Recent work has suggested that crater chains exist on Earth: (a) Eight circular depressions (3-17 km wide) distributed along a 700 km line across Kansas, Missouri, and Illinois may comprise part of a crater chain (Rampino and Volk, 1996, Geo. Res. Lett. 23, 49), and (b) Radar imagery of the 17 km diameter, 360 Myr old Aorounga impact crater in northern Chad suggests a ~ 10 km wide circular structure adjacent to the Aorounga crater and a second, less distinct feature somewhat displaced from the line defined by the other two (Ocampo and Pope, 1996, LPSC 27, 977). If these chains are real, one possible mechanism for their formation is that they were formed by weak asteroids or comets tidally stretched apart by the Moon (analogous to comet Shoemaker-Levy 9 (SL9) at Jupiter) with the resulting fragment trains proceeding directly to strike the Earth. To test this, we modeled close encounters with the Earth and Moon of strengthless particulate bodies held together by self-gravity. Our progenitor is an elongated rubble-pile with a bulk density of 2 g/cc and axis ratios of 2.8:1.7:1.5 km. We vary encounter parameters such as the close-approach distance q, the encounter velocity v_infty , and the rotation period P of the progenitor. The effects of other parameters, such as spin and long axis orientation at closest approach, are also investigated. Our results show that low values of q, v_infty , and P increase the likelihood of tidal disruption (e.g. a progenitor encountering Earth with q < 2 R_oplus , v_infty < 10 km/s, P < 6 hours, a prograde spin, and a favorable long axis orientation produces a SL9-type catastrophic disruption, with > 50% of the mass evolving into a ``string of pearls''; Note that non-catastrophic disruptions, which are more common, cannot produce crater chains). Tidal disruption events occur more frequently near the Earth than the Moon, a consequence of the Earth's comparatively larger size and density. We used these results to estimate the crater chain production frequency on the Earth and Moon. We found that the rate of formation of lunar crater chains is several times the corresponding terrestrial rate. The number of known lunar crater chains ( ~ 1) together with the relatively young age of the Earth's surface suggest that terrestrial crater chains, if real, form by other processes.
AAS/Division for Planetary Sciences Meeting Abstracts #28
- Pub Date:
- September 1996