Radar Investigation of Mars, Mercury, and Titan

Abstract

Radar astronomy is the study of the surfaces and near surfaces of Solar System objects using active transmission of modulated radio waves and the detection of the reflected energy. The scientific goals of such experiments are surprisingly broad and include the study of surface slopes, fault lines, craters, mountain ranges, and other morphological structures. Electrical reflectivities contain information about surface densities and, to some extent, the chemical composition of the surface layers. Radar probes the subsurface layers to depths of the order of 10 wavelengths, providing geological mapping and determinations of the object's spin state. Radar also allows one to study an object's atmosphere and ionic layers as well as those of the interplanetary medium. Precise measurements of the time delay to surface elements provide topographic maps and powerful information on planetary motions and tests of gravitational theories such as general relativity. In this paper, we limit our discussion to surface and near-surface probing of Mercury, Mars, and Titan and review the work of the past decade, which includes fundamentally new techniques for Earth-based imaging. The most primitive experiments involve just the measurement of the total echo power from the object. The most sophisticated experiments would produce spatially resolved maps of the reflected power in all four Stokes' parameters. Historically, the first experiments produced echoes from the Moon during the period shortly after World War II (see e.g. Evans 1962), but the subject did not really develop until the early 1960s when the radio equipment was sufficiently sensitive to detect echoes from Venus and obtain the first Doppler strip "maps" of that planet. The first successful planetary radar systems were the Continuous Wave (CW) radar at the Goldstone facility of the Caltech's Jet Propulsion Laboratory and the pulse radar at the MIT Lincoln Laboratory. All of the terrestrial planets were successfully studied during the following decade, yielding the spin states of Venus and Mercury, a precise value of the astronomical unit, and a host of totally new discoveries concerning the surfaces of the terrestrial planets and the Moon. This work opened up at least a similar number of new questions. Although the early work was done at resolution scales on the order of the planetary radii, very rapid increases in system sensitivities improved the resolution to the order of 100 km, but always with map ambiguities. Recently, unambiguous resolution of 100 m over nearly the entire surface of Venus has been achieved from the Magellan spacecraft using a side-looking, synthetic aperture radar. Reviews of the work up to the Magellan era can be found in Evans (1962), Muhleman et al (1965), Evans & Hagfors (1968, see chapters written by G Pettengill, T Hagfors, and J Evans), and Ostro (1993). The radar study of Venus from the Magellan spacecraft was a tour de force and is well described in special issues of Science (volume 252, April 12, 1991) and in the Journal of Geophysical Research (volume 97, August 25 and October 25, 1992). Venus will not be considered in this paper even though important polarization work on that planet continues at Arecibo, Goldstone, and the Very Large Array (VLA). In this paper we review the most recent work in Earth-based radar astronomy using new techniques of Earth rotation, super synthesis at the VLA in New Mexico (operated by the National Radio Astronomy Observatory), and the recently developed "long-code" techniques at the Arecibo Observatory in Puerto Rico (operated by Cornell University). [Note: It was recently brought to our attention that the VLA software "doubles" the flux density of their primary calibrators. Consequently, it is necessary to half the radar power and reflectivity numerical values in all of our published radar results from the VLA/Goldstone radar.] The symbiotic relationship in these new developments for recent advances in our understanding of Mercury and Mars is remarkable. VLA imaging provides for the first time, unambiguous images of an entire hemisphere of a planet and the long-code technique makes it possible to map Mars and Mercury using the traditional range-gated Doppler strip mapping procedure [which was, apparently, developed theoretically at the Lincoln Laboratory by Paul Green, based on a citation in Evans (1962)]. Richard Goldstein was the first to obtain range-gated planetary maps of Venus as reported in Carpenter & Goldstein (1963). Such a system was developed earlier for the Moon as reported by Pettengill (1960) and Pettengill & Henry (1962). We first discuss the synthesis mapping technique.

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