Showing papers in "Space Science Reviews in 1973"

Abundances of the elements in the solar system

Abstract: The present status of abundance information for elements in meteorites and in the Sun is reviewed, and a new table of abundances of the elements, which should be characteristic of the primitive solar nebula, is compiled and presented. Special attention is called to the elemental abundances in the silicon-to-calcium region, where many of the abundances are rather poorly determined, and where these abundances have an impact on theories of nucleosynthesis of the elements. To each elemental isotope is assigned a mechanism of nucleosynthesis which may have been responsible for production of most of that isotope, and brief comments are made concerning the present status of understanding of the different mechanisms of nucleosynthesis.

The magnetotail and substorms

Abstract: The tail plays a very active and important role in substorms. Magmetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary flares more, the field strength in the tail increases, and the currents strengthen and move closer to the earth. Further, the plasma sheet thins and the magnetic flux crossing the neutral sheet lessens. The experimental evidence for these processes is discussed and a phenomenological or qualitative model of the substorm sequence is presented. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet.

A survey of the outer planets Jupiter, Saturn, Uranus, Neptune, Pluto, and their satellites

Abstract: A survey of current knowledge about Jupiter, Saturn, Uranus, Neptune, Pluto, and their satellites is presented. The best available numerical values are given for physical parameters, including orbital and body properties, atmospheric composition and structure, and photometric parameters. The more acceptable current theories of these bodies are outlined with thorough referencing offering access to the details. The survey attempts to cover the literature through May 1, 1972.

The ionospheres of the major planets

Abstract: Physical and chemical processes which affect the equilibrium distribution of ionization in the atmospheres of Jupiter, Saturn, Uranus and Neptune are reviewed. Current models imply readily detectable ionospheres for all four planets and suggest that protons should represent the dominant positive ion. Attention is directed to the probable importance of dissociative ionization of H2 as a source of H+. A number of potentially important loss mechanisms for H+ are discussed including a possible reaction of H+ with vibrationally excited H2. Protons may be removed efficiently at lower altitudes by reaction with CH4 and this process may offer a simple remote means for location of the turbopause.

Analyses of techniques for measuring DC and AC electric fields in the magnetosphere

Abstract: Methods for measuring the amplitudes and directions of DC electric fields and the directions, power spectra, and dispersion relations of AC electric fields in the magnetosphere are discussed with emphasis on their applicability in various regimes of the magnetospheric plasma. The two classes of techniques that are discussed are measurement of the bulk flow of the plasma and the potential difference between pairs of separated conductors. The plasma bulk flow discussion includes measurements by ionospheric radar backscatter, whistler mode wave propagation, energetic or thermal particle trajectories, artificial ion cloud motion, probe measurements of bulk flow, vehicle wake analyses, effects of bulk flow on the coupling of antennas, and the bulk flow of an artificial electron beam.

Study of the magnetosphere using energetic solar particles

Abstract: A review is presented of the study of the magnetosphere using energetic solar particles. After a historical introduction, present knowledge about the geomagnetic field topology is shortly described in order to assess the contribution, which energetic particle measurements can make towards the study of the magnetosphere. The following historical review of satellite observations is discussed in terms of their contributions to a phenomenological classification of polar cap structures. Subsequent to about 1969 detailed trajectory calculations of particles in model magnetospheric fields were available which could be used in the interpretation of the measurements so that a strict historical treatment is not tractable. After a short description of the trajectory calculation method in various model magnetospheric fields the role of different interplanetary conditions in determining intensity structures in the geomagnetic tail and over the polar caps is discussed. Firstly, polar cap observations during different interplanetary conditions are described in terms of their contribution to the understanding of the magnetospheric topology, and secondly, observations in the distant magnetosphere and magnetotail are reported and compared with relevant theory.

Structure of Jupiter and Saturn

Abstract: Understanding of the planetary interiors depends upon our knowledge of the equations of state and of the transport properties of matter at high pressures and temperatures. The present status of this knowledge in relation to hydrogen and helium is discussed in detail including electrical and thermal conductivity, viscosity, diffusivity, etc. On this basis the various possible models of the internal structure of Jupiter and of Saturn are presented and their agreement with observational constraints such as the multipole gravitational coefficients analyzed. Relevance of planetary magnetic fields, basic atmospheric information and the Great Red Spot of Jupiter to the models of the interiors are discussed.

Helium in the terrestrial atmosphere

Abstract: After a brief historical review of the discovery of helium in the terrestrial atmosphere, the production mechanisms of the isotopes He4 and He3 are discussed. Although the radioactive production of He4 in the Earth is well understood, some uncertainty still exists for the degassing process leading to an atmospheric influx of (2.5 ±1.5) × 106 atoms cm−2 s−1. Different production mechanisms are possible for He3 leading to an influx of (7.5±2.5) atoms cm−2 s−1. Observations of helium in the thermosphere show a great variability of this constituent. The different mechanisms proposed to explain the presence of the winter helium bulge are discussed. Since helium ions are present in the topside ionosphere and in the magnetosphere, ionization mechanisms are analyzed. Owing to possible variations and uncertainties in the solar UV flux, the photoionization coefficient is (8±4) × 10−8 s−1. Finally, the helium balance between production in the earth and loss into the interplanetary space is discussed with respect to the different processes which can play an effective role.

The infrared spectrum of the airglow

Abstract: The techniques of studying the IR airglow are briefly discussed followed by a review of recent observations. The main emphasis is on the molecular emissions of OH and O2 from the mesosphere and lower thermosphere with a shorter discussion of the F-region atomic emissions. The theoretical interpretations of these emissions are reviewed. It is concluded that dynamic effects must play a dominant and as yet incompletely understood part in the photochemistry of the molecular emission region. An atlas of recently available airglow spectra is included.

The dynamics of the atmospheres of the major planets

Abstract: The literature on the dynamics of Jupiter's atmosphere is reviewed and used as a basis for suggesting what observations would yield useful information about Jovian Dynamics. The atmospheres of Saturn, Uranus and Neptune are discussed from the same point of view.

Magnetospheres of the planets

Abstract: Scaling laws for possible outer planet magnetospheres are derived. These suggest that convection and its associated auroral effects will play a relatively smaller role than at Earth, and that there is a possibility that the outer planets could have significant radiation belts of energetic trapped particles.

Chemistry of the outer solar system

Abstract: Data on the composition of the satellites of the outer planets and the composition and structure of planetary atmospheres are briefly reviewed in light of simple models for the origin of the solar system and the planets. Some crucial tests of present theories are suggested.

Neutron measurements in space

Abstract: The experimental measurements of the neutron flux and energy spectrum in space since 1964 are reviewed and related to the theoretical predictions. A discussion of the neutron sources is presented. The difficulties associated with neutron measurements of both the atmospheric neutron leakage flux and solar neutrons are included. Particular emphasis is placed upon the neutron leakage flux and energy measurements at energies greater than about 1 MeV. The possibilities of CRAND as a source for the energetic trapped protons are discussed in light of recent measurements of the 10–100 MeV neutron flux. The current status of the solar neutron flux observations is also presented.

Interaction of the interstellar medium with the solar wind

Abstract: The possibilities for making observations of the region of interaction between the solar wind and the interstellar medium, and of the interstellar medium itself, are discussed. It is emphasized that missions to the outer planets undertaken in 1977–1979 are ideally suited for making such observations.

Gas and Dust in Comets

Abstract: A comet becomes a spectacular phenomenon for a transient time only It is a starlike object at large distances from the Sun The cometary head typically appears at distances less than 3 astronomical units (AU), and the tail at less than 15 AU (although exceptions are known) This suggests that the transient phenomena appear only while a strong interaction prevails between the solar light or the solar wind, and something rather elusive that we call the cometary nucleus The nucleus, even if it is too tiny to be easily seen, must be the permanent feature that is the source and the origin of these transient phenomena

The Canyon Diablo meteorite

Abstract: The giant diamond-containing Canyon Diablo meteorite is in composition a typical representative of the widespread group of iron meteorites — the coarse octahedrites. But it is unique in a number of scientifically important aspects. When it fell, it formed the Arizona (Barringer meteorite) crater (1220 m in diam), which is of the explosive type. The investigations of the shock recrystallization of the crater rocks and the meteoritic material are of importance for planetology, and in particular for the eludication of matter recrystallization conditions during the collisions of large cosmic bodies. The study of the diamonds contained in the Canyon Diablo meteorite is of importance to various branches of carbon cosmochemistry. The Canyon Diablo meteorite fell in Arizona, U.S.A., some tens of thousands years ago. However, the Arizona meteorite crater is well preserved owing to the semi-arid climatic conditions. Signs of shock recrystallization of the rocks — shatter cones, impactites, dense and super-dense silica modifications were revealed in the Arizona meteorite crater. Around the crater many samples of the Canyon Diablo iron meteorite have been found (at distances of up to 9 km from the crater), together with a great amount of finely dispersed meteorite matter. The total weight of the material attributed to the meteorite is put at 30 tons. A number of meteorite fragments have been oxidized to different degrees during terrestrial weathering. Typical samples of the Canyon Diablo meteorite are represented by coarse octahedrite matter with kamacite band widths of 1.8–2.2 mm. In many meteorite fragments, especially the fragments found on the crater rim, the Widmanstatten pattern has been destroyed as a result of the explosion which occurred when the meteorite hit the Earth. The meteorite is rich in accessory minerals (cohenite, schreibersite, troilite.). The Ni content is, in typical samples of the meteorite, about 7.16%, in rare, atypical, medium octahedrite samples the Ni content reached 8.2 %. In the content of trace elements the meteorite may be classified with the I Ga-Ge group. In the content of stable isotopes of elements there is no substantial difference between the Canyon Diablo meteorite and other octahedrites. Radioactive cosmogenic isotopes are represented by isotopes with a large half-life. The diamonds in the Canyon Diablo meteorite are unevenly distributed and are found inside the highly recrystallized meteorite fragments at the rim of the crater. Diamonds are present in the form of intergrowths of microcrystals, crystallite sizes are < 1μ, the sizes of the intergrowths reach 2–5 mm. Admixtures of graphite and the hexagonal diamond lonsdaleite are present in the intergrowths. From the evidence of shock recrystallization of the meteorite matter it would seem that the diamond containing fragments of the Canyon Diablo meteorite have undergone shock pressures of from 400 kbar to 1 mgbar at impact, at these pressures the diamonds would crystallise. The diamond-containing sample of the Canyon Diablo meteorite investigated by the author has experienced a shock pressure of up to 1 mgbar during the explosion.

Elemental and isotopic abundances of the volatile elements in the outer planets

Abstract: Certain fundamental scientific problems of a cosmological as well as cosmogonic character, may be solved by the insertion of entry probes into the atmospheres of the outer planets. It is recommended that attempts be made to determine the elemental and isotopic abundances of H, D, He3, He4, C, N, O, S, and the rare gas elements. These determinations should cast much light on the processes which participated in the assembly of the giant planets. This would give powerful boundary conditions on theories of the origin of the solar system, and would also give additional experimental information bearing on cosmology.

Stellar and solar abundances

Abstract: Recent spectroscopic results on stellar and solar abundances are reviewed with special reference to (a) Standard abundance distribution (Sun, hot stars, diffuse nebulae); (b) Abundance peculiarities related to stellar evolution (red giants showing results of H-burning and s-process, peculiar and metallic-lined stars); and (c) Population effects that may be related to the evolution of the Galaxy (correlation between stellar age and metal abundance, differences in details of heavy-element mixture in atmospheric composition of normal stars that have not reached an advanced evolutionary stage).

Radio physics of the outer solar system

Abstract: The remote sensing of low frequency nonthermal radio emission is the astronomy of field and particle phenomena. Observations conducted from space lead to information about the composition and dynamic processes occurring in planetary magnetospheres as well as within the interplanetary and interstellar medium. The potential of this technique is demonstrated by considering observations obtained from earth orbit missions.

Formation of the outer planets

Abstract: A discussion is given of a number of physical processes which were probably important during the formation of the outer planets if these formed from a gaseous solar nebula in which magnetic effects were not important. Arguments are given that large-scale gravitational instabilities in the solar nebula did not occur. Qualitative consideration is given to the conditions in which dynamical capture of gas onto a planetary core may take place; this may have played a major role in the formation of Jupiter and Saturn. Because of the great difficulty of fractionating hydrogen from helium in the assembly of the outer planets, it is argued that a new approach should be made to the construction of planetary models. Conditions which may lead to the formation of the regular satellite systems are discussed, and the associated problem of removal of primordial angular momentum from Jupiter, Saturn, and Uranus.

Plasma theory of radio echoes from the Sun and its implications for the problem of the solar wind

Abstract: It is demonstrated that the solar radio echo data cannot be understood in the framework of conventional theories of reflections from those coronal layers, where the refraction index n = 0, from moving inhomogeneities or shock fronts. It was found that all peculiar and unexpected features of radio echo spectra can be explained by the theory of its formation and amplification by induced processes of scattering from turbulent pulsations of the coronal plasma. So, the structure of the radio echo spectra brings information, not only about the systematic large-scale motions of coronal plasma, but also about the plasma turbulent pulsation spectrum, its temperature and density as well. It was found also that the reflected signal originates from the disturbed coronal zones situated above active regions while reflections from the quiet corona are weak or completely absent. The analysis of the radio echo spectra shows that in comparatively thin layers of the corona situated above the active regions, rapid acceleration and heating of the solar wind sometimes take place. The velocity gradient is found to be changing in fairly wide limits. Solar wind velocities as high as 120 km/s were sometimes observed at a distance of only ≈ 1.5 R ⊙ from the Sun's centre. The source of energy for the acceleration and heating of solar wind is the dissipation of turbulent plasma pulsations. These regions where the high temperature gradient precludes the possibility of a continuous flow of coronal plasma, possess the character of weak detonation waves. It can be anticipated that the non-thermal heating of the coronal plasma which was postulated to remove discrepancies between the existing models and the observations of the solar wind, is localized mainly in these regions, thus playing an important role in the formation of fundamental properties of the interplanetary medium. So, for the further development of the theory of the solar wind, it is of prime importance to obtain complete information about the actual time-space distribution of these additional non-thermal energy sources acting in the ‘active’ corona. This information can be gained by radar probing of the Sun in the frequency range from 40 to 10 MHz with the scanning beam such that it would enable the continuous monitoring of the Sun. Combined radar experiments and simultaneous observations of the sporadic solar radio-emission may also be of a high value, since as a result of a comparison of the information about the state of the turbulent coronal plasma obtained through these two different channels, a full description of its state in a local region of rapid acceleration of the solar wind could be achieved.

The significance of atmospheric measurements for interior models of the major planets

Abstract: We present a discussion of proposed models for interior processes in Jupiter and Saturn, and discuss how these models can be tested by atmospheric measurements by space vehicles. The importance of measurements at Uranus and Neptune is also discussed.

The helium abundance on Jupiter

Abstract: Methods of determining helium on Jupiter (and the Jovian planets) are critically surveyed. Current information is consistent with solar abundance, He/H2 = 0.11 by number. The available lines of evidence are the mean density, spectral-line broadening, and stellar occultations. Methods usable from spacecraft flying by are discussed. Observation of far-infrared emission has great promise, but we may have to await the development of entry probes for the greatest assurance.

Chemistry of the Solar Nebula

Abstract: The composition, mineralogy and texture of chondritic meteorites suggest they are relatively unaltered relicts of the condensation and accretion processes which took place in the primitive solar nebula. Chondrites thus are thought to contain a unique record of the physico-chemical conditions which prevailed at the time and place (asteroid belt) of their origin. Elemental abundance patterns are an important clue to the events and processes. Most elements can be placed in one of four groups according to their observed fractionation behavior in chondritic material: refractory, siderophile, normally depleted and strongly depleted. This grouping can be explained in terms of four events which presumably took place during cooling, condensation and accretion in the nebula. In order of inferred occurrence these are: (1) partial loss of the initial condensates rich in refractory elements at T > 1300K, (2) partial loss of metallic Fe-Ni grains, perhaps because they were magnetic, at 1000 to 700K, (3) partial remelting and outgassing of the condensate (chondrule formation) at 600 to 350K, and (4) accretion, when the P — T conditions controlled the volatile content (500 to 350K). Total gas pressure at the time and place of accretion is estimated to fall between 10−6 and 10−4 atm.

A Cosmochemical View of Cosmic Rays and Solar Particles

Abstract: The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above ~15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.

The solar wind in the outer solar system

Abstract: The properties of the solar wind including magnetic fields, plasma, and plasma waves are briefly reviewed with emphasis on conditions near and beyond the orbit of Jupiter. An extrapolation of the steady-state wind to large distances, evolution of disturbances and structure, modulation of cosmic rays, interactions with planetary bodies (bow shocks and magnetosheaths), and interactions with interstellar neutral helium and hydrogen are briefly discussed. Some comments on instrumentation requirements to observationally define the above phenomena are also included.

Theories of Nucleosynthesis

Abstract: A review is presented of current theories of nucleosynthesis. The predicted contributions from (1) cosmological nucleosynthesis, (2) super-massive stars, (3) non-violent (quasi-static) stellar evolution, (4) supernova explosions, (5) cosmic ray interactions with the interstellar medium and (6) nova explosions to the observed solar system abundances are summarized. Recent studies of ‘explosive nucleosynthesis’ in supernovae and of the production of lithium, beryllium and boron by the interaction of cosmic rays with interstellar gas are emphasized. Observations of stellar spectra which either impose limitations upon or provide confirmation of various aspects of these theories are noted, as are several critical nuclear experiments. The general picture which emerges is incouraging in that most of the major abundance features appear to be at least qualitatively understood, but significant further research is required.

The gravitational fields of the major planets

Abstract: The constraints placed on models of the interiors of the major planets by the non-spherical components of their gravitational fields are explained, and several methods of determining these non-spherical components are described and evaluated.

Thermal radio emission from the major planets.

Abstract: The use of long-wavelength radio measurements of brightness temperature to remotely measure the thermal structure of the atmospheres of the major planets at great depths is discussed. Data are presented which show that the gross features of Jupiter's and Saturn's microwave spectra, as determined from ground based observations, can be explained in terms of thermal emission from ammonia in deep convective atmospheres of He and H2.

The Mighei meteorite

Abstract: The Mighei meteorite is generally considered to be unique amongst the group of stony meteorites known as the carbonaceous chondrites in a number of scientifically interesting aspects. The meteorite, which is related to the type II carbonaceous chondrites of Wiik's classification (or type C2 according to van Schmus and Wood), contains extraterrestrial organic compounds (general C content = 2.6%), and extraterrestrial water associated with iron-magnesium silicate crystals (general H2O content=12%).