Mapping rays and secondary craters from the Martian crater Zunil

Zunil, a 10.1 km rayed crater in Elysium Planitia, Mars, produced more than 7 × 10⁷ secondary craters ≥ 15 m in diameter. We mapped Zunil's rays from thermal IR THEMIS nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters (candidate secondaries) up to 3600 km range. Ray segments were mapped up to 450 km east of Zunil and up to 1700 km to the west. Both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia, which are abundant west of but not east of Zunil. Nevertheless, our interpretation is that Zunil was created by a moderately oblique impact from the east. Zunil secondaries are abundant over all terrain types except the Medusae Fossae Formation (MFF). Given the likely age of Zunil (<100 Ma), parts of the MFF must be eroding at ≥0.08 m/Ma. The size-frequency distribution of the secondaries in Zunil's rays and probable distant secondaries (750-1700 km to the west of Zunil) have a cumulative power law exponent near -5, whereas secondaries between the major rays have an exponent near -3.4. We modeled the sizevelocity relationship for Zunil's ejected fragments; it is consistent with the predictions of Melosh (1984) for spallation of a strong surface layer and demonstrates that the inverse size-velocity correlation continues up to at least 2 km/s.