bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐frequency electromagnetic exploration for groundwater on Mars
Authors: Grimm Robert E.
Published: October 2002

Water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near‐ideal exploration target on Mars for low‐frequency, diffusive electromagnetic methods. Models of the temperature‐ and frequency‐dependent electrical properties of rock‐ice‐water mixtures are used to predict the electromagnetic response of the Martian subsurface. Detection of ice is difficult unless it is massively segregated. In contrast, liquid water profoundly affects soundings, and even a small amount of adsorbed water in the cryosphere can be detected. Subcryospheric water is readily distinguishable at frequencies as low as 100 Hz for fresh water to 10 mHz for brines. These responses can be measured using either natural or artificial sources. ULF signals from solar wind and diurnal‐heating perturbations of the ionosphere are likely, and disturbances of regional crustal magnetic fields may also be observable. Spherics, or ELF‐VLF signals from lightning discharge, would provide optimal soundings; however, lightning may be the least likely of the possible natural sources. Among the active techniques, only the time‐domain electromagnetic (TDEM) method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more. A ground‐ or aircraft‐based TDEM system of several kilograms can detect water to a depth of several hundred meters, and a system of tens of kilograms featuring a large, fixed, rover‐ or ballistically deployed loop can detect water to several kilometers depth.
Global distribution of lunar composition: New results from Lunar Prospector
Authors: Feldman W. C. , Gasnault O. , Maurice S. , Lawrence D. J. , Elphic R. C. , Lucey P. G. , Binder A. B.
Published: October 2002

Maps of thorium [Th], [FeO], the ratio of epithermal to thermal neutrons (E/T), and fast neutrons (FN) from Lunar Prospector were studied to determine their global distribution on the Moon. These distributions are compared to that of the 750 nm lunar albedo from Clementine to aid in their interpretation. All distributions were parameterized using a spherical harmonic expansion out to order l = 30. Resultant harmonic coefficients generally decrease with increasing l value as a power law in l for all five variables. The axes of all dipole components (l = 1) cluster closely about a centroid given by +14.1° latitude and 16.4° west longitude. This location is very close to the symmetry axes of their quadrupole components (l = 2), which cluster about a centroid given by +24.6° latitude and 25.1° west longitude. Both centroids are near the center of a suggested Procellarum basin, given by Whitaker [1981] at +23° latitude and 15° west longitude. This suggestion is strengthened by a sharp decrease of the intensities of all three variables at ∼50° from the centroids, which is close to the boundary of the putative Procellarum basin. The coincidence of the global concentration of heat‐producing elements (through [Th]) and mare deposits on the lunar surface (through [FeO], [T/E], and [FN]) with the circular outline of a putative Procellarum basin suggests that the events that were responsible for producing Oceanus Procellarum (perhaps a single giant impact) must figure importantly in shaping the global distribution of surface composition. A second, apparently older circular structure having its center near −5° latitude and 65° east longitude and a radius of 50° is also hinted at in the data.
Structure and evolution of southeast Thetis Regio
Authors: Ghail Richard C.
Published: October 2002

The highland terrain in southeast Thetis Regio consists of a series of thrust faults and conjugate graben that would usually be classified as tesserae. Unusually, this small area of tesserae is younger than the adjacent plains, a conclusion strengthened by modification of Jumaisat crater, a parabolic dark‐halo crater >75 Myr old. Two structural belts are distinguished, the eastern ridged terrain and the central plateau, separated by conjugate graben. Unlike most terrestrial mountain belts, the thrusts in southeast Thetis are outward dipping, implying obduction of the eastern plains onto Thetis. This most likely results from the buoyancy of the Venusian lithosphere, which inhibits subduction. A simple two‐stage orogenic model is inferred. Plains crust, driven by extension in Diana Chasma, initially overrides higher‐standing tesserae crust in southeast Thetis. Through interaction with a plume, or perhaps through convective instability of the lower lithosphere, a plateau forms that resists further compression and that exhibits extensive volcanism. A new area of thrusting in the ridged terrain develops, while the plateau starts to collapse along graben, the larger of which are filled with lavas. The core of Thetis Regio appears to be surrounded by other tectonic units like southeast Thetis, indicating a piecemeal construction similar to terrestrial continents.
Effects of glass content and oxidation on the spectra of SNC‐like basalts: Applications to Mars remote sensing
Authors: Minitti Michelle E. , Mustard John F. , Rutherford Malcolm J.
Published: October 2002

We have investigated the effects of crystallization and oxidation on the visible and near‐infrared (VISNIR) and mid‐infrared (mid‐IR) spectra of materials with a Shergotty‐Nakhla‐Chassigny (SNC) basaltic composition in order to investigate potential links between Martian VISNIR spectra, Martian mid‐IR spectra, and SNC meteorite data. SNC basalt samples of varying crystallinities were synthesized and subsequently oxidized in air at 700°C for 1–7 days, and reflectance spectra of both fine (≤75 μm) and coarse (75–500 μm) basalt samples were obtained before and after oxidation. The effects of crystallization on the spectral properties are readily observed in the unoxidized products. In the VISNIR, increasing crystallization is accompanied by an increasing influence of pyroxene absorptions and decreasing influence of glass absorptions. In the mid‐IR, changes in spectral properties with crystallization are due to the variations in relative amounts of glass, pyroxene, and plagioclase. Oxidation alters the glass and pyroxenes within the samples, leading to changes in VISNIR and mid‐IR spectral shapes, although distinct signatures of oxidation products are not always observed. Both spectral and electron microscopy data indicate that hematite is the dominant oxidation product in the pigeonite and glass of the samples. In pigeonite, hematite is present in nanophase form and leads to the diminishment of the diagnostic 1‐ and 2‐μm pyroxene absorptions in the VISNIR as well as pyroxene absorptions expressed between 800 and 1100 cm−1 in the mid‐IR. In glass, hematite forms a near‐surface layer that is detected with varying efficiencies in the VISNIR and mid‐IR depending on the crystallinity and particle size of the analyzed sample. Oxidized SNC basalt samples of various crystallinities and fine grain sizes provide the best analogs to Mars remote‐sensing data in the VISNIR and mid‐IR. In the VISNIR, partially crystalline, oxidized SNC basalts can reproduce Martian dark region characteristics. In the mid‐IR, similarities exist between the spectra of a partially crystalline, oxidized SNC basalt and the basalt lithology detected by the Thermal Emission Spectrometer (TES). The andesite lithology detected by TES is best matched by the spectrum of a fully crystalline, oxidized SNC basalt. These results imply that plagioclase‐rich lithologies are not required by TES data and that observed variations in spectral character of dark regions across the Martian surface can be explained by SNC basalts influenced by oxidation coupled with variations in degree of crystallization.
Candidate subglacial volcanoes in the south polar region of Mars: Morphology, morphometry, and eruption conditions
Authors: Ghatan Gil J. , Head James W.
Published: October 2002

A number of isolated mountains mapped in the south polar region of Mars are documented using Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) data. These mountains have average separation distances of ∼175 km, they are typically 30–40 km in diameter and 1000–1500 m high, and their bases fall near an elevation of ∼1200 m. A significant number of the population are located on or very close to a 660 km long line extending toward the south pole. The summits of a number of these mountains have unusual shapes: flat‐topped, flat‐topped with a cone, and large summit craters relative to the summit diameter. Sinuous channels are found in association with the margins of several of the mountains. Several modes of origin are considered for these mountains, including impact, tectonic, and volcanic. These mountains occur in intimate association with the Hesperian‐aged Dorsa Argentea Formation, a unit containing features and structures interpreted to be related to a thick, areally extensive ice sheet. On the basis of these observations and analyses it is concluded that the most plausible origin for these mountains is volcanic and that they represent extrusion of lava from vents, many of which lay underneath an ice sheet. The unusual shape of many of the mountains is consistent with construction of a volcanic edifice underneath an ice sheet and melting of adjacent ice‐rich material, sometimes forming drainage channels. The topography of these mountains suggests that the ice sheet averaged at least 1.4 km thick at the time of eruption. These features appear to be associated with Early Hesperian‐aged ridged plains (Hr).
New insights into Martian dust distribution and water‐ice cloud microphysics
Authors: Montmessin Franck , Rannou Pascal , Cabane Michel
Published: October 2002

In this paper we use a direct method which combines microphysics and photometric simulations of the Martian atmosphere. This approach allows us to reproduce vertical profiles of the light scattered by the haze at the limb of Mars. Subsequent results are compared to a multiple‐color image provided by the Viking Orbiter camera. The “free parameters” of our study are related to vertical transport and dust size distribution. A sensitivity study has been conducted, encompassing a relevant range of parameters, in order to obtain the most satisfying multispectral modeled profile with respect to the one inferred from data. Except for an unlikely distribution of submicron particles (reff ∼ 0.2 μm), no consistent fit can be obtained. This implies that size distributions of dust suggested by previous studies are not in agreement with the present analysis. This apparent discrepancy can be resolved by adding a separate peak of submicron particles to the size distribution previously extracted from Viking Lander images. Only in that case can spectral and vertical structures of haze be successfully matched. While a bimodal distribution would change the current picture of Martian dust, such distribution is a common representation of soil‐derived aerosol size function in Earth deserts. We also attempted to derive information from the cloud shown in the Viking limb image. Our estimates suggest that cloud particle effective radius is around 1.2–1.8 μm, while the cloud visible opacity is 0.02.
Cataclysmic bombardment throughout the inner solar system 3.9–4.0 Ga
Authors: Kring David A. , Cohen Barbara A.
Published: October 2002

Cohen et al. [2000] recently confirmed the hypothesis that the Moon was resurfaced by an intense period of impact cratering ∼3.9 Ga ago and, by inference, that the Earth also sustained bombardment. Analyses of lunar impact melts indicate that at least one of the projectiles that hit the Moon was a differentiated iron‐rich core, implying the bombardment was caused by asteroids. Meteorite analyses indicate asteroids in the asteroid belt were also heavily cratered ∼3.9 Ga and that the ancient cratered highlands of Mars suffered impacts at this time. Collectively, these data suggest there was an impact cataclysm that affected the entire inner solar system, resurfacing the terrestrial planets, and that the source of the impacting debris was the asteroid belt. Comets do not appear to have been important.
Trajectories and distribution of material ejected from the Chicxulub impact crater: Implications for postimpact wildfires
Authors: Kring David A. , Durda Daniel D.
Published: October 2002

The trajectories of low‐ and high‐energy ejecta from the Chicxulub impact crater have been computed using a numerical code and compared with analyses of debris deposited in K/T boundary sections. These calculations indicate that low‐energy ejecta produces centimeter‐ to meter‐thick deposits within ∼4000 km of the point of impact, although its azimuthal distribution depends on the incident angle of the projectile with the surface of the Earth. The high‐energy ejecta or vapor plume rises through the atmosphere and isotropically expands at the top of the atmosphere. The velocity of this ejecta increases as it rises. Approximately 12% of the high‐energy ejecta is lost because it reaches escape velocities, while ∼25% of the material reaccretes within 2 hours, ∼55% reaccretes within 8 hours, and ∼85% reaccretes within 72 hours. This ejecta is distributed globally, but it is concentrated around the Chicxulub impact site and at the antipode (corresponding to India and the Indian Ocean 65 million years ago); it is also slightly smeared in a longitudinal direction because of Earth's rotation. Because this debris does not reaccrete all at once, the collisional shock heating of the atmosphere will be drawn out over a period of a few days and will occur in pulses. This could be an important factor when calculating postimpact chemical reactions in the stratosphere. The calculations also indicate the ejecta would have ignited wildfires on several continents around the world, although the distribution of those fires depends partly on the trajectory of the projectile.
Investigation of the nature and stability of the Martian seasonal water cycle with a general circulation model
Authors: Richardson Mark I. , Wilson R. John
Published: October 2002

We describe the first use of a general circulation model to study the Martian water cycle. Water is treated as a passive tracer, except for ice‐albedo coupling. The model is used to assess which mechanisms and water reservoirs are critical to the seasonal evolution of water and specifically the attainment of an interannually repeatable steady state. The model comes to a reasonable steady state with active surface ice and atmospheric vapor and ice reservoirs. A regolith is not necessary. The mechanism of equilibration results from independent parameters controlling the transport of water between the northern polar and the extratropical atmospheres at different seasons. Water export from the northern summer pole results from weak mixing across a strong vapor gradient, dependent upon northern cap temperatures. Import at other seasons depends on stronger mixing and weak vapor gradients, which are history dependent. Equilibration is achieved when the fluxes balance, minus a small net loss to the south. We find that with a southern residual CO2 cap, the water cycle cannot be completely closed. We conclude that the northern summer cap temperature determines the bulk humidity of the atmosphere, all else being equal. We proceed to show that a water cap exposed in southern summer would be unstable with respect to the north for dynamical as well as thermal reasons. At high obliquity (45°), much higher vapor abundances result in more widespread surface ice with seasonal ice caps overlapping in the equinoctial subtropics, producing year‐round stability of water ice just north of the equator.
Medusae Fossae Formation: New perspectives from Mars Global Surveyor
Authors: Bradley Bethany A. , Sakimoto Susan E. H. , Frey Herbert , Zimbelman James R.
Published: October 2002

The nature and origin of the Medusae Fossae Formation (MFF) on Mars has been debated since the return of the first Viking images. The MFF's young age, distinctive surface texture, and lack of obvious source have prompted multiple hypotheses for its origin. This study uses data from the Mars Global Surveyor (MGS) mission to examine the MFF at all available scales. We discuss and quantify observations from Mars Orbiter Laser Altimeter (MOLA) topography and Mars Orbiter Camera (MOC) images to better constrain the origin of the MFF. Topographic grid estimates yield a present extent of 2.1 × 106 km2 and a volume of 1.4 × 106 km3; however, remnant yardang deposits observed far from the thicker lobes of MFF material suggest that it may have once covered up to 5 × 106 km2. We do not find compelling evidence for extensive fluvial reworking of the MFF; however, in several regions, buried channels are apparent in the MFF because the formation is draped over underlying topography. Layering is apparent at all scales, from submeter to hundreds of meters, with variable resistance to weathering. Continuity of layers appears to be local to regional, but not likely formation‐wide. Yardangs form both parallel and bidirectional patterns, with resistant layers and jointing probably influencing their orientations. A comparative study of MFF regional topography and surface expression indicates that the MFF is quantitatively dissimilar to Martian polar layered deposits. The material is most likely a friable and irregularly consolidated air fall deposit of probable volcanic origin.
Mass flux in the ancient Earth‐Moon system and benign implications for the origin of life on Earth
Authors: Ryder Graham
Published: October 2002

The origin of life on Earth is commonly considered to have been negatively affected by intense impacting in the Hadean, with the potential for the repeated evaporation and sterilization of any ocean. The impact flux is based on scaling from the lunar crater density record, but that record has no tie to any absolute age determination for any identified stratigraphic unit older than ∼3.9 Ga (Nectaris basin). The flux can be described in terms of mass accretion [Hartmann, 1980], and various independent means can be used to estimate the mass flux in different intervals. The critical interval is that between the end of essential crustal formation (∼4.4 Ga) and the oldest mare times (∼3.8 Ga). The masses of the basin‐forming projectiles during Nectarian and early Imbrian times, when the last 15 of the ∼45 identified impact basins formed, can be reasonably estimated as minima. These in sum provide a minimum of 2 × 1021 g for the mass flux to the Moon during those times. If the interval was 80 million years (Nectaris 3.90 Ga, Orientale 3.82 Ga), then the flux was ∼2 × 1013 g/yr over this period. This is higher by more than an order of magnitude than a flux curve that declines continuously and uniformly from lunar accretion to the rate inferred for the older mare plains. This rate cannot be extrapolated back increasingly into pre‐Nectarian times, because the Moon would have added masses far in excess of itself in post‐crust‐formation time. Thus this episode was a distinct and cataclysmic set of events. There are ∼30 pre‐Nectarian basins, and they were probably part of the same cataclysm (starting at ∼4.0 Ga?) because the crust is fairly intact, the meteoritic contamination of the pre‐Nectarian crust is very low, impact melt rocks older than 3.92 Ga are virtually unknown, and ancient volcanic and plutonic rocks have survived this interval. The accretionary flux from ∼4.4 to ∼4.0 Ga was comparatively benign. When scaled to Earth, even the late cataclysm does not produce ocean‐evaporating, globally sterilizing events. The rooted concept that such events took place is based on the extrapolation of a nonexistent lunar record to the Hadean. The Earth from ∼4.4 to ∼3.8 Ga was comparatively peaceful, and the impacting itself could have been thermally and hydrothermally beneficial. The origin of life could have taken place at any time between 4.4 and 3.85 Ga, given the current impact constraints, and there is no justification for the claim that life originated (or reoriginated) as late as 3.85 Ga in response to the end of hostile impact conditions.
Low‐albedo surfaces and eolian sediment: Mars Orbiter Camera views of western Arabia Terra craters and wind streaks
Authors: Edgett Kenneth S.
Published: October 2002

High spatial resolution (1.5 to 6 m/pixel) Mars Global Surveyor Mars Orbiter Camera images obtained September 1997 to June 2001 show that each of the large, dark wind streaks of western Arabia Terra originate at a barchan dune field on a crater floor. The streaks consist of a relatively thin (<1 m) coating of sediment deflated from the dune fields and their vicinity. In most cases, this sediment drapes over a previous mantle that more thickly covers nearly all of western Arabia Terra. No concurrent eolian bedforms are found within the dark streaks, nor do any dunes climb up crater walls to deliver sand via saltation to the streaks. The relations between dunes, wind streak, and subjacent terrain imply that dark‐toned grains finer than those that comprise the dunes are lifted into suspension and carried out of the craters to be deposited on the adjacent terrain. Previous eolian physics and thermal inertia studies suggest that, under modern Martian conditions, such grains likely include silt (3.9–62.5 μm), very fine sand (62.5–125 μm), and possibly fine sand (to ∼210 μm). The streaks change in terms of extent, relative albedo, and surface pattern over periods measured in years; however, through June 2001, very little evidence for recent eolian activity (dust plumes, storms, dune movement) was observed.
Multiyear Mars Orbiter Camera (MOC) observations of repeated Martian weather phenomena during the northern summer season
Authors: Cantor Bruce , Malin Michael , Edgett Kenneth S.
Published: October 2002

The Mars Global Surveyor Mars Orbiter Camera (MGS MOC) has observed several atmospheric events repeat within a few degrees of Ls from one year to the next over 3 Mars years during a portion of the northern summer, southern winter seasons (Ls = 124°–140°). The repeatability suggests a previously unobserved lack of significant variation in the present Martian northern summer seasonal climate. Nonetheless, interannual variations in the seasonal south polar cap and in the residual north polar cap were observed. These appear to have had a negligible effect in perturbing Mars' weather during the northern summer season.
Effects of lithosphere on the long‐wavelength gravity anomalies and their implications for the formation of the Tharsis rise on Mars
Authors: Zhong Shijie
Published: October 2002

The Tharsis rise represents the most significant long‐wavelength gravity and topography anomalies on Mars. Two competing models have been proposed to explain the origin of these anomalies. In the first model the Tharsis rise is attributed to loading of volcanic construction on the lithosphere, while in the second model these anomalies are explained as dynamic effects of a one‐plume thermal structure below the Tharsis rise. In this study we seek to distinguish between these two models by formulating a generalized viscoelastic loading model that determines responses to loads at different depths. With this new formulation we found that the instantaneous viscous flow loading formulation used to compute responses from a plume may significantly overestimate the geoid anomalies if the elastic thickness of the Tharsis lithosphere is ∼150 km as inferred from gravity and topography data. We suggest that the one‐plume structure if currently existing below the Tharsis rise may be responsible for only a small fraction (<10%) of the Tharsis geoid anomalies and that other processes, including surface loading of volcanic construction, may be the primary cause of the geoid anomalies. The plume structure is inefficient in producing geoid anomalies because the long‐wavelength components of the plume structure are confined to a ∼300 km layer below the lithosphere in which the geoid response is minimal. Future studies on gravity and topography at different wavelengths may put constraints on possible contribution of a plume to the elevated Tharsis topography.
Pyroxene reflectance spectra: Minor absorption bands and effects of elemental substitutions
Authors: Cloutis Edward A.
Published: October 2002

Reflectance spectra of a suite of compositionally diverse pyroxenes exhibit variable spectral properties which are associated with various elemental substitutions. Those associated with transition series elements, such as Cr, Ti, V, and Mn, give rise to a number of minor absorption bands in the visible spectral region and, in some cases (e.g., Mn), at longer wavelengths. Substitutions by other cations, such as Li and Zr, do not result in distinct absorption bands. The spectra of these pyroxenes are dominated by the transition series elements that may be present in the samples. The visible wavelength region of low‐calcium pyroxene reflectance spectra exhibit a number of absorption bands which are attributable to Fe2+ spin‐forbidden crystal field transitions. Fe2+‐Fe3+ intervalence charge transfer absorption bands near 0.77 μm are common in terrestrial pyroxenes which contain both ferrous and ferric iron. Collectively, these results indicate that the presence of various transition series elements can be detected in reflectance spectra and that their oxidation states and site occupancies can also be determined on the basis of unique spectral characteristics present in their reflectance spectra, thereby greatly increasing the range of compositional information which can be derived from analysis of their reflectance spectra.
Controls on Martian hydrothermal systems: Application to valley network and magnetic anomaly formation
Authors: Harrison Keith P. , Grimm Robert E.
Published: October 2002

Models of hydrothermal groundwater circulation can quantify limits to the role of hydrothermal activity in Martian crustal processes. We present here the results of numerical simulations of convection in a porous medium due to the presence of a hot intruded magma chamber. The parameter space includes magma chamber depth, volume, aspect ratio, and host rock permeability and porosity. A primary goal of the models is the computation of surface discharge. Discharge increases approximately linearly with chamber volume, decreases weakly with depth (at low geothermal gradients), and is maximized for equant‐shaped chambers. Discharge increases linearly with permeability until limited by the energy available from the intrusion. Changes in the average porosity are balanced by changes in flow velocity and therefore have little effect. Water/rock ratios of ∼0.1, obtained by other workers from models based on the mineralogy of the Shergotty meteorite, imply minimum permeabilities of 10−16m2during hydrothermal alteration. If substantial vapor volumes are required for soil alteration, the permeability must exceed 10−15m2. The principal application of our model is to test the viability of hydrothermal circulation as the primary process responsible for the broad spatial correlation of Martian valley networks with magnetic anomalies. For host rock permeabilities as low as 10−17m2and intrusion volumes as low as 50 km3, the total discharge due to intrusions building that part of the southern highlands crust associated with magnetic anomalies spans a comparable range as the inferred discharge from the overlying valley networks.
Simulation of surface meteorology at the Pathfinder and VL1 sites using a Mars mesoscale model
Authors: Tyler Daniel , Barnes Jeffrey R. , Haberle Robert M.
Published: October 2002

The Fifth‐Generation Penn State/NCAR Mesoscale Model (MM5) has been adapted to study circulations in the Martian atmosphere. The NASA Ames Mars general circulation model (GCM) provides initial and boundary conditions. The meteorology of this Mars MM5 (the OSU MMM5) is compared with Pathfinder and Viking Lander 1 (VL1) data for late northern summer. The MMM5 uses an equator‐crossing semiglobal polar stereographic mother domain, significantly reducing the boundary reflections inherent in Martian mesoscale simulations. Using two‐way nests, simultaneous simulations of two regions are performed: (1) Chryse Planitia (Mars Pathfinder (MPF)/VL1) and (2) the central chasmas of Valles Marineris. Simulations are hydrostatic and dry. The MMM5 uses the same atmospheric radiation package as the GCM but a much enhanced near‐surface vertical resolution and a different Planetary Boundary Layer (PBL) scheme. Model topography, thermal inertia, and surface albedo maps have all been developed using the most recent data from the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) experiments. The diurnal cycles of surface air temperature, surface pressure, and surface wind all show improvement in comparison with the GCM. For certain regions the local surface pressure tidal amplitudes are strongly dependent on the resolution of the model and/or the topography; VL1 and MPF are in such a region. The diurnal cycles of wind are complex, and for many locations the near‐surface winds are dominated by slope flows of multiple scales. Comparison with data indicates that resolving these slope flows is very important for simulating the diurnal wind cycle. At specific locations these slope flows dramatically influence the diurnal surface pressure cycle.
An inquiry into the lunar interior: A nonlinear inversion of the Apollo lunar seismic data
Authors: Khan A. , Mosegaard K.
Published: October 2002

This study discusses in detail the inversion of the Apollo lunar seismic data and the question of how to analyze the results. The well‐known problem of estimating structural parameters (seismic velocities) and other parameters crucial to an understanding of a planetary body from a set of arrival times is strongly nonlinear. Here we consider this problem from the point of view of Bayesian statistics using a Markov chain Monte Carlo method. Generally, the results seem to indicate a somewhat thinner crust with a thickness around 45 km as well as a more detailed lunar velocity structure, especially in the middle mantle, than obtained in earlier studies. Concerning the moonquake locations, the shallow moonquakes are found in the depth range 50–220 km, and the majority of deep moonquakes are concentrated in the depth range 850–1000 km, with what seems to be an apparently rather sharp lower boundary. In wanting to further analyze the outcome of the inversion for specific features in a statistical fashion, we have used credible intervals, two‐dimensional marginals, and Bayesian hypothesis testing. Using this form of hypothesis testing, we are able to decide between the relative importance of any two hypotheses given data, prior information, and the physical laws that govern the relationship between model and data, such as having to decide between a thin crust of 45 km and a thick crust as implied by the generally assumed value of 60 km. We obtain a Bayes factor of 4.2, implying that a thinner crust is strongly favored.
Drainage evolution in the Margaritifer Sinus region, Mars
Authors: Grant John A. , Parker Timothy J.
Published: October 2002

Margaritifer Sinus, Mars, records a complex history of water transport, storage, and release extending from the late Noachian into at least the mid‐Hesperian. Collection, transport, and discharge of water were accomplished by systems of differing character flanking opposite sides of the Chryse Trough. Drainage on the western side of the trough was via the segmented Uzboi‐Ladon‐Margaritifer mesoscale outflow system that heads in Argyre Basin and incises and fills as it crosses ancient multiringed impact basins. By contrast, Samara and Parana‐Loire Valles, two of the largest and best integrated valley systems on Mars, dominated drainage on the eastern trough flank. Valley morphometry suggests formation was due to precipitation‐recharged groundwater sapping. All systems discharged into Margaritifer Basin, located along the Chryse Trough axis, and caused ponding that persisted into the early Hesperian. The Uzboi‐Ladon‐Margaritifer system dominated discharge that was coincident with widespread geomorphic activity on Mars. As channel and valley formation ended, some water in Margaritifer Basin infiltrated the subsurface. Collapse and release of water began shortly thereafter and persisted into mid‐Hesperian times, thereby forming Margaritifer and Iani Chaos and incising Ares Vallis.
Numerical simulations of the Mjølnir marine impact crater
Authors: Shuvalov Valery , Dypvik Henning , Tsikalas Filippos
Published: October 2002

The 40‐km‐diameter Mjølnir crater in the Barents Sea was created by a meteoroid impact into a 400 m shallow sea about 142 Ma. We have used a multimaterial hydrocode with depth‐depending target strength to model numerically the cratering and early modification stages of the Mjølnir impact. Our models are constrained by the observed crater morphology and structure. The best results were obtained for a composite target strength structure: very low strength for the upper 3 km of sedimentary rocks and gradual increase from 3 to 6.5 km depth before using strength values typical for granitic rocks at greater depths. The low strength of the target sediments at shallow depths led to exceptionally intensive slumping and gravitational collapse of the order of 2.0–2.5, considerably larger than the average expected values for typical terrestrial craters. As a result, we obtain a crater collapse in between simple and complex, where slumping and gravitational collapse counteract the crater floor uplift and lead to suppression and burial of the central high. The Mjølnir impact in a shallow sea was responsible for a major disturbance of the water column. Following the impact‐induced outward water surge, large‐amplitude tsunamis were formed, and the rapid resurge of seawater into the excavated crater transported large amounts of ejecta and crater wall material back into the crater, accounting for the redestribution of ejecta material and extensive infilling.
Carbon dioxide snow storms during the polar night on Mars
Authors: Colaprete Anthony , Toon Owen B.
Published: October 2002

The Mars Orbiter Laser Altimeter detected clouds during the polar night on Mars. While these clouds are associated with topographic features, and are clearly mountain wave clouds as suggest by Pettengill and Ford [2000], we suggest that they differ greatly from terrestrial mountain wave clouds. Uplift generated from flow over the mountains may initiate the clouds on both planets. Terrestrial wave clouds are generally compact features with sharp edges. However, we find that the relatively large mass of condensate on Mars leads to diffuse clouds with snow tails that may extend many kilometers downwind from the mountain and even reach the surface. Both the observations and the simulations suggest substantial carbon dioxide snow precipitation in association with the underlying topography. This precipitation deposits CO2, dust, and water ice to the surface and may lead to propagating geologic features in the Martian polar regions.
Meltback of Hesperian‐aged ice‐rich deposits near the south pole of Mars: Evidence for drainage channels and lakes
Authors: Milkovich Sarah M. , Head James W. , Pratt Stephen
Published: October 2002

Mars Orbiter Laser Altimeter (MOLA) topographic data support the presence of an extensive Hesperian‐aged volatile‐rich south polar deposit, the Dorsa Argentea Formation (DAF) and related deposits, underlying the present Amazonian‐aged cap. The eastern margin of these deposits displays further evidence for meltback, ponding, and drainage of the volatile‐rich deposit. Channels leading from the margins of the DAF deposit enter nearby craters and are interpreted to represent drainage of water, ice, and sediment from the DAF. Channels connecting these craters provide evidence for extensive crater flooding, ponding (minimum volumes ∼1012m3), overtopping, downcutting, and further drainage of material through a series of craters and into the Prometheus Basin near the edge of the current cap. Topography data show that water filled some craters to depths of at least 200 m and possibly as much as 600 m. Materials derived from melting of this lobe of the DAF drained over a lateral distance of ∼600 km and a vertical height of ∼800 m, partially emptying into the Prometheus Basin. These combined observations imply that meltback of the older, volatile‐rich deposit was a widespread event, occurring on both sides of the deposit over a distance of at least 1800 km, and in an area possibly as much as ∼2.9 × 106km2. Candidate top‐down and bottom‐up melting scenarios are examined; the presence of associated Hesperian‐aged volcanic deposits and possible subglacial edifices suggests that local and regional basal heating was a major factor in generating the meltwater that formed the drainage features.
Is asteroid 951 Gaspra in a resonant obliquity state with its spin increasing due to YORP?
Authors: Rubincam David Parry , Rowlands David D. , Ray Richard D.
Published: October 2002

Asteroid 951 Gaspra appears to be in an obliquity resonance with its spin increasing due to the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack effect (YORP effect, for short). Gaspra, an asteroid 5.8 km in radius, is a prograde rotator with a rotation period of 7.03 hours. A 3 × 106 year integration indicates that its orbit is stable over at least this time span. From its known shape and spin axis orientation and assuming a uniform density, Gaspra's axial precession period turns out to be nearly commensurate with its orbital precession period, which leads to a resonance condition with consequent huge variations in its obliquity. At the same time, its shape is such that the YORP effect is increasing its spin rate. YORP may be a reason for small asteroids entering resonances in the first place: they speed up or slow down and fall into resonances. The continued action of YORP probably ultimately causes asteroids to leave resonances, so that they are quasi‐stable states.
Drainage basin evolution in Noachian Terra Cimmeria, Mars
Authors: Irwin Rossman P. , Howard Alan D.
Published: October 2002

Geomorphic mapping of a ∼1 million square kilometer section of Terra Cimmeria on Mars (1:1M scale) indicates that prolonged, intense fluvial erosion occurred during the period of heavy bombardment. Crater counts date the termination of ubiquitous, intense erosion to the late Noachian, although some valleys may have continued downcutting into the early Hesperian. Stratigraphic and topographic relationships indicate that early erosional processes created large, integrated drainage basins, affected primarily by large impact basin structures and regional slopes. This terrain is not consistent with an origin purely by volcanic or impact processes. Cratering competed with drainage basin development, minimizing valley length, catchment area, and valley network integration. Drainage basin disruption resulted from impacts on valley thalwegs, or when the ejecta of large (usually >75 km) craters created low divides. Near‐level upland intercrater plains are lightly dissected, in part because smaller craters could interrupt flow paths on gently sloping or flat terrain. Some closed drainage basins became integrated by continued erosion of drainage divides and stream capture, overflow of the divides, or headward growth of valleys fed by groundwater collected in closed basins. Drainage divide breaching was most effective on steeper (>0.5°) regional slopes, where observed drainage density is also highest. This is due to greater runoff volumes and velocity encouraging valley incision, as well as steep slopes allowing valleys to bypass or breach superimposed craters. Valley systems commonly extend nearly to the crests of sharp drainage divides, and spatially ubiquitous valley source points throughout the higher elevations suggest that runoff derived largely from precipitation.
Geologic mapping of the Chaac‐Camaxtli region of Io from Galileo imaging data
Authors: Williams David A. , Radebaugh Jani , Keszthelyi Laszlo P. , McEwen Alfred S. , Lopes Rosaly M. C. , Douté Sylvain , Greeley Ronald
Published: October 2002

We produced a geologic/geomorphologic map of the Chaac‐Camaxtli region of Io's leading anti‐Jovian hemisphere using regional resolution (186 m/pixel) Galileo imaging data collected during orbit I27 (February 2000) integrated with lower resolution (1.4 km/pixel) color data, along with other Galileo imaging and spectral data. This is the first regional map of Io made from Galileo data. Nine color and geomorphologic units have been mapped, and the close proximity of dark and various colored bright materials suggests that there is an intimate interaction between (presumably) silicate magmas and sulfur bearing volatile materials that produced a variety of explosive and effusive deposits in the recent geologic past. This region of Io is dominated by 11 volcanic centers, most of which are paterae that are analogous in morphology to terrestrial calderas but larger in size. Mapping of structural features indicates that most of the active regions occur in topographic lows, and less active or inactive paterae are associated with topographic highs. This indicates that crustal thickness variations influence magma access to the surface. Surface changes in this region since the Voyager flybys (1979) are relatively minor (additional bright and dark flows, color changes), although several active vents have migrated within paterae. This observation, along with the identification of the relatively regular spacing of paterae (∼100–150 km) along a line, may indicate there are multiple interlacing fractures in the crust that serve as magma conduits from the interior. This connection between volcanism and tectonism may have implications for tidal heating mechanisms and their effect on Io's lithosphere. Some inactive patera floors may be evolving into bright plains material, which, if composed of silicates, might explain the strength of Io's crust to support steep patera walls and tall mountains.
Ejecta distribution patterns at Meteor Crater, Arizona: On the applicability of lithologic end‐member deconvolution for spaceborne thermal infrared data of Earth and Mars
Authors: Ramsey Michael S.
Published: October 2002

A spectral deconvolution using a constrained least squares approach was applied to airborne thermal infrared multispectral scanner (TIMS) data of Meteor Crater, Arizona. The three principal sedimentary units sampled by the impact were chosen as end‐members, and their spectra were derived from the emissivity images. To validate previous estimates of the erosion of the near‐rim ejecta, the model was used to identify the areal extent of the reworked material. The outputs of the algorithm reveal subtle mixing patterns in the ejecta, identified larger ejecta blocks, and were used to further constrain the volume of Coconino Sandstone present in the vicinity of the crater. The availability of the multialtitude data set also provided a means to examine the effects of resolution degradation and quantify the subsequent errors on the model. These data served as a test case for the use of image‐derived lithologic end‐members at various scales, which is critical for examining thermal infrared data of planetary surfaces. The model results indicate that the Coconino Ss. reworked ejecta is detectable over 3 km from the crater. This was confirmed by field sampling within the primary ejecta field and wind streak. The areal distribution patterns of this unit imply past erosion and subsequent sediment transport that was low to moderate compared with early studies and therefore places further constraints on the ejecta degradation of Meteor Crater. It also provides an important example of the analysis that can be performed on thermal infrared data currently being returned from Earth orbit and expected from Mars in 2002.
Laminations from the Palmer Deep: A diatom‐based interpretation
Authors: Leventer Amy , Domack Eugene , Barkoukis Athan , McAndrews Beth , Murray Jane
Published: October 2002

Highly laminated, Holocene age, diatomaceous sediments are characteristic of the Palmer Deep, western Antarctic Peninsula (Ocean Drilling Program (ODP) Leg 178, Site 1098). From ∼10,000 years B.P. to the present, laminations are comprised of several groups of diatoms. Chaetoceros resting spores, the dominant laminae former, result from intense spring blooms. Rhizosolenia, Proboscia, Thalassiothrix, and Corethron are common also, the consequence of summer production marked by a well‐stratified water column. Each lamination represents a single productivity event, but laminations are not necessarily annual. High concentrations of a subpolar form of Eucampia antarctica, in laminations between ∼9000 and 6700 years B.P., suggest early Holocene warmth, a consequence of southward intrusion of more subpolar waters. The glacial‐interglacial transition is distinguished by pairs of laminae most likely deposited annually. Laminations with an overwhelming dominance of Chaetoceros resting spores alternate with more “terrigenous” laminae, representing alternation of intense spring blooms, with more mixed deposition during the summer. Proximity to retreating glacial ice results in the supply of silt and sand that provides a marker bed between successive blooms.
Glacial‐interglacial sea surface temperature changes across the subtropical front east of New Zealand based on alkenone unsaturation ratios and foraminiferal assemblages
Authors: Sikes E. L. , Howard W. R. , Neil H. L. , Volkman J. K.
Published: October 2002

We present sea surface temperature (SST) estimates based on the relative abundances of long‐chain C37 alkenones (U37K′) in four sediment cores from a transect spanning the subtropical to subantarctic waters across the subtropical front east of New Zealand. SST estimates from U37K′ are compared to those derived from foraminiferal assemblages (using the modern analog technique) in two of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of ∼18°C in subtropical waters and ∼14°C in subpolar waters, with a 4°–5°C gradient across the front. Down core U37K′ SST estimates indicate that the regional summer SST was 4°–5°C cooler during the last glaciation with an SST of ∼10°C in subpolar waters and an SST of ∼14°C in subtropical waters. Temperature reconstructions from foraminiferal assemblages agree with those derived from alkenones for the Holocene. In subtropical waters, reconstructions also agree with a glacial cooling of 4° to ∼14°C. In contrast, reconstructions for subantarctic pre‐Holocene waters indicate a cooling of 8°C with glacial age warm season water temperatures of ∼6°C. Thus the alkenones suggest the glacial temperature gradient across the front was the same or reduced slightly to 3.5°–4°C, whereas foraminiferal reconstructions suggest it doubled to 8°C. Our results support previous work indicating that the STF remained fixed over the Chatham Rise during the Last Glacial Maximum. However, the differing results from the two techniques require additional explanation. A change in euphotic zone temperature profiles, seasonality of growth, or preferred growth depth must have affected the temperatures recorded by these biologically based proxies. Regardless of the specific reason, a differential response to the environmental changes between the two climate regimes by the organisms on which the estimates are based suggests increased upwelling associated with increased winds and/or a shallowing of the thermocline associated with increased stratification of the surface layer in the last glaciation.