Since the early 1960s, radar has evolved into an effective and powerful tool for investigating planetary surface and subsurface environments, helping to characterize the topography, geology, and subsurface physical properties of the Earth, Moon, Mars, and other bodies in the solar system. Of particular interest is the use of radar to investigate the hydrology and paleohydrology of planetary bodies, as well as its use for assessment of the distribution and state of subsurface water and the location of potential subsurface habitats capable of sustaining life.
 During the coming decade, a variety of terrestrial and planetary radar instruments will be used to address these tasks. Recent developments and capabilities of radar and results of radar data analysis were discussed by 58 scientists and engineers attending the Lunar and Planetary Institute's (LPI) Workshop on Radar Investigations of Planetary and Terrestrial Environments, which was held in Houston, Texas, 7–10 February 2005. This 4-day workshop was sponsored by LPI, the National Aeronautics and Space Administration (NASA), Jet Propulsion Laboratory (JPL), and Southwest Research Institute (SwRI).
 The program consisted of a mix of invited and contributed talks, panel and open discussions, and poster presentations that addressed many aspects of planetary exploration using both sounding and synthetic aperture radars. Presentation topics included investigations of geologic and volatile analogs (e.g., terrestrial paleochannels, craters, subsurface structure and stratigraphy, glaciers, ice sheets, permafrost, groundwater, and gas hydrates), laboratory measurements of the electromagnetic properties of extraterrestrial and analog materials, data analysis, and numerical simulations of radar propagation from orbital, aerial, and ground-based instruments (particularly with regard to the identification and mapping of subsurface water on the Moon, Mars, and Europa). Presenters also discussed ambiguities and the potential environmental complications associated with the interpretation of radar data, and the utility of other complementary geophysical investigations to resolve these ambiguities.
 A key focus of the discussion was the use of radar to map the global distribution and state of subsurface water on Mars. The search for water is motivated by the following three objectives: (1) the abundance and distribution of water has important implications for understanding the evolution of the planetary surface, atmosphere, and climate; (2) the presence of water appears vital to the origin and continued survival of life; and (3) water represents an invaluable in situ resource for sustaining future human explorers [Chyba et al., 2006; Mars Exploration Program Analysis Group (MEPAG), Scientific Goals, Objectives, Investigations, and Priorities: 2003, April 2004, available at http://mepag.jpl.nasa.gov/reports/index.html].
 The first radar investigation of Mars conducted as part of the search for water began in June 2005, when the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board the European Space Agency's (ESA) Mars Express orbiter, began its ambitious mission to identify the distribution of water and ice within the top several kilometers of the crust [Picardi et al., 2005]. MARSIS is a multifrequency, coherent pulse, synthetic aperture radar sounder whose operation resembles that of a conventional ground-penetrating radar (GPR). Because the depth of investigation achieved by a radar sounder is maximized at low frequencies, MARSIS was designed with an operational frequency range of 0.5–5 MHz, with the lower limit imposed by the potential for interference from Mars' ionosphere. At these frequencies, MARSIS will have a vertical subsurface resolution of approximately 100 m and a maximum projected penetration depth of as much as several kilometers. Subsurface reflections can originate from both dielectric contrasts and structural discontinuities. Given the high dielectric contrast between liquid water and ice or rock, MARSIS theoretically can detect a liquid water interface over a depth range of 0.3–5 km, although propagation losses due to the local electrical and magnetic properties of the crust may significantly impact its sounding performance in environments other than the polar layered deposits [Beaty et al., 2001]. Under the most favorable conditions of surface roughness, target geometry, and rock composition, MARSIS may also be capable of detecting the much smaller dielectric contrast between massive lenses of segregated ice and ice-free or ice-saturated frozen ground [Beaty et al., 2001]. Researchers hope that MARSIS will provide a “first look” at the lithology, structure, and volatile stratigraphy of the Martian crust, results that may help target more capable and higher-resolution surface investigations in the future. Initial results from the first five months of MARSIS observations were recently reported by Picardi et al. .
 In November 2006, a second orbital radar sounder, called SHARAD (for “SHAllow RADar”), will begin operation aboard NASA's Mars Reconnaissance Orbiter (MRO) [Seu et al., 2004]. Like MARSIS, the primary mission of SHARAD is to search for dielectric evidence of subsurface water and ice, but at shallower depth and higher resolution than MARSIS, a capability derived from SHARAD's higher (20 MHz) operating frequency and 10 MHz bandwidth. Because of the nearly circular 400-km-high polar orbit of MRO (versus the highly elliptical orbit of Mars Express), SHARAD is also expected to sound more of the planetary surface at better horizontal resolution.
 Workshop participants discussed the potential flight of synthetic aperture imaging radar as the next logical step in the search for evidence of past and present water on Mars, noting that, on Earth, such investigations have revealed buried paleodrainage systems several meters beneath the Sahara Desert, and the basal topography of glaciers and ice sheets, hundreds to thousands of meters thick. Yet, despite the insights that orbital imaging radar systems can provide, unambiguous identification of crustal H2O on Mars may prove difficult or impossible using orbital systems alone. Therefore, after the most promising potential occurrences of near-surface water have been identified by orbital reconnaissance, attention must also be given to the types of follow-up investigations that should be conducted from either aerial platforms or at the surface. Potential candidates include seismology (active or passive), and a variety of low-frequency electromagnetic and other geophysical methods such as magnetotellurics, time domain electromagnetics, electrical resistivity, and nuclear-magnetic resonance.
 The papers contained in this special section address a broad range of topics related to the use and expected performance of sounding and imaging radars to explore the geology, polar ice, and the subsurface geology and hydrogeology of planetary bodies. These papers complement the abstracts presented at the workshop (http://www.lpi.usra.edu/meetings/radar2005/pdf/program.pdf) as well as the 27 papers on planetary exploration geophysics contained in the special section on the Geophysical Detection of Subsurface Water on Mars, published in the April 2003 issue of JGR–Planets. The papers specifically address the potential and limitations of various radar techniques, ambiguities in data analysis and interpretation, and illustrate many similarities between the planetary and terrestrial radar investigations.
 With the ongoing analysis of data from Earth-based investigations, past spacecraft missions, and the influx of new data anticipated from the Cassini RADAR, Mars Express MARSIS, and Mars Reconnaissance Orbiter SHARAD instruments, plans are now underway for a follow-up meeting to be held in Autumn 2007.