Ostro, S. J., R. F. Jurgens, D. K. Yeomans, E. M. Standish, and W. Greiner (1989). Science243, 1584-1586.
Radar echoes from the martian satellite Phobos provide information about that object's surface properties at scales near the 3.5-cm observing wavelength. Phobos appears less rough than the moon at centimeter-to-decimeter scales. The uppermost few decimeters of the satellite's regolith have a mean bulk density within 20% of 2.0 g/cc. The radar signature of Phobos (albedo, polarization ratio, and echo spectral shape) differs from signatures measured for small, Earth-approaching objects, but resembles those of large (>= 100-km), C-class, mainbelt asteroids.