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PubMed1990

Triton: topside ionosphere and nitrogen escape.

The principal ion in the ionosphere of Triton is N+. Energetic electrons of magnetospheric origin are the primary source of ionization, with a smaller contribution due to photoionization. To explain the topside plasma scale height, we postulate that N+ ions escape from Triton. The loss rate is 3.4 x 10(7) cm-2 s-1 or 7.9 x 10(24) ions s-1. Dissociative recombination of N2+ produces neutral exothermic fragments that can escape from Triton. The rate is estimated to be 8.6 x 10(6) N cm-2 s-1 or 2.0 x 10(24) atoms s-1. Implications for the magnetosphere of Neptune and Triton's evolution are discussed.

Atmosphere↗
PubMed1987

Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system.

Methane clathrate is expected to be an important carbon-containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3 in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particles irradiation. Significant coloration occurs with doses of 10(9) erg cm-2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to approximately 1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G approximately equal to 1 molecule keV-1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 x 10(4) years (Miranda) to 3 x 10(6) years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy or irradiated H2O+CH4 frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet-visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 x 10(9) year time scales, > or = 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation-hardened ice-tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact-exposed carbon-bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.

Ammonia↗
PubMed1987

The excitation of the far ultraviolet electroglow emissions on Uranus, Saturn, and Jupiter.

We propose that the diffuse FUV emissions of H and H2 in excess of photoelectron excitation observed from the sunlit atmospheres of Uranus, Saturn, and Jupiter are produced by electric field acceleration of photoelectrons and ions locally in the upper atmospheres. This in situ acceleration is required to satisfy the many observational constraints on the altitude distribution, exciting particle energy, and total input energy requirements of the electroglow mechanism. We further suggest that a primary mechanism leading to this acceleration is an ionospheric dynamo, which is created in the same manner as the Earth's dynamo. The calculated altitude of charge separation by the neutral wind drag on ions across magnetic field lines is consistent with the observed peaks in electroglow emissions from the Voyager ultraviolet spectrometer limb scan data on both Saturn (near the homopause) and Uranus (just above the homopause). This dynamo action therefore appears to initiate the acceleration process, which must have the form of field-aligned potentials to accelerate the magnetized electrons. We propose that these field-aligned potentials are due to anomalous resistivity, which results from sufficiently high field-aligned currents in the ionosphere to generate plasma instabilities and therefore runaway electrons and ions above some critical lower initial energy. There are multiple candidate processes for inducing these currents, including polarization in the equivalent F regions and inner magnetospheric convection, and each of these processes should exhibit latitudinal structure. The acceleration of low-energy electrons in an H2 atmosphere preferentially results in FUV radiation and further ionization, whereas electron acceleration in a nitrogen/oxygen atmosphere such as the Earth's is dominated by elastic scattering and thus results in electric currents. Individual electron and proton collisions with H2 molecules will result in excitation, ionization, and heating, so that considerable enhancement of the ionospheric density and heating of the upper atmosphere will accompany the FUV emission.

Electrons↗
PubMed1987

The main source of radio emission from the magnetosphere of Uranus.

Observations of kilometric radiation from Uranus made with the planetary radio astronomy experiment on the Voyager 2 spacecraft are presented and discussed. Similarities between the auroral kilometric radiation from Earth and the observed Uranus emission are pointed out. A geometrical beaming model is developed in which a single distributed source is located above the darkside auroral region and emits in the extraordinary mode by the cyclotron maser process. The model can account for nearly all the Uranian kilometric radiation from the high-frequency limit near 850 kHz down to about 150 kHz and for much of it down to the lower limit of 20 kHz.

Astronomical Phenomena↗
PubMed1985

Fluid inclusions in stony meteorites--a cautionary note.

Newly discovered fluid inclusions in thin sections of Bjurbole chondrules, shergottite EETA79001, lunar meteorite ALHA81005, and Apollo 16 glasses possess physical properties similar to those fluid inclusions found in thin sections of five stony meteorites recently described by Warner et al. (1983). The distribution and physical properties of these new fluid inclusions indicate they may be artifacts of thin section preparation; we suggest that saw coolant was sucked into vacuum vesicles in glasses and minerals through submicroscopic fractures produced during sawing. The similarities between these fluid inclusions and fluid inclusions previously described by Warner et al. (1983) lead us to conclude that many of the fluid inclusions reported earlier may be artifacts. Consequently, the origin of any fluid inclusions observed in thin sections of extraterrestrial materials must be interpreted with caution. The most probable true extraterrestrial fluid inclusions are those that have been observed in grains prepared without exposure to liquids of any kind.

Extraterrestrial Environment↗
PubMed1985

Iron-montmorillonite: a spectral analog of Martian soil.

Light absorption and reflectance by smectite clays containing various adsorbed ions were measured in the UV, VIS, and NIR ranges and compared to Martian dust and surface soil spectra. Structural iron in the octahedral sheet of smectites is responsible for a characteristic absorption feature in the UV at 240-260 nm, resulting from an O2 --> Fe3+ charge transfer that is similar to one observed in the Martian spectrum. Adsorbed iron affects, via crystal field absorptions, the reflectance of montmorillonite in the VIS and NIR (to 1.3 micrometers), causing stronger absorption and higher opacity in the wavelength range 0.4-0.6 micrometer, without developing any specific pronounced absorption feature. In both general appearance and presence of, or lack of, spectral features, the iron-montmorillonite reflectance spectra in the VIS and NIR are similar to the Martian spectra. At present, however, spectral similarity cannot be used as the sole criterion for constraining Martian mineralogy since several other minerals, other than Fe-smectites, show sufficient similitude to the Martian spectra; other properties have to be explored and combined to obtain a definitive identification.

Bentonite↗
PubMed1987

Concepts for collection of aerosols in Titan's atmosphere.

In this paper we consider design of instruments for collection of aerosols during entry in Titan's atmosphere. Major constraints on designs are small sample collection time, low aerosol column density, and the need to collect 1-10 micrograms of aerosol for gas chromatographic analysis. Thus it is important to maximize aerosol collection through collector design, which includes consideration of various types of collectors and maximizing the collection efficiency of a given type of collector. Sampling systems discussed include inertial impactors, filters, electrostatic devices, and multistage instruments. Aerosol sampling is reviewed in the context of high-altitude (200-70 km) and low-altitude (60-30 km) regions of Titan's atmosphere.

Aerosols↗
PubMed1987

Gas chromatographic instrumentation for the analysis of aerosols and gases in Titan's atmosphere.

During the next decade or so, NASA, in conjunction with the European Space Agency, plans to send a spacecraft to the Saturnian system so that local studies of Saturn and its satellite, Titan, can be made. In order to study the atmosphere of Titan, analysis of both aerosols and gases will have to be made. To accomplish this, gas chromatographic instrumentation for the collection and analysis of organic gases and aerosols in Titan's atmosphere is being developed. The aerosols will be collected and then subjected to pyrolysis-gas chromatography. Results using a simple pyrolysis-GC system and tholin, made by subjecting a nominal Titan mixture (96.8% N2, 3% CH4, 0.2% H2) to laser-supported shocks, show that many compounds, including hydrocarbons and simple nitriles, can be identified by this technique. Atmospheric gases will be collected using large volume (>10 cm3) sample loops and then analyzed by gas chromatography. Large volume samples are required because the ambient pressures, where the probe instruments are first deployed, will be low (<10 mbar). Preliminary studies using a 20 cm3 sampling system and a very sensitive meta-stable ionization detector show that hydrocarbon components at the 10 ppb level can be detected. Work will continue to improve GC sensitivity, minimize analysis time, and develop interfaces with suitable sample collectors for analysis of atmospheres by future spacecraft.

Aerosols↗
PubMed1989

Drainage pits in cohesionless materials: implications for surface of Phobos.

Viking orbiter images show grooves and chains of pits crossing the surface of Phobos, many of which converge toward the large crater Stickney or its antipode. Although it has been proposed that the pits and grooves are chains of secondary craters, their morphology and geometric relations suggest that they are the surface traces of fractures in the underlying solid body of Phobos. Several models have been proposed to explain the pits, of which the most plausible are gas venting and drainage of regolith into open fractures. the latter mechanism is best supported by the image data and is the mechanism studied in this investigation. Drainage pits and fissures are modeled experimentally by using two rigid substrate plates placed edge to edge and covered by uniform thicknesses of dry fragmental debris (simulated regolith). Fracture extension is simulated by drawing the plates apart, allowing drainage of regolith into the newly created void. A typical drainage experiment begins with a shallow depression on the surface of the regolith, above the open fissure. Increased drainage causes local drainage pits to form; continued drainage causes the pits to coalesce, forming a cuspate groove. The resulting experimental patterns of pits and grooves have pronounced similarities to those observed on Phobos. Characteristics such as lack of raised rims, linearity of grooves and chains of pits, uniform spacing of pits, and progression from discrete pits to cuspate grooves are the same in the experiments and on Phobos. In contrast, gas-venting pits occur in irregular chains and have raised rims. These experiments thus indicate that the Phobos grooves and pits formed as drainage structures. The pit spacing in an experiment is measured at the time that the maximum number of pits forms, prior to groove development. The average pit spacing is compared to the regolith thickness for each material. Regression line fits indicate that the average spacing of drainage pits in unconsolidated, noncohesive regolith is nearly equal to the thickness of regolith and appears to gbe independent of the angle of repose, within the resolution of our experiments. This provides a simple means of estimating regolith thickness where drainage pits are present. On Phobos, two locations differing by 90 degrees in longitude have average pit spacings that suggest regolith thicknesses of 290 and 300 m, suggesting that large areas of Phobos have a nearly uniform regolith thickness of approximately 300 m.

Extraterrestrial Environment↗
PubMed1990

A numerical simulation of climate changes during the obliquity cycle on Mars.

A one-dimensional seasonal energy balance climate model has been developed for the Martian surface and coupled to a model of CO2 distribution between atmosphere, regolith, and polar caps. This model takes into account the greenhouse warming of carbon dioxide, the meridional transport of heat, the CO2 condensation and sublimation cycle, and its adsorption in the regolith. The model takes into consideration the diurnal variation of solar irradiation, since it is shown that disregard of this effect yields temperatures too high by several degrees. The yearly-averaged temperatures calculated from this climate model at different obliquities are used to estimate the importance of CO2 exchanges between the regolith and atmosphere-cap systems during the obliquity cycle. For this purpose, the equation of thermal diffusion into the ground is solved for each latitude belt. The results differ substantially from those of previous studies, due in part to the consideration of the diurnal and seasonal variations of the solar irradiance. The model shows the importance of taking these short-period variations into account instead of using yearly-averaged quantities, due to the strong nonlinearity of the climate system on Mars. The roles of meridional heat transport and greenhouse warming are analyzed and shown to be important. For example, a permanent polar cap of carbon dioxide is destroyed by heat transport when the obliquity is high, while at low obliquity, high-pressure systems without permanent cap can exist if enough exchangeable carbon dioxide is available. Further, the results show the possible existence of hysteresis cycles in the formation and sublimation of permanent deposits during the course of the obliquity cycle.

Atmosphere↗
PubMed1993

Close encounters: an examination of UFO experiences.

Ss who reported UFO experiences were divided into those whose experiences were nonintense (e.g., seeing lights and shapes in the sky) and those whose experiences were intense (e.g., seeing and communicating with aliens or missing time). On a battery of objective tests Ss in these 2 groups did not score as more psychopathological, less intelligent, or more fantasy prone and hypnotizable than a community comparison group or a student comparison group. However, Ss in the UFO groups believed more strongly in space alien visitation than did comparison Ss. The UFO experiences of Ss in the intense group were more frequently sleep-related than the experiences of Ss in the nonintense group. Among the combined UFO Ss, intensity of UFO experiences correlated significantly with inventories that assessed proneness toward fantasy and unusual sensory experiences. Implications are discussed.

Adult↗
PubMed1982

The moon illusion: II. A reference theory.

The present theory provides explanations for the moon illusion and related issues involving size and distance perception in natural, outdoor settings. Although some assumptions of previous theories are rejected, other pivotal aspects are retained in this formulation. In particular, the present theory states that both the sky and ground are important referents in judging the spatial extent of the moon. Neither factor alone can account for all the available data, but quantitative models incorporating both factors do quite well when applied to the parametric findings of Holway and Boring, as well as to the results obtained by Kaufman and Rock. The reference theory and its associated class of specific models suggest new theoretical directions and experimental tests to narrow yet further the selection of appropriate explanations for one of visual perception's oldest unsolved puzzles.

Astronomical Phenomena↗
PubMed1999

The generation of martian floods by the melting of ground ice above dykes.

The surface of Mars is cut by long linear faults with displacements of metres to kilometres, most of which are thought to have been formed by extension. The surface has also been modified by enormous floods, probably of water, which often flowed out of valleys formed by the largest of these faults. By analogy with structures on Earth, we propose here that the faults are in fact the surface expression of dykes, and not of large-scale tectonic movements. We use a numerical model to show that the intrusion of large dykes can generate structures like Valles Marineris. Such dykes can provide a heat source to melt ground ice, and so provide a source of water for the floods that have been inferred to originate in some of the large valleys.

Disasters↗