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NASA NTRS2006

Aerocapture Technology Development for Planetary Science - Update

Within NASA's Science Mission Directorate is a technological program dedicated to improving the cost, mass, and trip time of future scientific missions throughout the Solar System. The In-Space Propulsion Technology (ISPT) Program, established in 2001, is charged with advancing propulsion systems used in space from Technology Readiness Level (TRL) 3 to TRL6, and with planning activities leading to flight readiness. The program's content has changed considerably since inception, as the program has refocused its priorities. One of the technologies that has remained in the ISPT portfolio through these changes is Aerocapture. Aerocapture is the use of a planetary body's atmosphere to slow a vehicle from hyperbolic velocity to a low-energy orbit suitable for science. Prospective use of this technology has repeatedly shown huge mass savings for missions of interest in planetary exploration, at Titan, Neptune, Venus, and Mars. With launch vehicle costs rising, these savings could be the key to mission viability. This paper provides an update on the current state of the Aerocapture technology development effort, summarizes some recent key findings, and highlights hardware developments that are ready for application to Aerocapture vehicles and entry probes alike. Description of Investments: The Aerocapture technology area within the ISPT program has utilized the expertise around NASA to perform Phase A-level studies of future missions, to identify technology gaps that need to be filled to achieve flight readiness. A 2002 study of the Titan Explorer mission concept showed that the combination of Aerocapture and a Solar Electric Propulsion system could deliver a lander and orbiter to Titan in half the time and on a smaller, less expensive launch vehicle, compared to a mission using chemical propulsion for the interplanetary injection and orbit insertion. The study also identified no component technology breakthroughs necessary to implement Aerocapture on such a mission. Similar studies of Aerocapture applications at Neptune, Venus, and Mars were studied in 2003 through 2005. All showed significant performance improvements for the missions studied. Findings from these studies were used to guide the technology development tasks originally solicited in a 2002 NASA ROSS Research Announcement. The tasks are now in their final year and have provided numerous improvements in modeling and hardware, for use in proposals or new mission starts. Major Accomplishments: Since validation of the Aerocapture maneuver requires a space flight, ground developments have focused on modeling and environment prediction, materials, and sensors. Lockheed Martin has designed and built a 2-meter Carbon-Carbon aeroshell "hot structure." The article utilizes co-cured stiffening ribs and advanced insulation to achieve large scale, and up to a 40% reduction in areal density over the Genesis probe construction. This concept would be an efficient solution for probes that experience heat rates near 800-1000 W/cm(exp 2), such as at Venus and Earth. Applied Research Associates has extensively tested a family of efficient ablative TPS materials that provide solutions for a range of heating conditions. These materials are being applied to high-temperature structures built by ATK Space Systems, led by Langley Research Center. One-meter aeroshells will be thermally tested to validate construction and demonstrate higher bondline temperatures, which can lead to mass savings of up to 30% over traditional heatshields. Ames Research Center has developed aeroshell instrumentation that could measure environmental conditions and material performance during atmospheric entry. Instruments to measure TPS recession, heat flux, and catalycity could be combined with traditional sensors to provide a "plug-and-play" system for minimal mass and power, that would acquire flight data for model improvement and risk reduction on future missions. Improved atmospheric and aerothermodynamic models ha also been a major focus of the program. Next Steps: Aerocapture is one of five technologies in competition for a flight validation opportunity through the New Millennium Program. If selected, a fully autonomous vehicle will perform an Aerocapture at Earth in 2010, and flight data will be used to validate the guidance system and the TPS material for science mission infusion.

Munk, Michelle M.↗
NASA NTRS1990

Venus atmospheric circulation - Known and unknown

Temporally and longitudinally averaged circulation determined from images acquired since 1979 from the Pioneer Venus orbiter has shown significant changes in th meridional flow. The solar-locked structure in the zonal and meridional components of the cloud motions is also observed to change with time. Mechanisms for such changes remain unknown, although planetary waves and a slight modulation of the cloud level have been suggested as explanations. Decomposition of the solar-locked structure into diurnal and semidiurnal components suggests that nightside zonal flow may be 10-20 m/s faster than the average dayside flow. Many aspects of the circulation remain unknown, including the horizontal and vertical structure on the nightside and at levels higher and lower than the UV cloud features, sources, and sinks of UV absorbers, and meridional transports of heat, momentum, and water vapor at different levels.

Limaye, Sanjay Shridhar↗
NASA NTRS2020

An evolutionary system of mineralogy. Part II: Interstellar and solar nebula primary condensation mineralogy (>4.565 Ga)

The evolutionary system of mineralogy relies on varied physical and chemical attributes, including trace elements, isotopes, solid and fluid inclusions, and other information-rich characteristics, to understand processes of mineral formation and to place natural condensed phases in the deep-time context of planetary evolution. Part I of this system reviewed the earliest refractory phases that condense at T > 1000 K within the turbulent expanding and cooling atmospheres of highly evolved stars. Part II considers the subsequent formation of primary crystalline and amorphous phases by condensation in three distinct mineral-forming environments, each of which increased mineralogical diversity and distribution prior to the accretion of planetesimals >4.5 billion years ago. (1) Interstellar molecular solids: Varied crystalline and amorphous molecular solids containing primarily H, C, O, and N are observed to condense in cold, dense molecular clouds in the interstellar medium (10 < T < 20 K; P < 10−13 atm). With the possible exception of some nanoscale organic condensates preserved in carbonaceous meteorites, the existence of these phases is documented primarily by telescopic observations of absorption and emission spectra of interstellar molecules in radio, microwave, or infrared wavelengths. (2) Nebular and circumstellar ice: Evidence from infrared observations and laboratory experiments suggest that cubic H2O (“cubic ice”) condenses as thin crystalline mantles on oxide and silicate dust grains in cool, distant nebular and circumstellar regions where T ~100 K. (3) Primary condensed phases of the inner solar nebula: The earliest phase of nebular mineralogy saw the formation of primary refractory minerals that solidified through high-temperature condensation (1100 < T < 1800 K; 10−6 < P < 10−2 atm) in the solar nebula more than 4.565 billion years ago. These earliest mineral phases originating in our solar system formed prior to the accretion of planetesimals and are preserved in calcium-aluminum-rich inclusions, ultra-refractory inclusions, and amoeboid olivine aggregates.

Shaunna M. Morrison↗
NASA NTRS2003

High Resolution Spectroscopy to Support Atmospheric Measurements

Spectroscopic parameters (such as line position, intensity, broadening and shifting coefficients and their temperature dependences, line mixing coefficients etc.) for various molecular species of atmospheric interest are determined. In order to achieve these results, infrared spectra of several molecular bands are obtained using high-resolution recording instruments such as tunable diode laser spectrometer and Fourier transform spectrometers. Using sophisticated analysis routines (Multispectrum nonlinear least squares technique) these high-resolution infrared spectra are processed to determine the various spectral line parameters that are cited above. Spectra were taken using the McMath-Pierce Fourier transform spectrometer (FTS) at the National Solar Observatory on Kitt Peak, Arizona as well as the Bruker FTS at the Pacific Northwest National Laboratory (PNNL) at Richland, Washington. Most of the spectra are acquired not only at room temperature, but also at several different cold temperatures. This procedure is necessary to study the variation of the spectral line parameters as a function of temperature in order to simulate the Earth's and other planetary atmospheric environments. Depending upon the strength or weakness of the various bands recorded and analyzed, the length(s) of the absorption cells in which the gas samples under study are kept varied from a few centimeters up to several meters and the sample temperatures varied from approximately +30 C to -63 C. Research on several infrared bands of various molecular species and their isotopomers are undertaken. Those studies are briefly described.

Venkataraman, Malathy Devi↗
NASA NTRS1999

Initial Test Firing Results for Solid CO/GOX Cryogenic Hybrid Rocket Engine for Mars ISRU Propulsion Applications

ORBITEC is developing methods for producing, testing, and utilizing Mars-based ISRU fuel/oxidizer combinations to support low cost, planetary surface and flight propulsion and power systems. When humans explore Mars we will need to use in situ resources that are available, such as: energy (solar); gases or liquids for life support, ground transportation, and flight to and from other surface locations and Earth; and materials for shielding and building habitats and infrastructure. Probably the easiest use of Martian resources to reduce the cost of human exploration activities is the use of the carbon and oxygen readily available from the CO2 in the Mars atmosphere. ORBITEC has conducted preliminary R&D that will eventually allow us to reliably use these resources. ORBITEC is focusing on the innovative use of solid CO as a fuel. A new advanced cryogenic hybrid rocket propulsion system is suggested that will offer advantages over LCO/LOX propulsion, making it the best option for a Mars sample return vehicle and other flight vehicles. This technology could also greatly support logistics and base operations by providing a reliable and simple way to store solar or nuclear generated energy in the form of chemical energy that can be used for ground transportation (rovers/land vehicles) and planetary surface power generators. This paper describes the overall concept and the test results of the first ever solid carbon monoxide/oxygen rocket engine firing.

Rice, Eric E.↗
NASA NTRS2020

Utilizing a Database of Simulated Geometric Albedo Spectra for Photometric Characterization of Rocky Exoplanet Atmospheres

In anticipation of future flagship missions focused on the goal of achieving direct imaging of rocky exoplanets, we have developed a database of models to help the community examine the potential spectral characteristics of a broad range of rocky planet atmospheres. Using the publicly available Planetary Spectrum Generator, we have computed a grid of 141,600 rocky exoplanet geometric albedo spectra across a seven-dimensional parameter space. Using this grid, we have performed a color–color analysis seeking to identify the most useful near-UV and red or near-infrared photometric follow-up channels to combine with a green-optical(discovery)spectral channel. We found that a combination of filters at 0.4, 0.58, and∼0.8μm were able to distinguish between atmospheres with moderate-to-high concentrations of four different dominant absorbing constituents, given at least 10 hr of observation on a star at 10 parsec with a 15 m class space telescope; however, more moderate abundances similar to those of solar system rocky bodies would be more challenging to detect. We recommend that future missions seeking to characterize directly imaged rocky exoplanets by colors alone further consider multiband photometry as a first discriminator for planetary characteristics.

Adam J R W Smith↗
NASA NTRS2014

Deep Mapping of Small Solar System Bodies with Galactic Cosmic Ray Secondary Particle Showers

Galactic cosmic rays rain steadily from all directions onto asteroids and comets. The interaction of these high-energy ions produces a cascade of secondary particles, including muons, which can penetrate the solid interiors of small solar system bodies. Muons, which are produced in abundance in Earth's atmosphere, have been used to image large structures on Earth, including the Great Pyramids and the interior of volcanoes. In this study, we demonstrate that the transmitted flux of muons is sensitive to the interior density structure of asteroids and comets, less than a few hundred meters in diameter. Muonography has the potential to fill a critical gap in our knowledge of the deep interiors of small bodies, providing information needed for planetary defense, in situ resource utilization, and planetary science. We use Monte Carlo codes (MCNPX and FLUKA), which accurately model galactic cosmic ray showers, to explore systematic variations in the production of muons in solid surfaces. Results of these calculations confirm the scaling of muon production in Earth's atmosphere to solid regolith materials, as predicted by a simple, semi-empirical model. Muons are primarily produced in the top meter of the regoliths of asteroids and comets. Their rate of production is over three orders of magnitude lower than in Earth's atmosphere and depends strongly on regolith density. In practice, the use of muonography to characterize the interiors of small solar system bodies must overcome their low rate of production and their dependence on regolith density, which can vary over the surface of asteroids and comets. We show that interior contrast can be resolved using a muon telescope (hodoscope) with about 1 sq m aperture with integration times ranging from hours to weeks. Design concepts for a practical hodoscope that could be deployed in situ or on an orbiting spacecraft, are described. Regolith density within the top meter of an asteroid can be determined from radar observations. A concept for a pilot mission that combines remote radar measurements with in situ muonography of a near-Earth asteroid is presented. Perceived challenges and next steps for the development of the concept are described.

Prettyman, T. H.↗
NASA NTRS1974

Possible H2/+/ ultraviolet spectrum of Jupiter

An ultraviolet spectral probe for a hydrogen-rich planetary atmosphere, such as that of Jupiter, is suggested, utilizing discrete lines in the H2(+) 2p pi u - /s sigma g electronic transition. For the Jovian atmosphere, the dominant mechanism for exciting H2(+) to its 2p pi u state appears to be photoexcitation, principally through absorption of the solar Lyman-alpha line. The critical role of corrections to the Born-Oppenheimer approximation in the use of an H2(+) probe is discussed.

Wu, F.-M.↗
NASA NTRS2013

Interstellar and Planetary Analogs in the Laboratory

We present and discuss the unique capabilities of the laboratory facility, COSmIC, that was developed at NASA Ames to investigate the interaction of ionizing radiation (UV, charged particles) with molecular species (neutral molecules, radicals and ions) and carbonaceous grains in the Solar System and in the Interstellar Medium (ISM). COSmIC stands for Cosmic Simulation Chamber, a laboratory chamber where interstellar and planetary analogs are generated, processed and analyzed. It is composed of a pulsed discharge nozzle (PDN) expansion that generates a free jet supersonic expansion in a plasma cavity coupled to two ultrahigh-sensitivity, complementary in situ diagnostics: a cavity ring down spectroscopy (CRDS) system for photonic detection and a Reflectron time-of-flight mass spectrometer (ReTOF-MS) for mass detection. This setup allows the study of molecules, ions and solids under the low temperature and high vacuum conditions that are required to simulate some interstellar, circumstellar and planetary physical environments providing new fundamental insights on the molecular level into the processes that are critical to the chemistry in the ISM, circumstellar and planet forming regions, and on icy objects in the Solar System. Recent laboratory results that were obtained using COSmIC will be discussed, in particular the progress that have been achieved in monitoring in the laboratory the formation of solid particles from their gas-phase molecular precursors in environments as varied as circumstellar outflow and planetary atmospheres.

Salama, Farid↗
NASA NTRSDate not supplied

Active Solar-Like Stars and the Production of Prebiotically Relevant Molecules in Young Earth Like Exoplanet

The magnetic activity of young G type stars produces elevated ionizing radiation fluxes for its planetary system. X-ray, Extreme UV, flare radiation and the associated precipitating energetic particles accelerated in coronal mass ejection driven shocks can drive complex chemistry in the exoplanetary atmosphere, and ignite the production of complex molecules (Airapetian et al., 2016; Hayworth et al. 2021). To address the efficiency of the production of prebiotic molecules in young Earth-like environments, we developed and coupled a set of atmospheric models that account for the determination of the flux of precipitating particle, its ionization/dissociation effects in the planetary atmosphere, and the subsequent chemistry that leads to the creation of prebiotic molecules and greenhouse gases. We show that the productions rates are high enough to provide critical input for the origin of prebiotic molecules on the early Earth. Overall, the impact of the space weather on the atmospheric photochemistry should not be neglected when assessing the factors of habitability of the environments of young rocky (exo)planets.

Guillaume Gronoff↗
NASA NTRS1988

Interiors of the giant planets

This theoretical/observational project constrains interior structure of Jovian planets through observational data. Researchers continue to concentrate on Neptune in support of the 1989 Voyager encounter. Occultations of stars by Neptune are observed from the Tucson area and from Chile to obtain information about Neptune's atmosphere and to continue to search for Neptune arcs. Occultations by other solar system objects are also observed as part of collaborative efforts from time to time. New results on the structure of scintillations in the central flash occultation by Neptune on 20 August 1985 were derived. Analysis shows that scintillations are present throughout the lightcurve, both near the half-intensity points (at a pressure of 1 microbar) and near the central flash (at 0.4 mbar). Near the planetary limb, the scintillations are extended parallel to the limb; near the shadow center, they are extended parallel to the limb; near the shadow center, they are extended in a radial direction. Researchers collaborated with Ramesh Narayan to derive a theory relating the scintillations to density fluctuations in Neptune's atmosphere. The theory will ultimately enable researchers to test whether the scintillations are caused by internal gravity waves in Neptune's upper atmosphere.

Hubbard, W. B.↗
NASA NTRS2002

Building a virtual planet

The virtual Planetary Laboratory (VPL) is a recently funded 5-yr project, which seeks toimprove our understanding of the range of plausible environments and the likely signatures for life on extrasolar terrestrial planets. To achieve these goals we are developing a suite of innovative modeling tools to simulate the environments and spectra of extrasolar planets. The core of the VPL IS a coupled radiative transfer/climate/chemistry model, which is augmented by interchangeable modules which characterize geological, exogenic, atmospheric escape, and life processes. The VPL is validated using data derived from terrestrial planets within our own solar system. The VPL will be used to explore the plausible range of atmospheric composittions and globally averaged spectra for extrasolar planets and for early Earth, and will improve our understanding of the effect of life on a planet's atmospheric spectrum and composition. The models will also be used to create a comprehensive spectral catalog to provide recommendations on the optimum wavelength range, spectral resolution, and instrument sensitivity required to characterize extrasolar terrestrial planets. Although developed by our team, the VPL is envisioned to be a comprehensive and flexible tool, which can be collaboratively used by the broader planetary science and astrobiology communities. This presentation will describe the project concept, the tasks involved, and will outline current progress to date. This work is funded by the NASA Astrobiology Institute.

astrobiology↗
NASA NTRS2021

Intercomparison of Middle Atmospheric Meteorological Analyses for the Northern Hemisphere Winter 2009-2010

Detailed meteorological analyses based on observations extending through the middle atmosphere (~15 to 100 km altitude) can provide key information to whole atmosphere modelling systems regarding the physical mechanisms linking day-to-day changes in ionospheric electron density to meteorological variability near the Earth’s surface. However, the extent to which independent middle atmosphere analyses differ in their representation of wave-induced coupling to the ionosphere is unclear. To begin to address this issue, we present the first intercomparison among four such analyses, JAGUAR-DAS, MERRA-2, NAVGEM-HA, and WACCMX+DART, focusing on the Northern Hemisphere (NH) 2009-2010 winter, which includes a major sudden stratospheric warming (SSW). This intercomparison examines the altitude, latitude, and time dependences of zonal mean zonal winds and temperatures among these four analyses over the 1 December 2009 – 31 March 2010 period, as well as latitude and altitude dependences of monthly mean amplitudes of the diurnal and semidiurnal migrating solar tides, the eastward propagating diurnal zonal wave number 3 nonmigrating tide, and traveling planetary waves associated with the quasi-5-day and quasi-2-day Rossby modes. Our results show generally good agreement among the four analyses up to the stratopause (~50 km altitude). Large discrepancies begin to emerge in the mesosphere and lower thermosphere owing to (1) differences in the types of satellite data assimilated by each system and (2) differences in the details of the global atmospheric models used by each analysis system. The results of this intercomparison provide initial estimates of uncertainty in analyses commonly used to constrain middle atmospheric meteorological variability in whole atmosphere model simulations.

John P Mccormack↗
NASA NTRS1974

Atlas of radiation budget measurements from satellites, 1962 - 1970

Radiation budget data measured at the top of the atmosphere by earth-orbiting satellites for various time periods in the years 1962 through 1970 are presented in the form of contoured maps which were derived from 5 seasonal and 25 monthly and semimonthly data sets. The data are values of planetary albedo, infrared radiant exitance, absorbed solar energy, net radiation balance. Documentation and contoured maps are included together with data tabulation at each 10 deg latitude and longitude. Additional data published by other authors are referenced.

Vonderhaar, T. H.↗
NASA NTRS2009

The Magnetic Field of Mars and its Interaction with the Solar Wind

The outermost layers of the Martian atmosphere are thought to be scientifically unique due to the large influences exerted by the highly dynamic lower atmosphere and the direct input of the solar wind from above. The nature of the solar wind interaction with the upper atmosphere is of particular interest because Mars lacks a global magnetic field, but is well shielded over some regions by strong crustal magnetic fields. Under such circumstances, the direct impact of solar wind plasma may have resulted in enhanced loss of volatiles over the ages including the components of water. The history of upper atmosphere and solar wind interaction measurements at Mars will be reviewed, recent results from the Mars Global Surveyor and Mars Express summarized, and prospects for new scientific advances enabled by the measurements that will be made by planned orbiter and penetrator missions. Special attention will be given to planetary magnetic field measurements, the measurement of ionospheric currents driven by the solar wind, and the role of space weather modeling and forecasting in the future of Mars exploration.

Le, Guan↗
NASA NTRS2016

Advanced Technologies to Improve Closure of Life Support Systems

As NASA looks beyond the International Space Station toward long-duration, deep space missions away from Earth, the current practice of supplying consumables and spares will not be practical nor affordable. New approaches are sought for life support and habitation systems that will reduce dependency on Earth and increase mission sustainability. To reduce launch mass, further closure of Environmental Control and Life Support Systems (ECLSS) beyond the current capability of the ISS will be required. Areas of particular interest include achieving higher degrees of recycling within Atmosphere Revitalization, Water Recovery and Waste Management Systems. NASA is currently investigating advanced carbon dioxide reduction processes that surpass the level of oxygen recovery available from the Sabatier Carbon Dioxide Reduction Assembly (CRA) on the ISS. Candidate technologies will potentially improve the recovery of oxygen from about 50% (for the CRA) to as much as 100% for technologies who's end product is solid carbon. Improving the efficiency of water recycling and recovery can be achieved by the addition of advanced technologies to recover water from brines and solid wastes. Bioregenerative technologies may be utilized for water reclaimation and also for the production of food. Use of higher plants will simultaneously benefit atmosphere revitalization and water recovery through photosynthesis and transpiration. The level at which bioregenerative technologies are utilized will depend on their comparative requirements for spacecraft resources including mass, power, volume, heat rejection, crew time and reliability. Planetary protection requirements will need to be considered for missions to other solar system bodies.

Barta, Daniel J.↗
NASA NTRS2017

Multi-Wavelength Laser Transmitter for the Two-Step Laser Time-of-Flight Mass Spectrometer

Missions to diverse Outer Solar System bodies will require investigations that can detect a wide range of organics in complex mixtures, determine the structure of selected molecules, and provide powerful insights into their origin and evolution. Previous studies from remote spectroscopy of the Outer Solar System showed a diverse population of macromolecular species that are likely to include aromatic and conjugated hydrocarbons with varying degrees of methylation and nitrile incorporation. In situ exploration of Titan's upper atmosphere via mass and plasma spectrometry has revealed a complex mixture of organics. Similar material is expected on the Ice Giants, their moons, and other Outer Solar System bodies, where it may subsequently be deposited onto surface ices. It is evident that the detection of organics on other planetary surfaces provides insight into the chemical and geological evolution of a Solar System body of interest and can inform our understanding of its potential habitability. We have developed a prototype two-step laser desorption/ionization time-of-flight mass spectrometer (L2MS) instrument by exploiting the resonance-enhanced desorption of analyte. We have successfully demonstrated the ability of the L2MS to detect hydrocarbons in organically-doped analog minerals, including cryogenic Ocean World-relevant ices and mixtures. The L2MS instrument operates by generating a neutral plume of desorbed analyte with an IR desorption laser pulse, followed at a delay by a ultraviolet (UV) laser pulse, ionizing the plume. Desorption of the analyte, including trace organic species, may be enhanced by selecting the wavelength of the IR desorption laser to coincide with IR absorption features associated with vibration transitions of minerals or organic functional groups. In this effort, a preliminary laser developed for the instrument uses a breadboard mid-infrared (MIR) desorption laser operating at a discrete 3.475 μm wavelength, and a breadboard UV ionization laser operating at a wavelength of 266 nm. The MIR wavelength was selected to overlap the C-H stretch vibrational transition of certain aromatic hydrocarbons, and the UV wavelength provides additional selectivity to aromatic species via UV resonance-enhanced multiphoton ionization effects. The use of distinct laser wavelengths allows efficient coupling to the vibrational and electronic spectra of the analyte in independent desorption and ionization steps, mitigating excess energy that can lead to fragmentation during the ionization process and leading to selectivity that can aid in data interpretation.

Yu, Anthony W.↗
NASA NTRS1992

Results of a zonally truncated three-dimensional model of the Venus middle atmosphere

Although the equatorial rotational speed of the solid surface of Venus is only 4 m s(exp-1), the atmospheric rotational speed reaches a maximum of approximately 100 m s(exp-1) near the equatorial cloud top level (65 to 70 km). This phenomenon, known as superrotation, is the central dynamical problem of the Venus atmosphere. We report here the results of numerical simulations aimed at clarifying the mechanism for maintaining the equatorial cloud top rotation. Maintenance of an equatorial rotational speed maximum above the surface requires waves or eddies that systematically transport angular momentum against its zonal mean gradient. The zonally symmetric Hadley circulation is driven thermally and acts to reduce the rotational speed at the equatorial cloud top level; thus wave or eddy transport must counter this tendency as well as friction. Planetary waves arising from horizontal shear instability of the zonal flow (barotropic instability) could maintain the equatorial rotation by transporting angular momentum horizontally from midlatitudes toward the equator. Alternatively, vertically propagating waves could provide the required momentum source. The relative motion between the rotating atmosphere and the pattern of solar heating, which as a maximum where solar radiation is absorbed near the cloud tops, drives diurnal and semidiurnal thermal tides that propagate vertically away from the cloud top level. The effect of this wave propagation is to transport momentum toward the cloud top level at low latitudes and accelerate the mean zonal flow there. We employ a semispectral primitive equation model with a zonal mean flow and zonal wavenumbers 1 and 2. These waves correspond to the diurnal and semidiurnal tides, but they can also be excited by barotropic or baroclinic instability. Waves of higher wavenumbers and interactions between the waves are neglected. Symmetry about the equator is assumed, so the model applies to one hemisphere and covers the altitude range 30 to 110 km. Horizontal resolution is 1.5 deg latitude, and vertical resolution is 1.5 km. Solar and thermal infrared heating, based on Venus observations and calculations drive the model flow. Dissipation is accomplished mainly by Rayleigh friction, chosen to produce strong dissipation above 85 km in order to absorb upward propagating waves and limit extreme flow velocities there, yet to give very weak Rayleigh friction below 70 km; results in the cloud layer do not appear to be sensitive to the Rayleigh friction. The model also has weak vertical diffusion, and very weak horizontal diffusion, which has a smoothing effect on the flow only at the two grid points nearest the pole.

Newman, M.↗