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Interface between atomic and molecular physics and atmospheric physics; middle atmospheric research; and atmospheric exchange are discussed. Also included are proposals for research from 1982 through 1985, priorities, and available support.
Results of a numerical simulation of the diurnal redistribution of temperature by radiative and molecular-conductive processes in the Martian soil-atmosphere system. An attempt is made to assess the importance of atmospheric molecular conduction near the surface and to estimate the characteristic depth of the diurnal temperature wave. The computational results are found to indicate a dual structure in the diurnal temperature wave propagation pattern, with a diffusive-type wave in the lowest 150 m superimposed on a radiatively induced disturbance with a characteristic scale of 1.8 km. Atmospheric molecular thermal conduction typically accounts for about 15% of the total heating/cooling in the lowest 25 m. Thermal conduction in both the soil and atmosphere appears to be an important factor in the thermal coupling of these subsystems. A free-convection regime in the conduction layer is predicted by the model for about five hours of the Martian day.
A popular, as well as scientifically rigorous, scenario for the origin of life on Earth involves the production of organic molecules by interaction of lightning (or other forms of energy) with a chemically reducing atmosphere in the early history of Earth. Experiments since the 1950's have convincingly demonstrated that the yield of organic molecules is high when the atmosphere contains molecular hydrogen, methane, ammonia, and water vapor. Additional work has also shown that such a highly reducing atmosphere might not, however, have been sufficiently long-lived in the presence of intense solar ultraviolet radiation for life to have formed from it. One way of maintaining such an atmosphere would be to have a continual replenishment of the reduced gases by prolonged volcanic outgassing from a reducing of Earth's interior. The length of time that this replenishment might need to continue is in part constrained by the flux of asteroids onto the Earth's surface containing sufficient energy to destroy most, if not all, life that had developed up to that point in time. If a reducing atmosphere is a key ingredient for the origin of life on Earth, the time of the last environmental sterilization due to large impacts would be an important constraint. In a deep marine setting (e.g., hydrothermal vent), the last global sterilization might have occurred at 4200-4000 Ma. On the Earth's surface, the last global sterilization event might have occurred at 4000-3700 Ma. If these are meaningful constraints, how likely is it that a reducing atmosphere could have survived on the Earth until about 3800 Ma ago? Due to the importance of replenishing this atmosphere with reducing components by volcanic outgassing from the mantle, geochemical information on the history of the mantle's oxidation state would be useful for addressing this question. Geochemical and experimental data discussed in this abstract suggest that extrusive mafic volcanics derived from the upper mantle have had oxidation states near the fayalite-magnetite quartz buffer throughout the last 3800 Ma. At magmatic temperatures, the gases released from volcanoes having this oxidation state would have been, as they are today, composed dominantly of carbon dioxide and water vapor, and would not contain the ingredients for maintaining a reducing atmosphere. Consequently, geochemical data do not favor the survival of a reducing atmosphere until about 3800 Ma. Alternative venues and pathways for the origin of life need to be investigated (e.g., hydrothermal vents along oceanic ridges).
Structure and mean molecular weight measurement of unknown atmosphere assessed from high altitude tests in earth atmosphere
The M3V dwarf star L 98-59 hosts three small (R < 1.6 Rꚛ) planets. The host star is bright (K = 7.1) and nearby (10.6 pc), making the system a prime target for follow-up characterization with the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST). Herein, we use simulated transmission spectroscopy to evaluate the detectability of spectral features with HST and JWST assuming diverse atmospheric scenarios (e.g., atmospheres dominated by H2, H2O, CO2, or O2). We find that H2O and CH4 present in a low mean molecular weight atmosphere could be detected with HST in one transit for the two outermost planets, while H2O in a clear steam atmosphere could be detected in six transits or fewer with HST for all three planets. We predict that observations using JWST/NIRISS would be capable of detecting a clear steam atmosphere in one transit for each planet and H2O absorption in a hazy steam atmosphere in two transits or less. In a clear, desiccated atmosphere, O2 absorption may be detectable for all three planets with NIRISS. If the L 98-59 planets possess a clear, Venus-like atmosphere, NIRSpec could detect CO2 within 26 transits for each planet, but the presence of H2SO4 clouds would significantly suppress CO2 absorption. The L 98-59 system is an excellent laboratory for comparative planetary studies of transiting multiplanet systems, and observations of the system via HST and JWST would present a unique opportunity to test the accuracy of the models presented in this study.
A great deal of information can be obtained about aerosol and Rayleigh scattering by observing the polarization state for both reflected and transmitted radiation. Results of neutral point behavior are presented for (1) a Rayleigh or molecular scattering atmosphere; (2) a haze L distribution of aerosols typical of those found in the earth's atmosphere; and (3) two actual models of the earth's atmosphere which combine Rayleigh scattering with a normal aerosol content and with a one-third normal aerosol content.
With the commissioning of powerful, new-generation telescopes such as the James Webb Space Telescope (JWST) and the ground-based Extremely Large Telescopes, the first characterization of a high molecular weight atmosphere around a temperate rocky exoplanet is imminent. Atmospheric simulations and synthetic observables of target exoplanets are essential to prepare and interpret these observations. Here we report the results of the first part of the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project, which compares 3D numerical simulations performed with four state-of-the-art global climate models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for the potentially habitable target TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection), and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in good agreement. The intermodel spread in the global mean surface temperature amounts to 7 K (6 K) for the N2-dominated (CO2-dominated) atmosphere. The radiative fluxes are also remarkably similar (intermodel variations less than 5%), from the surface (1 bar) up to atmospheric pressures ∼5 mbar. Moderate differences between the models appear in the atmospheric circulation pattern (winds) and the (stratospheric) thermal structure. These differences arise between the models from (1) large-scale dynamics, because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped, and (2) parameterizations used in the upper atmosphere such as numerical damping.
Titan is the largest moon of Saturn, with a thick (1.45 bar) atmosphere composed primarily of molecular nitrogen and methane. Atmospheric photochemistry results in the production of a wide range of complex organic molecules, including hydrocarbons, nitriles, aromatics and species of possible pre-biotic relevance. Studies of Titan's atmospheric chemistry thus provide a unique opportunity to explore the origin and evolution of complex organic matter in a primitive (terrestrial) planetary atmosphere. Underpinned by laboratory measurements, remote and in-situ observations of hydrocarbons, nitriles and oxygen-bearing species provide important new insights in this regard. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new facility, well suited to the study of molecular emission from Titan's upper and middle-atmosphere. This presentation will focus on detection and mapping of rotational emission lines from molecules including HNC, CO, HC3N, CH3CN, C2H3CN and C2H5CN, as well minor isotopologues. Possible chemical formation pathways for these species will be discussed, and the scope for improved understanding of non-aqueous organic chemistry through laboratory experiments and atmospheric/liquid-phase simulations under Titan-like conditions will be examined [5].
Observations of Io suggest that it may have an atmosphere in which sodium vapor, ammonia, and nitrogen are important constituents. Several atmospheric models consisting of these gases are tested as a function of total content against the Pioneer 10 observations and for stability against escape. The results suggest that the atmosphere is very tenuous and that the interpretation of the ionosphere detected by Pioneer 10 by a static model may be inconsistent with the sodium-cloud observations. It is postulated that ionization may also be escaping and that sodium may be comparable in content in the atmosphere with some molecular constituent such as NH3 or N2. Sodium and this molecular component then dominate the atmosphere. It is also suggested that particle precipitation contributes to heating of the atmosphere and to the production of ionization; the difference between day- and nighttime ionospheres and possible trailing and leading side effects may relate to the nature of the particle energy distributions. These distributions may result from the peculiar interaction of Io with the Jovian magnetosphere.
The atmosphere of Saturn’s moon, Titan, is unlike any other. Photochemistry and ion chemistry in the upper atmosphere process the major atmospheric constituents, molecular nitrogen (N2) and methane (CH4), into larger and more complex molecules including hydrocarbons and nitriles. To date there are ten hydrocarbons heavier than methane that have been detected, including large quantities of the saturated hydrocarbons (alkanes) ethane (C2H6) and propane (C3H8). In addition to being significant trace gases of the atmosphere, these species condense, rain out and add to the volume of Titan’s hydrocarbon lakes and seas. The fourth lightest alkane, butane (C4H10),comes in two structural forms: an unbranched carbon chain (n-butane) and a branched chain (i-butane). The relative production rates of both these isomers are presently unknown, although the circumstantial case for butane to be present in detectable amounts is strong, based on photochemical models and from Cassini low-resolution mass spectrometry of the ionosphere. Butane, like its lighter siblings ethane and propane, is thought to be present in Titan’s lakes and seas where it may be active in forming molecular ‘co-crystals’ with acetylene (C2H2). We report on our recent attempts to detect both forms of butane in Titan’s atmosphere by analysis of Cassini infrared spectra collected during 2004-2017 by the Composite Infrared Spectrometer(CIRS) instrument. This multi-year effort has encompassed the collection of new laboratory spectra of both isomers at several institutions, followed by a concerted attempt to find their signatures in CIRS spectra. We search for butane by high precision modeling and removal of overlying emissions due to other, already-known gases to reveal signatures that match those of the butane isomers. The current status of the search will be discussed, including improved upper limits for both isomers, and directions of future research to detect both species.
Molecular absorption bands and surface structure of Jupiter atmosphere
The mixing ratio of molecular oxygen and other species that do not condense at martian ambient conditions will vary as the mass of atmosphere oscillates over a martian year due to alternate condensation and vaporization of carbon dioxide at the poles. This variation can be estimated utilizing measurements of mixing ratio made in-situ together with multi-year measurements of atmospheric pressure, both available from the Viking landers starting in 1976. The concentration of non-condensables, on average, will be approximately an inverse function of the atmospheric pressure.
During the period April 29 through May 2, 1985, the Atmospheric Trace Molecular Spectroscopy experiment was operated as part of the Spacelab-3 payload of the shuttle Challenger. The instrument, a modified Michelson Interferometer covering the frequency range from 600 to 5000/cm, at a spectral resolution of 0.01/cm, recorded infrared spectra of the Sun and of the Earth's atmosphere at times close to entry into and exit from occultation by the Earth's limb as seen from the shuttle orbit of 360 km. Spectra were obtained that are free from absorptions due to constituents of the atmosphere (i.e., solar pure spectra), as well as spectra of the atmosphere itself, covering line-of-sight tangent altitudes that span the range from the lower thermosphere to the bottom of the troposphere. This atlas, believed to be the first record of observations of the continuous high resolution infrared spectrum of the Sun and the Earth's atmosphere from space, provides a compilation of these spectra arranged in a hardcopy format suitable for quick-look reference purposes; the data are also available in digital form.
Infrared photometry of Titan, Saturn, and Saturn's Rings at 3.5, 4.9, 17.8, and 18.4 microns is reported. Comparison of the albedo of Titan in the 4.9 micron 'window' with the albedo of the rings and with laboratory spectra suggests that frost, possibly water ice, could be a major constituent. If thick clouds are present they must be very dark at 4.9 microns. The 17.8 and 18.4 micron data are not consistent with a clear, dense molecular hydrogen atmosphere.
Sensitivity studies related to the effects of line cutoff, spectral resolution, and temperature and pressure interpolations in radiative transfer have been performed so that a data set of absorption coefficients for water vapor, CO2, and O3 may be created efficiently. Results show that computations of absorption coefficients are affected only slightly by cutting a line off at a wave number 190 times the Lorentz half width from the center, or equivalently, cutting off 0.33 percent of the line intensity from the wings. To achieve a relative cooling rate error smaller than 2 percent, it is sufficient to precompute the absorption coefficient at three temperatures (210, 250, and 290 K) and 19 pressures with Delta (log 10 p) = 0.2. The absorption coefficient at other conditions can be interpolated linearly with pressure and exponentially with a quadratic in temperature. For the spectral resolution the absorption coefficients can be adequately computed at 0.01, 0.002, 0.005, and 0.025/cm intervals in the thermal water vapor, the CO2 and O3 bands, and the solar water vapor bands, respectively, which limits the error to only a few percent in the cooling and heating rates. Using the precomputed absorption coefficients, repeated monochromatic calculations of atmospheric heating/cooling rates for radiation model developments and for comparison with less detailed calculations are no longer difficult.
Thermal escape of neutral hydrogen and distribution in Earth thermosphere, noting collision in transition regions
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