Eagle: a Synthetic Aperture Radar Mapper for the Mars Scout Program

Background: The Mars Express and Mars Reconnaissance Orbiter spacecraft both carry radar sounder systems (MARSIS and SHARAD). These instruments transmit a radar signal with wavelength of meters to tens of meters, and penetrate to depths of several km in typical dry crustal rocks. Sounders, however, provide relatively coarse horizontal resolution (1-10 km), and cannot reveal detail of the near-surface (upper 10 m) geologic environment. Synthetic aperture radar (SAR) systems operate at higher frequencies (shorter wavelengths), and use a side-looking geometry to achieve high spatial resolution. At wavelengths of a few tens of cm (“L” or “P” band in radar terminology), the radar signal can penetrate to significant depths (meters to tens of meters), depending upon the rock abundance and bulk chemistry of the target region, and be scattered back to the sensor by subsurface rocks, voids, or interfaces. These reflections can be used to map geologic features beneath surface mantling deposits. For example, in lunar highlands material, which has a low abundance of radar-absorbing minerals, 70-cm radar signals propagate up to 50 m to reveal buried mixed zones of feldspathic and basaltic debris [1]. In much less “dry” conditions, such as those found in terrestrial deserts, penetration of 1-2 m has been used to reveal ancient drainage patterns [2]. Both cases show that it is not always necessary for the radar to encounter a discrete “bedrock” interface – often there are significant geologic unit boundaries delineated by changes in the bulk scattering properties (rock abundance or loss tangent) of deep deposits. The surface of Mars is covered, to varying degrees, by surficial sediments deposited and modified by aeolian, volcanic, fluvial, impact, or other processes. These surficial sediments obscure the underlying geologic setting and context, leaving very limited outcrops of bedrock in some areas (Fig. 1). Nearinfrared and thermal infrared sensors can reveal the properties of the upper few cm, at best. A SAR can map the subsurface geology, and the properties of the overlying sediments, across much of the planet.