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Compaction Craters on (486958) Arrokoth

Presentation #111.03 in the session “Arrokoth in Context”.

Published onOct 03, 2021
Compaction Craters on (486958) Arrokoth

Compaction cratering requires both high porosity (≳50%) and, depending on scale, low enough crush strength. Evidence from Arrokoth (distribution of surface slopes, neck strength limits, bilobate shape energy) and from cometary analogues (SL9 tidal breakup, 67P radio tracking) implies a density in the range 150-600 kg/m3. For a Pluto- or 67P-like composition, this implies porosities in the 70-85% range. Laboratory estimates of crush strengths at these high porosities, and for icy compositions, suggest values of order 100 kPa or less. If so, we expect the largest crater on Arrokoth, “Maryland,” to have formed largely by crushing of pore space and material displacement, without substantial excavation of material. This is consistent with Maryland’s stereo-revealed conical shape and lack of raised rim and ejecta blanket. In contrast, photoclinometric topographic profiles across craters an order of magnitude smaller on Arrokoth appear to possess such rims, suggesting a crush strength >10 kPa. High porosity reduces cratering efficiency in the gravity regime while compaction moves it towards strength scaling (controlled by the crush strength). Compaction also guarantees that most of a given impactor’s kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. For typical cold classical encounter velocities, impactor and near-field target temperatures should reach ~100 K, warm enough to mobilize hypervolatile ices, but little else (whereas faster, hot classical or scattered disk impacts can melt water ice). Monte Carlo simulations of Maryland-forming conditions indicate that, while Arrokoth’s small lobe (SL) is protected from catastrophic disruption by crush-up, the momentum imparted to SL is sufficient to break the relatively narrow neck between the 2 lobes, for typical cometary compressive and shear strengths (and assuming the Maryland impact postdates lobe merger of course). From geometry it is more likely that the SL was driven obliquely into the large lobe (LL), rather than away, with shear and compressive dissipation at the disrupted neck limiting the relative motion of SL with respect to LL to under 1 km. Unusual strength properties are not required to preserve Arrokoth’s bilobate configuration.


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