L. Marinangeli

SHARAD (SHAllow RADar) is a sounding radar provided by Agenzia Spaziale Italiana (ASI) as a Facility Instrument on the Mars Reconnaissance Orbiter mission. Its 20‐MHz center frequency and 10‐MHz bandwidth complement the lower‐frequency, relatively narrower bandwidth capability of the MARSIS sounding radar. A joint Italian‐U.S. team has guided the experiment development and is responsible for data analysis and interpretation. The radar transmits signals at a 700 Hz pulse repetition frequency (PRF) and collects reflections from both the surface and near subsurface of Mars. Vertical and horizontal resolutions are, respectively, 15 m (free‐space) and 3–6 km (cross‐track) by 0.3–1 km (along‐track). The scientific objective of SHARAD is to map, in selected locales, dielectric interfaces to at least several hundred meters depth in the Martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials, including competent rock, soil, water, and ice. A signal‐to‐noise ratio of ∼50 dB (for a specular surface return) is achieved with 10 W of radiated power by using range and azimuth focusing in ground data processing. Preprocessed data as well as range‐ and azimuth‐focused data will be formatted according to Planetary Data System (PDS) standards and be made available from the ASI Science Data Center (ASDC) and from the Geosciences Node of the Planetary Data System (PDS). Important targets for SHARAD include the polar layered deposits, sedimentary stacks (especially in Terra Meridiani), buried channel systems, buried impact craters, volcanic complexes, and shallow ice deposits in equilibrium with the atmosphere.

The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter has imaged the internal stratigraphy of the north polar layered deposits of Mars. Radar reflections within the deposits reveal a laterally continuous deposition of layers, which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice. The packet/interpacket structure can be explained by approximately million-year periodicities in Mars' obliquity or orbital eccentricity. The observed approximately 100-meter maximum deflection of the underlying substrate in response to the ice load implies that the present-day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers. Alternatively, the response to the load may be in a transient state controlled by mantle viscosity. Both scenarios probably require that Mars has a subchondritic abundance of heat-producing elements.