Radar: The Cassini Titan Radar Mapper

The Cassini RADAR instrument is a multimode 13.8 GHz multiple-beam sensor that can operate as a synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer. The principal objective of the RADAR is to map the surface of Titan. This will be done in the imaging, scatterometer, and radiometer modes. The RADAR altimeter data will provide information on relative elevations in selected areas. Surfaces of the Saturn’s icy satellites will be explored utilizing the RADAR radiometer and scatterometer modes. Saturn’s atmosphere and rings will be probed in the radiometer mode only. The instrument is a joint development by JPL/NASA and ASI. The RADAR design features significant autonomy and data compression capabilities. It is expected that the instrument will detect surfaces with backscatter coefficient as low as −40 dB.RADAR Team LeaderThis revised version was published online in July 2005 with a corrected cover date.     

MIRO: Microwave Instrument for Rosetta Orbiter

The European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a relatively small and lightweight millimeter-submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively, the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids. The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species – CO, CH₃OH, NH₃ and three, oxygen-related isotopologues of water, H₂ ¹⁶O, H₂ ¹⁷O and H₂ ¹⁸O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system.     

Saturn's Rings: pre-Cassini Status and Mission Goals

Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized. This revised version was published online in August 2006 with corrections to the Cover Date.     

Planetary characteristics from radar observations

Conclusions Long wavelength radar observations of Venus yield a surface reflectivity of about 15%. Total power measurements at 12.5 cm and 3.6 cm strongly suggest that significant atmospheric absorption is operative in this wavelength region. If the observed low value of reflectivity at 3.6 cm is attributed to atmospheric absorption alone an opacity of = 1.14 is implied at this wavelength rather independently from assumptions concerning the surface scattering characteristics of Venus. An inverse ² opacity law for the atmosphere is consistent with the reflectivity measurements over the complete range of observations wavelengths.The mathematical characteristics of the Venusian backscatter law are the same as for the moon but wavelength-dependent mean effective slopes indicate that Venus appears smoother than the moon at all radar wavelengths.Considerable progress has been made toward obtaining a precise value for the Venusian axial rotation vector which is found to be oriented to within 10 degrees of the planet's orbital plane. The period of (retrograde) rotation lies within the range 242–250 days with the lower value favored by the statistics of the data. Regions of enhanced radar return fixed to the surface have been found and verified at a later conjunction. Measurements of the surface radar depolarization support the hypothesis that the prominences are due to increased surface roughness as opposed to regional increases of dielectric constant.Observations of Mercury strongly suggest that the rotation period of the planet is about 59 days, a conclusion which has been supported, a posteriori, by theoretical tidal calculations and rediscussions of optical observations of surface markings. Mercury has radar backscatter characteristics more similar to the moon than Venus and exhibits a reflectivity of about 5%.Mars has demonstrated strong variations of radar backscatter characteristics which appear correlated with the Martian longitude and, in turn, with the dark surface markings in its north equatorial zone. Particularly reliable correlations have been discovered with the positions of Trivium Charontis and Syrtis Major. The observed variations appear to be primarily manifested in terms of the Martian radar backscatter law or surface roughness as opposed to variations in the intrinsic surface material reflectivities although the observations are not sufficiently precise to resolve this question. Variations in surface materials are apparently also present but their degree is currently unassayable. The reflectivity of the average surface has been crudely determined to be about 7% which suggests that the surface of Mars is composed of underdense materials. The 7% value is consistent with the values of 7.5% and 5% for the moon and Mercury, respectively, and is significantly different from the 15% value for Venus,No unequivocal radar detection of Jupiter has been made although a statistically weak detection has been reported for a single opposition which could not be verified in succeeding attempts.