bluuebunny/crossref_metadata_2025 huggingface dataset:
Subgrid variability and stochastic downscaling of modeled clouds: Effects on radiative transfer computations for rainfall retrieval
Authors: Harris Daniel , Foufoula‐Georgiou Efi
Published: February 2004

The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) Goddard Profiling (GPROF) rainfall retrieval algorithm is an inversion type algorithm, which uses numerical cloud models and radiative transfer schemes to simulate the brightness temperatures that the TMI would see, thereby allowing one to relate hydrometeor profiles to brightness temperature. The variability in modeled hydrometeor fields is known to have an important effect on simulated brightness temperatures, and while the TMI instrument sees all the variability down to scales of a few meters, cloud models are typically run at resolutions of 1–3 km. This paper is an illustrative investigation into the importance of subgrid variability (scales below 1–3 km), which is ignored when simulating brightness temperatures. Previous studies on the importance of subgrid variability have been based on comparisons of simulated brightness temperatures computed from hydrometeor fields of a high resolution model and spatially aggregated hydrometeor fields from the same model run. It is argued that numerical cloud models have reduced small‐scale variability due to model artifacts such as computational mixing, and this may lead to an underestimation of the importance of including subgrid variability. To address this problem, stochastic downscaling developed in a wavelet‐based framework is used to reintroduce the variability reduced by computational mixing. In particular, a high resolution model is spatially aggregated (i.e., upscaled) over the scales affected by computational mixing and stochastically downscaled back to the original resolution of the model. The higher degree of variability introduced by the downscaling (which is a closer approximation to the variability observed in hydrometeor concentrations as compared to that produced by high resolution models) is found to result in larger biases in estimated brightness temperature. This points to the potential for a significant source of bias in microwave‐sensed precipitation retrievals that requires further study.
Tests of halogen photochemistry using in situ measurements of ClO and BrO in the lower polar stratosphere
Authors: Avallone Linnea M. , Toohey Darin W.
Published: February 2004

In situ observations of the halogen oxides ClO and BrO made from the NASA ER‐2 during the Airborne Arctic Stratospheric Expedition (AASE) I and II missions are used to test current understanding of photochemical parameters. Measurements of ClO obtained during AASE I in the dark perturbed polar vortex are analyzed with respect to temperature to derive the equilibrium expression for the ClO/Cl2O2 system. Assuming photochemical steady state and complete activation of chlorine (ClO + 2Cl2O2 = Cly), observations of ClO made during AASE II are used to derive the photolysis rate of Cl2O2. The photolysis rate derived from atmospheric observations is compared to J values calculated with a photochemical model and various values for the absorption cross section of Cl2O2. The photolysis rate calculated with the cross section of Huder and DeMore [1995] is shown to be systematically too small, while those of Burkholder et al. [1990] and Cox and Hayman [1988] are too large to be consistent with atmospheric observations. Observations of BrO made during AASE II indicate that our understanding of the inorganic bromine budget in the polar regions is incomplete. A possible role for the adduct BrOOCl is investigated.
Pacific Exploratory Mission in the Tropical Pacific: PEM‐Tropics B, March‐April 1999
Authors: Raper James L. , Kleb Mary M. , Jacob Daniel J. , Davis Douglas D. , Newell Reginald E. , Fuelberg Henry E. , Bendura Richard J. , Hoell James M. , McNeal Robert J.
Published: February 2004

The Pacific Exploratory Mission ‐ Tropics B (PEM‐Tropics B) was conducted by the NASA Global Tropospheric Experiment (GTE) over the tropical Pacific Ocean in March‐April 1999. It used the NASA DC‐8 and P‐3B aircraft equipped with extensive instrumentation for measuring numerous chemical compounds and gases. Its central objective was to improve knowledge of the factors controlling ozone, OH, aerosols, and related species over the tropical Pacific. Geographical coverage ranged from 38°N to 36°S and 148°W to 76°E. Major deployment sites included Hilo, Hawaii, Christmas Island, Tahiti, Fiji, and Easter Island. PEM‐Tropics B was a sequel to PEM‐Tropics A, which was conducted in September‐October 1996 and encountered considerable biomass burning. PEM‐Tropics B, conducted in the wet season of the southern tropics, observed an exceedingly clean atmosphere over the South Pacific but a variety of pollution influences over the tropical North Pacific. Photochemical ozone loss over both the North and the South Pacific exceeded local photochemical production by about a factor of 2, implying a major deficit in the tropospheric ozone budget. Dedicated flights investigated the sharp air mass transitions at the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Extensive OH observations permitted the first large‐scale comparisons with photochemical model predictions. High concentrations of oxygenated organics were observed ubiquitously in the tropical Pacific atmosphere and may have important implications for global HOx and NOx budgets. Extensive equatorial measurements of dimethyl sulfide and OH suggest that important aspects of marine sulfur chemistry are still poorly understood.
Cometary dust streams at Mars: Preliminary predictions from meteor streams at Earth and from periodic comets
Authors: Treiman Allan H. , Treiman Jay S.
Published: February 2004

Spacecraft are at risk from impacts of interplanetary dust particles, and those particles are concentrated near the orbits of comets and of meteor streams, which are (for the most part) inferred to derive from comets. To explore potential dangers to Mars‐orbiting spacecraft from cometary dust, we screened known comets and meteor showers for those with orbits that closely approach Mars' orbit. Of the 135 periodic Mars‐crossing comets, the orbits of 50 approach within 0.1 AU of Mars and so are potential current sources of dust and meteor streams at Mars. Among these, IP/Halley, IP/Tempel 1, and 1991D1 P/Hermann seem the most promising targets for further study. Past orbits of all Mars‐crossing comets might also yield meteor streams at Mars. Among known meteor streams, the daytime Arietids and Geminids could produce significant numbers of meteors at Mars but have not produced (so far) meteor storms that could present significant dangers to spacecraft.
Thermal convection in the outer shell of large icy satellites
Authors: Deschamps Frédéric , Sotin Christophe
Published: February 2004

Evolution of large icy satellites is controlled by heat transfer across the outer ice I layer. After the core overturn a possible structure consists of a silicate core and a shell of molten ices. As the satellite cools down, the primordial ocean crystallizes. If the outer layer is thick enough, convection is very likely to occur in it. We have used the results of a recent two‐dimensional numerical model of convection including variable viscosity to estimate the vigor and the efficiency of convection in this layer. Viscosity variations induce the apparition of a stagnant lid at the top of the fluid, which reduces the efficiency of heat transfer. In the present study, the Rayleigh number Ra and the heat flux Φ are computed as a function of the thickness of the layer, assuming that the ice flow is Newtonian. Calculations are first made for a generic satellite of radius R = 2500 km and mean density 〈ρ〉 = 1.9 g/cm3. It is then shown that variations of ±500 km on the radius and ±0.5 g/cm3 on the mean density do not induce significant differences in the values of Ra and Φ. On the other hand, variations of the reference viscosity μ0, and of the activation energy E induce major differences. The reference viscosity is equal to the viscosity close to the melting point, and its possible value yields around μ0 = 5×1013 Pa s. A possible value of E is 60 kJ/mol. For these values of the rheological parameters we find that the initial ocean may crystallize completely in ∼3.6 Gyr. Higher values of μ0 and/or E reduce significantly the vigor and the efficiency of convection. The influence of the composition of the initial ocean is also investigated. The presence of ammonia reduces the convective strength and the heat flux. The upper structure of icy satellites is discussed as a function of the rheological and compositional parameters. The presence of a sub‐surface ocean could be explained by either the presence of volatiles in the initial ocean or the presence of additional heat sources, such as tidal dissipation.
Strength of Martian lithosphere beneath large volcanoes
Authors: Arkani‐Hamed Jafar
Published: February 2004

This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics truncated at degrees 10, 11, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11. Therefore intermediate‐scale surface topography and gravity anomalies of Mars, specified by harmonics of degree 11–50, are considered in this study. These harmonics are dominated by shield volcanoes, Alba Patera, and Isidis basin. The central part of Valles Marineris is also well defined by these harmonics. The strong correlation of the topography and gravity anomalies over these harmonics and the positive Bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses. This indicates that the Martian lithosphere has been strong enough to support both the topography and the internal excess masses. We determine the rigidity of the lithosphere beneath a given surface feature. The lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Valles Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes cannot. Internal excess masses of at least 40%, and more likely 50%, of the topographic masses are required, and the lithosphere beneath must have an elastic core of ∼100–80 km to support these volcanoes. The mean thermal gradient beneath this region of Tharsis is estimated to be ∼7–10 K/km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle.
Pluto and Charon: A case of precession‐orbit resonance?
Authors: Rubincam David Parry
Published: February 2004

Pluto may be the only known case of precession‐orbit resonance in the solar system. The Pluto‐Charon system orbits the Sun with a period of 1 Plutonian year, which is 250.8 Earth years. The observed parameters of the system are such that Charon may cause Pluto to precess with a period near 250.8 Earth years. This gives rise to two possible resonances, heretofore unrecognized. The first is due to Pluto's orbit being highly eccentric, giving solar torques on Charon with a period of 1 Plutonian year. Charon, in turn, drives Pluto near its precession period. Volatiles, which are expected to shuttle across Pluto's surface between equator and pole as the insolation varies, might change the planet's dynamical flattening enough so that Pluto crosses the nearby resonance, forcing the planet's equatorial plane to depart from Charon's orbital plane. The mutual tilt can reach as much as 3° after integrating over 8.4×106 years, depending upon how close Pluto is to the resonance and the supply of volatiles. The second resonance is due to the Sun's traveling above and below Charon's orbital plane; it has a period half that of the eccentricity resonance. Reaching this half‐Plutonian‐year resonance requires a much larger but still theoretically possible amount of volatiles. In this case the departure of Charon from an equatorial orbit is ∼1° after integrating for 5.6×106 years. The calculations ignore libration and tidal friction. It is not presently known how large the mutual tilt can grow over the age of the solar system, but it is probably less than ∼4°. If so, then observing such small angles from a Pluto flyby mission would be difficult.
Evidence for episodicity in the magma supply to the large Tharsis volcanoes
Authors: Wilson Lionel , Scott Evelyn D. , Head James W.
Published: February 2004

The volumes and estimated total lifetimes of the Tharsis shield volcanoes on Mars imply that they were built with an average magma supply rate within a factor of 2–3 of ∼0.05 m3 s−1. The morphologies and positions of the summit calderas of these volcanoes, specifically the overlapping relationships between the calderas, are evidence that multiple magma reservoirs existed within them, centered on distinctly different locations at various times. The requirement that the magma in an older reservoir must have cooled below its solidus temperature in order to change the local stresses in such a way that a new reservoir grew at a different location argues for long time gaps (probably tens of Myr) during which the supply rate of magma from the mantle was much smaller than at other times. Conversely, the need to offset conductive cooling to the surface once an active magma reservoir was established implies that the magma supply rate was large, ∼1–10 m3 s−1, for times of order 0.1 Myr. An initial pulse involving rates of more than 150 m3 s−1 acting for at least a few weeks was needed to initiate each new reservoir. The lengths of numerous long lava flows on the flanks of the Tharsis Montes imply that lava must have been erupted at rates of ∼100–300 m3 s−1 for periods of a few years to a few tens of years. The volumes of many of these flows exceed the amounts of magma that could be released from overpressured magma reservoirs of the sizes implied by the dimensions of the summit calderas with the surrounding rocks behaving elastically. A maintained high magma supply rate from the mantle (leading to buffered eruption conditions) is almost certainly required for such flows, whereas inelastic caldera collapse events (unbuffered eruption conditions) can explain the emplacement of other flows. Taken together, the various lines of evidence suggest that the Tharsis volcanoes were built episodically with active phases lasting less than 1 Myr alternating with ∼100 Myr quiet phases. We comment on the implications of these issues for the episodicity of the generation of magma in the mantle and for the mode of transport of the magma in dikes or diapirs.
A characterization of crystalline ice nanoclusters using transmission electron microscopy
Authors: Delzeit Lance , Blake David
Published: February 2004

The preparation of crystalline ice nanoclusters from a mixture of 0.5–2.0% water vapor in N2 carrier gas produces clusters that range in diameter from ∼10 to several hundred nanometers, with a mode of 15–30 nm. By varying the percent water vapor in the mixture or the aliquot size, the average cluster size and the size distribution of the clusters can be controlled. If a second strong hydrogen‐bonding molecule such as methanol is added to the mixture, the size distribution of the clusters is changed. The spherical shape of the crystalline ice nanoclusters observed in transmission electron microscopy (TEM) bright field imaging suggests that the clusters form from liquid droplets that crystallize upon further cooling. TEM dark field imaging of individual clusters reveals a structure in which crystalline domains are oriented one to another in mosaic fashion. This structural relationship appears to propagate from the centers of the clusters, indicating that nucleation and growth proceed from the centers outward. Water ice nanoclusters prepared in this way are useful analogs for ices from a variety of natural environments, including cold interstellar molecular clouds, comets, and terrestrial polar stratospheric clouds. The ice nanoclusters have a large surface area relative to their bulk that allows for the rapid reaction of the ice with other species present in the water‐carrier gas mixture or deposited on the clusters subsequent to their formation.
Measurements of temperature‐dependent absorption cross sections of C2H2 in the VUV‐UV region
Authors: Wu C. Y. Robert , Chen F. Z. , Judge D. L.
Published: February 2004

Using a synchrotron radiation as a continuum light source, we have carried out laboratory measurements of the temperature‐dependent photoabsorption cross sections of C2H2 from 120 to 230 nm region at 295 K and from 120 to 215 nm region at 150 K with a spectral resolution of 0.007 nm. In the presently studied spectral region the cross section values vary by 6 orders of magnitude, and the absorption features vary from sharp rovibronic structure, shaded bands, to diffuse bands. High‐temperature cross sections of C2H2 in the 120–140 nm region has also been measured for the purpose of identifying possible hot bands. The previous assignments of several hot bands in the 130–140 nm region were crucial in the determination of the electronic structure of a valence E state. However, we find that the previously assigned hot bands at 135.09 and 136.24 nm are not hot bands at all because they are insensitive to temperature change from 150 to 370 K. The Galileo spacecraft has made incredibly successful observations of the Jovian system including the prominent C2H2 auroral features, and the Jupiter flyby of the Cassini spacecraft is yet to come. Equipped with higher‐resolution spectrometers, the Cassini mission promises to provide higher scientific returns regarding the Jovian auroral observations. Solar and stellar occultation experiments at Saturn by the Cassini spacecraft will provide more detailed observations of the Saturnian upper atmosphere than ever before. The data presented in the present work are an extension of our effort to provide the required data to the scientific community and will have a significant impact on our understanding of the atmospheres of Jupiter and Saturn, in particular, and other outer planets and satellites, in general.
Infrared and vapor flux studies of vapor‐deposited amorphous and crystalline water ice films between 90 and 145 K
Authors: La Spisa Sabrina , Waldheim Matthew , Lintemoot Jaime , Thomas Tiffany , Naff Janelle , Robinson Marin
Published: February 2004

Three methods were simultaneously employed to investigate vapor‐deposited water ice films (90–145 K) in a high‐vacuum chamber: grazing‐angle Fourier transform infrared (FTIR)‐reflection absorption spectroscopy to characterize ice phase; optical interference methods to monitor ice density, growth rate, and film thickness; and gas phase mass spectrometry to monitor sublimation flux during annealing. The results of this work were used to further describe the physical properties of amorphous and crystalline ice, with the goal of achieving greater consensus regarding the structure, density, and vapor flux of amorphous ice. Amorphous and crystalline ice were clearly distinguishable using grazing‐angle FTIR spectroscopy. Vapor‐deposited films (deposited between 0.1 and 10 nm s−1) appeared amorphous at temperatures below 105 K and crystalline above 120 K. Between 110 and 115 K, amorphous ice, crystalline ice, or a mixture of both was observed, inconsistent with the prediction that ice will be crystalline in this regime [Kouchi et al., 1994]. Upon annealing (1–2 K min−1), amorphous ice underwent a phase change to crystalline ice between 147 and 154 K. The density of ice at 90 K was slightly greater (0.94±0.01 g cm−3) than the density of ice above 110 K (0.93±0.01 g cm−3), consistent with literature values for bulk amorphous (excluding micropores) and crystalline ice. Sublimation flux was examined in light of the proposed existence of two metastable phases of amorphous ice: low‐density amorphous and restrained amorphous [Jenniskens et al., 1998]. No supporting evidence was provided; however, our techniques cannot rule out their proposed existence.
Water detection on atmosphereless celestial bodies: Alternative explanations of the observations
Authors: Starukhina Larissa
Published: February 2004

Alternative explanations are proposed for the results of the three types of remote sensing experiments in which water ice is supposed to be found on the surfaces of atmosphereless celestial bodies: (1) measurements of hydrogen content by the neutron spectrometer on Lunar Prospector, (2) observations of the absorption bands near 3 μm in reflectance spectra of asteroids, and (3) radar observations of polar regions of Mercury. Calculations have shown that in the first two types of observations solar wind protons chemically trapped by oxygen atoms in hydroxyl groups or other radiation defects in oxygen‐bearing particles on the surfaces of atmosphereless bodies can be mistaken for water. Higher hydrogen content in the lunar polar regions, especially in polar craters, as compared to equatorial zones can be due to sharp decrease of escape probability with temperature. Spots of high hydrogen content in equatorial zones of the Moon can be explained by variations of degassing rates in different materials. Solar wind origin of OH groups may account for 3‐μm absorption by asteroids of M and E classes thought to be differentiated. Strong, highly depolarized radar echoes from polar and lower latitude craters of Mercury can be explained by decrease of the dielectric loss of silicate material with temperature, which solves the problems of delivery and thermal stability of low‐loss material on Mercury surface, being consistent with the observed regional variations of radar brightness. The possibilities considered in the paper should be taken into account in interpretation of the observations aimed on search of water.
Lava fountains from the 1999 Tvashtar Catena fissure eruption on Io: Implications for dike emplacement mechanisms, eruption rates, and crustal structure
Authors: Wilson Lionel , Head James W.
Published: February 2004

The first direct evidence that patterns of basaltic eruptive activity may be very similar on Io and Earth was seen on November 26, 1999, when the solid‐state imaging system on board the Galileo spacecraft obtained high‐resolution images of a fissure eruption taking place in one of the calderas of the Tvashtar Catena complex. We analyzed the dynamics of the fissure eruption from the morphology of the lava fountains and pyroclastic deposits and derived an estimate of the eruption rate. Our best estimate of the volume eruption rate per unit length of fissure lies in the range 0.7 to 7 m3 s−1 m−1, and multiplying this rate by the fissure length of ∼25 km yields a total volume eruption rate of magma of ∼2×104 to 2×105 m3 s−1. The upper end of the inferred eruption rate is very close to the upper end of the range of eruption rates observed or inferred for fissure eruptions on Earth. Theoretical models of the possible dike systems feeding the fissure show that for eruptions of positively buoyant magma, the elastic properties of rocks are such that a magma flux of 6 to 7 m3 s−1 m−1 is the largest value that can ever be realized in such eruptions, this limiting value being independent of planetary gravity and thus the same on all silicate bodies. The eruption rate inferred for Tvashtar can also be achieved by a negatively buoyant magma, but in that case the source depth cannot be much greater than a few tens of kilometers. We find that some eruptive events are consistent with the effusion of magmas which are buoyant relative to the crust, whereas others appear to involve magmas which are not. This implies that the distribution of crustal volatiles is laterally very inhomogeneous. Our analyses suggest that the depths from which magmas make their final ascent to the surface of Io are generally of order tens, rather than hundreds, of kilometers.
Radiolysis of water ice in the outer solar system: Sputtering and trapping of radiation products
Authors: Bahr D. A. , Famá M. , Vidal R. A. , Baragiola R. A.
Published: February 2004

We performed quantitative laboratory radiolysis experiments on cubic water ice between 40 and 120 K, with 200 keV protons. We measured sputtering of atoms and molecules and the trapping of radiolytic molecular species. The experiments were done at fluences corresponding to exposure of the surface of the Jovian icy satellites to their radiation environment up to thousands of years. During irradiation, O2 molecules are ejected from the ice at a rate that grows roughly exponentially with temperature; this behavior is the main reason for the temperature dependence of the total sputtering yield. O2 trapped in the ice is thermally released from the ice upon warming; the desorbed flux starts at the irradiation temperature and increases strongly above 120 K. Several peaks in the desorption spectrum, which depend on irradiation temperature, point to a complex distribution of trapping sites in the ice matrix. The yield of O2 produced by the 200 keV protons and trapped in the ice is more than 2 orders of magnitude smaller than used in recent models of Ganymede. We also found small amounts of trapped H2O2 that desorb readily above 160 K.
The summer 1997 eruption at Pillan Patera on Io: Implications for ultrabasic lava flow emplacement
Authors: Williams David A. , Davies Ashley G. , Keszthelyi Laszlo P. , Greeley Ronald
Published: February 2004

Galileo data and numerical modeling were used to investigate the summer 1997 eruption at Pillan Patera on Io. This event, now defined as “Pillanian” eruption style [Keszthelyi et al., this issue], included a high‐temperature (>1600°C), possibly ultrabasic, 140‐km‐high plume eruption that deposited dark, orthopyroxene‐rich pyroclastic material over >125,000 km2, followed by emplacement of dark flow‐like material over >3100 km2 to the north of the caldera. We estimate that the high‐temperature, energetic episode of this eruption had a duration of 52–167 days between May and September 1997, with peak eruption temperatures around June 28, 1997. Galileo 20 m pixel−1 images of part of the Pillan flow field show a widespread, rough, pitted surface that is unlike any flow surface we have seen before. We suggest that this surface may have resulted from (1) a fractured lava crust formed during rapid, low‐viscosity lava surging, perhaps including turbulent flow emplacement; (2) disruption of the lava flow by explosive interaction with a volatile‐rich substrate; or (3) a combination of 1 and 2 with or without accumulation of pyroclastic materials on the surface. Well‐developed flow lobes are observed, suggesting that this is a relatively distal part of the flow field. Shadow measurements at flow margins indicate a thickness of ∼8–10 m. We have modeled the emplacement of putative ultrabasic flows from the summer 1997 Pillan eruption using constraints from new Galileo data. Results suggest that either laminar sheet flows or turbulent channelized flows could have traveled 50–150 km on a flat, unobstructed surface, which is consistent with the estimated length of the Pillan flow field (∼60 km). Our modeling suggests low thermal erosion rates (<0.1 m d−1), and that the formation of deep (>20 m) erosion channels was unlikely, especially distal to the source. We calculate a volumetric flow rate of ∼2–7×103 m3 s−1, which is greater than those for typical Mauna Loa/Kilauea flows but comparable to those for the (1783) Laki eruption and the inferred flow rates of the Roza flows in the Columbia River flood basalts. The differences in ultrabasic eruption styles on Earth and Io appear to be controlled by the different eruption environments: Plumes at sites of ultrabasic eruptions on Io suggest strong magma‐volatile interactions on a low‐gravity body lacking an atmosphere, whereas the geology at sites of komatiite eruptions on Earth suggest mostly submarine emplacement of thick flows with a pronounced lack of subaerial explosive activity.
Chemical variation of the large Apollo 15 olivine‐normative mare basalt rock samples
Authors: Ryder Graham , Schuraytz Benjamin C.
Published: February 2004

Most chemical analyses of Apollo 15 olivine‐normative mare basalts have been conducted on subsamples of <1 g, and dominantly rather less than 0.5 g, very small by the standards of analyses of terrestrial rocks. Most analyses have been partial (a small number of elements) rather than comprehensive. They show considerable scatter, but the general inference has been that the rock samples represent a single suite dominated by olivine control. However, because of the scatter, some workers have suggested more complex petrogeneses and even that the rock samples do not form a single comagmatic suite. We analyzed the 25 largest rock samples identified as Apollo 15 olivine‐normative mare basalts using subsamples that were mainly 5 g (and all but three >4 g) to obtain greater whole‐rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X‐ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short‐range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine‐normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine‐accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.
Density structure of Io and the migration of magma through its lithosphere
Authors: Leone Giovanni , Wilson Lionel
Published: February 2004

We model the density structure of the outer several tens of kilometers of Io on the basis of the assumption that condensed volatiles, mainly SO2, exist at shallow depths in a mafic to ultramafic lithosphere, which has a finite porosity due to its accumulation as a mixture of pyroclastic and effusive volcanic deposits, which include condensed volatiles. The porosity decreases with increasing depth due to pressure‐driven compaction and temperature‐driven melting of the volatiles, and this controls the bulk density structure of the lithosphere. We also model the densities of ascending magmas as a function the types and amounts of volatiles that are available in their source regions. The relative densities of the accumulated volcanic deposits and of the magmas rising through them control whether the magmas will erupt at the surface or will stall to form intrusions which can grow into magma reservoirs. We find that the depths at which magma reservoirs may exist are strongly influenced by the void space fraction in surface deposits and only weakly controlled by the mass fraction of SO2 in the deposits and the mass fraction of SO2 in the magmas. Once nucleated, a reservoir has the potential to grow to a very large vertical extent (∼15 km total height) before the magma immediately beneath its roof is again buoyant relative to the surrounding rocks. The presence of large‐volume reservoirs may explain the great extent of the larger lava flow fields seen on Io and the longevity of some eruptions. The presence of very soluble volatiles, such as water, in magmas would have virtually no effect on these findings because these volatiles would influence the densities of magmas only very near the surface (though they would, even in very small amounts, then have a dramatic effect on the explosivities of eruptions).
Statistics of Mars' topography from the Mars Orbiter Laser Altimeter: Slopes, correlations, and physical Models
Authors: Aharonson Oded , Zuber Maria T. , Rothman Daniel H.
Published: February 2004

Data obtained recently by the Mars Orbiter Laser Altimeter (MOLA) were used to study the statistical properties of the topography and slopes on Mars. We find that the hemispheric dichotomy, manifested as an elevation difference, can be described by long baseline tilts but in places is expressed as steeper slopes. The bimodal hypsometry of elevations on Mars becomes unimodal when referenced to the center of figure, contrary to the Earth, for which the bimodality is retained. However, ruling out a model in which the elevation difference is expressed in a narrow equatorial topographic step cannot be done by the hypsometry alone. Mars' slope distribution is longer tailed than those of Earth and Venus, indicating a lower efficiency of planation processes relative to relief‐building tectonics and volcanics. We define and compute global maps of statistical estimators, including the interquartile scale, RMS and median slope, and characteristic decorrelation length of the surface. A correspondence between these parameters and geologic units on Mars is inferred. Surface smoothness is distinctive in the vast northern hemisphere plains, where slopes are typically <0.5°. Amazonis Planitia exhibits a variation in topography of <1 m over 35‐km baselines. The region of hematite mineralization in Sinus Meridiani is also smooth, with median slopes lower than 0.4°, but does not form a closed basin. The shallower long‐wavelength portion of the lowlands' topographic power spectrum relative to the highlands' can be accounted for by a simple model of sedimentation such as might be expected at an ocean's floor. The addition of another process such as cratering is necessary to explain the spectral slope in short wavelengths. Among their application, these MOLA‐derived roughness measurements can help characterize sites for landing missions.
Observations and initial modeling of lava‐SO2 interactions at Prometheus, Io
Authors: Milazzo M. P. , Keszthelyi L. P. , McEwen A. S.
Published: February 2004

We present observations and initial modeling of the lava‐SO2 interactions at the flow fronts in the Prometheus region of Io. Recent high‐resolution observations of Prometheus reveal a compound flow field with many active flow lobes. Many of the flow lobes are associated with bright streaks of what is interpreted to be volatilized and recondensed SO2 radiating away from the hot lava. Lower‐resolution color data show diffuse blue to violet areas, also near the active flow front, perhaps from active venting of SO2. Not clearly visible in any of the images is a single source vent for the active plume. While the size of the proposed vent is probably near the limit of the resolution, we expected to see radial or concentric albedo patterns or other evidence for gas and entrained particles above the flow field. The lack of an obvious plume vent, earlier suggestions that the Prometheus‐type plumes may originate from the advancing flow lobes, and the high‐resolution images showing evidence for large‐scale volatilization of the SO2‐rich substrate at Prometheus encouraged us to develop a model to quantify the heat transfer between a basaltic lava flow and a substrate of SO2 snow. We calculate that the vaporization rate of SO2 snow is 2.5×10−6 m s−1 per unit area. Using an estimated 5 m2 s−1 lava coverage rate (from change detection images), we show that the gas production rate of SO2 at the flow fronts is enough to produce a resurfacing rate of ∼0.24 cm yr−1 at the annulus of Prometheus. This is much less than other estimates of resurfacing by the Prometheus plume. While not easily explaining the main Prometheus plume, our model readily accounts for the bright streaks.
Thermal inertia of crater‐related wind streaks on Mars
Authors: Pelkey Shannon M. , Jakosky Bruce M. , Mellon Michael T.
Published: February 2004

Data from the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor spacecraft are used to analyze the structure of and materials comprising crater‐related wind streaks. Comparisons of high‐resolution TES albedo and TES‐based thermal inertia values within and adjacent to the streaks allow us to constrain the properties of the streaks and gain insight into their physical structure. The majority of the analyzed Type I bright, depositional streaks are distinct from the surrounding terrain in albedo, but not in thermal inertia. We conclude that these streaks consist of thin deposits of bright material greater than ∼1 μm thick but less than 1–3 mm thick. The majority of the analyzed Type I dark, erosional streaks identified from Viking images are now indistinct from the surrounding terrain in both albedo and thermal inertia. We conclude that these streaks have been covered by a thin deposit of bright material greater than ∼1 μm thick but less than 1–3 mm thick. All of the analyzed Type II dark, depositional streaks are distinct from the surrounding terrain in both albedo and thermal inertia. We conclude that these streaks consist of deposits greater than a few centimeters thick. In this case we were also able to use the thermal inertia values to estimate the particle sizes of the deposited material; the values correspond to a broad range of sand‐sized particles easily mobilized by wind, supporting the deflation/deposition theory of Type II streak formation.
Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission
Authors: Malin Michael C. , Edgett Kenneth S.
Published: February 2004

More than 3 years of high‐resolution (1.5–20 m/pixel) photographic observations of the surface of Mars have dramatically changed our view of that planet. Among the most important observations and interpretations derived therefrom are that much of Mars, at least to depths of several kilometers, is layered; that substantial portions of the planet have experienced burial and subsequent exhumation; that layered and massive units, many kilometers thick, appear to reflect an ancient period of large‐scale erosion and deposition within what are now the ancient heavily cratered regions of Mars; and that processes previously unsuspected, including gully‐forming fluid action and burial and exhumation of large tracts of land, have operated within near‐contemporary times. These and many other attributes of the planet argue for a complex geology and complicated history.
Variability of the western Mediterranean Sea surface temperature during the last 25,000 years and its connection with the Northern Hemisphere climatic changes
Authors: Cacho Isabel , Grimalt Joan O. , Canals Miquel , Sbaffi Laura , Shackleton Nick J. , Schönfeld Joachim , Zahn Rainer
Published: February 2004

Sea surface temperature (SST) profiles over the last 25 kyr derived from alkenone measurements are studied in four cores from a W‐E latitudinal transect encompassing the Gulf of Cadiz (Atlantic Ocean), the Alboran Sea, and the southern Tyrrhenian Sea (western Mediterranean). The results document the sensitivity of the Mediterranean region to the short climatic changes of the North Atlantic Ocean, particularly those involving the latitudinal position of the polar front. The amplitude of the SST oscillations increases toward the Tyrrhenian Sea, indicating an amplification effect of the Atlantic signal by the climatic regime of the Mediterranean region. All studied cores show a shorter cooling phase (700 years) for the Younger Dryas (YD) than that observed in the North Atlantic region (1200 years). This time diachroneity is related to an intra‐YD climatic change documented in the European continent. Minor oscillations in the southward displacement of the North Atlantic polar front may also have driven this early warming in the studied area. During the Holocene a regional diachroneity propagating west to east is observed for the SST maxima, 11.5–10.2 kyr B.P. in the Gulf of Cadiz, 10–9 kyr B.P. in the Alboran Sea, and 8.9–8.4 kyr B.P. in the Thyrrenian Sea. A general cooling trend from these SST maxima to present day is observed during this stage, which is marked by short cooling oscillations with a periodicity of 730±40 years and its harmonics.
Punctuated evolution of global climate cooling during the Late Middle to Early Late Miocene: High‐resolution planktonic foraminiferal and oxygen isotope records from the Mediterranean
Authors: Turco E. , Hilgen F. J. , Lourens L. J. , Shackleton N. J. , Zachariasse W. J.
Published: February 2004

High‐resolution planktonic foraminiferal and oxygen isotope records are presented from a Mediterranean deep marine succession, dated astronomically between 12.12 and 9.78 Ma. Planktonic and benthic oxygen isotope records are punctuated by two episodes of δ18O increase, which have astronomical ages of 11.4 and 10.4 Ma and correspond to the Mi5 and Mi6 events ofMiller et al.[1991a]. These ice growth events coincide with low‐amplitude variations in the 1.2 Myr obliquity cycle and are accompanied by significant faunal changes in the Mediterranean, such as the arrival of neogloboquadrinids, the increase in abundance of theG. apertura‐G. obliquusgroup, and the areal differentiation betweenN. atlanticaandN. acostaensis. Short‐term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession‐controlled sedimentary cycles. Features of the sapropel/grey marl layers indicate that the short‐term astronomically controlled circum‐Mediterranean climate changes remained basically the same over the last 12 Myr.
High‐resolution temperature and evaporation changes during the Late Holocene in the northeastern Arabian Sea
Authors: Doose‐Rolinski Heidi , Rogalla Ulf , Scheeder Georg , Lückge Andreas , von Rad Ulrich
Published: February 2004

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone‐derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone‐producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high‐(decadal)‐resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short‐term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes (“Little Ice Age”) and confirm global effects. Surface salinity values, reconstructed from δ18O records after correction for temperature‐related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher‐than‐recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon‐controlled areas.
Os‐Sr‐Nd results from sediments in the Bay of Bengal: Implications for sediment transport and the marine Os record
Authors: Pierson‐Wickmann Anne‐Catherine , Reisberg Laurie , France‐Lanord Christian , Kudrass Herman R.
Published: February 2004

Sr, Nd, and Os isotopic data are presented for sediments from diverse locations in the Bay of Bengal. These data allow the samples to be divided into three groups, related to their sedimentary contexts. The first group, mainly composed of sediments from the shelf off Bangladesh and the currently active fan, has Sr and Nd characteristics consistent with a dominantly Himalayan source. Their 187Os/188Os ratios (∼1.2‐1.5) show that the average detrital material delivered by the Ganga‐Brahmaputra (G‐B) river system is not unusually radiogenic. A large difference in 187Os/188Os ratio exists between these Bengal Fan sediments and Ganga bedloads (187Os/188Os ∼2.5, Pierson‐Wickmann et al. [2000]). This difference mainly reflects addition of a less radiogenic Brahmaputra component, though mineralogical sorting and loss of radiogenic Os during transport may also play some role. The second sample group contains sediments from elsewhere in the Bay, particularly those located on the continental slope. They display Os isotopic compositions (0.99–1.11) similar to that of present seawater and higher Os and Re concentrations. These characteristics suggest the presence of a large hydrogenous contribution, consistent with the lower sedimentation rate of these samples. Sr and Nd ratios indicate that a significant fraction of these sediments is derived from erosion of non‐Himalayan sources, such as the Indo‐Burman range. These observations could be explained by the deflection of sediments from the G‐B river system by westward currents in the head of the Bay. The third group contains only one sample, but shows that in addition to a Himalayan source, sediment discharge from Sri Lanka may influence the detrital component in the distal part of the fan. The similarity between the isotopic compositions of the group I R/V Sonne samples and those of Ocean Drilling Program Leg 116 [France‐Lanord et al., 1993; Reisberg et al., 1997] suggests that the material eroding in the Himalayas has been roughly constant since the Miocene. The high Os isotopic ratios of leachates of both Sonne group I and Miocene Leg 116 sediments imply that much of the leachable highly radiogenic Os component was conserved during transport through the estuary or interaction with seawater. In constrast, samples with lower, but still relatively high, sedimentation rates (Sonne groups II and III and Pliocene Leg 116) seem to have significantly adsorbed or exchanged Os and Re with seawater. This suggests that in some cases the Os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient marine Os record, or conversely, to date unfossiliferous sediments.