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PubMed1994

Spectroscopy of Mars from 2.04 to 2.44 micrometers during the 1993 opposition: absolute calibration and atmospheric vs mineralogic origin of narrow absorption features.

We present moderate-resolution (lambda/delta lambda = 300 to 370) reflectance spectra of Mars from 2.04 to 2.44 micrometers that were obtained at UKIRT during the 1993 opposition. Seven narrow absorption features were detected and found to have a Mars origin. By comparison with solar and Mars atmospheric spectra, five of these features were attributed all or in part to Mars atmospheric CO2 or CO(2.052 +/- 0.003, 2.114 +/- 0.002, 2.150 +/- 0.003, 2.331 +/- 0.001, and 2.357 +/- 0.002 micrometers). Two of the bands (2.331 +/- 0.001 and 2.357 +/- 0.002 micrometers) appear to have widths and depths that are consistent with additional, nonatmospheric absorptions, although a solar contribution cannot be entirely ruled out. Two other weak bands centered at 2.278 +/- 0.002 and 2.296 +/- 0.002 micrometers may be at least partially mineralogic in origin. The data provide no conclusive identification of the mineralogy responsible for these absorption features. However, examination of terrestrial spectral libraries and previous moderate spectral resolution mineral studies indicates that the most likely origin of these features is either (bi)carbonate or (bi)sulfate anions in framework silicates or (Fe, Mg)-OH bonds in sheet silicates. If the bands are caused by phyllosilicate minerals, then an explanation must be found for the extremely narrow widths of the cation-OH features in the Mars spectra as compared to terrestrial minerals.

Astronomy↗
PubMed1994

A photochemical model of the martian atmosphere.

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.

Atmosphere↗
PubMed1994

Infrared spectroscopy of Triton and Pluto ice analogs: the case for saturated hydrocarbons.

The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton and Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1 (2.25 micrometers) and extends to lower frequencies, may be due to alkanes (C(n)H2n+2) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemistry of N2:CH4 and N(2):CH4:CO ices was explored demonstrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances. For example, using these A values the abundance ratios CH4/N2 approximately 1.3 x 10(-3), C2H4/N2 < or = 9.5 x 10(-7) and H2CO/N2 < or = 7.8 x 10(-7) are deduced for Triton and CH4/N2 approximately 3.1 x 10(-3), C2H4/N2 < or = 4.1 x 10(-6), and H2CO/N2 < or = 5.2 x 10(-6) deduced for Pluto. The small amounts of C2H4 and H2CO in the surface ices of these bodies are in disagreement with the large abundances expected from many theoretical models.

Alkanes↗
PubMed1994

The Titan haze revisited: magnetospheric energy sources and quantitative tholin yields.

We present laboratory measurements of the radiation yields of complex organic solids produced from N2/CH4 gas mixtures containing 10 or 0.1% CH4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton tholin. High-energy magnetospheric electrons responsible for tholin production represent a class distinct from the plasma electrons considered in models of Titan's airglow. Electrons with E > 20 keV provide an energy flux approximately 1 x 10(-2) erg cm-2 sec-1, implying from our measured tholin yields a mass flux of 0.5 to 4.0 x 10(-14) g cm-2 sec-1 of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m.) This figure is in agreement with required mass fluxes computed from recent radiative transfer and sedimentation models. If, however, these results, derived from experiments at approximately 2 mb, are applied to lower pressure levels toward peak auroral electron energy deposition and scaled with pressure as the gas-phase organic yields, the derived tholin mass flux is at least an order of magnitude less. We attribute this difference to the fact that tholin synthesis occurs well below the level of maximum electron energy deposition and to possible contributions to tholins from UV-derived C2-hydrocarbons. We conclude that Titan tholin, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze--a result consistent with the fact that the optical properties of Titan tholin, among all proposed materials, are best at reproducing Titan's geometric albedo spectrum from near UV to mid-IR in light-scattering models.

Aerosols↗
PubMed1994

Temperature of nitrogen ice on Pluto and its implications for flux measurements.

Previous work by K.A. Tryka et al. (Science 261, 751-754, 1993) has shown that the profile of the 2.148-micrometers band of solid nitrogen can be used as a "thermometer" and determined the temperature of nitrogen ice on Triton to be 38(+2)-1 K. Here we reevaluate that data and refine the temperature value to 38 +/- 1 K. Applying the same technique to Pluto we determine that the temperature of the N2 ice on that body is 40 +/- 2 K. Using this result we have created a nonisothermal flux model of the Pluto-Charon system. The model treats Pluto as a body with symmetric N2 polar caps and an equatorial region devoid of N2. Comparison with the infrared and millimeter flux measurements shows that the published fluxes are consistent with models incorporating extensive N2 polar caps (down to +/- 15 degrees or +/- 20 degrees latitude) and an equatorial region with a bolometric albedo < or = 0.2.

Astronomical Phenomena↗
PubMed1995

Photochemical modeling of Titan's atmosphere

We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984, Astrophys. J. Suppl. 55, 465-506), the most significant recent change in the reactions used is the updated methane photodissociation scheme (Mordaunt et al. 1993, J. Chem. Phys. 98(3), 2054-2065). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990, Icarus, 85, 43-57) using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992, Icarus 100, 512-526). We have run the model in both steady-state and diurnal modes, with 62 speices involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.

Atmosphere↗
PubMed1995

Titan's stratospheric temperature asymmetry: a radiative origin?

During the 1981 Voyager encounter, Titan's stratosphere exhibited a large thermal asymmetry, with high northern latitudes being colder than comparable southern latitudes. Given the short radiative time constant, this asymmetry would not be expected at the season of the Voyager observations (spring equinox), if the infrared and solar opacity sources were distributed symmetrically. We have investigated the radiative budget of Titan's stratosphere, using two selections of Voyager IRIS spectra recorded at symmetric northern and southern latitudes. In the region 0.1-1 mbar, temperatures are 7 K colder at 50 degrees N than at 53 degrees S and the difference reaches approximately 13 K at 5 mbar. On the other hand, the northern region is strongly enriched in nitriles and hydrocarbons, and the haze optical depth derived from the continuum emission between 8 and 15 micrometers is twice as large as in the south. Cooling rate profiles have been computed at the two locations, using the gas and haze abundances derived from the IRIS measurements. We find that, despite lower temperatures, the cooling rate profiles in the pressure range 0.15-5 mbar are 20 to 40% larger in the north than in the south, because of the enhanced concentrations of infrared radiators. Because the northern hemisphere appears darker than the southern one in the Voyager images, enhanced solar heating is also expected to take place at 50 degrees N. Solar heating rate profiles have been calculated, with two different assumptions on the origin of the hemispheric asymmetry. In the most likely case where it results from a variation in the absorbance of the haze material, the heating rates are found to be 12-15% larger at the northern location than at the southern one, a smaller increase than that in the cooling rates. If the lower albedo in the north results from an increase in the particle number density, a 55 to 75% difference is found for the pressure range 0.15-5 mbar, thus larger than that calculated for the cooling rates. Considering the uncertainties in the haze model, dynamical heat transport may significantly contribute to the meridional temperature gradients observed in the stratosphere. On the other hand, the latitudinal variation in gas and haze composition may be sufficient to explain the entire temperature asymmetry observed, without invoking a lag in the thermal response of the atmosphere due to dynamical inertia.

Atmosphere↗
PubMed1995

Comets, impacts, and atmospheres.

We are proposing a model for the delivery of volatiles to the inner planets by icy planetesimals (comets). Laboratory studies of the trapping of gases in ice forming at low temperatures simulate the formation of comet nuclei at various distances from the Sun in the solar nebula. The total gas content as well as the relative proportions of gases trapped in the ice are strong functions of temperature. As they trap N2 inefficiently, all planetesimals formed interior to Neptune are deficient in nitrogen, acquiring values of C/N resembling those found in the inner planet volatile inventories. A mixture of three basic types of comets appears capable of accounting for the observed volatile inventories on Venus, Earth, and Mars, with the caveat that impact erosion is necessary to explain the present condition of the martian atmosphere. The model includes the possibility of several epochs of clement conditions on early Mars. Some tests of these ideas are suggested, including measurements in Jupiter's atmosphere by the Galileo probe.

Atmosphere↗
PubMed1995

Reflectance spectroscopy of ferric sulfate-bearing montmorillonites as Mars soil analog materials.

Spectroscopic analyses have shown that smectites enhanced in the laboratory with additional ferric species exhibit important similarities to those of the soils on Mars. Ferrihydrite in these chemically treated smectites has features in the visible to near-infrared region that resemble the energies and band strengths of features in reflectance spectra observed for several bright regions on Mars. New samples have been prepared with sulfate as well, because S was found by Viking to be a major component in the surface material on Mars. A suite of ferrihydrite-bearing and ferric sulfate-bearing montmorillonites, prepared with variable Fe3+ and S concentrations and variable pH conditions, has been analyzed using reflectance spectroscopy in the visible and infrared regions, Mössbauer spectroscopy at room temperature and 4 K, differential thermal analysis, and X-ray diffraction. These analyses support the formation of ferrihydrite of variable crystallinity in the ferrihydrite-bearing montmorillonites and a combination of schwertmannite and ferrihydrite in the ferric sulfate-bearing montmorillonites. Small quantities of poorly crystalline or nanophase forms of other ferric materials may also be present in these samples. The chemical formation conditions of the ferrihydrite-bearing and ferric sulfate-bearing montmorillonites influence the character of the low temperature Mössbauer sextets and the visible reflectance spectra. An absorption minimum is observed at 0.88-0.89 micrometers in spectra of the ferric sulfate-bearing samples, and at 0.89-0.92 micrometers in spectra of the ferrihydrate-bearing montmorillonites. Mössbauer spectra of the ferric sulfate-bearing montmorillonites indicate variable concentrations of ferrihydrite and schwertmannite in the interlaminar spaces and along grain surfaces. Dehydration under reduced atmospheric pressure conditions induces a greater effect on the adsorbed and interlayer water in ferrihydrite-bearing montmorillonite than on the water in ferric sulfate-bearing montmorillonite. Reflectance spectra of ferric sulfate-bearing montmorillonite include a strong 3-micrometers band that is more resistant to dry atmospheric conditions than the 3-micrometers band in spectra of similarly prepared ferrihydrite-bearing montmorillonites.

Aluminum Silicates↗
PubMed1995

Numerical simulation of the general circulation of the atmosphere of Titan.

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

Atmosphere↗
PubMed1996

Hydrocarbon photochemistry in the upper atmosphere of Jupiter.

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.

Astronomy↗
PubMed1996

A matched filter method for ground-based sub-noise detection of terrestrial extrasolar planets in eclipsing binaries: application to CM Draconis.

The photometric detection of extrasolar planets by transits in eclipsing binary systems can be significantly improved by cross-correlating the observational light curves with synthetic models of possible planetary transit features, essentially a matched filter approach. We demonstrate the utility and application of this transit detection algorithm for ground-based detections of terrestrial-sized (Earth-to-Neptune radii) extrasolar planets in the dwarf M-star eclipsing binary system CM Draconis. Preliminary photometric observational data of this system demonstrate that the observational noise is well characterized as white and Gaussian at the observational time steps required for precision photometric measurements. Depending on planet formation scenarios, terrestrial-sized planets may form quite close to this low-luminosity system. We demonstrate, for example, that planets as small as 1.4 Earth radii with periods on the order of a few months in the CM Draconis system could be detected at the 99.9% confidence level in less than a year using 1-m class telescopes from the ground. This result contradicts commonly held assumptions limiting present ground-based efforts to, at best, detections of gas giant planets after several years of observation. This method can be readily extended to a number of other larger star systems with the utilization of larger telescopes and longer observing times. Its extension to spacecraft observations should also allow the determination of the presence of terrestrial-sized planets in nearly 100 other known eclipsing binary systems.

Algorithms↗
PubMed1996

The 825-1110 angstroms EUV spectrum of Venus.

On 15 August 1994 we launched the EUVS sounding rocket payload to observe the 825-1110 angstrom region of Venus's far ultraviolet airglow spectrum. The EUVS telescope/spectrograph obtained good data at five times higher spectral resolution than was previously available in the far ultraviolet. We present these data and compare our results to those obtained by the Galileo UVS and Venera 11/12 UV spectrophotometers. We identify several new spectral emission features, including both singly ionized nitrogen and molecular nitrogen in Venus's spectrum. We also see evidence for electron-impact-induced emission from CO. Finally, the EUVS data indicate that the "Ar" emissions detected in Venus's far ultraviolet spectrum by Venera 11/12 spectrophotometers are in fact not due to argon, thus eliminating the discrepancy between in situ and remote sensing measurements.

Astronomy↗
PubMed1996

Potential Jupiter astmospheric constituents: candidates for the mass spectrometer in the Galileo atmospheric probe.

An array of carbon-hydrogen-nitrogen (CHN) compounds are formed when mixtures of ammonia and acetylene are photolyzed with ultraviolet (UV) light. Some of these compounds, formed in our investigation of pathways for HCN synthesis on Jupiter, have not been encountered previously in observational, theoretical or laboratory photochemical studies. Knowledge of these structures may facilitate the interpretation of the mass spectral data obtained by the Galileo probe as it passed through the atmosphere of Jupiter.

Acetylene↗
PubMed1996

Titan's 5 micrometers spectral window: carbon monoxide and the albedo of the surface.

We have measured the spectrum of Titan near 5 micrometers and have found it to be dominated by absorption from the carbon monoxide 1-0 vibration-rotation band. The position of the band edge allows us to constrain the abundance of CO in the atmosphere and/or the location of the reflecting layer in the atmosphere. In the most likely case, 5 micrometers radiation is reflected from the surface and the mole fraction of CO in the atmosphere is qCO=10(+10/-5) ppm, significantly lower than previous estimates for tropospheric CO. The albedo of the reflecting layer is approximately 0.07(+0.02/-0.01) in the 5 micrometers continuum outside the CO band. The 5 micrometers albedo is consistent with a surface of mixed ice and silicates similar to the icy Galilean satellites. Organic solids formed in simulated Titan conditions can also produce similar albedos at 5 micrometers.

Astronomical Phenomena↗
PubMed1997

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations.

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.

Cosmic Dust↗
PubMed1997

Titan haze: structure and properties of cyanoacetylene and cyanoacetylene-acetylene photopolymers.

The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosphere of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolysis) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase HC3N at 185 and 254 nm. The resulting brown to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon-carbon double bonds, which, on exposure to heat, cyclizes to form a graphitic structure. Similar methods of analysis were used to show that when HC3N is photolyzed in the presence of Titan's other atmospheric constituents (CH4, C2H6, C2H2, and CO), a copolymer is formed in which the added gases are incorporated as substituents on the polymer chain. Of special significance is the copolymer of HC3N and acetylene (C2H2). Even in experiments where C2H2 was absorbing nearly all of the incident photons, the ratio of C2H2 to HC3N found in the resulting polymer was only 2:1. Scanning electron microscopy was used to visually examine the polymer particles. While pure polyacetylene particles are amorphous spheres roughly 1 micrometer in diameter, polycyanoacetylenes appear to be strands of rough, solid particles slightly smaller in size. The copolymer of HC3N and C2H2 exhibits characteristics of both pure polymers. This is particularly important as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorporation of other minor atmospheric constituents, like HC3N, into the polyacetylenes are expected to have optical constants more comparable to those of the Titan haze.

Acetylene↗