Small Impact Craters on Triton: Evidence for a Turn-up in the Size-frequency Distribution of Small (sub-km) KBOs, and Arguments Against a Planetocentric Origin

Details Small Impact Craters on Triton: Evidence for a Turn-up in the Size-frequency Distribution of Small (sub-km) KBOs, and Arguments Against a Planetocentric Origin Small Impact Craters on Triton: Evidence for a Turn-up in the Size-frequency Distribution of Small (sub-km) KBOs, and Arguments Against a Planetocentric Origin

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2009m&link_type=abstract

American Astronomical Society, DPS meeting #42, #20.09; Bulletin of the American Astronomical Society, Vol. 42, p.984

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We report crater counts on the 10-frame, highest-resolution (350-420 m/px) Voyager 2 Triton mosaic (kindly MTF-sharpened and photometrically corrected by P.M. Schenk). Despite variable degrees of smear, crater counting on portions of these images is straightforward, and on the smoothest, presumably cryovolcanic plains, abundant craters are seen down to the resolution limit (craters 1 km in diameter). These counts complement those by Schenk & Zahnle (2007) on their 1.65 km/px global basemap. We find a fairly steep cumulative crater distribution (-3.2 slope), which implies a differential impactor index q ≈ 3.5. This is consistent with the classic Dohnanyi slope for a collisionally evolved population; more importantly, the transition from q = 2.8 (Schenk & Zahnle 2007) to 3.5 (or higher on Cipango Planum, which while less cratered, offers the "cleanest” counting surface and the sharpest MTF), as crater diameters drop below 10 km (and thus impactor diameters drop below 1 km), may be a signature of the strength-gravity transition in terms of disruption energy for KBOs (O'Brien & Greenberg 2003). These craters cannot be secondaries, as they are larger than any plausible "crossover diameter” for Triton's young surface. But do Triton's craters represent the impacts of small KBOs? Schenk & Zahnle (and others) have argued for a prograde planetocentric origin, based mostly on the geographic concentration of Triton's craters on its leading hemisphere. We argue that the trailing hemisphere cantaloupe terrain, a unique and topographically complex terrain of unknown compositional and crater retention properties, should be sparsely cratered. Conversely, their proposal for inner satellite sesquinaries would require, e.g., a fortuitous, >60-km-diameter impact crater on Proteus, which would launch ice blocks to speeds ≥1.6 km/s in order to reach Triton. Scaling from Pwyll secondaries on Europa (a best case analogy) indicates the ice blocks will not be big enough.

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