Mars Express MARSIS radargram simulations over Lucus Planum (Mars) This archive contains simulations of radar surface scattering for the MARSIS low-frequency radar over the area of Lucus Planum. Ground Penetrating Radar (GPR) is a well-established geophysical technique employed for more than five decades to investigate the terrestrial subsurface. It is based on the transmission of radar pulses at frequencies in the MF, HF and VHF portions of the electromagnetic spectrum into the surface, to detect reflected signals from subsurface structures (see e.g. Bogorodsky et al. 1985). Orbiting GPR have been successfully employed in planetary exploration (Phillips et al. 1973, Picardi et al. 2004, Seu et al. 2007, Ono et al. 2009), and are often called subsurface radar sounders. By detecting dielectric discontinuities associated with compositional and/or structural discontinuities, radar sounders are the only remote sensing instruments allowing the study of the subsurface of a planet from orbit. MARSIS is a synthetic-aperture, orbital sounding radar carried by the European Space Agency spacecraft Mars Express (Picardi et al. 2005). MARSIS is optimized for deep penetration, having detected echoes down to a depth of 3.7 km over the South Polar Layered Deposits (Plaut el al. 2007). MARSIS transmits through a dipole, which has negligible directivity, with the consequence that the radar pulse illuminates the entire surface beneath the spacecraft and not only the near-nadir portion from which subsurface echoes are expected. The electromagnetic wave can then be scattered by any roughness of the surface. If the surface of the body being sounded is not smooth at the wavelength scale, i.e. if the r.m.s. of topographic heights is greater than a fraction of the wavelength, then part of the incident radiation will be scattered in directions different from the specular one. This means that areas of the surface that are not directly beneath the radar can scatter part of the incident radiation back towards it, and thus produce surface echoes that will reach the radar after the echo coming from nadir, which can mask, or be mistaken for, subsurface echoes. This surface backscattering from off-nadir directions is called "clutter". To validate the detection of subsurface interfaces, numerical electromagnetic models of surface scattering, such as those by Nouvel et al. (2004), Russo et al. (2008) or Spagnuolo et al. (2011), have been used to produce simulations of surface echoes, which are then compared to real echoes detected by the radar. A code for the simulation of radar wave surface scattering has been developed, based on the work of Nouvel et al. (2004), using the MOLA topographic dataset (Smith et al. 2001) to represent the Martian surface as a collection of flat plates called facets. The radar echo is computed as the coherent sum of reflections from all facets illuminated by the radar. The computational burden of every simulation is very high but, thanks to a collaboration with CINECA, the code has been parallelized and ported for use in a Blue Gene/Q system. The code has been tested and its performance evaluated on the Fermi machine at CINECA. The data included in this archive simulate MARSIS observations over the area of Lucus Planum, in the equatorial region of Mars, and have been used in the paper "Radar sounding of Lucus Planum, Mars, by MARSIS" by Orosei, Rossi, Cantini, Caprarelli, Carter, Papiano, Cartacci, Cicchetti and Noschese, accepted for pubblication on Journal of Geophysical Research - Planets. A direct comparison of simulations with actual observations allows to unambiguously identify subsurface echoes in MARSIS radargrams. Echoes reaching the radar after nadir surface echoes will be identified as coming from the subsurface if they are not present in simulations. Conversely, any secondary echo that is present in both real and simulated data must be interpreted as coming from the surface. The numerical code for the simulation of surface scattering was developed at the Consorzio Interuniversitario per il Calcolo Automatico dell'Italia Nord-Orientale (CINECA) in Bologna, Italy. Simulations were produced thanks to the Partnership for Advanced Computing in Europe (PRACE), awarding us access to the SuperMUC computer at the Leibniz-Rechenzentrum, Garching, Germany through project 2013091832. Test simulations were run Jacobs University CLAMV HPC cluster, and we are grateful to Achim Gelessus for his support. REFERENCES Bogorodsky, V., Bentley, C., Gudmandsen, P. 1985. 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