bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of giant Martian polygons
Authors: McGill George E. , Hills L. Scott
Published: May 2009

Extensive areas of the Martian northern plains in Utopia and Acidalia planitiae are characterized by “polygonal terrane.” Polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks, columnar joints, or frost‐wedge polygons on Earth. However, the Martian polygons are orders of magnitude larger than these potential Earth analogues, leading to severe mechanical difficulties for genetic models based on simple analogy arguments. Plate‐bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons. Although trough widths and depths relate primarily to shrinkage, the large scale of the polygonal pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material. Geological relationships favor a sedimentary origin for polygonal terrane material, but our model is not dependent on the specific genesis. Our analysis also suggests that the polygons must have formed at a geologically rapid rate.
Secondary electron yields of solar system ices
Authors: Suszcynsky David M. , Borovsky Joseph E. , Goertz Christoph K.
Published: May 2009

The secondary electron yields of H2O, CO2, NH3 (ammonia), and CH3OH (methanol) ices have been measured as a function of electron beam energy in the 2‐ to 30‐keV energy range. The ices were produced on a liquid‐nitrogen‐cooled cold finger and transferred under vacuum to a scanning electron microscope where the yield measurements were made. The imaging capabilities of the scanning electron microscope provide a means of correlating the yield measurements with the morphology of the ices and are also used to monitor charging effects. The yields were determined by measuring the amplified current from a secondary electron detector and calibrating this current signal with the amplified current signal from samples of metals with known secondary electron yields. Each of the measured yields is found to decrease with an increase in energy in the 2‐ to 30‐keV range. Estimates are given for the maximum secondary electron yield Ymax of each ice and the energy at which this maximum yield occurs. Implications for the charging of solar system ice grains are discussed.
Historical review of a long‐overlooked paper by R. A. Daly concerning the origin and early history of the Moon
Authors: Baldwin Ralph B. , Wilhelms Don E.
Published: May 2009

In 1946 the great geologist R.A. Daly published an important paper in which he discussed a great many problems concerning the Moon and its features and origin. His paper was almost completely ignored by the scientists of the day and was “lost” for nearly half a century. The present paper marks an attempt to outline Daly's contributions to the interpretation of these lunar problems, in particular the origin of the Moon. One of the major ideas, which probably was the incentive for him to write the paper, was that the Moon was born as a result of a very early glancing collision of the Earth and a planet‐sized body. Other subjects covered are the origin of the craters from Earth fragments, although meteoritic impact is also presented; the nature of the maria as lava from the body of the Moon; and origin of the lines of small craters as produced by gas escaping from the Moon. Daly rejected all non impact models for crater origin except for the tiny gas‐made aligned pits. He vigorously stated that the Moon was largely created from the body of the Earth and discussed three methods by which this could be accomplished, one internal and two external, before settling on the glancing collision model. Daly clearly was the pioneer in presenting the impact model of the origin of the Moon. Much later, works by others have modified the hypothesis, but this is only natural evolution. Two other “lost” papers will be mentioned to show that this is an all too frequent occurrence. In 1893 Gilbert wrote a milestone paper that was generally unrecognized for more than 50 years. He espoused the impact theory of the origin of lunar basins and craters. He was wrong about the mechanism involved, but he was right about the impact idea. Similarly, Öpik in 1916 showed that impact craters must be formed by explosions due to the high energies of striking meteorites. He showed that such impacts, even at low angles of fall would result in circular craters, thus correcting Gilbert, whom he did not mention. His paper also “disappeared” for many years. Early recognition and understanding of these three papers would have advanced lunar studies by many years.
Volatiles in interplanetary dust particles: A review
Authors: Gibson Everett K.
Published: May 2009

The paper presents a review of the volatiles found within interplanetary dust particles. These particles have been shown to represent primitive material from early in the solar system's formation and also may contain records of stellar processes. The organogenic elements (i.e., H, C, N, O, and S) are among the most abundant elements in our solar system, and their abundances, distributions, and isotopic compositions in early solar system materials permit workers to better understand the processes operating early in the evolutionary history of solar system materials. Interplanetary dust particles have a range of elemental compositions, but generally they have been shown to be similar to carbonaceous chondrites, the solar photosphere, Comet Halley's chondritic cores, and matrix materials of chondritic chondrites. Recovery and analysis of interplanetary dust particles have opened new opportunities for analysis of primitive materials, although interplanetary dust particles represent major challenges to the analyst because of their small size.
The Mars Observer laser altimeter investigation
Authors: Zuber M. T. , Smith D. E. , Solomon S. C. , Muhleman D. O. , Head J. W. , Garvin J. B. , Abshire J. B. , Bufton J. L.
Published: May 2009

The primary objective of the Mars Observer laser altimeter (MOLA) investigation is to determine globally the topography of Mars at a level suitable for addressing problems in geology and geophysics. Secondary objectives are to characterize the 1064‐nm wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes, to assist in addressing problems in atmospheric circulation, and to provide geodetic control and topographic context for the assessment of possible future Mars landing sites. The principal components of MOLA are a diode‐pumped, neodymium‐doped yttrium aluminum garnet laser transmitter that emits 1064‐nm wavelength laser pulses, a 0.5‐m‐diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10‐ns resolution. MOLA will provide measurements of the topography of Mars within approximately 160‐m footprints and a center‐to‐center along‐track footprint spacing of 300 m along the Mars Observer subspacecraft ground track. The elevation measurements will be quantized with 1.5 m vertical resolution before correction for orbit‐ and pointing‐induced errors. MOLA profiles will be assembled into a global 0.2° × 0.2° grid that will be referenced to Mars' center of mass with an absolute accuracy of approximately 30 m. Other data products will include a global grid of topographic gradients, corrected individual profiles, and a global 0.2° × 0.2° grid of 1064‐nm surface reflectivity.
Past and present water budget of Venus
Authors: Donahue T. M. , Hodges R. R.
Published: May 2009

A detailed analysis of Pioneer Venus large‐probe neutral mass spectrometer (LNMS) data confirms an earlier report that the abundance of Venus deuterium relative to hydrogen is 2 orders of magnitude larger than that of terrestrial deuterium. These results have recently received confirmation from ground‐based spectroscopic observations. The LNMS data can be analyzed to obtain height profiles for hydrogen compounds because of the very different isotopic signatures of Venus hydrogen and terrestrial hydrogen. The mixing ratio for water increases by a factor of 4 between 0 and 10 km to a value of 2.2 times the 36Ar mixing ratio (67 ppm) and drops to 10 ppm above 50 km. Preliminary analysis has not revealed the presence of any hydrogen constituent that can compensate for the decrease in water vapor mixing ratio below 10 km. No combination of gas phase and surface chemistry has been found to account for the strange behavior, and vertical transport alone is not feasible. These results are inexplicable unless they can be interpreted in terms of large‐scale temporal and spatial variations (for which other evidence also exists). It is argued that present‐day deuterium and hydrogen cannot be in a steady state with stochastic cometary inputs but that these results call for an early Venus endowed with at least 2 orders of magnitude more water above the surface than is presently there. From the isotope ratio measurement alone, however, how the water got there or how much was originally present cannot be discerned.
Reflectivity (visible and near IR), Mössbauer, static magnetic, and X ray diffraction properties of aluminum‐substituted hematites
Authors: Morris Richard V. , Schulze Darrell G. , Lauer Howard V. , Agresti David G. , Shelfer Tad D.
Published: May 2009

Because aluminum substitution for iron occurs in polymorphs of Fe2O3 and FeOOH in terrestrial (and by inference Martian) environments, it is important for the mineralogical remote sensing of both planets at visible and near‐IR wavelengths to know the effects of Al substitution on their reflectivity spectra. Diffuse reflectivity (350–2200 nm), Mössbauer, static magnetic, and X ray diffraction data are reported for a series of aluminum‐substituted hematites α‐(Fe,Al)2O3 for compositions having values of Als. (mole ratio Al/(Al+Fe)) up to 0.61. Samples were prepared by oxidation of magnetite, dehydroxylation of goethite, and direct precipitation. Unit cell dimensions decrease with Als but at a rate less than that predicted by the Vegard rule. At 293 K, Mössbauer spectra are sextets (negative quadrupole splitting, QS) for Als up to ∼0.5 and doublets for larger Als. At 21 K, all compositions are sextets; however, there is a discontinuity of ∼0.5 T in the magnetic hyperfine field (Bhf) and QS changes sign at Als = 0.06(2) (Morin transition). Al‐poor compositions have positive QS and higher Bhf. Negative and positive quadrupole splittings are indicative of the weakly ferromagnetic and antiferromagnetic states of hematites, respectively. The position of the least energetic crystal‐field transition (6A1→4T1g) of ferric iron shifts to longer wavelengths with increasing Als. The magnitude of the shift is a linear function of (1/a)5, where a is the hexagonal unit cell dimension. For geologically reasonable amounts of Al substitution (Als < 0.33), the magnitude of the shift is small (∼20 nm), so that it is problematical (based on reflectivity data alone) to uniquely ascribe shifts in 4T1g band positions to different degrees of Al substitution. On the basis of Martian spectral data, the range in Als for Martian hematites is 0 < Als < 0.19.
Plains tectonism on Venus: The deformation belts of Lavinia Planitia
Authors: Squyres Steven W. , Jankowski David G. , Simons Mark , Solomon Sean C. , Hager Bradford H. , McGill George E.
Published: May 2009

High‐resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. A number of geologic units can be distinguished, including volcanic plains units with a range of ages. Transecting these plains over much of the Lavinia region are two types of generally orthogonal features that we interpret to be compressional wrinkle ridges and extensional grooves. The dominant tectonic features of Lavinia are broad elevated belts of intense deformation that transect the plains with complex geometry. They are many tens to a few hundred kilometers wide, as much as 1000 km long, and elevated hundreds of meters above the surrounding plains. Two classes of deformation belts are seen in the Lavinia region. “Ridge belts” are composed of parallel ridges, each a few hundred meters in elevation, that we interpret to be folds. Typical fold spacings are 5–10 km. “Fracture belts” are dominated instead by intense faulting, with faults in some instances paired to form narrow grabens. There is also some evidence for modest amounts of horizontal shear distributed across both ridge and fracture belts. Crosscutting relationships among the belts show there to be a range in belt ages. In western Lavinia in particular, many ridge and fracture belts appear to bear a relationship to the much smaller wrinkle ridges and grooves on the surrounding plains: Ridge morphology tends to dominate belts that lie more nearly parallel to local plains wrinkle ridges, and fracture morphology tends to dominate belts that lie more nearly parallel to local plains grooves. We use simple models to explore the formation of ridge and fracture belts. We show that convective motions in the mantle can couple to the crust to cause horizontal stresses of a magnitude sufficient to induce the formation of deformation belts like those observed in Lavinia. We also use the small‐scale wavelengths of deformation observed within individual ridge belts to place an approximate lower limit on the venusian thermal gradient in the Lavinia region at the time of deformation.
Pervasive large‐scale magnetic fields in the Venus nightside ionosphere and their implications
Authors: Luhmann J. G.
Published: May 2009

When the solar wind dynamic pressure at Venus was extraordinarily high during the primary mission of the Pioneer Venus Orbiter (PVO), “disappearing ionospheres” occurred on the nightside, with accompanying pervasive near‐periapsis magnetic fields of tens of nanoteslas. These nightside counterparts of the generally horizontal large‐scale magnetic fields in the dayside ionosphere are found to exhibit some dependence of field magnitude on the solar wind pressure but not on solar zenith angle. Their statistical behavior suggests a global configuration in which the low‐altitude field wraps around the planet, while the field at higher altitudes is draped like the induced magnetotail field. The toroidal low‐altitude field geometry implies the possible existence of magnetic x points in the low‐altitude wake.
Atmosphere and climate studies of Mars using the Mars Observer pressure modulator infrared radiometer
Authors: McCleese D. J. , Haskins R. D. , Schofield J. T. , Zurek R. W. , Leovy C. B. , Paige D. A. , Taylor F. W.
Published: May 2009

Studies of the climate and atmosphere of Mars are limited at present by a lack of meteorological data having systematic global coverage with good horizontal and vertical resolution. The Mars Observer spacecraft in a low, nearly circular, polar orbit will provide an excellent platform for acquiring the data needed to advance significantly our understanding of the Martian atmosphere and its remarkable variability. The Mars Observer pressure modulator infrared radiometer (PMIRR) is a nine‐channel limb and nadir scanning atmospheric sounder which will observe the atmosphere of Mars globally from 0 to 80 km for a full Martian year. PMIRR employs narrow‐band radiometric channels and two pressure modulation cells to measure atmospheric and surface emission in the thermal infrared; a visible channel (0.39–4.7 μm) is used to measure solar radiation reflected from the atmosphere and surface. Vertical profiles of atmospheric temperature, the infrared extinction of dust suspended in the atmosphere, atmospheric water vapor, and condensate hazes will be retrieved from infrared measurements having a vertical resolution of 5 km, which is half an atmospheric scale height. PMIRR infrared and visible measurements will be combined to determine the radiative balance of the polar regions, where a sizeable fraction of the global atmospheric mass annually condenses onto and sublimes from the surface. Derived meteorological fields, including diabatic heating and cooling and the vertical variation of horizontal winds, will be computed from the globally mapped fields retrieved from PMIRR data. Analyses of these observed and derived fields will address many key questions regarding the atmosphere and climate of Mars.
Martian parent craters for the SNC meteorites
Authors: Mouginis‐Mark P. J. , McCoy T. J. , Taylor G. J. , Keil K.
Published: May 2009

The young ages (∼1.3 Ga) and the basaltic to ultramafic compositions of the shergottites, nakhlites, and chassignites meteorites severely restrict their potential source regions on Mars. We have used this age and compositional information, together with geologic data derived from Viking Orbiter images, to identify 25 candidate impact craters in the Tharsis region of Mars that could be the source crater for these meteorites. None of these craters are close to the size (∼100 km diameter) implied by the dynamical study of SNC ejection developed by Vickery and Melosh (1987). The craters in our study were selected because they are >10 km in diameter, have morphologies indicative of young craters, and satisfy both the petrologic criteria of the SNCs and the proposed 1.3 Ga crystallization ages. Of these 25 craters, only nine are found on geologic units believed to be young (crater density is less than 570 craters of greater than 1 km diameter per 106 km2). No crater exists to satisfy well the criteria of sampling both a 1.3 Ga surface (nakhlites and Chassigny) and a 180 Ma surface (shergottites) without at the same time imposing significant constraints on the chronology of Mars as inferred from the cumulative crater curves. The relatively young age (based on their inferred position in the stratigraphic column of Tharsis (Scott et al., 1981)) of the SNCs implies that volcanic activity on the plains of the Tharsis region extended well past 1.3 Ga.
Labile trace elements in carbonaceous chondrites: A survey
Authors: Xiao Xiaoyue , Lipschutz Michael E.
Published: May 2009

We report radiochemical neutron activation analysis data for Co, Au, Ga, Rb, Sb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by increasing putative volatility in primary nebular processes) in 42 C2–C6 chondrites, all but three from Antarctica. From these and literature data for 19 additional chondrites, Cl‐normalized concentrations of the nine most volatile elements (Ag → In) are quite constant in most meteorites. Trace element trends in 39 Antarctic and 22 non‐Antarctic carbonaceous chondrites are similar: no evidence exists for substantial alteration by weathering of samples in Antarctica, nor do the data reflect modification by open‐system, parent body metamorphism at ≥ 500°C. Volatile element concentrations and siderophile ratios (Au/Co and Ga/Co) define continua which correlate at statistically significant levels. Carbonaceous chondrites sample not a few, compositionally distinct parents but rather a compositional continuum in which parent materials forming under more oxidizing conditions incorporated lesser complements of volatiles, essentially unfractionated from cosmic composition. This may well reflect the range of formation conditions (temperature, duration, and water/rock ratios) represented by oxygen isotope variations during preterrestrial aqueous alteration of parent materials.
Properties of haze in the atmosphere of Triton
Authors: Krasnopolsky V. A. , Sandel B. R. , Herbert F.
Published: May 2009

Modeling the formation of haze by methane photolysis products requires knowledge of the methane altitude profile and optical data at two wavelengths to determine the two free parameters: the cross‐section‐weighted mean radius rc and the particle material density ρ. Using Voyager ultraviolet spectrometer measurements of CH4 and haze combined with the haze brightness profile determined by the narrow angle camera, we infer a haze optical thickness of 0.024 at 1500 Å and 7.8 × 10−3 in the spectral range of the narrow angle camera centered at 4700 Å, ρ/γ = 0.36±0.1 g cm−3 (γ is the quantum yield of condensate), and values of rc varying from 0.1±0.02 μm at 30 km to 0.15 ± 0.03 μm near the surface. Other auxiliary properties of the haze are also determined. The value found for ρ/γ corresponds to a packing coefficient of 0.6γ if C2H4 is the main condensible species.
An unusual volcano on Venus
Authors: Moore H. J. , Plaut J. J. , Schenk P. M. , Head J. W.
Published: May 2009

Materials that issued from an unusual Venusian volcano produced (1) a complex domical structure about 100 km across with thick, broad flow lobes up to 41 km wide, (2) an extensive sheet of thick flows, and (3) radar‐bright surfaces that extend to 360–400 km from the volcano. Altimetry indicates that the relief of the domical structure is about 0.5–1.1 km. The lobes and flows have prominent regularly spaced ridges ∼ 686–820 m apart. Thick flows with large ridge separations and broad lobes are rare on Venus. We suggest that the viscosities these flows were larger than those of most lava flows on Venus. Comparisons of the dimensions of the volcano's lobes with lava flows on Earth suggest that the Venusian lavas may have large silica contents. Radar‐bright surfaces around the volcano may represent the result of an explosive eruption or very thin deposits of low‐viscosity lavas. Thus we suggest that the radar‐bright surfaces and lavas of the volcano were derived from a magma that differentiated within the crust or mantle of Venus. The differentiation produced (1) a gas‐rich low‐viscosity phase, (2) high‐viscosity lavas, and (3) a residual primary magma.
Seismic evidence for a low‐velocity lunar core
Authors: Sellers Paul C.
Published: May 2009

The seismic evidence for a low‐velocity lunar core has been reexamined. A relocation for a meteorite impact on day 263 in 1973, which was the event used to first estimate the size of the lunar core, has been found which can explain the arrival time and signal amplitude data without requiring the presence of a lunar core. However, tentative identification has been made of arrivals in the seismograms of two of the largest, most distant impacts detected by the lunar seismic network during its period of operation which may have travelled through a lunar core about 400 to 450 km in radius, and with a P wave velocity of about 5 km/s. Thus the lunar seismic data remain ambiguous but can be interpreted in terms of there once being a small liquid core, as required by palaeomagnetic observations.
Critical taper wedge mechanics of fold‐and‐thrust belts on Venus: Initial results from Magellan
Authors: Suppe John , Connors Chris
Published: May 2009

Fold‐and‐thrust belts exist on Venus at the margins of crystal blocks such as plateaus, tesserae, and coronae and as ridge belts within the plains. These fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics, a mechanics that is well known for fold‐and‐thrust belts and accretionary wedges on Earth. For example, an analysis of fold geometry at the toe of the Artemis Chasma fold belt indicates fault‐bend folding above a regionally extensive decollement horizon at a depth of about 1.5 km. Near‐surface deformation on Venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1–2 km. Critical taper wedge mechanics under anticipated Venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10–20°, which is similar to some estimated slopes in the steep parts of the fold belts. The low taper toes of fold belts may be cohesion‐dominated toes on either brittle or plastic decollement horizons. Once the base of the wedge undergoes the brittle‐plastic transition, the surface slope is expected to flatten to near horizontal, in qualitative agreement with many topographic profiles of fold‐and‐thrust belts on Venus. The estimated depth of the brittle‐plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric‐temperature gradient. The relief of fold belts (measured between the toe of the wedge and the flat crest) displays a remarkable linear dependence on absolute elevation (Figure 15), ranging from 6 km for Maxwell Montes at an elevation of +10 km to a few hundred meters at the lowest planetary elevations (0 to −2 km). This remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle‐plastic transition, possibly controlled by an isostatic coupling of elevation, lithospheric thickness, and geothermal gradient.
Regional topographic rises on Venus: Geology of Western Eistla Regio and comparison to Beta Regio and Atla Regio
Authors: Senske D. A. , Schaber G. G. , Stofan E. R.
Published: May 2009

Regional topographic rises in the equatorial region of Venus (Western Eistla Regio, Beta Regio, and Atla Regio) form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes. The geologically least complex of these regions is Western Eistla Regio, an elongated domical rise that contains the volcanoes Sif and Gula montes. Sif Mons is a large volcano (diameter > 100 km) whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing, widespread outpouring of lava, additional fracturing along its northern flank, and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion. Sif contains a 40‐km‐diameter summit caldera within which are nested smaller calderas and pit structures. Theoretical considerations suggest multiple levels of magma emplacement within the volcano, with the primary reservoir residing in the crust below the edifice. Gula Mons is made up of multiple volcanic source regions and is associated with Guor Linea, a zone of lithospheric stretching, faulting, and rifting. The geologically complex highland of Beta Regio is a tectonic junction with three arms of Devana Chasma intersecting at Theia Mons. Magellan data reveal (1) the details of rifting and volcanism concentrated at Theia Mons, (2) that Rhea Mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question, (3) that tesserae are a major unit, arrayed in a pattern around Theia from east to west starting on the eastern flank of Beta and continuing westward to the northern part of Asteria Regio, (4) that the northern periphery of the highland contains coronae connected by fractures forming chains, and (5) the presence of a deformed crater within Devana Chasma, split and extended in an east‐west direction, indicating that part of the rift has undergone as much as 20–30% extension. Like Beta, Atla Regio is a complex area of converging rifts (five separate arms) that are centered on the volcano Ozza Mons. The Magellan images show that volcanism and rifting have occurred contemporaneously at Ozza, with lava flows both covering and being crosscut by faults. A second volcano, Maat Mons, contributes to the infilling of the rift Dali Chasma and does not appear to be deformed by faulting. Two of the rifts at Atla contain chains of coronae, suggesting that upwelling along linear zones of extension has occurred. Beta, Atla, and Western Eistla are similar in that they are broad regional rises, are associated with major rift zones, have large apparent depths of isostatic compensation, and contain large volcanoes. Their associations with features mapped as coronae and tesserae are quite variable, suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment.
The morphology and evolution of coronae on Venus
Authors: Squyres Steven W. , Janes D. M. , Baer Gidon , Bindschadler Duane L. , Schubert Gerald , Sharpton Virgil L. , Stofan Ellen R.
Published: May 2009

Coronae on Venus are large, circular to ovoidal surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. New data from the Magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail. Typical coronae are dominated by concentric tectonic features and have a raised rim, a central region higher than the surounding plains but in many instances lower than the rim, and, commonly, a peripheral depression or “moat”. Some coronae also show significant amounts of radial tectonic structure, and in most cases this predates the concentric features. In addition, there are other features on Venus, recognized for the first time in Magellan data, that consist of domical rises with intense radial tectonic patterns and little or no concentric structure. All of these features commonly are associated with moderate to large quantities of volcanism. In fact, some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography. We explore a model of corona formation that links these forms into a genetic sequence. The model begins with the ascent of a mantle diapir. Upward mantle flow driven by its ascent forces the lithosphere above the diapir upward, producing a gentle dome with a radiating pattern of extensional fractures. As the diapir impinges on the underside of the lithosphere it flattens and spreads, transforming the uplift to a more flat‐topped shape. In this flattened, near‐surface configuration the diapir can cool rapidly. With the resultant loss of buoyancy the raised plateau can relax to form a central sag, a raised rim, and a depressed moat. Concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae. Volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs. Our analyses to date suggest that this scenario is broadly consistent with many of the coronae on Venus. However, there is enormous diversity in corona morphology, and features are present that require substantial deviations from this simple model. In particular, some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas. Some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the Venusian lithosphere.
Mid‐infrared spectroscopy of Pahala ash palagonite and implications for remote sensing studies of Mars
Authors: Crisp Joy , Bartholomew Mary Jane
Published: May 2009

Remote sensing of Mars at mid‐infrared wavelengths could be used to identify different silicate rocks and minerals. Current knowledge suggests that the primary components of the surface of Mars are basaltlike rocks and a palagonitelike dust. To study the mid‐infrared (7–20 μm) spectroscopy of this combination of materials, we examined the influence of Pahala ash palagonite on the biconical reflectance of quartzite and basalt. The reflectance of the palagonite is flat and low (1–2%), unlike the reflectance of quartzite and basalt. The reflectance of quartzite or basalt is significantly reduced by the addition of small amounts of the palagonite dust. A dusting of 1–20 μm palagonite particles and 20–250 μm particle clumps, covering 50% of the quartzite host, reduces the quartzite mid‐infrared reflectance by 50%, as expected for an essentially black coating. Similar data were obtained for basalt. Even for extremely small amounts of palagonite (2–100 μm thickness, averaging 5 μm), the reflectance of the underlying rock is linearly related to exposed surface area. To confirm the opaque behavior of the Pahala ash palagonite coatings, we estimated the imaginary part (k) of the refractive index from transmission measurements. The strong absorption and low reflectance (high emissivity) of the palagonite at 8–11 μm and 17–25 μm explain its opaque behavior. Estimates of the extinction coefficient indicate that even a 5‐μm thickness of Pahala ash palagonite is optically thick in these wavelength ranges where silicate rock identification is critical. Thus mid‐infrared remote sensing of rocks on Mars will be practical only for dust‐free areas.
Global distribution and characteristics of coronae and related features on Venus: Implications for origin and relation to mantle processes
Authors: Stofan Ellen R. , Sharpton Virgil L. , Schubert Gerald , Baer Gidon , Bindschadler Duane L. , Janes Daniel M. , Squyres Steven W.
Published: May 2009

Coronae on Venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies. The annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both. Topographically, coronae are domes, plateaus, plateaus with interior lows, and rimmed depressions. A subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large‐scale calderas. A number of features have been identified with many of the basic characteristics of coronae (similar interior deformation, associations with volcanism, high topography) but lacking a distinct tectonic annulus. These features tend to be somewhat smaller than coronae and may represent “failed” coronae or coronae in an early stage of evolution. The size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form N(D) = kD−α, where N is the number of coronae with maximum widths greater than D (km) and α = 3.05. The spatial distribution of coronae is not random; the features are concentrated in a few groups and along several chains. Coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as Western Eistla Regio. The largest corona, Artemis, is actually larger than several volcanic rises on Venus. Coronae and volcanic rises appear to be surface manifestations of mantle plumes. There is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on Earth. Instead, a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume. The morphology and size distribution of coronae, highlands, and major shields suggest that mantle upwelling on Venus operates either on several spatial scales, with coronae representing smaller‐scale upflows and major volcanic rises representing larger convective upwellings, or on several temporal scales, with coronae representing shorter duration upflows and major volcanic rises representing long‐term upwellings.
Magellan observations of Alpha Regio: Implications for formation of complex ridged terrains on Venus
Authors: Bindschadler Duane L. , DeCharon Annette , Beratan Kathi K. , Smrekar Suzanne E. , Head James W.
Published: May 2009

Magellan images of Alpha Regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain. We examine the complex ridged terrain of Alpha Regio, using morphology and crosscutting relationships between structures to derive a sequence of tectonic events. Structures include broad (∼10–20 km wide) linear and arcuate ridges, fine‐scale (<3 km wide) ridges, linear disruption zones (LDZs) up to several kilometers wide, and numerous grabens (∼5 km wide) and associated scarps and troughs. Based on their morphology, we interpret the broad and fine‐scale ridges as compressional structures, possibly folds. LDZs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric. Graben and associated structures are interpreted as extensional features. They crosscut ridges and LDZs and thus appear to be the youngest structures in Alpha Regio. This sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain (and highlands on Venus in general): a hotspot model and a coldspot model. The presence of compressional features along and parallel to the margins of Alpha Regio and the lack of any high‐elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling. Mantle downwelling appears to be the most likely mode of formation of the upland of Alpha Regio and is likely to be important in other highland regions, such as Ovda and Thetis regiones, which are also dominated by complex ridged terrain.
Features on Venus generated by plate boundary processes
Authors: McKenzie Dan , Ford Peter G. , Johnson Catherine , Parsons Barry , Sandwell David , Saunders Stephen , Solomon Sean C.
Published: May 2009

Various observations suggest that there are processes on Venus that produce features similar to those associated with plate boundaries on Earth. Synthetic aperture radar images of Venus, taken with a radar whose wavelength is 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound source whose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis region of Venus but are not clearly visible elsewhere. The composition of the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraft corresponds to that expected from adiabatic decompression, like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of Earth can also be applied to Venus.
Small volcanic edifices and volcanism in the plains of Venus
Authors: Guest John E. , Bulmer Mark H. , Aubele Jayne , Beratan Kathi , Greeley Ronald , Head James W. , Michaels Gregory , Weitz Catherine , Wiles Charles
Published: May 2009

The most widespread terrain type on Venus consists of volcanic lowland plains. Several styles of volcanism are represented in the plains. The most extensive volcanic units consist of flood lavas, the largest of which have volumes of the order of thousands of cubic kilometers. As with terrestrial flood lavas, they are inferred to have erupted at high effusion rates. They show a range of radar backscatter characteristics indicating different surface textures and ages. Small edifices on the plains occur mainly in clusters associated with fracture belts. The majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter. Volcanic cones have the same size range. Volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3. These are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava. For many domes, the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep, scalloped outer margins. Because of the high atmospheric pressures on Venus, explosive activity is less likely to occur than on Earth. However, n a few plains areas there is evidence of pyroclastic deposits surrounding craters, indicating that volatile contents in some of the magmas may be high in comparison to Earth. The clusters of small volcanic edifices are considered to be analogous to plains volcanism, similar to that of the Snake River Plain of Idaho. There may also be analogues with terrestrial volcanic clusters associated with mid‐oceanic ridges.
Dynamic fragmentation in impacts: Hydrocode simulation of laboratory impacts
Authors: Melosh H. J. , Ryan E. V. , Asphaug E.
Published: May 2009

Impacts into solids are invariably accompanied by the abundant formation of cracks and fragments created by crack linkage. In contrast to the dominantly compressive nature of impact‐generated stress waves, these cracks form in a tensile stress regime created either by reflection of the compressive waves from free surfaces or by displacement of material outward from the impact site. The formation of cracks is thus a complex process, which is also highly nonlinear because crack formation strongly affects the propagation of subsequent stress waves. In this paper we generalize a continuum damage model of dynamic fragmentation originally proposed by Grady and Kipp to two and higher dimensions. We develop algorithms that permit efficient computer implementation of this model in the context of a Lagrangian hydrocode and compare the code predictions to an extensive suite of laboratory impact fragmentation experiments. We find that both the largest fragment size and many details of the fragment size‐number distribution are faithfully reproduced by the code, including the previously enigmatic segmentation of the cumulative size‐number distributions. We also argue that the mode of failure may be different for laboratory size scales and geologically interesting problems such as multikilometer‐scale impact cratering or asteroid fragmentation, making it imperative to use physical modeling rather than empirical scaling laws to address fragmentation at large size scales.
Magellan observations of extended impact crater related features on the surface of Venus
Authors: Campbell D. B. , Stacy N. J. S. , Newman W. I. , Arvidson R. E. , Jones E. M. , Musser G. S. , Roper A. Y. , Schaller C.
Published: May 2009

A total of 57 parabolic‐shaped and 9 approximately circular extended, impact crater related features have been found in Magellan synthetic aperture radar (SAR) and thermal emissivity data covering 92% of the surface of Venus. The parabolic features are, with seven exceptions, oriented E‐W with the apex to the east and the impact crater located just west of the apex. They were first identified in the surface emissivity data derived from Magellan radiometry measurements, but the great majority are only clearly visible in the SAR imagery. The overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the “parent” craters. The floors of almost all these craters have high specific radar backscatter cross sections (i.e., they are bright in the SAR imagery) relative to their surroundings and tend to have low emissivities. Approximately one‐third of the impact craters with diameters ≥15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the SAR or emissivity data. No features have been found which overlie the parabolic features, indicating that they are among the youngest features on the surface of the planet. This suggests that radar‐bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature. A model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the E‐W zonal winds. Fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0.6 cm on the wavelength scale (12.6 cm) surface roughness and, hence, of 1 to 2 cm on the largest particle sizes of interest. Fallout times from 50 km in the Venus atmosphere for particles of this size are about 2 hours, allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s−1. Measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits, are consistent with deposit depths of a few centimeters to 1 or 2m.
Styles of deformation in Ishtar Terra and their implications
Authors: Kaula William M. , Bindschadler Duane L. , Grimm Robert E. , Hansen Vicki L. , Roberts Kari M. , Smrekar Suzanne E.
Published: May 2009

Ishtar Terra, the highest region on Venus, appears to have characteristics of both plume uplifts and convergent belts. Magellan imagery over longitudes 330°–30°E indicates a great variety of tectonic and volcanic activity, with large variations within distances of only a few 100 km. The most prominent terrain types are the volcanic plains of Lakshmi and the mountain belts of Maxwell, Freyja, and Danu. The belts appear to have marked variations in age. There are also extensive regions of tessera in both the upland and outboard plateaus, some rather featureless smooth scarps, flanking basins of complex extensional tectonics, and regions of gravitational or impact modification. Parts of Ishtar are the locations of contemporary vigorous tectonics and past extensive volcanism. Ishtar appears to be the consequence of a history of several 100 m.y., in which there have been marked changes in kinematic patterns and in which activity at any stage has been strongly influenced by the past. Ishtar demonstrates three general properties of Venus: (1) erosional degradation is absent, leading to preservation of patterns resulting from past activity; (2) many surface features are the responses of a competent layer less than 10 km thick to flows of 100 km or broader scale; and (3) these broader scale flows are controlled mainly by heterogeneities in the mantle. Ishtar Terra does not appear to be the result of a compression conveyed by an Earthlike lithosphere. But there is still doubt as to whether Ishtar is predominantly the consequence of a mantle upflow or downflow. Upflow is favored by the extensive volcanic plain of Lakshmi and the high geoid: topography ratio; downflow is favored by the intense deformation of the mountain belts and the absence of major rifts. Both could be occurring, or have recently occurred, with Lakshmi the most likely locus of upflow and Maxwell the main locus of downflow. But doubts about the causes of Ishtar will probably never be resolved without circularization of the Magellan orbit to obtain a more detailed gravity field.