Internal structure of Planum Boreum, from Mars advanced radar for subsurface and ionospheric sounding data

摘要

An investigation of the internal structure of the ice-rich Planum Boreum (PB) deposit at the north pole of Mars is presented, using 178 orbits of Mars advanced radar for subsurface and ionospheric sounding data. For each radargram, bright, laterally extensive surface and subsurface reflectors are identified and the time delay between them is converted to unit thicknesses, using a real dielectric constant of 3. Results include maps of unit thickness, for PB and its two constituent units, the stratigraphically older basal unit (BU) and the stratigraphically younger north polar layered deposits (NPLD). Maps of the individual units' surface elevation are also provided. Estimates of water ice volume in each unit are (1.3 ± 0.2) × 10~6 km~3 in PB, (7.8 ± 1.2) × 10~5 km~3 in the NPLD, and (4.5 ± 1.0) × 10~5 km~3 in the BU. No lithospheric deflection is apparent under PB, in agreement with previous findings for only the Gemina Lingula lobe, which suggests that a thick elastic lithosphere has existed at the north pole of Mars since before the emplacement of the BU. The extent of BU material in the Olympia Planum lobe of PB is directly detected, providing a more accurate map of BU extent than previously available from imagery and topography. A problematic area for mapping the BU extent and thickness is in the distal portion of the 290°F-300°E region, where MARSIS data show no subsurface reflectors, even though the BU is inferred to be present from other lines of evidence.