bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption of CO on oxide and water ice surfaces: Implications for the Martian atmosphere
Authors: Leu M.‐T. , Blamont J. E. , Anbar A. D. , Keyser L. F. , Sander S. P.
Published: May 2009

The adsorption of carbon monoxide (CO) on water ice and on the oxides Fe2O3, Fe3O4, Al2O3, SiO2, CaO, MgO, and TiO2 (rutile and anatase) has been investigated in a flow reactor. A mass spectrometer was employed as a detector to monitor the temporal concentrations of CO. We have measured adsorption coefficients as large as 1 × 10−4 for CO on TiO2 solids in helium at 196 K. The fractional surface coverage for CO on TiO2 solids in helium was also determined to be approximately 10% at 196 K. The upper limits of the fractional surface coverage for the other oxides (Fe2O3, Fe3O4, Al2O3, SiO2, CaO, and MgO) and water ice were also measured to be less than 1%. The implications for the stability of CO2 in the Martian atmosphere and the “CO hole” observed by the Phobos/ISM (infrared spectrometer) experiment are discussed.
Quantitative subpixel spectral detection of targets in multispectral images
Authors: Sabol Donald E. , Adams John B. , Smith Milton O.
Published: May 2009

Spectral mixture analysis was used to determine threshold detection limits of target materials in the presence of background materials within the field of view under various simulated but realistic compositional, instrumental, and topographical conditions. Detection thresholds were determined for the cases where the target is detected as (1) a component of a spectral mixture (continuum threshold analysis) and (2) residuals (residual threshold analysis). In continuum threshold analysis, the target was included as a component during unmixing thereby permitting evaluation of target detectability. In residual threshold analysis, the unmodeled target was detected as wavelength‐dependent deviations of the spectral mixture (target included) from the predicted spectrum (mixtures of the modeled background spectra). High resolution laboratory spectra were used to test the “best case” for target detection in spectral mixtures. Data quality was then decreased to simulate the effects of various imaging instruments (spectral sampling and noise) and changes in lighting geometry. The results show that the contrast of the target spectrum, relative to mixtures of background spectra, determines which level of spectral mixture analysis (continuum or residual analysis) detects the target at the lower threshold. Continuum analysis provides lower thresholds when there is contrast between the target and mixtures of the background spectra throughout much of the spectrum, whereas residual analysis improves detectability when the uniqueness of the target is found in narrow absorption feature(s). Varying spectral sampling (by wavelength) changes the contrast (and detectability) of the target and background materials. Although simple examples of target and background materials are used in this paper, they illustrate a general approach for evaluating the spectral detectability of terrestrial and planetary targets at the subpixel scale. Using spectral mixing analysis as a framework makes it possible to specify the conditions that are necessary to detect a particular material and the most appropriate imaging system for a given application.
Mars secular obliquity change due to the seasonal polar caps
Authors: Rubincam David Parry
Published: May 2009

There is a weak positive feedback mechanism between the astronomy and meteorology of Mars. The mechanism is this: the seasonal waxing and waning polar caps cause small changes in Mars' dynamical flattening. Because the changes in flattening are out of phase with the Sun, there is a net annual solar torque on the planet which increases the angle between the equatorial and orbital planes. On the basis of Viking observations of the present climate and simple atmospheric models of past climates these seasonal shifts of mass between the atmosphere and polar caps are capable of secularly increasing Mars' obliquity by about 1° or 2° since the origin of the solar system. Thus the climate, driven largely by the axial tilt, reacts back on the planet and slightly enhances the seasons on Mars as time progresses. More sophisticated models will probably not change this result much; therefore this mechanism probably produced only minor changes in Mars' climate. It causes negligible changes in the axial tilt and climate of the Earth.
Mars Observer camera
Authors: Malin M. C. , Danielson G. E. , Ingersoll A. P. , Masursky H. , Veverka J. , Ravine M. A. , Soulanille T. A.
Published: May 2009

The Mars Observer camera (MOC) is a three‐component system (one narrow‐angle and two wide‐angle cameras) designed to take high spatial resolution pictures of the surface of Mars and to obtain lower spatial resolution, synoptic coverage of the planet's surface and atmosphere. The cameras are based on the “push broom” technique; that is, they do not take “frames” but rather build pictures, one line at a time, as the spacecraft moves around the planet in its orbit. MOC is primarily a telescope for taking extremely high resolution pictures of selected locations on Mars. Using the narrow‐angle camera, areas ranging from 2.8 km × 2.8 km to 2.8 km × 25.2 km (depending on available internal digital buffer memory) can be photographed at about 1.4 m/pixel. Additionally, lower‐resolution pictures (to a lowest resolution of about 11 m/pixel) can be acquired by pixel averaging; these images can be much longer, ranging up to 2.8 × 500 km at 11 m/pixel. High‐resolution data will be used to study sediments and sedimentary processes, polar processes and deposits, volcanism, and other geologic/geomorphic processes. The MOC wide‐angle cameras are capable of viewing Mars from horizon to horizon and are designed for low‐resolution global and intermediate resolution regional studies. Low‐resolution observations can be made every orbit, so that in a single 24‐hour period a complete global picture of the planet can be assembled at a resolution of at least 7.5 km/pixel. Regional areas (covering hundreds of kilometers on a side) may be photographed at a resolution of better than 250 m/pixel at the nadir. Such images will be particularly useful in studying time‐variable features such as lee clouds, the polar cap edge, and wind streaks, as well as acquiring stereoscopic coverage of areas of geological interest. The limb can be imaged at a vertical and along‐track resolution of better than 1.5 km. Different color filters within the two wide‐angle cameras permit color images of the surface and atmosphere to be made to distinguish between clouds and the ground and between clouds of different composition.
Mars Observer magnetic fields investigation
Authors: Acuña M. H. , Connerney J. E. P. , Wasilewski P. , Lin R. P. , Anderson K. A. , Carlson C. W. , McFadden J. , Curtis D. W. , Réme H. , Cros A. , Médale J. L. , Sauvaud J. A. , d'Uston C. , Bauer S. J. , Cloutier P. , Mayhew Michael , Ness N. F.
Published: May 2009

The Mars Observer magnetic fields investigation will provide fast vector measurements of the Martian magnetic field over a wide dynamic range. The fundamental objectives of this investigation are (1) to establish the nature of the magnetic field of Mars, (2) to develop appropriate models for its representation, which take into account the internal sources of magnetism and the effects of the interaction with the solar wind, and (3) to map the Martian crustal remanent field to a resolution consistent with the Mars Observer orbit altitude and ground track separation. The basic instrumentation complement implemented for this mission is a synergistic combination of a dual, triaxial, flux gate magnetometer system and an electron reflectometer with sensors mounted on a spacecraft boom. The dual magnetometer system allows the real‐time estimation and correction of spacecraft‐generated fields, while the electron reflectometer provides remote magnetic field sensing capabilities. These instruments have an extensive spaceflight heritage, and similar versions of the same have been flown in numerous missions like Voyager, Magsat, International Solar Polar mission (ISPM), Giotto, Active Magnetospheric Particle Tracer Explorers, and Global Geospace Science (GGS). Depending on the telemetry rate supported, a minimum of 2–16 vector samples per second will be acquired. The instrument is microprocessor controlled, can be partially reprogrammed in flight, and supports the packet telemetry protocol implemented for Mars Observer.
Thermal emission spectrometer experiment: Mars Observer mission
Authors: Christensen Philip R. , Anderson Donald L. , Chase Stillman C. , Clark Roger N. , Kieffer Hugh H. , Malin Michael C. , Pearl John C. , Carpenter James , Bandiera Nuno , Brown F. Gerald , Silverman Steven
Published: May 2009

Thermal infrared spectral measurements will be made of the surface and atmosphere of Mars by the thermal emission spectrometer (TES) on board Mars Observer. By using these observations the composition of the surface rocks, minerals, and condensates will be determined and mapped. In addition, the composition and distribution of atmospheric dust and condensate clouds, together with temperature profiles of the CO2 atmosphere, will be determined. Broadband solar reflectance and thermal emittance measurements will also be made to determine the energy balance in the polar regions and to map the thermophysical properties of the surface. The specific science objectives of this investigation are to determine (1) the composition and distribution of surface materials, (2) the composition, particle size, and spatial and temporal distribution of suspended dust, (3) the location, temperature, height, and water abundance of H2O clouds, (4) the composition, seasonal behavior, total energy balance, and physical properties of the polar caps, and (5) the particle size distribution of rocks and fines on the surface. The instrument consists of three subsections: a Michelson interferometer, a solar reflectance sensor, and a broadband radiance sensor. The spectrometer covers the wavelength range from 6 to 50 μm (∼1600–200 cm−1) with nominal 5 and 10 cm−1 spectral resolution. The solar reflectance band extends from 0.3 to 2.7 μm; the broadband radiance channel extends from 5.5 to 100 μm. There are six 8.3‐mrad fields of view for each sensor arranged in a 3 × 2 array, each with 3‐km resolution at the nadir. Uncooled deuterated triglycine sulphate (DTGS) pyroelectic detectors provide a signal‐to‐noise ratio (SNR) of over 500 at 10 μm for daytime spectral observations at a surface temperature of 270 K. The SNR of the albedo and thermal bolometers will be approximately 2000 at the peak signal levels expected. The instrument is 23.6 × 35.5 × 40.0 cm, with a mass of 14.4 kg and an average power consumption of 14.5 W. The approach will be to measure the spectral properties of thermal energy emitted from the surface and atmosphere. Emission phase angle studies and day‐night observations will be used to separate the spectral character of the surface and atmosphere. The distinctive thermal infrared spectral features present in minerals, rocks, and condensates will be used to determine the mineralogic and petrologic character of the surface and to identify and study aerosols and volatiles in the atmosphere.
Densities and vibrational distribution of H3+ in the Jovian auroral ionosphere
Authors: Kim Y. H. , Fox J. L. , Porter H. S.
Published: May 2009

Observations of the H3+ infrared emission at 2 and 4 μm have suggested that H3+ is in local thermodynamic equilibrium (LTE) in the region of the Jovian ionosphere from which the emissions originate. We have tested this assumption by calculating the vibrational distribution of H3+ over the altitude range of 350 to 1500 km above the methane cloud tops (1 to 4 × 10−3 μbar). We have constructed a model of the Jovian auroral ionosphere in which the neutral temperatures are enhanced over those of the mid‐latitude ionosphere, as suggested by observations and models of the auroral region. We have modeled the precipitation of 10‐keV electrons with an energy flux of 1 erg cm−2 s−1. Both the energy and energy flux are less than those that are implicated in the production of the UV aurora. We have computed the densities and vibrational distribution of H3+ and find that the distribution of the six lowest states of H3+ can be determined fairly well in spite of uncertainties in the atomic and molecular data. Since the nearly resonant transfer of vibration from H2(υ=1) is an important process in populating the H3+(υ1=0,υ2=2) state, it is necessary to model the vibrational distribution of H2 as well. The computed altitude profiles and vibrational distributions of H3+ and H2 are consistent with the observations of infrared emission in the 2‐and 4‐μm regions. The H3+ is not in LTE near and above the H3+ peak, since loss of the H3+(υ1=0,υ2=1) and H3+(υ1=0,υ2=2) states by radiation is approximately equal to the collisional loss rate.
Science applications of the Mars Observer gamma ray spectrometer
Authors: Boynton W. V. , Trombka J. I. , Feldman W. C. , Arnold J. R. , Englert P. A. J. , Metzger A. E. , Reedy R. C. , Squyres S. W. , Wänke H. , Bailey S. H. , Brückner J. , Callas J. L. , Drake D. M. , Duke P. , Evans L. G. , Haines E. L. , McCloskey F. C. , Mills H. , Shinohara C. , Starr R.
Published: May 2009

The Mars Observer gamma ray spectrometer will return data related to the elemental composition of Mars. The instrument has both a gamma ray spectrometer and several neutron detectors. The gamma ray spectrometer will return a spectrum nominally every 20 s from Mars permitting a map of the elemental abundances to be made. The gamma rays are emitted from nuclei involved in radioactive decay, from nuclei formed by capture of a thermal neutron, and from nuclei put in an excited state by a fast‐neutron interaction. The gamma rays come from an average depth of the order of a few tens of centimeters. The spectrum will show sharp emission lines whose intensity determines the concentration of the element and whose energy identifies the element. The neutron detectors, using the fact that the orbital velocity of the Mars Observer spacecraft is similar to the velocity of thermal neutrons, determine both the thermal and epithermal neutron flux. These parameters are particularly sensitive to the concentration of hydrogen in the upper meter of the surface. By combining the results from both techniques it is possible to map the depth dependence of hydrogen in the upper meter as well. These data permit a variety of Martian geoscience problems to be addressed including the crust and mantle composition, weathering processes, volcanism, and the volatile reservoirs and processes. In addition, the instrument is also sensitive to gamma ray and particle fluxes from non‐Martian sources and will be able to address problems of astrophysical interest including gamma ray bursts, the extragalactic background, and solar processes.
Scattering properties of the Venusian surface: Preliminary results from Magellan
Authors: Tyler G. Leonard , Simpson Richard A. , Maurer Michael J. , Holmann Edgar
Published: May 2009

Radar backscatter functions
for incidence angles 0≤ϕ≤4°–10° have been derived from Magellan altimetry radar echoes. The procedure includes constrained solution of a system of simultaneous equations for which the echo spectrum and echo time profile are inputs. An initially practical and workable set of constraints has been applied; optimization and improved results are expected as the analysis matures. The scattering functions yield information on small scale surface structure (tens of centimeters to tens of meters) but averaged over hundreds of square kilometers. RMS surface slopes derived from fits of analytic functions to the
results have been converted to map form and show patterns similar to those reported using other techniques. A scattering law of exponential form matches the data better than either the Hagfors or Gaussian form in most areas, but the Hagfors function is generally best at the smoothest sites. Limited study of radar image data indicates that average backscatter cross section, and possibly its derivative with incidence angle, can be derived at oblique angles (17°≤ϕ≤45°). The altimetry results in combination with those derived from the synthetic aperture radar will strongly constrain the form of
over the range 0≤ϕ≤45°. Offsets of the echo peak in altimetry spectra from those expected of nadir echoes are surprisingly common and are loosely correlated with Venus topography; to date no specific cause for this phenomenon has been identified.
Origins of the rings of Uranus and Neptune: 1. Statistics of satellite disruptions
Authors: Colwell Joshua E. , Esposito Larry W.
Published: May 2009

Stochastic simulations of the collisional fragmentation of the small moons of Neptune and Uranus confirm the conclusions of Smith et al. [1986, 1989] that many of these moons cannot have survived intact since the end of planetary formation. We perform two types of stochastic simulations of the collisonal history of small moons. Monte Carlo simulations in which only the largest surviving fragment from each disruption is followed show bimodal probability distributions for the size of the largest fragment. Once the moon is destroyed the first time, the collisional cascade to smaller sizes proceeds relatively quickly. A Markov chain approach allows us to follow the size distribution from each disruption to arbitrarily small sizes. These simulations show that the more numerous smaller fragments can outlive the largest fragment followed in the Monte Carlo formalism. We find that the evolution of moon populations from catastrophic fragmentation is more complex than can be described by a simple break‐up “time scale.” We rederive cratering rates using the method of Smith et al. [1986, 1989] for all the satellites of Uranus and Neptune with an improved crater scaling and modified impactor distributions. These changes produced a higher number of predicted craters larger than the radii of the small moons of Uranus and Neptune than derived by Smith et al. [1986, 1989]. Our more detailed simulations of the catastrophic fragmentation process also show a higher rate of disruptions than estimated by Smith et al. [1986, 1989]. The smallest observed moons at Uranus and Neptune have calculated lifetimes against catastrophic fragmentation of less than 5 × 108 years. If we require that half the mass of a satellite is given escape velocity we find lifetimes for Cordelia (1986U7), Naiad (1989N6), and Thalassa (1989N5) of 9 × 108, 2 × 109, and 4 × 109 years, respectively. We conclude that we are observing a collisionally evolved small satellite population around Neptune and Uranus and that some observed moons are gravitationally bound rubble piles that have undergone multiple disruptions. In order for the satellites to be primordial the population of planet family comets in the outer solar system would have to be smaller than current estimates by at least a factor of 5.
Magnetic fields in Venus nightside ionospheric holes: Collected Pioneer Venus Orbiter magnetometer observations
Authors: Luhmann J. G. , Russell D. S.
Published: May 2009

The magnetic fields detected by the Pioneer Venus Orbiter (PVO) magnetometer within the electron density depletions called “holes” in the nightside ionosphere are typically larger and more organized than the fields in the surrounding ionosphere. Moreover, they have substantial sunward/antisunward components which cause them to appear as near‐radial fields near the antisolar point. The collection of observations presented here illustrates the variety of appearances of the fields in holes. Some new results which summarize their average properties, their dependence on solar wind conditions, and their lack of geographical control are also presented. These results are potentially pertinent to the interpretation of data from the PVO entry at the end of 1992 and from the impending Mars Observer mission, which will probe the magnetic fields in the low‐altitude wake of weakly magnetized Mars.
Channels and valleys on Venus: Preliminary analysis of Magellan data
Authors: Baker V. R. , Komatsu G. , Parker T. J. , Gulick V. C. , Kargel J. S. , Lewis J. S.
Published: May 2009

A preliminary survey of Magellan imagery reveals more than 200 newly discovered relic channel and valley landform complexes. For purposes of discussion the channels can be classed as simple, complex, and compound. Integrated valleys also occur. Simple channels include (1) sinuous rules that closely resemble their lunar counterparts and (2) a newly recognized long sinuous form of high width‐to‐depth ratio and remarkably constant width. Herein designated canali, the most spectacular of these channels is 6800 km long. One of the compound channels, an outflow complex in Lada Terra, extends over 1200 km and is up to 30 km wide. Streamlined hills and spill relationships at a cross‐axial ridge are similar to features in flood channels. Venusian channels have a global distribution with most of the large canali‐type channels developed on volcanic plains. Alternative hypotheses for the channel‐forming processes include genesis by the following erosive fluids: ultramafic silicate melts, sulfur, and carbonate lavas. Each of these causative agents has profound implications for Venusian planetology. The remote possibility of an aqueous origin, indicated by apparent regime behavior of the active channeling process, cannot be excluded with absolute certainty.
Voyager 2 ultraviolet spectrometer solar occultations at Neptune: Constraints on the abundance of methane in the stratosphere
Authors: Bishop James , Atreya Sushil K. , Romani Paul N. , Sandel Bill R. , Herbert Floyd
Published: May 2009

Voyager 2 ultraviolet spectrometer (UVS) solar occultation lightcurves at wavelengths 125–138 nm acquired during the Neptune encounter are compared with one‐dimensional methane photochemical‐transport models. Photoabsorption by methane (the photochemical parent) is expected to be the major source of opacity at these wavelengths, in which case the UVS lightcurves can be used to infer the strength of eddy mixing at altitudes above the methane photolysis peak and compatible CH4mixing ratios in the lower stratosphere. For thep‐Tmodels adopted in this study, acceptable fits to the UVS lightcurves are obtained with eddy mixing coefficient values (K1/2) near the half‐light altitudes (placed 550–600 km above the respective 1 bar levels) of 2–15 × 106cm2s−1(ingress) and 4–35 × 106cm2s−1(egress) and lower stratospheric (p> 1.0 mbar) methane mixing ratio values (ƒT(CH4)) of 5 – 100 × 10−5(ingress and egress). The ranges in ƒT(CH4) andK1/2reflect uncertainties in the backgroundp‐Tstructure and the application of different criteria for deciding what constitutes a “good fit.” For the nominalp‐Tmodels and a criterion based on replicating the spacing in altitude of the 125–138 nm UVS lightcurves,K1/2values (near 0.2 μbar) of 107cm2s−1(ingress) and 1–2 × 107cm2s−1(egress) and ƒT(CH4) values of ∼1 × 10−4(ingress and egress) are indicated, with the egress results being more uncertain. These results are insensitive to photochemical details of the models.
Aeolian features on Venus: Preliminary Magellan results
Authors: Greeley Ronald , Arvidson Raymond E. , Elachi Charles , Geringer Maureen A. , Plaut Jeffrey J. , Saunders R. Stephen , Schubert Gerald , Stofan Ellen R. , Thouvenot Eric J. P. , Wall Stephen D. , Weitz Catherine M.
Published: May 2009

Magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian, or wind processes. Wind streaks are the most common aeolian feature. They consist of radar backscatter patterns that are high, low, or mixed in relation to the surface on which they occur. A data base of more than 3400 wind streaks shows that low backscatter linear forms (long, narrow streaks) are the most common and that most streaks occur between 17°S to 30°S and 5°N to 53°N on smooth plains. Moreover, most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains. We infer that both of these geological settings provide fine particulate material that can be entrained by the low‐velocity winds on Venus. Turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks. Thus, some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock; other high backscatter streaks may be lag deposits of dense materials from which low‐density grains have been removed (dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns). Wind streaks generally occur on slopes < 2° and tend to be oriented toward the equator, consistent with the Hadley model of atmospheric circulation. In addition to wind streaks, other aeolian features on Venus include yardangs(?) and dune fields. The Aglaonice dune field, centered at 25°S, 340°E, covers ∼1290 km2 and is located in an ejecta flow channel from the Aglaonice impact crater. The Meshkenet dune field, located at 67°N, 90°E, covers ∼17,120 km2 in a valley between Ishtar Terra and Meshkenet Tessera. Wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds. Dunes on Venus signal the presence of sand‐size (∼60 to 2,000 μm) grains. The possible yardangs are found at 9°N, 60.5°E, about 300 km southeast of the crater Mead. Although most aeolian features are concentrated in smooth plains near the equator, the occurrence of wind streaks is widespread, and some have been found at all latitudes and elevations. They demonstrate that aeolian processes operate widely on Venus. The intensity of wind erosion and deposits, however, varies with locality and is dependent on the wind regime and supply of particles.
Fluid outflows from Venus impact craters: Analysis from Magellan data
Authors: Asimow Paul D. , Wood John A.
Published: May 2009

Many impact craters on Venus have unusual outflow features originating in or under the continuous ejecta blankets and continuing downhill into the surrounding terrain. These features clearly resulted from flow of low‐viscosity fluids, but the identity of those fluids is not clear. In particular, it should not be assumed a priori that the fluid is an impact melt. A number of candidate processes by which impact events might generate the observed features are considered, and predictions are made concerning the rheological character of flows produced by each mechanism. A sample of outflows was analyzed using Magellan images and a model of unconstrained Bingham plastic flow on inclined planes, leading to estimates of viscosity and yield strength for the flow materials. It is argued that at least two different mechanisms have produced outflows on Venus: an erosive, channel‐forming process and a depositional process. The erosive fluid is probably an impact melt, but the depositional fluid may consist of fluidized solid debris, vaporized material, and/or melt.
Venus topography and kilometer‐scale slopes
Authors: Ford Peter G. , Pettengill Gordon H.
Published: May 2009

During the first 8 months of the Magellan mission, the radar altimeter has made some three million measurements of the surface of Venus covering the latitude range from 85°N to 80°S. Methods involving range correlation, Doppler filtering, multi‐burst summation, and range migration are used to focus the observations and to achieve high surface resolution. Results are presented as maps of the global distribution of topography, meter‐scale roughness, and power reflection coefficient. The results are similar to those reported in previous experiments (surface heights exhibit a unimodal distribution with more than 80% of the surface lying within 1 km of the 6051.84‐km mean radius) but the higher resolution of the Magellan altimeter has disclosed several surprisingly steep features, e.g. the southwest face of the Maxwell Montes, the southern face of the Danu Montes, and the chasmata to the east of Thetis Regio, where average kilometer‐scale slopes of greater than 30° are not uncommon. This conclusion is corroborated by close inspection of synthetic aperture radar imagery.
Fractal analysis of Venus topography in Tinatin Planitia and Ovda Regio
Authors: Kucinskas Algis B. , Turcotte Donald L. , Huang Jie , Ford Peter G.
Published: May 2009

We have applied a one‐dimensional fractal analysis to Magellan altimetry data for two 20° × 20° equatorial regions on Venus. For the Ovda Regio highlands the power spectra of the altimetry data correlate well with power law (fractal) statistics over the full range of wavelengths considered 36 km ≤ λ ≤ 703 km; the mean fractal dimension is DOvda = 1.64. For the Tinatin Planitia lowlands the power spectra consistently show a rollover at long wavelengths, λ > 200 km; the long‐wavelength topography in this region either never developed or has subsequently relaxed. The mean of the best fit fractal dimension for Tinatin is DTinatin = 1.41. The measure of roughness (RA) correlates well with variations in relief, especially at short wavelengths λ ∼ 50 km. Our quantitative analysis of Venus topography can be applied on a global scale. The one‐dimensional method can be extended to two‐dimensional space, yielding two‐dimensional maps of regional D and RA, allowing a more complete use of regional two‐dimensional altimetry data.
Reflection and emission properties on Venus: Alpha Regio
Authors: Tryka Kimberly A. , Muhleman Duane O.
Published: May 2009

The retrieval of geophysical surface properties from radar measurements is relatively straightforward when dealing with near‐normal incidence angles and physical surfaces which are relatively smooth. Procedures developed from Hagfors' (1964) scattering model have been used with success to extract various properties (reflectivity, emissivity, slope, etc.) from both Pioneer Venus (Pettengill et al., 1988) and Magellan (Tyler et al., 1991; Pettengill et al., 1991) data. However, under certain circumstances, such as physically rough areas or areas of nonhomogeneous composition, Hagfors' model may lead to serious errors in the interpretation of radar and radiometry data. We examine the Magellan‐derived surface properties for some areas of below average emissivity in Alpha Regio and in an area to the east of Alpha, Crater Stuart, in order to determine if Hagfors' model is applicable. We also apply methods of classical multiple scattering theory to show enhanced radar backscattering and depressed thermal emission can be expected with a matrix of silicic or mafic rocks embedded in low loss soils.
Coldspots and hotspots: Global tectonics and mantle dynamics of Venus
Authors: Bindschadler Duane L. , Schubert Gerald , Kaula William M.
Published: May 2009

Magellan radar images and altimetry data and Pioneer Venus gravity data of major highlands and lowlands on Venus are examined with the objective of relating these tectonic and volcanic landforms to convection within Venus' mantle. Two roughly circular lowlands, Atalanta and Lavinia planitiae, are bowl‐shaped depressions which contain compressional tectonic features and lack hotspot‐related features such as coronae and shield volcanoes. They are identified as surface expressions of young mantle coldspots or regions of mantle downwelling. Highlands can be divided into two distinct groups: volcanic rises and plateau‐shaped highlands. Volcanic rises are domical, circular to elongate regions characterized by volcanic construction; extensional tectonism; large, positive gravity and geoid anomalies centered on the highlands; and large apparent depths of compensation. Volcanic rises include Beta, Bell, Atla, and Western Eistla regiones and are identified as surface expressions of large mantle hotspots or plumes. Plateau‐shaped highlands are steep‐sided elevated regions that range from circular (Alpha Regio) to elongate or irregular (Ovda Regio, Tellus Regio) in shape. Their surfaces are dominated by complex ridged terrain, rather than the shield volcanoes and flows that dominate volcanic rises. Most plateau‐shaped highlands contain compressional structures, some of which commonly lie along their margins, and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises. Plateau‐shaped highlands, which include those mentioned above and Western Ishtar Terra, Thetis Regio, and Phoebe Regio, are identified as surface expressions of mantle coldspots or regions of downwelling. The highland coldspot features are elevated as a consequence of crustal thickening; crustal thickening is absent or minor at lowland coldspots. The importance of mantle downwelling in the tectonic deformation of Venus suggests that its mantle, like the mantle of Earth, is strongly heated from within. The major Venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet's core.
The rotation period, direction of the North Pole, and geodetic control network of Venus
Authors: Davies M. E. , Colvin T. R. , Rogers P. G. , Chodas P. W. , Sjogren W. L. , Akim E. L. , Stepanyantz V. A. , Vlasova Z. P. , Zakharov A. I.
Published: May 2009

Before the Magellan mission, the best estimates of the rotation rate and pole direction of Venus were derived from Earth‐based radar measurements. A new determination of these rotational parameters has now been made from an analysis of Magellan radar images. Control points were selected from the north polar region and measured on full‐resolution radar strips. The measurements were entered into a least squares adjustment to solve for the pole direction and rotation rate of Venus, as well as the coordinates of the control points themselves. The current data set contains 3893 measurements of 571 points on 560 radar strips. Spacecraft ephemeris errors dominate the measurement errors. One technique used to remove ephemeris errors is to adjust the averaged orbital inclination and argument of periapsis for each orbit. In a more precise technique used for selected blocks of orbits, an improved spacecraft ephemeris is computed by optimally fitting measurements of additional points at all latitudes of the radar strips together with Earth‐based spacecraft radiometric tracking measurements. In a separate experiment, measurements were made of a few points common to both Venera 15/16 and Magellan images. The long time baseline between the images should have led to an accurate determination of the rotation period and pole direction of Venus. However, the measurement residuals were unexpectedly large, and these solutions are not currently considered reliable. A rotation period estimate of 243.0185 ± 0.0001 days was determined via the ephemeris improvement technique applied simultaneously to two overlapping orbit blocks with many common points and separated by two full Venus rotations. Using this period value, the control network computations estimated the north pole direction as right ascenson = 272.76° ± 0.02° and declination = 67.16° ± 0.01° in the J2000 frame.
Mylitta Fluctus, Venus: Rift‐related, centralized volcanism and the emplacement of large‐volume flow units
Authors: Roberts Kari Magee , Guest John E. , Head James W. , Lancaster Michael G.
Published: May 2009

Mylitta Fluctus is a volcanic flow field that covers approximately 300,000 km2 in southern Lavinia Planitia. The flows are typically radar‐bright with uniform surface textures. Central channels are common. Maximum flow lengths range from 400 to 1000 km; flow widths range 30–100 km in the medial and distal portions of the flow field. The total volume of the flow field, based on estimates of flow thickness, is of the order of 2 × 104 km3. The flow field is composed of six smaller flow fields that are interpreted to represent major eruption events in the evolution of Mylitta. An asymmetric shield volcano 200 km in radius with a central caldera has been identified as the single major source. It is located along a possible rift zone at the northern edge of Lada Terra. No evidence for fissure‐fed eruptions is observed, although eruptions may have occurred along fissures within the rift before the main vent centralized. Mylitta is similar in scale to many terrestrial flood basalt provinces, although it is lower in total estimated volume and is distinguished by the presence of a central source edifice. The origin of Mylitta is proposed to be linked to regional extension and possible hotspot activity in a manner similar to that suggested for the origin of terrestrial flood basalts. The lower apparent volume of Mylitta relative to terrestrial flood basalts suggests that the amount of material upwelling from the mantle may have been lower in this case or that the formation of the flow field has not yet finished. Detailed studies of the morphology and distribution of flow fields similar to Mylitta should yield insight into the variation and evolution of hotspot‐related volcanism and the formation of possible flood basalts on Venus.
Flexural ridges, trenches, and outer rises around coronae on Venus
Authors: Sandwell David T. , Schubert Gerald
Published: May 2009

High‐resolution altimetry collected by the Magellan spacecraft reveals trench and outer rise topographic signatures around major coronae (e.g. Eithinoha, Heng‐O, Artemis, and Latona). In addition, Magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward. Both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere. We have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim. The approach was tested at northern Freyja Montes where the best fit elastic thickness is 18 km, in agreement with previously published results. The elastic thicknesses determined by modelling numerous profiles at Eithinoha, Heng‐O, Artemis, and Latona are 15, 40, 37, and 35 km, respectively. At Eithinoha, Artemis, and Latona where the plates appear to be yielding, the maximum bending moments and elastic thicknesses are similar to those found at the Middle America, Mariana, and Aleutian trenches on Earth, respectively. Estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients. At Heng‐O, Artemis, and Latona, temperature gradients are less than 10 K/km, which correspond to conductive heat losses of less than one half the expected average planetary value. We propose two scenarios for the creation of the ridge, trench, and outer rise topography: differential thermal subsidence and lithospheric subduction. The topography of Heng‐O is well matched by the differential thermal subsidence model. However, at Artemis and Latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone. In these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts (rolls back) and the corona diameter increases.
Gravitational spreading of high terrain in Ishtar Terra, Venus
Authors: Smrekar Suzanne E. , Solomon Sean C.
Published: May 2009

Magellan altimetry measurements indicate that the mountain belts and plateau margins of Ishtar Terra have topographic slopes in the range 2–30°. Magellan radar images reveal that numerous sets of narrow, closely spaced troughs and lineations, which we interpret as graben and normal faults, are located in many of these regions of high slope. The orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading. In several of the mountain belts, the extensional structures are parallel to the apparent direction of shortening, consistent with the interpretation that gravitational spreading occurred while mountain building was still active. To explore the implications of this spreading for the tectonic evolution of Ishtar Terra, we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range. Flow in the crust is assumed to be governed by a nonlinear, depth‐dependent rheology. We predict brittle failure and relaxation of topographic relief as functions of time, for ranges of crustal thickness (10–30 km) and thermal gradient (5–25 K/km) and for observed ranges in topographic elevation (1–6 km) and slope (1–30°). This parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief. For a crustal thickness greater than 10 km, a thermal gradient of 15 K/km or more, and average values of relief (3 km) and slope (3°), the topography relaxes to 25% of its original height in less than 10 m.y. Although initial failure, in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands, is predicted to occur much earlier, values of horizontal surface strain that are likely to be observable (∼1%) do not accumulate until significant relaxation of relief begins. Given the predicted rate of topographic relaxation, either the crust of Ishtar Terra must be anomalous in some way (either much stronger than predicted by standard flow laws or very thin or cold) or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for Venus of 500 Ma. The large apparent depth of compensation, the abundance of volcanism, and the uncertainty in the local age favor the view that mountain building in Ishtar Terra has occurred until times at least as recent as 10 Ma.
Mass movements on Venus: Preliminary results from Magellan cycle 1 observations
Authors: Malin Michael C.
Published: May 2009

Mass movements on Venus, seen in radar images acquired by the Magellan spacecraft during its first mapping, are easily interpreted within the scheme commonly used to classify terrestrial landslides. Rock slumps, rock and/or block slides, rock avalanches, debris avalanches, and possibly debris flows are seen in areas of high relief and steep slope gradients and are most abundant in the tectonic troughs that crisscross much of the equatorial region of Venus. Many classes of regolith and sediment movements are not seen; such features might be too small to resolve in the 75 m per picture element radar images, or their absence may reflect the relatively thin cover of fine sediments inferred from emissivity measurements and other observations. Venusian landslides, like those found within the Valles Marineris on Mars, tend to come from escarpments typically higher than those on Earth. They appear to fall between the terrestrial and Martian height to length trends; they are also somewhat larger (using length as a surrogate for volume) than terrestrial subaerial landslides but smaller than their Martian counterparts. Good morphologic analogs can be found in terrestrial volcanic slides (both subaerial and submarine): oversteepening of volcanic edifices by intrusion and subsequent lateral collapse appears responsible for shaping a number of large, isolated volcanos on Venus. Faulting and seismically induced accelerations are probably responsible for the majority of nonvolcanic mass movements. The atmosphere may participate in promoting the movement of some of the landslide debris, but environmental factors (e.g., rainfall, temperature cycling) do not appear to play as dominant a role as they do on Earth. Based on the types and locations of landslides seen in the Magellan data, it is possible to scale the terrestrial occurrence rate to Venus: if Venus is as seismically and volcanically active as Earth, than of order one major landslide (i.e., discernable in Magellan images or ∼5–10 km in runout distance) should occur per year, which careful reexamination of Magellan images acquired during later mapping cycles may be able to detect.
Venus surface radiothermal emission as observed by Magellan
Authors: Pettengill Gordon H. , Ford Peter G. , Wilt Robert J.
Published: May 2009

The Magellan radiometer experiment has observed the 12.6‐cm‐wavelength radio emissivity of more than 91% of the Venus surface during the first 8 months of its operation. The global mean value of emissivity seen using horizontal linear polarization is 0.845, a value that corresponds to a dielectric permittivity of between 4.0 and 4.5, depending on the surface roughness. These values are consistent with the dry basaltic minerals thought to compose the bulk of the Venus surface. The 2° beam width of the Magellan high‐gain antenna yields surface resolutions that vary from 15 by 23 km at periapsis (10°N latitude) to about 85 km at the north pole; measurements of emissivity carry an absolute error of about 0.02 and can distinguish local variations as small as 0.005. The observations have confirmed earlier findings that a few regions on Venus, primarily located at high elevations, possess unexpectedly low values of radiothermal emissivity, occasionally reaching as low as 0.3. Some members of each of five classes of features are found to display anomalously low values of emissivity: highlands, volcanoes, novae, ridges, and impact craters. Two possible explanations for these low values have been advanced: (1) emission from a highly reflective single interface between the atmosphere and a surface material having a bulk dielectric permittivity of order 80; or (2) emission from the surface of a low‐loss medium having a more usual permittivity (of order 5) but which contains many voids permitting efficient internal multiple scattering. Distinguishing between these hypotheses is difficult with the data presently in hand.
Surface modification of Venus as inferred from Magellan observations of plains
Authors: Arvidson Raymond E. , Greeley Ronald , Malin Michael C. , Saunders R. Stephen , Izenberg Noam , Plaut Jeffrey J. , Stofan Ellen R. , Shepard Michael K.
Published: May 2009

In Sedna Planitia, clear stratigraphic relations can be discerned among volcanic flow units. Young flows exhibit SAR specific cross section values similar to fresh terrestrial basalt flows, whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows. Total degradation of ∼1 m depth over ∼0.6 b.y. is inferred for the Sedna area from radar signatures, impact crater abundances, and ejecta superposition relations with respect to volcanic flow units. Analyses of parabolic ejecta deposits associated with the crater Stuart imply that the material is typically centimeters in thickness. A relatively small fraction (∼10%) of Venusian impact craters exhibit prominent parabolic ejecta deposits. These craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years. The inferred dispersal rate (<10−3 μm/yr) is too low to produce the degradation of flows at Sedna Planitia, and it is concluded that the dominant flow modification process is in situ weathering. In addition, elevation dependent weathering is inferred in western Ovda Regio, where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations. The inferred rate of generation of high reflection coefficient materials is no more than ∼10−2μm/yr, based on the inability of aeolian activity to cover high‐reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high‐reflectivity surfaces at high elevations. Surface modification rates on Venus are orders of magnitude lower than on Earth. Venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on Mars but are comparable to the estimated rate of weathering and erosion of Martian bedrock. Low surface modification rates imply that it will be possible to determine regional‐scale age variations on Venus based on the degree of preservation of volcanic landforms and microwave signatures.