Touring the saturnian system: the atmospheres of titan and saturn

This report follows the presentation originally given in the ESA Phase A Study for the Cassini Huygens Mission. The combination of the Huygens atmospheric probe into Titan's atmosphere with the Cassini orbiter allows for both in-situ and remote-sensing observations of Titan. This not only provides a rich harvest of data about Saturn's famous satellite but will permit a useful calibration of the remote-sensing instruments which will also be used on Saturn itself. Composition, thermal structure, dynamics, aeronomy, magnetosphere interactions and origins will all be investigated for the two atmospheres, and the spacecraft will also deliver information on the interiors of both Titan and Saturn. As the surface of Titan is intimately linked with the atmosphere, we also discuss some of the surface studies that will be carried out by both probe and orbiter. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ionic mobility of the middle atmosphere

Positive ion mobilities are calculated for 40–75 km by computing the theoretical positive ion composition and combining it with laboratory-determined mobilities. Theoretical determinations for mobility appear to be especially apt for the 40–65 km region since oxonium ions are observed to be the principal positive ions and they should be subject to thermodynamic equilibrium. We compute a mean reduced mobility of 2.1 ± 0.1 cm²/V-s for 40–65 km using the 1976 U.S. Standard Atmosphere and a water vapor mixing ratio of 5 ppmv. The results are compared with atmospheric data for mobility. Observations of a lower mobility from about 35 km down to ground level are qualitatively compatible with the onset of the so-called non-proton hydrate ions at about this altitude and extending to lower heights. We note that the laboratory determined mobilities for oxonium ions average about 12 % less than the theoretical Langevin values. The total positive ion conductivity is determined also and compared with in-situ results.     

An imaging telescope for high energy gamma-ray astronomy

Recent tests of a γ-ray imaging telescope, which incorporated a coded aperture mask and multi-wire proportional counter system produced good images of a tritium target source which was used to generate the 20 MeV protons at a proton Van de Graaff accelerator. This paper indicates what performance one might expect if a large area drift chamber were used in conjunction with a coded aperture mask. The prospects for achieving significant scientific results if such a system were flown on a variety of space vehicles are discussed.     

Interplanetary scintillation

The use of interplanetary scintillations for probing otherwise inaccessible regions of the solar wind is reviewed. A comparison with space-craft observations in the ecliptic is used as a calibration for the scintillation observations. Recent observations at high latitudes and near the Sun are discussed from this viewpoint. A new analysis which uses both scintillation and angular scattering observations to estimate the electron density spectrum is introduced. The spectrum appears to have a high frequency cutoff which varies slowly with solar distances and may also have a relatively flat region just below the cutoff frequency.     

The Rotation and Interior Structure Experiment on the InSight Mission to Mars

The Rotation and Interior Structure Experiment (RISE) on-board the InSight mission will use the lander’s X-band (8 GHz) radio system in combination with tracking stations of the NASA Deep Space Network (DSN) to determine the rotation of Mars. RISE will measure the nutation of the Martian spin axis, detecting for the first time the effect of the liquid core of Mars and providing in turn new constraints on the core radius and density. RISE will also measure changes in the rotation rate of Mars on seasonal time-scales thereby constraining the atmospheric angular momentum budget. Finally, RISE will provide a superb tie between the cartographic and inertial reference frames. This paper describes the RISE scientific objectives and measurements, and provides the expected results of the experiment.     

Measurement and assessment of wind tunnel flow quality

For decades, wind tunnel testing has been conducted in test section environments that have not been adequately or consistently documented. Since wind tunnel flow quality can adversely affect test results, accurate and consistent flow quality measurements are required, along with an understanding of the sources, characteristics, and management of flow turbulence. This paper will review turbulence measurement techniques and data obtained in subsonic, transonic, and supersonic test facilities as they relate to the determination and assessment of wind tunnel flow quality. The principles and practical application of instrumentation used in the measurement and characterization of wind tunnel turbulence will be described. Techniques used for the identification of the sources of wind tunnel disturbances, and the performance of turbulence suppression devices will be outlined. These test techniques will be illustrated with extensive measurements obtained in a number of test facilities. The measurements will provide comprehensive turbulence data that are vital to the assessment and management of flow quality. Procedures designed to assess the potential influence of adverse flow quality on wind tunnel model test performance will also be discussed.     

The Geology of Mercury: The View Prior to the MESSENGER Mission

Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth’s Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (∼1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ∼3.8 Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions about the geologic evolution of Mercury; the depth and breadth of the MESSENGER data will permit the confident reconstruction of the geological history and thermal evolution of Mercury using new imaging, topography, chemistry, mineralogy, gravity, magnetic, and environmental data.     

Martian Seeps and their Relation to Youthful Geothermal Activity

Gullies found on Martian hillsides by Malin and Edgett (2000) appear in many cases to be formed by water seeps produced by underground aquifers. It is proposed that these aquifers result from geologically recent melting of permafrost ice by sporadic, localized geothermal activity. This is consistent with evidence from crater counts and Martian meteorites that much higher-temperature geothermal activity has produced volcanic activity and lava flows within the last 200 Myr, and perhaps within the last 10 Myr. This hypothesis explains an aspect initially described as surprising, namely concentration of the gullies at high latitudes and on shadowed slopes. Similar features are found on Icelandic basaltic hillsides, which may be ideal analogs for further studies that may clarify the Martian phenomena.