2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 117-5
Presentation Time: 9:00 AM-6:00 PM

TYCHO SECONDARY CRATERS IDENTIFIED VIA EJECTED BOULDERS

BART, Gwendolyn D., NICKERSON, Ryan, and JOHNSON, Andrew, Dept. of Physics, Univ. of Idaho, Campus Box 440903, Moscow, ID 83844-0903, gbarnes@uidaho.edu

We are conducting a survey of distant secondary craters of the lunar impact crater Tycho. We compare the sizes of the boulders ejected from these distant secondary craters with the boulders ejected from similar lunar primary craters. Previous work (Bart and Melosh, 2007, Geophys. Res. Lett.) has shown that secondary craters produce larger ejected boulders than primary craters do. Hence, boulder size might provide a method for distinguishing very distant secondary craters from primary craters.

This investigation will reveal the influence of distant secondary craters on the small crater population and the primary crater production function. Some small craters are secondary craters, meaning that the impactor was launched as ejecta from another crater, rather than falling from interplanetary space. Secondary craters near their primary crater were formed by impact velocities <1 km/s and hence can be distinguished by a characteristic morphology, including irregular shape, shallow depth, location in crater chains, and a herringbone ejecta pattern. Distant secondary craters, however, are launched at higher velocities, and thus are morphologically similar to small primary craters, making them more difficult to identify.

The source (primary or secondary) of a crater is important for surface age determination. Crater distributions are used to determine the age of a surface; the distribution for a surface is shown graphically as a Size-Frequency Distribution. An older surface plots higher (more craters) and a younger surface plots lower (fewer craters). This method for surface age determination assumes that crater production is random in time and space, and has been shown to be reliable for large craters over broad surface areas, where those assumptions hold true. But because distant secondary craters are a result of rare large impacts, their production is neither random in time (only being produced during a rare large impact) nor random in space (being distributed as ejecta from the large impact.) This distribution violates the assumptions of the crater dating method, and would preclude small craters from being a reliable chronometer.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 117--Booth# 335
Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies (Posters)
Minneapolis Convention Center: Hall C
9:00 AM-6:00 PM, Monday, 10 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 305

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