Cratering on Nix and Hydra William Bottke (SwRI)

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

Impacting Populations Craters can also tell you about the evolution of the 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.

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. 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): Nix (D = 88 km) and Hydra (D = 72 km): a) Impacts may disrupt the bodies or hit them hard enough to “jolt” them into new orbits. b) They reside close to dynamical resonances; orbital periods of Hydra, Nix, and Charon are ratios of ± ; ± c) Have very low eccentricities (e < 0.02) and inclinations (< 0.02 deg from Charon’s inclination). Do (b) & (c) have implications for (a)?

Craters on Nix and Hydra To discuss craters on Nix and Hydra, we need to know: 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

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

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

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 times smaller than parent body. SPH results suggest that cratering events produce very steep size distributions. The largest fragments are times smaller than parent body.

Craters on Nix and Hydra To discuss craters on Nix and Hydra, we need to know: 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

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

Collisional Evolution of Pluto Debris Disk The energy coupling between bodies is surprisingly high when impact velocities are only several tens of m/s. 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. Leinhardt and Stewart (2007)

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

Craters on Nix and Hydra To discuss craters on Nix and Hydra, we need to know: 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

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

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

Modeling the Primordial Disk Primordial disk was set to > 35 M Earth Primordial disk was set to > 35 M Earth 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

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

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

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

Size Distributions of Primordial Disk Nix and Hydra-sized objects decrease over 600 My 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.

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

Size Distributions of Primordial Disk Extreme case with no D < 80 km objects. Extreme case with no D < 80 km objects. Nix and Hydra-sized objects still disrupt, but more slowly in outer disk. 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.

Problems Making Pluto System Early! If Pluto system formed very early in inner primordial disk: 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. T ~ Few My after CAIs T ~ 600 My after CAIs Survival Probability of Nix/Hydra is only ~2%

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

Problems Making Pluto System Late! Assume the Pluto system formed in the excited inner disk over first 600 My: 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 ~ My after CAIs Late Pluto System Impact Events Unlikely!

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…

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

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

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. 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.

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

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

It is difficult to construct a scenario where the Pluto system was created over first ~600 My. 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: 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. 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. Other Nix/Hydra craters come from D < 4 km comets from Kuiper belt/scattered disk.Conclusions