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THERMODYNAMIC and photochemical arguments1–4 suggest that Titan, the largest satellite of Saturn, has a deep ocean of liquid hydrocarbons. At visible wavelengths, Titan's surface is obscured by a thick stratospheric haze, but radar observations5–7 have revealed large regions of high surface reflectivity that are inconsistent with a global hydrocarbon ocean. Titan's surface has also been imaged at infrared wavelengths8–10, and the highest-resolution data (obtained by the Hubble Space Telescope) show clear variations in surface albedo and/or topography10. The natural interpretation of these observations is that Titan, like the Earth, has continents and oceans. But Titan's high orbital eccentricity poses a problem for this interpretation, as the effects of oceanic tidal friction would have circularized Titan's orbit for most configurations of oceans and continents1,11. Here we argue that a more realistic topography, in which liquid hydrocarbons are confined to a number of disconnected seas or crater lakes, may satisfy both the dynamical and observational constraints.

In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence. These observations constitute a control experiment for the serach for extraterrestrial life by modern interplanetary spacecraft.

Sources of organic molecules on the early Earth divide into three categories: delivery by extraterrestrial objects; organic synthesis driven by impact shocks; and organic synthesis by other energy sources (such as ultraviolet light or electrical discharges). Estimates of these sources for plausible end-member oxidation states of the early terrestrial atmosphere suggest that the heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of organics comparable to those produced by other energy sources. Which sources of prebiotic organics were quantitatively dominant depends strongly on the composition of the early terrestrial atmosphere. In the event of an early strongly reducing atmosphere, production by atmospheric shocks seems to have dominated that due to electrical discharges. Organic synthesis by ultraviolet light may, in turn, have dominated shock production, but only if a long-wavelength absorber such as H2S were supplied to the atmosphere at a rate sufficient for synthesis to have been limited by ultraviolet flux, rather than by reactant abundance. In the apparently more likely case of an early terrestrial atmosphere of intermediate oxidation state, atmospheric shocks were probably of little importance for direct organic production. For [H2]/[CO2] ratios of approximately 0.1, net organic production was some three orders of magnitude lower than for reducing atmospheres, with delivery of intact exogenous organics in interplanetary dust particles (IDPs) and ultraviolet production being the most important sources. At still lower [H2]/[CO2] ratios, IDPs may have been the dominant source of prebiotic organics on the early Earth. Endogenous, exogenous and impact-shock sources of organics could each have made a significant contribution to the origins of life.

Computer visualization, including graphical, imaging, and animation methods, is an essential tool for spacecraft exploration of the solar system. Its applications range from planning trajectories and orbital tours, and animation of spacecraft encounters, to a variety of applications involving the visual display and scientific analysis of returned data. Scientific visualization can take forms as varied as the suite of instruments carried by the spacecraft, but includes representation of motion and fields, animation of imaging sequences, and the effective communication of scientific results by the use of novel computer-generated graphics, images, and animated sequences. We present a few examples drawn from spacecraft flybys of the outer planets and the ongoing scientific analysis of the wealth of data they have returned.

The implications and limitations of program ACRETE were tested. The program is a scheme based on Newtonian physics and accretion with unit sticking efficiency, devised to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, the ratio of gas to dust in the solar nebula, the total nebular mass, and the orbital eccentricity of the accreting grains was explored. Only for a small subset of conceivable cases are planetary systems closely like our own generated. Many models have tendencies towards one of two preferred configurations: multiple star systems, or planetary systems in which Jovian planets either have substantially smaller masses than in our system or are absent altogether. But for a wide range of cases recognizable planetary systems are generated - ranging from multiple star systems with accompanying planets, to systems with Jovian planets at several hundred AU, to single stars surrounded only by asteroids.

During its active lifetime Mariner 9 obtained 32 high-resolution pictures of Phobos, and 9 of Deimos. The spacecraft orbit was adequate for about 80% complete coverage of Phobos. However, the coverage of the outer satellite was restricted to the Mars-facing side. The approaches used in processing the pictures are discussed. For each satellite picture a coordinate grid of the satellite was obtained with the aid of a computer.

Voyager 1 images of Titan, when normalized to remove limb darkening, reveal an axially symmetric brightness pattern with significant north-south asymmetry. This interhemispheric contrast seems to be a response to seasonal solar heating variations resulting from Titan's inclined spin axis. The contrast significantly lags the solar forcing, indicating that its production involves the atmosphere well below the unit optical depth level. The contrast has a significant effect on Titan's disk-integrated brightness as seen from earth, and probably accounts for most of the observed long term variation, with solar UV variations accounting for the remainder.

The overall objective of this experiment is exploratory reconnaissance of Jupiter, Saturn, their satellites, and Saturn's rings. Such reconnaissance, at resolutions and phase angles unobtainable from Earth, can be expected to provide much new data relevant to the atmospheric and/or surface properties of these bodies. The experiment also has the following specific objectives

The cameras aboard Voyager 1 have provided a closeup view of the Jupiter system, revealing heretofore unknown characteristics and phenomena associated with the planet's atmosphere and the surfaces of its five major satellites. On Jupiter itself, atmospheric motions-the interaction of cloud systems-display complex vorticity. On its dark side, lightning and auroras are observed. A ring was discovered surrounding Jupiter. The satellite surfaces display dramatic differences including extensive active volcanismn on Io, complex tectonism on Ganymnede and possibly Europa, and flattened remnants of enormous impact features on Callisto.

Martian surface temperature distributions based on IR scans of disk compared with predictions of thermal properties

Orbiting Astronomical Observatory data between 2000 and 3600 A, obtained by the Wisconsin Experimental Package objective grating scanning spectrometer, are used to set upper limits to the abundances of many minor constituents in the atmospheres of Mars, Jupiter, Saturn, and Venus.

Ice clouds existence on Venus from Venera 4 vapor observations consistent with near IR spectra

Analyses of the Mariner 9 pictures of the Martian satellites have yielded much new information: improved ephemerides, estimates of their principal axes, information about the texture of their surfaces, and estimates of the structural strength of their interiors. Both satellites are found to be in synchronous rotation, as was expected from tidal theory. A close examination of the pre-orbital satellite pictures has failed to show any unknown satellite. The photometric behavior of Phobos and Deimos indicates that they have intricate surface layers consistent with the presence of a regolith. Morphological features on Phobos and the expected collisional history of both satellites imply that both are made of well-consolidated material.

Statistical correlation of latitude with time of maximum darkening of Mars dark areas, noting consistency of 2 models

Analysis of non-gray radiative equilibrium and gray convective equilibrium on Titan suggests that a massive molecular-hydrogen greenhouse effect may be responsible for the disagreement between the observed IR temperatures and the equilibrium temperature of an atmosphereless Titan. Calculations of convection indicate a probable minimum optical depth of 14 which corresponds to a molecular hydrogen shell of substantial thickness with total pressures of about 0.1 bar. It is suggested that there is an equilibrium between outgassing and blow-off on the one hand and accretion from the protons trapped in a hypothetical Saturnian magnetic field on the other, in the present atmosphere of Titan. It is believed that an outgassing equivalent to the volatilization of a few kilometers of subsurface ice is required to maintain the present blow-off rate without compensation for all geological time. The presence of an extensive hydrogen corona around Titan is postulated, with surface temperatures up to 200 K.

Figure and inhomogeneities relationship for moon, Mercury, Venus, Mars and earth, discussing circular basins and mascons

Structure, composition and temperature of Venus atmosphere from spacecraft measurements and ground based observations

Anisotropic nonconservative scattering in modified Schuster-Schwarzschild approximation and Venus cloud layer

Martian surface topography effects on mean wind from time independent and frictionless thermal wind equation solution for radiative-convective atmospheres at scaling analysis level

In the ionospheric model of Venus, the observed microwave radiation is attributed to free-free emission of electrons in a dense Cytherean ionosphere. The present paper discusses the mechanisms for formation of such a dense ionosphere, both in the original formulation of Jones and in later formulations which introduce holes in the ionosphere to achieve consistency with the observed radar reflectiveness. Ionization by solar ultra-violet radiation and by the solar proton wind, as measured near Venus by Mariner 2, are considered, assuming that the Cytherean surface magnetic field strength is ≤10−3 gauss. Both sources of ionization are primarily effective at the level where the neutral particle density is ∼1011 cm−3, somewhat above the level at which diffusive equilibrium is established. Three-body electron-ion recombination processes are ineffective at these densities. Radiative recombination is plausible only if all the N2 at this level is converted to N by the Herzberg-Herzberg photodissociation mechanism. But the times for N to diffuse to dense levels where recombination to N2 occurs are so much shorter than the time for Herzberg-Herzberg photodissociation on Venus, that N2 must be a predominant atmospheric constituent at the level of the Cytherean ionosphere. If dissociative recombination therefore prevails, characteristics values of the electron density are ne ∼ 106 cm−3 over a region of 50-km thickness. If radiative recombination prevailed, ne ∼ 108 cm−3 would result, but even this is too small for the ionospheric model to be a valid explanation of the Venus microwave emission. The ionospheric model is accordingly rejected. We conclude that the observed microwave emission arises from the Cytherean surface.

Lunar gravity data and orbital photography indicate that there is a mascon basin approximately 1000 kilometers in diameter on the farside of the moon and that Mare Marginis is the flooded fraction of a mascon basin approximately 900 kilometers in diameter.

The small magnetic field strength observed near Mars by Mariner IV suggests that protons from the solar wind may enter the Martian atmosphere and produce ionization in addition to that produced by ultraviolet light and x-rays. It is found that solar protons produce a thin ionized layer at a rate of the order of 3 x 10(3) per cubic centimeter per second at a depth corresponding to the F(1) region in the terrestrial atmosphere. Unless the effective recombinative coefficient is very large (greater than 10(-5) centimeter cubed per second) or unless unusual diffusion effects are present, this layer should have been detected by Mariner IV, and therefore must be present in one of the observed ionized regions. Because of its very compact shape, the subsidiary maximum near 95 kilometers discovered in the Mariner-IV occultation experiment may be the proton ionization peak. If so, the major 120-kilometer maximum is an F(2) layer. Distinction between photon and proton ionization regions can be made by microwave occultation experiments aboard planetary orbiters.

Observations of Comets Halley and Wilson reveal an emission feature peaking near 3.4 micrometers, characteristic of C-H stretching in hydrocarbons. We have previously (Chyba and Sagan 1987a, Nature (London) 330, 350-353) fit this feature with a simple two-component thermal emission model for dust in the cometary coma (one component corresponding to large, cool, optically thick particles, the other due to smaller, hotter, organic grains) by employing laboratory spectra of the organic residue produced by the irradiation of carbon-bearing ices. This procedure yields optical depths in agreement with limits from spacecraft data. One remarkable result of such modeling is that at approximately 1 AU emission features at wavelengths longer than 3.4 micrometers are largely overwhelmed (or "diluted") by continuum emission. The large particle optical depth is approximately 10(2) times that of the emitting organics, so that, relative to the continuum, only near the continuum minimum can the emitting organics make a significant contribution. At approximately 1 AU, the 3.4-micrometers feature is the sole feature near that minimum, lying at the intersection of the curves for particle thermal emission and scattered sunlight. Thus, since as a comet moves away from perihelion the intersection of the scattered solar spectrum and the comet's thermal emission spectrum will move to longer wavelengths, we predicted (Chyba and Sagan 1987a) that the 3.4-micrometers feature is diluted while those at longer wavelengths are progressively revealed--so long as the comet retains its coma. We now quantitatively develop this model and find agreement with observational data for Comet Halley for certain plausible values of optical constants. Thus the observed heliocentric evolution of the 3.4-micrometers feature provides information on the composition, and perhaps structure, of the organic grains in Comet Halley. In addition, we argue that the heliocentric evolution of organic features will differ in the cases of thermal emission from small grains and gas-phase fluorescence. Therefore observations of cometary spectral evolution can in principle distinguish between solid or gas-phase origins for these features.

Exobiological survey of solar system beyond Mars, considering asteroids, Jovian planets and moons, Pluto and comets

Because of the high temperatures prevailing in the lower atmosphere of Venus, its chemistry is dominated by the tendency toward thermodynamic equilibrium. From the atomic composition deduced spectroscopically, the thermodynamic equilibrium composition of the atmosphere of Venus is computed, and the following conclusions drawn. (i) There can be no free carbon, hydrocarbons, formaldehyde, or any other organic molecule present in more than trace amounts. (ii) The original atomic composition of the atmosphere must have included much larger quantities of hydrogen and a carbon/oxygen ratio/= 0.5. (This ratio is now almost precisely 0.5.) (iii) The present atomic proportions of the atmosphere of Venus are so unique that an evolutionary mechanism involving two independent processes seems necessary, as follows. Water, originally present in large quantities, has been photodissociated in the upper atmosphere, and the resulting atomic hydrogen has been lost in space. The resulting excess oxygen has been very effectively bound to the surface materials. (iv) There must be some weathering process, for example, violent wind erosion, to disturb and expose a sufficient quantity of reduced surface material to react with the oxygen produced by photodissociation.

Some recent papers on solar neutrinos, Martian rivers, and Praesepe stars are reviewed. Possible causes of solar neutrino detection rates being below theoretical expectations are quoted. The widespread presence of sinuous dendritic channels on Mars is noted. The occurrence of earth-like epochs on Mars is indicated as a plausible explanation for many such channels in terms of surface liquid water flows. The roughly uniform distribution of the Praesepe stars through the main sequence width is viewed as the indication that the excursion time off the main sequence is comparable to the time between mixings.

The hypothesis that solar protons produce the Martian blue haze is examined. To reach the atmospheric depth at which the observed haze layer is localized, proton energies in excess of 2 Mev are required. In order to produce sufficient molecular ionization to obscure surface detail in the blue, the proton fluxes must be greater than 1011 cm−2 sec−1 at Mars. So that magnetic deflection of solar protons by the terrestrial magnetic field can explain the clearing of the blue haze which is observed at opposition, the interplanetary magnetic field strength must be less than 10−8 gauss. Recent observations argue strongly against these required interplanetary magnetic field strengths and solar proton energies and fluxes. Several further observational tests are suggested, but at the present time it appears that this hypothesis should be rejected.

Doppler radar spectroscopy of Mars surface - elevation differences between bright and dark areas

Thermodynamic equilibrium composition calculations on carbon, hydrogen, oxygen, and nitrogen in Earth, Venus, Mars, and Jupiter atmospheres

Mercury brightness temperatures from contact conduction and radiative transfer equation solutions, noting dependence on heliocentric longitude and phase angle

International audience; In this study, the hierarchical stability of the seven known large size ratio triple asteroids is investigated. The effect of the solar gravity and primary's J 2 are considered. The force function is expanded in terms of mass ratios based on the Hill's approximation and the large size ratio property. The empirical stability parameters are used to examine the hierarchical stability of the triple asteroids. It is found that the all the known large size ratio triple asteroid systems are hierarchically stable. This study provides useful information for future evolutions of the triple asteroids.

International audience; Dust devils - convective vortices made visible by the dust and debris they entrain - are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites. We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements. Daily (10:00-16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction. The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain. (C) 2012 Elsevier Inc. All rights reserved.

International audience; Around 83 B.C., give or take a century, comet Kiess (C/1911 N1) passed by the Sun, ejecting a cloud of dust particles. The comet returned in 1911, after completing one orbit. The dust particles were pushed by solar radiation pressure into slightly wider orbits and have been returning ever since, forming a thin ongoing stream of dust that usually passes just outside Earth's orbit. On occasion, the combined gravity of the solar system's planets moves this dust trail into Earth's path. Earth encountered this 2000-year-old dust in 1935, 1986, and 1994, causing a meteor shower known as the Aurigids.