3D radar wavefield migration of comet interiors

Imaging the interior structure of small planetary bodies facilitates a deep understanding of their origin and evolution, thus addressing fundamental questions about the formation of the Solar System. We show that high resolution 3D imaging of their interior structure is possible using radar waves that reflect from internal discontinuities of dielectric properties. A radar imaging mission at a comet nucleus would have the benefit of orbiting all around a finite and transparent body, collecting echoes that derive only from the target, and processing them collectively in phase. As is the case in the medical field, imaging a comet nucleus requires its illumination from multiple directions, which can be accomplished with a spacecraft in slow polar orbit around the studied object. Long acquisition time leads to a dense acquisition array resembling that of conventional synthetic aperture radar systems, but completely surrounding the nucleus (4π steradians). Acquisition with an orbiter at large distance from the comet nucleus (5× the mean diameter) results in relatively coarse data sampling relative to the radar wavelength, thus enabling 3D imaging with a short (<90 days) mission. Radar migration is performed using techniques adapted from terrestrial exploration seismology. Wavefield migration identifies interior reflectors by time reversal of monostatic radar data redatumed to the known comet surface for computational speed-up. This technique is applicable to nuclei of arbitrary shape and interior complexity, and it is complementary to wavefield tomography tasked with constraining physical properties in-between the interior interfaces. Migration resolution is identical in all directions (range and cross-range) and it is equivalent to the range resolution of the radar system. Least-squares migration enhances resolution further by deconvolution of the radar wavelet from the reflectivity image.