Radio reflection imaging of asteroid and comet interiors II: Results and recommendations

Abstract

We modeled orbital surface-penetrating radar of an asteroid and comet using two-dimensional finite-difference wavefield migration, in order to assess key target properties and experiment parameters required to fully image internal structure. Migration places radar echoes in their correct positions in a complex subsurface and is a complementary tool to travel-time tomography. The target shape was scaled from 433 Eros to 0.5-km mean diameter for an asteroid and 10km for a comet. The interiors were populated with a power-law distribution of spherical blocks. We used an image structural similarity index to compare the internal surfaces reconstructed under different assumptions to a “best” image using optimum parameters. We found that successful internal imaging of the asteroid was not sensitive to whether the block interstices were regolith or void. Frequency dependence between 5 and 15MHz was also minor. Internal interfaces could also be imaged if the attenuation was higher than that inferred within volcanic plains on Mars, but not as high as measured in a strongly fractured volcanic tuff on Earth. The overall imaging quality for the comet was statistically similar to the asteroid, but there was less variability due to smaller internal contrasts. A key finding is that imaging was vastly improved by using a second spacecraft as a radar receiver. A subsatellite with a different orbit will eventually provide a range of different illumination geometries over each part of the target. Finally, the results depend strongly on the specified internal velocity distribution, representing partial progress in complementary tomographic velocity estimation. The modeled impedance contrasts within the asteroid are larger than those typically encountered in exploration seismology and very much larger than in medical imaging, and so the velocity used to migrate the reflections must be close to the actual distribution. This again emphasizes the need for joint traveltime tomography and wavefield migration for asteroid imaging, which is optimized using two orbiters.