Crater Density Predictions for New Horizons Flyby Target 2014 MU69

Abstract

In preparation for the Jan 1/2019 encounter between the New Horizons spacecraft and the Kuiper Belt object 2014 MU69, we provide estimates of the expected impact crater surface density on the Kuiper Belt object. Using the observed crater fields on Charon and Pluto down to the resolution limit of the 2015 New Horizons flyby of those bodies and estimates of the orbital distribution of the crater-forming projectiles, we calculate the number of craters per unit area formed as a function of the time a surface on 2014 MU69 has been exposed to bombardment. We find that if the shallow cratersize distribution from 1-15 km exhibited on Pluto and Charon is indeed due to the sizes of Kuiper Belt projectiles, 2014 MU69 should exhibit a surface that is only lightly cratered below 1 km scale, despite being bombarded for ∼ 4 billion years. Its surface should therefore be more clearly indicative of its accretionary environment. In addition, this object may the first observed for which the majority of the bombardment is from exogenic projectiles moving less than or near the speed of sound in the target materials, implying morphologies more akin to secondary craters elsewhere in the Solar System.Lastly, if the shallow Kuiper Belt size distribution implied from the Pluto and Charon imaging is confirmed at MU69, then we conclude that this size distribution is a preserved relic of its state ≃4.5 Gyr ago and provides a direct constraint on the planetesimal formation process itself.

The Contribution of Dwarf Planets to the Origin of Jupiter Family Comets

“…We found that the large objects have a significant effect on long-term time scales, leading to a modest enhancement of the injection rate of new comets into the inner solar system. The reservoir requirements implied by our injection rate remain in broad agreement with recent reservoir estimations done from observations of the trans-Neptunian regions, as well as with those estimated from the analysis of cratering records on Pluto and 2014 MU 69 (Greenstreet et al 2015(Greenstreet et al , 2019Singer et al 2019).…”

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

“…Crater morphology varies with the impactor and target characteristics. The low and high velocity tails of the expected impact velocity distribution for Arrokoth reach down to a few m s -1 and as high as a few km s -1 , but the mode, ~300 m s -1 (5), is slow compared to primary cratering velocities on the surfaces of both icy and rocky bodies closer to the Sun, and is more typical of secondary cratering velocities on those bodies (85,86). These impacts often form craters with similar morphological characteristics to primary craters, although secondary craters are often shallower than the same size primary impact, and may be elongated in the direction radial to the primary crater.…”

“…The interior shape of a crater is expected to be bowl-like with a depth/diameter ratio typically not higher than ~0.2 (31). The predicted modal impact velocity onto Arrokoth is ~300 m s -1 (5), which is sufficient to form craters with typical morphologies (see Supplementary Text). In the case of Arrokoth, the lowest velocity impacts (≲ 20 m s -1 ) are unlikely to leave conspicuous depressions, but these impacts are expected to be a small fraction of the total (5).…”

“…The predicted modal impact velocity onto Arrokoth is ~300 m s -1 (5), which is sufficient to form craters with typical morphologies (see Supplementary Text). In the case of Arrokoth, the lowest velocity impacts (≲ 20 m s -1 ) are unlikely to leave conspicuous depressions, but these impacts are expected to be a small fraction of the total (5). The formation of a crater on a slope or modification by later geologic processes (such as mass wasting or a subsequent fault near the crater) may also alter the crater's appearance.…”

“…Parenthetical numbers are the total number of craters/pits in each category. The Arrokoth crater data are compared to crater densities on Charon's Vulcan Planitia (39) without diameter adjustments for gravity or velocity scaling, and to predictions based on an impactor flux model for six different ages of surfaces on Arrokoth and gravity regime scaling (blue curves with different line styles, (5)). The LL_Term and LL_Bright distributions are offset horizontally by ± 9% for clarity.…”