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NASA NTRS1991

Planetary astronomy

The authors profile the field of astronomy, identify some of the key scientific questions that can be addressed during the decade of the 1990's, and recommend several facilities that are critically important for answering these questions. Scientific opportunities for the 1990' are discussed. Areas discussed include protoplanetary disks, an inventory of the solar system, primitive material in the solar system, the dynamics of planetary atmospheres, planetary rings and ring dynamics, the composition and structure of the atmospheres of giant planets, the volcanoes of IO, and the mineralogy of the Martian surface. Critical technology developments, proposed projects and facilities, and recommendations for research and facilities are discussed.

Morrison, David↗

NASA NTRS1990

Ion composition measurement techniques for space plasmas

Plasmas found in space range from the solar wind with a typical temperature of 100,000-1,000,000 K, about 400 km/s bulk flow speed, and high ionization (charge states) of ions, to the hot, slowly moving plasmas in the outer magnetospheres of the giant planets, to the cold, corotating plasmas in inner magnetospheres. Space plasma instruments and techniques are reviewed, with an emphasis on hot plasma composition measurements. Starting with Faraday Cup detectors some 30 years ago, plasma instruments have evolved to the present time-of-flight systems with excellent mass resolution and three-dimensional viewing capabilities.

Gloeckler, George↗

NASA NTRS1991

Organic matter in meteorites and comets - Possible origins

At least six extraterrestrial environments may have contributed organic compounds to meteorites and comets: solar nebula, giant-planet subnebulae, asteroid interiors containing liquid water, carbon star atmospheres, and diffuse or dark interstellar clouds. The record in meteorites is partly obscured by pervasive reheating that transformed much of the organic matter to kerogen; nonetheless, it seems that all six formation sites contributed. For comets, the large abundance of HCHO, HCN, and unsaturated hydrocarbons suggests an interstellar component of 50 percent or more, but the contributions of various interstellar processes, and of a solar-nebula component, are hard to quantify. A research program is outlined that may help reduce these uncertainties.

Anders, Edward↗

NASA NTRS1991

Planetary rings

This review discusses each planetary ring system and describes recent theoretical developments and models of ring evolution. The distribution of the inner rings and satellites of all four giant planets reduced to a uniform scale of 'planetary units', normalized by the equatorial radius of each planet, is summarized. Only major ring subunits are indicated, with the density of cross-hatching representing in a crude way the relative optical depths of the different components of each system. One result of the spacecraft observations has been the discovery at each planet of several small (10-100 km in radius) satellites that appear to be intimately related to the rings, dynamically and possibly genetically as well.

Nicholson, Philip D.↗

NASA NTRS1990

Pioneers 10 and 11 deep space missions

Pioneers 10 and 11 were launched from earth, 2 March 1972, and 5 April 1973, respectively. The Pioneers were the first spacecraft to explore the asteroid belt and the first to encounter the giant planets, Jupiter and Saturn. The Pioneer 10 spacecraft is now the most distant man-made object in our solar system and is farther from the sun than all nine planets. It is 47 AU from the sun and is moving in a direction opposite to that of the sun's motion through the galaxy. Pioneer 11 is 28 AU from the sun and is traveling in the direction opposite of Pioneer 10, in the same direction as the sun moves in the galaxy. These two Pioneer spacecraft provided the first large-scale, in-situ measurements of the gas and dust surrounding a star, the sun. Since launch, the Pioneers have measured large-scale properties of the heliosphere during more than one complete 11-year solar sunspot cycle, and have measured the properties of the expanding solar atmosphere, the transport of cosmic rays into the heliosphere, and the high-energy trapped radiation belts and magnetic fields associated with the planets Jupiter and Saturn. Accurate Doppler tracking of these spin-stabilized spacecraft was used to search for differential gravitational forces from a possible trans-Neptunian planet and to search for gravitational radiation. Future objectives of the Pioneer 10 and 11 missions are to continue measuring the large-scale properties of the heliosphere and to search for its boundary with interstellar space.

Dyal, P.↗

NASA NTRS1992

The ultraviolet albedo of Mars

Ultraviolet observations have proven successful at studying the composition and structure of the upper atmospheres of the giant planets and Venus. In the case of Mars, the analysis of UV data has concentrated on the spatial distribution and temporal behavior of ozone. The potential of far ultraviolet (FUV) data (shortward of 200 nm) for measuring the atmospheric structure, in particular the vertical distribution of aerosols, is demonstrated. For illustrative purposes, a simple two layer model (atmosphere and surface) is computed using the atmospheric composition measured at the Viking landing sites. This model is then compared to the albedo observed with the International Ultraviolet Explorer (IUE). This comparison clearly shows that a clear atmosphere with the nominal Viking mixing ratios of CO2, O2, and H2O is incompatible with the IUE data shortward of 200 nm. To obtain a better match between data and model, an aerosol has to be introduced that effectively scatters at these wavelengths. The required reduction in effective CO2 pathlength is a factor of 5. The comparison also points out the inadequacy of the IUE to provide data in the critical wavelength region between 195 nm and 210 nm. Such data will be forthcoming from the Hubble Space Telescope and will provide the opportunity to infer the vertical distribution of aerosols on Mars on a global scale.

Wagener, R.↗

NASA NTRS1992

The role of the molecular-metallic transition of hydrogen in the evolution of Jupiter, Saturn, and brown dwarfs

An equation of state for hydrogen which predicts a molecular-metallic phase transition at finite temperatures has become available recently. The effect of this phase transition on the cooling histories of these two giant planets and of substellar brown dwarfs is studied. The phase transition alters the present age of Jupiter and of Saturn by a few percent. Interestingly, the cooling of brown dwarfs is most strongly affected at the time when the interior adiabat crosses the critical point of the phase transition.

Saumon, Didier↗

NASA NTRS1991

The Next Generation Space Telescope - A large UV-IR successor to HST

Attention is given to the science goals of the Next Generation Space Telescope (NGST), namely, detection and spectroscopy of gas giant planets to earthlike planets around nearby stars, study of nearby star-forming complexes with resolutions of 5-50 AU in the visible and IR, measurement of stellar populations in a variety of environments, spatial resolution and mapping of complex structures in the inner narrow line regions for AGN and QSOs, and investigation of the structure and evolution of galaxies at redshifts greater than 1. The NGST characteristics, milestones, long-term strategy, and launch capability are also discussed.

Illingworth, Garth↗

NASA NTRS1992

Cassini: Mission to Saturn and Titan

The Cassini Mission to Saturn and Titan represents an important step into the exploration of the outerplanets. It will expand on the flyby encounters of Pioneer and Voyager and parallel the detailed exploration of the Jupiter system to be accomplished by the Galileo Mission. By continuing the study of the two giant planets and enabling detailed comparisons of their structure and behavior, Cassini will provide a tremendous insight into the formation and evolution of the solar system. In addition, by virtue of its focus on the Saturnian satellite Titan, Cassini will return detailed data on an environment whose atmospheric chemistry may resemble that of the primitive Earth. The scientific objectives can be divided into five categories: Titan, Saturn, rings, icy satellites, and magnetospheres. The key area of interest to exobiologists is Titan; the other four scientific categories will be discussed briefly to provide a comprehensive overview of the Cassini Mission.

Kerridge, Stuart J.↗

NASA NTRS1993

Plasma, magnetic, and electromagnetic measurements at nonmagnetic bodies

The need to explore the magnetospheres of the Earth and the giant planets is widely recognized and is an integral part of our planetary exploration program. The equal need to explore the plasma, magnetic, and electromagnetic environments of the nonmagnetic bodies is not so widely appreciated. The previous, albeit incomplete, magnetic and electric field measurements at Venus, Mars, and comets have proven critical to our understanding of their atmospheres and ionospheres in areas ranging from planetary lightning to solar wind scavenging and accretion. In the cases of Venus and Mars, the ionospheres can provide communication paths over the horizon for low-altitude probes and landers, but we know little about their lower boundaries. The expected varying magnetic fields below these planetary ionospheres penetrates the planetary crusts and can be used to sound the electrical conductivity and the thermal profiles of the interiors. However, we have no knowledge of the levels of such fields, let alone their morphology. Finally, we note that the absence of an atmosphere and an ionosphere does not make an object any less interesting for the purposes of electromagnetic exploration. Even weak remanent magnetism such as that found on the Moon during the Apollo program provides insight into the present and past states of planetary interiors. We have very intriguing data from our space probes during times of both close and distant passages of asteroids that suggest they may have coherent magnetization. If true, this observation will put important constraints on how the asteroids formed and have evolved. Our planetary exploration program must exploit its full range of exploration tools if it is to characterize the bodies of the solar system thoroughly. We should especially take advantage of those techniques that are proven and require low mass, low power, and low telemetry rates to undertake.

Russell, C. T.↗

NASA NTRS1993

Acoustic oscillations of Jupiter

We calculate low-degree l acoustic (p-mode) oscillations of the giant planet Jupiter. We use two Jupiter models, both of which contain a rocky core, an ice mantle, and a hydrogen-rich envelope. One of the models incorporates the 'plasma phase transition' of hydrogen, a first-order phase transition that yields a density discontinuity in the hydrogen-rich envelope. Our calculations of the acoustic oscillation spectrum of Jupiter have taken account of the effects of the rapid rotation of the planet: the Coriolis force, centrifugal force, and deformation of the equilibrium state. The p-mode frequency spectra of the Jupiter models are summarized in several echelle diagrams. We show that the frequency spectra of the low-degree p-modes are strongly affected by the existence of the rocky core and by avoided crossings among high radial-order p-modes with different l's.

Lee, Umin↗

NASA NTRS1993

New laboratory measurements on ammonia's inversion spectrum, with implications for planetary atmospheres

Microwave spectral measurements have been performed on pure room-temperature gaseous ammonia at frequencies from 1.75 to 18 GHz (1.7-17 cm), at 50-, 100-, and 300-torr pressures. These measurements are part of a laboratory program to measure the microwave absorption spectrum of ammonia, under conditions applicable to giant planet atmospheres, now in progress at the Jet Propulsion Laboratory. The pure ammonia data reported here agree well with previous data by Bleaney and Loubser (1950) at 100 and 300 torrs, and with predictions of the absorptivity formalism published by Berge and Gulkis. Success with pure ammonia but failure with mixtures of ammonia in hydrogen and helium (Spilker, 1990) indicates that the Berge and Gulkis formalism does not correctly handle foreign-gas effects on ammonia inversion lines. This may require modifying conclusions of radio astronomical and radio occultation studies that used this formalism. Notably, a suggested depletion of ammonia and superabundance of hydrogen sulfide may have been exaggerated as a result of inaccuracies in the Berge and Gulkis formalism.

Spilker, Thomas R.↗

NASA NTRS1992

Robotic vehicles for planetary exploration

A program to develop planetary rover technology is underway at the Jet Propulsion Laboratory (JPL) under sponsorship of the National Aeronautics and Space Administration. Developmental systems with the necessary sensing, computing, power, and mobility resources to demonstrate realistic forms of control for various missions have been developed, and initial testing has been completed. These testbed systems and the associated navigation techniques used are described. Particular emphasis is placed on three technologies: Computer-Aided Remote Driving (CARD), Semiautonomous Navigation (SAN), and behavior control. It is concluded that, through the development and evaluation of such technologies, research at JPL has expanded the set of viable planetary rover mission possibilities beyond the limits of remotely teleoperated systems such as Lunakhod. These are potentially applicable to exploration of all the solid planetary surfaces in the solar system, including Mars, Venus, and the moons of the gas giant planets.

Wilcox, Brian↗

NASA NTRS1994

McIDAS-eXplorer: A version of McIDAS for planetary applications

McIDAS-eXplorer is a set of software tools developed for analysis of planetary data published by the Planetary Data System on CD-ROM's. It is built upon McIDAS-X, an environment which has been in use nearly two decades now for earth weather satellite data applications in research and routine operations. The environment allows convenient access, navigation, analysis, display, and animation of planetary data by utilizing the full calibration data accompanying the planetary data. Support currently exists for Voyager images of the giant planets and their satellites; Magellan radar images (F-MIDR and C-MIDR's, global map products (GxDR's), and altimetry data (ARCDR's)); Galileo SSI images of the earth, moon, and Venus; Viking Mars images and MDIM's as well as most earth based telescopic images of solar system objects (FITS). The NAIF/JPL SPICE kernels are used for image navigation when available. For data without the SPICE kernels (such as the bulk of the Voyager Jupiter and Saturn imagery and Pioneer Orbiter images of Venus), tools based on NAIF toolkit allow the user to navigate the images interactively. Multiple navigation types can be attached to a given image (e.g., for ring navigation and planet navigation in the same image). Tools are available to perform common image processing tasks such as digital filtering, cartographic mapping, map overlays, and data extraction. It is also possible to have different planetary radii for an object such as Venus which requires a different radius for the surface and for the cloud level. A graphical user interface based on Tel-Tk scripting language is provided (UNIX only at present) for using the environment and also to provide on-line help. It is possible for end users to add applications of their own to the environment at any time.

Limaye, Sanjay S.↗

NASA NTRS1994

Comets, impacts, and atmospheres

Studies of element abundances and values of D/H in the atmospheres of the giant planets and Titan have emphasized the important role of icy planetesimals in the formation of these bodies. In these atmospheres, C/H and D/H increase as the relative masses of the 'cores' of the planets increase. N/H appears to deviate from this trend in an interesting way. In the inner solar system, the traditional approach of using carbonaceous chondrites as the source of planetary volatiles is in serious trouble because of the depletion of xenon and the unusual pattern of xenon isotopes found in the atmospheres of Earth and Mars, and because of the solar-type abundance ratios of argon, krypton and xenon and the large amounts of neon and argon on Venus. Recent studies of elemental abundances in comets, especially P/Halley, coupled with laboratory studies of the trapping of gas in ice formed at low temperatures by A. Bar-Nun et al. provide a consistent interpretation of all of these results. This interpretation emphasizes the fundamental importance of icy planetesimals (comets) and the randomness of early impacts in the formation of planetary systems. Cometary delivery by itself will not explain the noble gas abundances on the inner planets. There is good evidence for at least one additional source, which presumably consists of the rocky material making up the bulk of the planets. The existence of this rocky reservoir is manifested in the nucleogenic isotopes and in the neon which is found in all these atmospheres and is also present in the Earth's mantle. This neon may well be a relic of the planets' earliest, accretional atmospheres.

Owen, Tobias↗

NASA NTRS1994

Prebiotic chemistry in space; Proceedings of Symposia B1.4 and F3.3 of the COSPAR Scientific Assembly, 30th, Hamburg, Germany, July 11-21, 1994

A two-part symposium was held concerning topics in Solar System chemistry. The first part covered the organic chemistry ofsmall bodies of the interplanetray medium. It produced papers on the evolution, spectral properties and composition of organic matter in comets, interplanetary dust and asteroids. The second part covered cryochemistry and exobiology in planetary atmospheres (gas giant planets and their satellites) and in various astronomical ices.

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NASA NTRS1995

What's new, Voyager: The discoveries continue

The twin Voyager spacecraft, launched nearly two decades ago, continue to operate and are now searching for the edge of our solar system, the heliopause. Voyager's giant-planet flybys of Jupiter, Saturn, Uranus, and Neptune have provided data that are likely to remain the definitive data set for the foreseeable future and have led to many ongoing discoveries. As the spacecraft move toward the heliopause, they are also providing data on the structure of the heliosphere. This article discusses the discoveries resulting from the flyby and heliosphere data that have been made within the past five years.

Miner, Ellis D.↗

NASA NTRS1996

Experiments on Thermal Convection in Rotating Spherical Shells With Radial Gravity: The Geophysical Fluid Flow Cell

Experiments designed to study the fluid dynamics of buoyancy driven circulations in rotating spherical shells were conducted on the United States Microgravity Laboratory 2 spacelab mission. These experiments address several aspects of prototypical global convection relevant to large scale motions on the Sun, Earth, and on the giant planets. The key feature is the consistent modeling of radially directed gravity in spherical geometry by using dielectric polarization forces. Imagery of the planforms of thermally driven flows for rapidly-rotating regimes shows an initial separation and eventual merger of equatorial and polar convection as the heating (i.e. the Rayleigh number) is increased. At low rotation rates, multiple-states of motion for the same external parameters were observed.

Hart, John E.↗