by density instabilities within the layers. In this model, early global compression would have given way to later global expansion.
A Monte Carlo ray‐tracing model for scattering and polarization by large particles with complex shapes
Authors: Grundy W. M. , Douté S. , Schmitt B.
Published: February 2004

We present a new model based on Monte Carlo ray tracing which simulates scattering and linear polarization by particles with arbitrary shapes and sizes much larger than the wavelength. The model, called S‐Scat, provides a powerful tool for exploring the relationship between actual physical properties of large particles and the single‐particle parameters used in multiple‐scattering radiative transfer models, and will be particularly valuable for studies of icy outer solar system surfaces. We describe the model algorithm and apply the model to examine absorption and scattering behavior of single, irregular particles as functions of particle size, shape, and optical constants. Single‐scattering albedos are investigated first, with results used to test the validity of the widely used equivalent slab model. Single‐scattering phase functions are examined next, along with possibilities of parameterization via simple analytic expressions. Finally, the behaviors of linear polarization functions are explored, along with internal path lengths and the spatial distribution of scattered light in close proximity to the particle.
Theoretical modeling of eruption plumes on Mars under current and past climates
Authors: Hort M. , Weitz C. M.
Published: February 2004

We have used a combined conduit transport/eruption column model to explore the evolution of volcanic eruption plumes on Mars under different atmospheric conditions. In the calculations we consider a volatile phase composed of H2O, CO2, and SO2 and take into account that the magmatic water erupted at the vent may condense as the eruption column rises into the Martian atmosphere. As two end‐member models, we explore the eruption of rhyolitic and basaltic melt compositions containing different amounts of volatiles as well as having different eruption temperatures. Under current Martian atmospheric conditions eruption plumes are found to rise as high as 100 km for a mass eruption rate of 5×107 kg s−1, which is consistent with model calculations by Wilson and Head [1994]. In contrast, under a dense atmosphere (105 Pa on the Martian surface) which may have existed earlier in Martian history, the same eruption plume reaches only about 25 km height. All magmatic water released during the eruption is found to freeze as it rises in the eruption column, which means that fallout from the plume will contain water ice which can be subsequently deposited in near surface layers. This ice may then suddenly melt due to higher surface heat flow or shallow intrusions leading to rapid release of water on the flanks of volcanoes. Only if the atmosphere were hotter in the past could the water in the eruption plume condense and produce rain rather than ice. Furthermore, the calculations show that smaller micron‐size particles would be distributed globally from eruption plumes under current Martian conditions but would not have been as widely dispersed from plumes erupted into an earlier dense atmosphere.
Mass movement slope streaks imaged by the Mars Orbiter Camera
Authors: Sullivan Robert , Thomas Peter , Veverka Joseph , Malin Michael , Edgett Kenneth S.
Published: February 2004

Narrow, fan‐shaped dark streaks on steep Martian slopes were originally observed in Viking Orbiter images, but a definitive explanation was not possible because of resolution limitations. Pictures acquired by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft show innumerable examples of dark slope streaks distributed widely, but not uniformly, across the brighter equatorial regions, as well as individual details of these features that were not visible in Viking Orbiter data. Dark slope streaks (as well as much rarer bright slope streaks) represent one of the most widespread and easily recognized styles of mass movement currently affecting the Martian surface. New dark streaks have formed since Viking and even during the MGS mission, confirming earlier suppositions that higher contrast dark streaks are younger, and fade (brighten) with time. The darkest slope streaks represent ∼10% contrast with surrounding slope materials. No small outcrops supplying dark material (or bright material, for bright streaks) have been found at streak apexes. Digitate downslope ends indicate slope streak formation involves a ground‐hugging flow subject to deflection by minor topographic obstacles. The model we favor explains most dark slope streaks as scars from dust avalanches following oversteepening of air fall deposits. This process is analogous to terrestrial avalanches of oversteepened dry, loose snow which produce shallow avalanche scars with similar morphologies. Low angles of internal friction typically 10–30¡for terrestrial loess and clay materials suggest that mass movement of (low‐cohesion) Martian dusty air fall is possible on a wide range of gradients. Martian gravity, presumed low density of the air fall deposits, and thin (unresolved by MOC) failed layer depths imply extremely low cohesive strength at time of failure, consistent with expectations for an air fall deposit of dust particles. As speed increases during a dust avalanche, a growing fraction of the avalanching dust particles acquires sufficient kinetic energy to be lost to the atmosphere in suspension, limiting the momentum of the descending avalanche front. The equilibrium speed, where rate of mass lost to the atmosphere is balanced by mass continually entrained as the avalanche front descends, decreases with decreasing gradient. This mechanism explains observations from MOC images indicating slope streaks formed with little reserve kinetic energy for run‐outs on to valley floors and explains why large distal deposits of displaced material are not found at downslope streak ends. The mass movement process of dark (and bright) slope streak formation through dust avalanches involves renewable sources of dust only, leaving underlying slope materials unaffected. Areas where dark and bright slope streaks currently form and fade in cycles are closely correlated with low thermal inertia and probably represent regions where dust currently is accumulating, not just residing.
Radiation chemical alterations in solar system ices: An overview
Authors: Hudson R. L. , Moore M. H.
Published: February 2004

Many solar system objects, such as planets, satellites, rings, and comets, are known to either be made of ices or to have icy surfaces. Because these ices are subjected to radiation processing by keV‐MeV ions and UV photons, their chemical and physical properties evolve over time. We have employed a laboratory approach to investigate these radiolytic and photolytic changes. Ices, either pure or mixtures, are subjected to ionizing radiation and UV photons and then probed by IR spectroscopy. It has been found that radiolysis and photolysis destroy reactant molecules, synthesize new molecules, cause changes of phase in pure materials, and eject molecules from ices. Although our laboratory approach initially focused on product identification, it has become increasingly necessary to work toward a comprehensive understanding of ice chemistry. To this end, we describe recent radiation experiments on some of the most important solar system and interstellar ice molecules, such as H2O, CO, CO2, and hydrocarbons. Chemical reactions are given to account for our observations, and these reactions include such general types as acid‐base, electron‐transfer, radical‐radical combination, and H‐atom addition. Comparisons of ion and UV processing are described for pure H2O and for two binary mixtures, H2O + CO2 and NH3 + CO. Predictions of molecular evolution in complex ices, both H2O‐ and N2‐dominated, are made. Applications are described for cometary organic chemistry, ion processing in interstellar ices, and the recent discovery of H2O2 on the surface of Europa.
Structure, spectra, and mobility of low‐pressure ices: Ice I, amorphous solid water, and clathrate hydrates at T < 150 K
Authors: Devlin J. Paul
Published: February 2004

Recent advances in the study of low‐pressure water ices, including clathrate hydrates, are examined, highlighting aspects of modern science possibly related to the behavior of water ices in extraterrestrial environments. An effort has been made to identify properties of ice that are likely to be important in the conditions that exist in such environments and to review advances in understanding these properties. The basic science of crystalline ice I is relatively mature, but attention is given to concepts, such as point defects, which continue to evolve and which relate in an important manner to the low‐temperature behavior of ice I as well as of amorphous ice and the clathrate hydrates. A concept of amorphous and microporous amorphous ice, developed in the early 1980s, is presented, but more recent characterizations are emphasized. Similarly, fundamental properties of clathrate hydrates are summarized and used in discussions of more recently appreciated characteristics that may be important in space science. Particular emphasis is placed on the formation, infrared spectroscopy, and behavior of clathrate hydrates in vacuo at temperatures below 150 K. The review closes with a section on the properties of ice nanoparticles, which have only recently emerged as a subject of molecular science. Modern experimental and theoretical studies of ice particles have focused on the infrared spectra and structure of the ice surface, and the interaction of the surface with molecular adsorbates. Since, with the exception of high‐energy radiation, ice normally interacts with its environment through surface processes, such research may enlighten future studies.
Infrared spectra of N2‐rich ice mixtures
Authors: Satorre M. A. , Palumbo M. E. , Strazzulla G.
Published: February 2004

We present experimental results obtained by IR spectroscopy in the 1.75–25 μm (5700–400 cm−1) range of ice mixtures containing nitrogen, water, and methane. Some of the studied mixtures have also been irradiated with energetic ions and their spectra collected at different temperatures. Irradiation produces molecules not present in the original mixture and leaves over a complex refractory residue whose color, because of progressive carbonization, becomes darker and darker as the irradiation dose increases. The results are discussed with a view of their relevance to the chemistry of the surfaces of Pluto and Triton. It is suggested that isolated water molecules should be searched for; many not yet detected molecular species (in particular some containing CN groups) should be present. A hypothesis to explain the observation of CO2 on Triton (but not on Pluto) is also proposed.
Assimilation of Mars Global Surveyor atmospheric temperature data into a general circulation model
Authors: Zhang Kate Q. , Ingersoll Andrew P. , Kass David M. , Pearl John C. , Smith Michael D. , Conrath Barney J. , Haberle Robert M.
Published: February 2004

We examined the observed temperature data from Thermal Emission Spectrometer (TES) between heliocentric longitude Ls = 141° and 146° (∼10 Martian days in northern summer) during the mapping phase, then compared them with the simulated results using the NASA/Ames Mars general circulation model. Both show a strong polar vortex at the winter pole, higher equatorial temperatures near the ground and larger tropospheric lapse rates during daytime than at night. However, the simulation is colder than the observation at the bottom and top of the atmosphere and warmer in the middle. The highest wave activities are found in the polar front in both the simulations and the observations, but it is at a much higher altitude in the former. Experiments show that larger dust opacity improves the temperature field in the lower atmospheric levels. Using a steady state Kalman filter, we attempted to obtain a model state that is consistent with the observations. The assimilation did achieve better agreement with the observations overall, especially over the north pole. However, it is hard to make any further improvement. Dust opacity is the key factor in determining the temperature field; correcting temperature alone improves the spatial and temporal variations, it degrades the mean state in the south pole. Assimilation cannot improve the simulation further, unless more realistic dust opacity and its vertical profile are considered.
Physical state of ices in the outer solar system
Authors: Roush T. L.
Published: February 2004

Comparison of the identity and abundances of ices observed around protostars and those associated with comets clearly suggests that comets preserve the heritage of the interstellar materials that aggregrated to form them. However, the ability to identify these same species on icy satellites in the outer solar system is a complex function of the composition of the original ices, their subsequent thermal histories, and their exposure to various radiation environments. Our ability to identify the ices currently present on objects in the outer solar system relies upon observational and laboratory, and theoretical efforts. To date there is ample observational evidence for crystalline water ice throughout the outer solar system. In addition, there is growing evidence that amorphous ice may be present on some bodies. More volatile ices, e.g., N2, CH4, and CO, and other species, e.g., ammonia hydrate, are identified on objects lying at and beyond Uranus. Both photolysis and radiolysis play important roles in altering the original surfaces due to chemical reactions and erosion of the surface. Ultraviolet photolysis appears to dominate alteration of the upper few hundred angstroms, although sputtering of the surface can sometimes be a significantly competitive process, dominating on icy surfaces embedded in a strong planetary magnetospheric field. There is growing observational evidence that the by‐products of photolysis and radiolysis, suggested on a theoretical basis, are present on icy surfaces.
Galileo imaging of SO2 frosts on Io
Authors: Geissler P. , McEwen A. , Phillips C. , Simonelli D. , Lopes R. M. C. , Douté S.
Published: February 2004

Io's visible appearance changes dramatically with solar phase angle. The polar regions and some plume deposits near active volcanic centers become comparatively bright with increasing phase angle, while the equatorial band grows relatively dark. We suggest that the areas of Io that appear unusually bright at high phase are covered by thin frosts of SO2 that are transparent under normal illumination. A global disk‐resolved photometric analysis indicates that the frosts exhibit more nearly isotropic or forwardscattering behavior and less opposition brightening than average Ionian materials. Comparison with Near‐Infrared Mapping Spectrometer (NIMS) results suggests that these frosts have relatively strong 4.1 μm absorptions indicative of fine‐grained SO2.
Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data
Authors: Hood L. L. , Zakharian A. , Halekas J. , Mitchell D. L. , Lin R. P. , Acuña M. H. , Binder A. B.
Published: February 2004

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian‐aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin‐forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl‐like albedo markings of the Reiner Gamma class visible on available orbital photography.
Landform degradation and slope processes on Io: The Galileo view
Authors: Moore Jeffrey M. , Sullivan Robert J. , Chuang Frank C. , Head James W. , McEwen Alfred S. , Milazzo Moses P. , Nixon Brian E. , Pappalardo Robert T. , Schenk Paul M. , Turtle Elizabeth P.
Published: February 2004

The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on Io. We recognize four major slope types observed on a number of intermediate resolution (∼250 m pixel−1) images and several additional textures on very high resolution (∼10 m pixel−1) images. Slopes and scarps on Io often show evidence of erosion, seen in the simplest form as alcove‐carving slumps and slides at all scales. Many of the mass movement deposits on Io are probably mostly the consequence of block release and brittle slope failure. Sputtering plays no significant role. Sapping as envisioned by McCauley et al. [1979] remains viable. We speculate that alcove‐lined canyons seen in one observation and lobed deposits seen along the bases of scarps in several locations may reflect the plastic deformation and “glacial” flow of interstitial volatiles (e.g., SO2) heated by locally high geothermal energy to mobilize the volatile. The appearance of some slopes and near‐slope surface textures seen in very high resolution images is consistent with erosion from sublimation‐degradation. However, a suitable volatile (e.g., H2S) that can sublimate fast enough to alter Io's youthful surface has not been identified. Disaggregation from chemical decomposition of solid S2O and other polysulfur oxides may conceivably operate on Io. This mechanism could degrade landforms in a manner that resembles degradation from sublimation, and at a rate that can compete with resurfacing.
Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission
Authors: Keszthelyi L. , McEwen A. S. , Phillips C. B. , Milazzo M. , Geissler P. , Turtle E. P. , Radebaugh J. , Williams D. A. , Simonelli D. P. , Breneman H. H. , Klaasen K. P. , Levanas G. , Denk T.
Published: February 2004

The Solid‐State Imaging (SSI) instrument provided the first high‐ and medium‐resolution views of Io as the Galileo spacecraft closed in on the volcanic body in late 1999 and early 2000. While each volcanic center has many unique features, the majority can be placed into one of two broad categories. The “Promethean” eruptions, typified by the volcanic center Prometheus, are characterized by long‐lived steady eruptions producing a compound flow field emplaced in an insulating manner over a period of years to decades. In contrast, “Pillanian” eruptions are characterized by large pyroclastic deposits and short‐lived but high effusion rate eruptions from fissures feeding open‐channel or open‐sheet flows. Both types of eruptions commonly have ∼100‐km‐tall, bright, SO2‐rich plumes forming near the flow fronts and smaller deposits of red material that mark the vent for the silicate lavas.
Characteristics of the Pathfinder APXS sites: Implications for the composition of Martian rocks and soils
Authors: Bridges Nathan T. , Crisp Joy A. , Bell James F.
Published: February 2004

An integrated study of spectral, elemental abundance, and image data from the Pathfinder alpha proton X‐ray spectrometer (APXS) measurement sites has led to a better understanding of the nature of Martian surface materials. This rigorous analysis provides a new level of detail that forms the basis for the results reported here and that can be used by future scientists trying to understand rocks and soils on Mars. Each APXS site has been precisely located by analyzing stereo Imager for Mars Pathfinder (IMP) and rover camera images to determine the position of the APXS during deployment. Rover and IMP images are used to assess local geology, namely, the presence of pebbles, clods, and aeolian features in soils and surface textures and dust concentrations on rocks. IMP spectra using the latest calibrations for radiance and reflectance are analyzed at a number of different illumination geometries for each APXS site. The three‐dimensional orientation of the sites has been determined and then used to compute the true sunlight (incidence) and viewing (emission) angles. Where suitable photometric coverage is available, reflectances are fit to the Hapke‐Irvine function. This is then used to adjust spectral reflectances to a normalized illumination geometry common to all rocks and soils. Comparing all the data sets shows a positive correlation among red/blue reflectance ratio, SO3 content, and dust abundance on rocks, indicating that rocks are coated with varying amounts of sulfur‐rich, red dust. Other elements, such as silicon and iron, are poor tracers for dust because their abundances within soil are similar to those within rock. Rock APXS targets that are bluer, poorer in sulfur, and have minimal dust coverage face toward the northeast, the direction in which winds may be capable of efficiently removing loose material under current Martian conditions. The IMP spectral properties of the soils show poor correlations with APXS elemental abundances. On the basis of currently released APXS data, bulk soils at the Pathfinder landing site are richer in silica and sulfur than the Viking landing sites, suggesting a different composition. The APXS soils with the most apparent pebbles are the poorest in SiO2, indicating that either the pebbles are more mafic than the APXS rocks or the pebble‐free soil component is inherently enriched in SiO2 compared to Viking soils. The mixture of materials at the APXS sites is a reflection of the overall complexity of Martian surface materials, a detailed understanding of which should drive the choice of future instruments and missions.
Evidence for space weather at Mercury
Authors: Killen R. M. , Potter A. E. , Reiff P. , Sarantos M. , Jackson B. V. , Hick P. , Giles B.
Published: February 2004

Mercury's sodium atmosphere is known to be highly variable both temporally and spatially. During a week‐long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind‐magnetosphere interactions. We assume that photon‐stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercury's magnetic moments are required to confirm the results.
Lunar Mare Soils: Space weathering and the major effects of surface‐correlated nanophase Fe
Authors: Taylor Lawrence A. , Pieters Carlé M. , Keller Lindsay P. , Morris Richard V. , McKay David S.
Published: February 2004

Lunar soils form the “ground truth” for calibration and modeling of reflectance spectra for quantitative remote sensing. The Lunar Soil Characterization Consortium, a group of lunar sample and remote sensing scientists, has undertaken the extensive task of characterization of lunar soils, with respect to their mineralogical and chemical makeup. This endeavor is aimed at deciphering the effects of space weathering of soils from the Moon, and these results should apply to other airless bodies. Modal abundances and chemistries of minerals and glasses in the <45 μm size fractions of nine selected mare soils have been determined, along with the bulk chemistry of each size fraction, and their IS/FeO values. These data can be addressed at http:/web.utk.edu/∼pgi/data.html. As grain size decreases, the bulk composition of each size fraction continuously changes and approaches the composition of the agglutinitic glasses. Past dogma had it that the majority of the nanophase Fe0 resides in the agglutinitic glasses. However, as grain size of a soil decreases, the percentage of the total iron present as nanophase‐sized Fe0 increases dramatically, while the agglutinitic glass content rises only slightly. This is evidence for a large contribution to the IS/FeO values from surface‐correlated nanophase Fe0, particularly in the <10 μm size fraction. This surficial nanophase Fe0 is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils. It is proposed that these vapor‐deposited, nanophase Fe0‐bearing patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.
High‐obliquity simulations for the Archean Earth: Implications for climatic conditions on early Mars
Authors: Jenkins Gregory S.
Published: February 2004

Global climate model simulations using high‐obliquity (70°), higher CO2 and CH4 but reduced O3 concentrations produce warm climate conditions for Archean boundary conditions (reduced solar forcing). Solar radiation under high‐obliquity is distribution in such a way as to warm high latitudes and cool low‐latitudes. Global mean air temperatures of approximately 280°K are found with a 25% reduction in the solar forcing, 4760 ppmv CO2 and 25 ppmv CH4. High‐obliquity could also explain why polar oceans on Mars may have occurred in its history. A substantial increase in the amount of insolation is accounted for if Mars had an obliquity of 70°.
Evidence for water ice near the lunar poles
Authors: Feldman W. C. , Maurice S. , Lawrence D. J. , Little R. C. , Lawson S. L. , Gasnault O. , Wiens R. C. , Barraclough B. L. , Elphic R. C. , Prettyman T. H. , Steinberg J. T. , Binder A. B.
Published: February 2004

Improved versions of Lunar Prospector thermal and epithermal neutron data were studied to help discriminate between potential delivery and retention mechanisms for hydrogen on the Moon. Improved spatial resolution at both poles shows that the largest concentrations of hydrogen overlay regions in permanent shade. In the north these regions consist of a heavily cratered terrain containing many small (less than ∼10‐km diameter), isolated craters. These border circular areas of hydrogen abundance ([H]) that is only modestly enhanced above the average equatorial value but that falls within large, flat‐bottomed, and sunlit polar craters. Near the south pole, [H] is enhanced within several 30‐km‐scale craters that are in permanent shade but is only modestly enhanced within their sunlit neighbors. We show that delivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permanently shaded craters near both poles is sufficiently low that a solar wind origin cannot explain their differences. We conclude that a significant portion of the enhanced hydrogen near both poles is most likely in the form of water molecules.
High‐resolution digital elevation models derived from Viking Orbiter images: Method and comparison with Mars Orbiter Laser Altimeter Data
Authors: Baratoux D. , Delacourt C. , Allemand P.
Published: February 2004

The purpose of this paper is to present a method to derive local and high‐resolution digital elevation models (DEM) of Mars from Viking Orbiter images. We focus on two aspects that appear to be new in photogrammetry applied to Mars: the correlation method and the relative orientation method based on the analysis of the coplaneity. We demonstrate that a DEM from two Viking Orbiter images selected according to the criteria of parallax height ratio, resolution, and illumination conditions can be obtained with a better spatial lateral resolution than the interpolated Mars Orbiter Laser Altimeter (MOLA) data. Two images on the Deuteronilus Mensae area satisfy these criteria. The algorithm of reconstruction of the topography is divided into two steps. First, a correlation method at subpixel level based on Fourier transform has been adapted to extract image location of homologous points. As initial camera positions and angles are known with poor accuracy, an adjustment is then carried out by the method of relative orientation which requires no absolute control points and takes advantage of the subpixel accuracy of our matching algorithm. Finally, the DEM is projected into a Martian reference frame and registered to MOLA data for validation. The spatial resolution depends on the roughness of the area and is locally close to the distance between individual MOLA footprints along one profile (300 m), as demonstrated from the comparison with MOLA profiles. These first results of high‐resolution DEM from Viking Orbiter images should be used with MOLA data for many geological studies that require high‐resolution topographic data.
Introduction to the special section: The Marsokhod Silver Lake field experiment
Authors: Stoker Carol
Published: February 2004

The Mars Surveyor program is planning a series of rover missions to Mars to perform in situ exploration and to collect samples for return to Earth. A successful program will require capable rovers carrying sophisticated payloads with instruments for in situ analysis as well as sample collection, handling, and preservation capability. Numerous critical technological, engineering, and scientific issues need to be addressed to insure a successful program. Rovers must be capable of carrying the required instrumentation and must travel long distances while surviving lengthy exposure to the extreme Martian environment.
Persistent suborbital climate variability in marine isotope stage 5 and termination II
Authors: Oppo Delia W. , Keigwin Lloyd D. , McManus Jerry F. , Cullen James L.
Published: February 2004

New surface water records from two high sedimentation rate sites, located in the western subtropical North Atlantic near the axis of the Gulf Stream, provide clear evidence of suborbital climate variations through marine isotope stage (MIS) 5 persisting even into the warm peak of the interglaciation (substage 5e). We found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of MIS 5, implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region. Although some records suggest that longer suborbital variations (4–10 kyr) during MIS 5 are linked to deepwater changes, none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr. However, when examined in conjunction with published data from the Norwegian Sea, new evidence from the subpolar North Atlantic suggests that coupled surface‐deepwater oscillations occurred during the penultimate deglaciation. This supports the hypothesis that during glacial and deglacial times, ocean‐ice interactions and deepwater variability amplify suborbital climate change at higher latitudes. We suggest that during the penultimate deglaciation the North Atlantic deepwater source varied between Nordic Sea and open North Atlantic locations, in parallel with surface temperature oscillations.
Antarctic sea ice and the control of Pleistocene climate instability
Authors: Keeling Ralph F. , Stephens Britton B.
Published: February 2004

A hypothesis is presented for the origin of Pleistocene climate instability, based on expansion of Antarctic sea ice and associated changes in the oceans' salinity structure. The hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of Toggweiler and Samuels [1993b], in which deepwater upwelling is restricted to high southern latitudes. The reconfigured conveyor is shown to be potentially stabilized in an “on” mode by precipitation at high southern latitudes and potentially destabilized into “on” and “off” modes by the counteracting influence of Antarctic sea ice. The mechanism is clarified by the use of a hydraulic analogue. We hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between Pleistocene warm and cold periods. The hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods, including aspects of interhemispheric timing.
North Pacific response to millennial‐scale changes in ocean circulation over the last 60 kyr
Authors: Kiefer T. , Sarnthein M. , Erlenkeuser H. , Grootes P. M. , Roberts A. P.
Published: February 2004

Foraminifera‐based sea surface temperature estimates in the northwest Pacific (Ocean Drilling Program Site 883; 51°N) varied by 4°C on millennial timescales over the last 60,000 calendar (cal) years, with the most prominent amplitudes during marine isotope stage 3. Age control is based on benthic δ18O records, 14C ages, and on geomagnetic intensity variations at Site 883 tuned to the North Atlantic paleointensity stack since 75 ka (NAPIS‐75), in turn, tied to the Greenland Ice Sheet Project 2 (GISP2) ice core chronology. On the basis of this tuning, northwest Pacific warm phases parallel Greenland cold stadials and viceversa. This contrasts with atmospheric heat transfer, expected to produce quasi‐coeval signals across the Northern Hemisphere. The antiphasing may instead stem from variations in global thermohaline circulation. At its North Pacific terminus in the subarctic gyre, any slowdown or shutoff of North Atlantic Deep Water formation during Dansgaard‐Oeschger stadials led to a turnoff or reduction of the upwelling of cold Pacific deepwater and thus, to striking, instantaneous, short‐term warmings of surface water.
Millennial‐scale dynamics of the east Asian winter monsoon during the last 200,000 years
Authors: de Garidel‐Thoron Thibault , Beaufort Luc , Linsley Braddock K. , Dannenmann Stefanie
Published: February 2004

The primary productivity dynamics of the last 200,000 years in the Sulu Sea was reconstructed using the abundance of the coccolithophore Florisphaera profunda in the IMAGES MD97‐2141 core. We find that primary productivity was enhanced during glacial periods, which we suggest is due to a stronger East Asian winter monsoon. During the last 80 kyr, eight significant increases in primary productivity (PP) in the Sulu Sea are similar to East Asian winter monsoon changes recorded in Chinese loess. The PP maxima are not linked with Heinrich events (HE) in the North Atlantic, although four PP peaks are synchronous with HE. The PP oscillations have frequencies near those of the Dansgaard‐Oeschger cycles in Northern Hemisphere ice records and indicate a teleconnection of the East Asian winter monsoon with Greenland climate. In this Sulu Sea record the East Asian winter monsoon oscillates with periodicities of ∼6, 4.2–3.4, 2.3, and 1.5 kyr. In particular, the 1.5 kyr cycle exhibits a strong and pervasive signal from stage 6 to the Holocene without any ice volume modulation. This stationarity suggests that the 1.5 kyr cycle is not driven by some high‐latitude forcing.
Derivation of δ18O from sediment core log data: Implications for millennial‐scale climate change in the Labrador Sea
Authors: Weber M. E. , Mayer L. A. , Hillaire‐Marcel C. , Bilodeau G. , Rack F. , Hiscott R. N. , Aksu A. E.
Published: February 2004

Sediment core logs from six sediment cores in the Labrador Sea show millennial‐scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard‐Oeschger cycles. The same millennial‐scale climate change is documented for surface water δ18O records of Neogloboquadrina pachyderma (left coiled); hence the surface water δ18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 years). For the Labrador Sea, sediment core logs contain important information about deepwater current velocities and also reflect the variable input of ice‐rafted debris from different sources as inferred from grain‐size analysis, the relation of density and P wave velocity, and magnetic susceptibility. For the last glacial, faster deepwater currents, which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted from several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deepwater currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly thereafter, while the abrupt atmospheric temperature rise happened after a larger time lag of ≥ 1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial timescales but decoupling at orbital timescales.
An evaluation of Lohmann's foraminifera weight dissolution index
Authors: Broecker Wallace , Clark Elizabeth
Published: February 2004

Down water column traverses of core top weights for three planktonic species confirm Lohmann's [1995] relationship between foraminifera shell weight loss and bottom water carbonate ion content. However, they also suggest that the initial shell thickness varies with growth habitat and that the offset between bottom water and pore water carbonate ion concentration varies even on small space scales.