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Cratering on Nix and Hydra

Cratering on Nix and Hydra. William Bottke ( SwRI ). Craters. Craters are found on nearly every solid body in the solar system. If properly interpreted, craters can help us understand how surfaces of solar system bodies have evolved over the last 4.5 Gy. Impacting Populations.

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Cratering on Nix and Hydra

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  1. Cratering on Nix and Hydra William Bottke (SwRI)

  2. Craters • Craters are found on nearly every solid body in the solar system. • If properly interpreted, craters can help us understand how surfaces of solar system bodies have evolved over the last 4.5 Gy.

  3. Impacting Populations • Craters can also tell you about the evolution of the impacting populations. • Ancient populations are more massive; they may have experienced more collisional evolution. • Younger populations have less mass; their size distributions may not have changed for some time.

  4. Summary of Some Key Parameters • Pluto (D = 2,300 km) and Charon (D = 1,200 km) are very big bodies. They are very hard to destroy by impact. • Nix (D = 88 km) and Hydra (D = 72 km): • Impacts may disrupt the bodies or hit them hard enough to “jolt” them into new orbits. • They reside close to dynamical resonances; orbital periods of Hydra, Nix, and Charon are ratios of 6.064 ± 0.006 ; 3.991 ± 0.007. • Have very low eccentricities (e < 0.02) and inclinations (< 0.02 deg from Charon’s inclination). Do (b) & (c) have implications for (a)?

  5. Craters on Nix and Hydra • To discuss craters on Nix and Hydra, we need to know: • Nature of Pluto-system forming event. • Evolution of debris in Pluto system. • Timing of Pluto-system formation event • Nature and evolution of “outside” impacting populations over time

  6. Formation of Charon (and Nix/Hydra) • Charon(and Nix/Hydra) presumably made by a giant impact. Impact velocities need to be < 0.9 km/s. Canup (2005)

  7. Formation of Nix and Hydra • In many ways, the Pluto-Charon event is equivalent to a giant “cratering event”. • Nix and Hydra probably formed from small eject fragments delivered to region outside the orbit of Charon.

  8. SFD Morphologies Cratering Event Supercatastrophic Event “Concave” SFD Durda et al. (2006) • SPH results suggest that cratering events produce very steep size distributions. The largest fragments are 10-50 times smaller than parent body.

  9. Craters on Nix and Hydra • To discuss craters on Nix and Hydra, we need to know: • Nature of Pluto-system forming event. • Evolution of debris in Pluto system. • Timing of Pluto-system formation event • Nature and evolution of “outside” impacting populations over time

  10. Collisional Evolution of Pluto Debris Disk • At distances of > 20,000 km from Pluto, fragments with moderate eccentricities and low inclinations may take 104 - 105 years to hit one another. • Impact velocities are on the order of ~50 m/s. • The collisional physics in this velocity regime are very strange when compared to “typical” collisions among asteroids or comets at > 1 km/s.

  11. Collisional Evolution of Pluto Debris Disk Leinhardt and Stewart (2007) • The energy coupling between bodies is surprisingly high when impact velocities are only several tens of m/s. • The debris in the Pluto/Charon system may evolve by a mixture of accretion on big bodies and substantial collisional grinding among small ones. • Only largest objects (Nix/Hydra) may survive bombardment.

  12. Collisional Evolution of Pluto Debris Disk Leinhardt and Stewart (2007) • Craters produced in the “strength regime” at ~50 m/s on a small icy target have not been investigated. • Nix and Hydra may help us understand a whole new realm of cratering physics!

  13. Craters on Nix and Hydra • To discuss craters on Nix and Hydra, we need to know: • Nature of Pluto-system forming event. • Evolution of debris in Pluto system. • Timing of Pluto-system formation event • Nature and evolution of “outside” impacting populations over time

  14. Early Evolution in the Nice Model Scenario • The primordial disk of comets is dynamically excited by planetary perturbations and embedded Plutos. Gomes et al. (2005)

  15. Early Evolution in the Nice Model Scenario Inner Disk Outer Disk • The inner disk gets more excited than outer disk, with impact velocities of 0.83 km/s vs. 0.25 km/s, respectively. Gomes et al. (2005)

  16. Modeling the Primordial Disk • Primordial disk was set to > 35 MEarth Assume ~1000 Plutos based on Nice model depletion factors and shallow SFD like “hot” KB population Assume outer disk has same shape as Trojans + “Cold” Classical KB

  17. Collisional Disruption of Captured Comets Impact into “Rubble-Pile” Object • We need to model how KBOs disrupt. • Comets are likely weak. • No adequate collisonal disruption models yet exist that account for all relevant physics. Durda, Bottke et al. (2006) Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)

  18. Collisional Disruption of Captured Comets • We tested disruption laws between strong and weak ice. Asteroids Strong ice Weak ice Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)

  19. Collisional Disruption of Captured Comets • We tested disruption laws between strong and weak ice. Asteroids Strong ice Weak ice Model Comets Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)

  20. Size Distributions of Primordial Disk • Nix and Hydra-sized objects decrease over 600 My • 15% of inner disk objects make it 600 My • 30% of outer disk objects make it 600 My.

  21. Size Distributions of Primordial Disk • Extreme case with no D < 80 km objects.

  22. Size Distributions of Primordial Disk • Extreme case with no D < 80 km objects. • Nix and Hydra-sized objects still disrupt, but more slowly in outer disk. • 15% of inner disk objects make it to end • 40% of outer disk objects make it to end.

  23. Problems Making Pluto System Early! T ~ Few My after CAIs T ~ 600 My after CAIs T ~ Few My after CAIs • If Pluto system formed very early in inner primordial disk: • We assume that most Pluto-size bodies experience a Pluto-system class impact event • Pluto system formed by oblique impact from object 30-50% mass of Pluto at V < 0.8 km/s (Canup 2005). • But…Nix and Hydra each have ~15% probability of survival against collisions over 600 My. Survival Probability of Nix/Hydra is only ~2%

  24. Impact “Jolting” of Nix and Hydra Pluto Pluto Pluto • Key factor: Nix/Hydra have orbital velocities around Pluto of ~ 100 m/s. Impact velocity with Nix/Hydra is ~ 800 m/s! • Projectile ~0.01 × mass of Nix/Hydra leads to Δa / a ~ 0.1. • Nix/Hydra can only tolerate Δa / a < 0.01 if they are to stay close to 4:1 and 6:1 resonances with Charon. Nix Nix (100 m/s) Nix Impactor (800 m/s) (1) (2) (3)

  25. Problems Making Pluto System Late! • Assume the Pluto system formed in the excited inner disk over first 600 My: • Roughly ~1000 Plutos and standard inner disk collision probabilities. • If Pluto system formation event happens too early, Nix and Hydra can be destroyed or jolted. • Only a small fraction (< few %) of the Plutos experience Pluto system formation event over 600 My. • Getting our Pluto system appears to be an amazing fluke! T ~ 100-600 My after CAIs Late Pluto System Impact Events Unlikely!

  26. We Are Missing Something… • If the Pluto system cannot be made early or late, we must be making an bad assumption somewhere. • New models of planetesimal formation indicate bodies can be “born big”. Perhaps this provides a way out…

  27. Hypothesis For Creating the Pluto System Companion Pluto (3) Companion Put Into Kozai Resonance; Eventually Hits Pluto. (1) Pluto Forms in Distant Binary with Pluto-Sized Companion Companion Pluto (4) Pluto System Created When Primordial Disk Depleted! (2) After 600 My, Distant Encounter with Uranus/Neptune in “Nice Model”

  28. Largest Craters Expected on Nix and Hydra Burchell and Leliwa-Kopystynski (2009) • Assuming scaling law with crater/projectile diameter is ~10, the objects making craters on Nix/Hydra are D < 3-4 km. Nix and Hydra Rocky Bodies Icy Bodies

  29. Crater Populations on Nix and Hydra • If largest crater on Nix/Hydra is < 40 km, the crater SFD should resemble disk objects with D < 4 km. • A partial “wave” seen near 2-3 km. • Shallow SFD for D < 4 km.

  30. Crater Populations on Nix and Hydra • The Nice model erases the primordial disk and send comets into current reservoirs. • Depletion factor of ~1000. • Not enough mass to change SFD, so we expect same SFD now as we had during first 600 My for D < 4 km.

  31. Nix/Hydra Crater Production Populationand Cratering Rates • Present-day impact population based on fresh craters/observations. For Nix and Hydra, use Dprojectile < 4 km and Dcrater < 40 km. • These estimates probably hold for the last ~4 Gy or so. Zahnle et al. (2003)

  32. Conclusions • It is difficult to construct a scenario where the Pluto system was created over first ~600 My. • If it was formed during the Nice model via a binary impact, several constraints make sense: • Nix and Hydra look surprisingly undisturbed. • We can explain how the Pluto impact occurred in the first place. • Early Nix/Hydra craters may come from “system debris” striking at ~50 m/s. • Other Nix/Hydra craters come from D < 4 km comets from Kuiper belt/scattered disk.

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