Cratering on Nix and HydraCratering on Nix and HydraCratering on Nix and HydraCratering on Nix and Hydra
William Bottke (SwRI) William Bottke (SwRI) William Bottke (SwRI) William Bottke (SwRI)
CratersCraters
Craters are found on nearly Craters are found on nearly every solid body in the solar every solid body in the solar system. system.
If properly interpreted, If properly interpreted, craters can help us craters can help us understand how surfaces of understand how surfaces of solar system bodies have solar system bodies have evolved over the last 4.5 Gy.evolved over the last 4.5 Gy.
Impacting PopulationsImpacting Populations
Craters can also tell you Craters can also tell you about the evolution of the about the evolution of the impacting populations.impacting populations.– Ancient populations are more Ancient populations are more
massive; they may have massive; they may have experienced more collisional experienced more collisional evolution.evolution.
– Younger populations have less Younger populations have less mass; their size distributions may mass; their size distributions may not have changed for some time.not have changed for some time.
Summary of Some Key ParametersSummary of Some Key Parameters
Pluto (Pluto (DD = 2,300 km) and Charon ( = 2,300 km) and Charon (DD = 1,200 km) are very big bodies. = 1,200 km) are very big bodies. They are very hard to destroy by impact.They are very hard to destroy by impact.
Nix (Nix (DD = 88 km) and Hydra ( = 88 km) and Hydra (DD = 72 km): = 72 km):
a)a) Impacts may disrupt the bodies or hit them hard enough to Impacts may disrupt the bodies or hit them hard enough to “jolt” them into new orbits. “jolt” them into new orbits.
b)b) They reside close to dynamical resonances; orbital periods of They reside close to dynamical resonances; orbital periods of Hydra, Nix, and Charon are ratios of 6.064 ± 0.006 ; 3.991 ± 0.007.Hydra, Nix, and Charon are ratios of 6.064 ± 0.006 ; 3.991 ± 0.007.
c)c) Have very low eccentricities (e < 0.02) and inclinations (< 0.02 deg Have very low eccentricities (e < 0.02) and inclinations (< 0.02 deg from Charon’s inclination). from Charon’s inclination).
Do (b) & (c) have implications for (a)? Do (b) & (c) have implications for (a)?
Craters on Nix and HydraCraters 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.Nature of Pluto-system forming event.
– Evolution of debris in Pluto system.Evolution of debris in Pluto system.
– Timing of Pluto-system formation eventTiming of Pluto-system formation event
– Nature and evolution of “outside” impacting populations over timeNature and evolution of “outside” impacting populations over time
Formation of Charon (and Nix/Hydra)Formation of Charon (and Nix/Hydra)
Charon (and Nix/Hydra) presumably made by a giant Charon (and Nix/Hydra) presumably made by a giant impact. Impact velocities need to be < 0.9 km/s.impact. Impact velocities need to be < 0.9 km/s.
Canup (2005)Canup (2005)
Formation of Nix and HydraFormation of Nix and Hydra
In many ways, the Pluto-Charon event is equivalent to a In many ways, the Pluto-Charon event is equivalent to a giant “cratering event”.giant “cratering event”.
Nix and Hydra probably formed from small eject fragments Nix and Hydra probably formed from small eject fragments delivered to region outside the orbit of Charon. delivered to region outside the orbit of Charon.
SFD Morphologies
Cratering EventCratering Event Supercatastrophic EventSupercatastrophic Event
“Concave” SFD
Durda et al. (2006)
SPH results suggest that cratering events produce very SPH results suggest that cratering events produce very steep size distributions. The largest fragments are 10-50 steep size distributions. The largest fragments are 10-50 times smaller than parent body.times smaller than parent body.
Craters on Nix and HydraCraters 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. Nature of Pluto-system forming event.
– Evolution of debris in Pluto system.Evolution of debris in Pluto system.
– Timing of Pluto-system formation eventTiming of Pluto-system formation event
– Nature and evolution of “outside” impacting populations over timeNature and evolution of “outside” impacting populations over time
Collisional Evolution of Pluto Debris DiskCollisional Evolution of Pluto Debris Disk
At distances of > 20,000 km from Pluto, fragments with At distances of > 20,000 km from Pluto, fragments with moderate eccentricities and low inclinations may take moderate eccentricities and low inclinations may take 101044 - 10 - 1055 years to hit one another. 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 The collisional physics in this velocity regime are very strange when compared to “typical” collisions among strange when compared to “typical” collisions among asteroids or comets at > 1 km/s.asteroids or comets at > 1 km/s.
Collisional Evolution of Pluto Debris DiskCollisional Evolution of Pluto Debris Disk
The energy coupling between bodies is surprisingly high when impact The energy coupling between bodies is surprisingly high when impact velocities are only several tens of m/s.velocities are only several tens of m/s.
– The debris in the Pluto/Charon system may evolve by a mixture of accretion The debris in the Pluto/Charon system may evolve by a mixture of accretion on big bodies and substantial collisional grinding among small ones. on big bodies and substantial collisional grinding among small ones.
– Only largest objects (Nix/Hydra) may survive bombardment.Only largest objects (Nix/Hydra) may survive bombardment.
Leinhardt and Stewart (2007)Leinhardt and Stewart (2007)
Collisional Evolution of Pluto Debris DiskCollisional Evolution of Pluto Debris Disk
Craters produced in the “strength regime” at ~50 m/s on a Craters produced in the “strength regime” at ~50 m/s on a small icy target have not been investigated. small icy target have not been investigated.
Nix and Hydra may help us understand a whole new realm Nix and Hydra may help us understand a whole new realm of cratering physics!of cratering physics!
Leinhardt and Stewart (2007)Leinhardt and Stewart (2007)
Craters on Nix and HydraCraters 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. Nature of Pluto-system forming event.
– Evolution of debris in Pluto system.Evolution of debris in Pluto system.
– Timing of Pluto-system formation eventTiming of Pluto-system formation event
– Nature and evolution of “outside” impacting populations over timeNature and evolution of “outside” impacting populations over time
Early Evolution in the Nice Model ScenarioEarly Evolution in the Nice Model Scenario
The primordial disk of comets is dynamically excited by The primordial disk of comets is dynamically excited by planetary perturbations and embedded Plutos.planetary perturbations and embedded Plutos.
Gomes et al. (2005)Gomes et al. (2005)
Early Evolution in the Nice Model ScenarioEarly Evolution in the Nice Model Scenario
The inner disk gets more excited than outer disk, with The inner disk gets more excited than outer disk, with impact velocities of 0.83 km/s vs. 0.25 km/s, respectively. impact velocities of 0.83 km/s vs. 0.25 km/s, respectively.
Gomes et al. (2005)Gomes et al. (2005)
Inner Disk
Outer Disk
Modeling the Primordial DiskModeling the Primordial Disk
Primordial disk was Primordial disk was set to > 35 set to > 35 MMEarthEarth
Assume ~1000 Plutos Assume ~1000 Plutos based on Nice model based on Nice model depletion factors and depletion factors and
shallow SFD like “hot” shallow SFD like “hot” KB populationKB population
Assume outer disk has Assume outer disk has same shape as Trojans + same shape as Trojans +
“Cold” Classical KB “Cold” Classical KB
Collisional Disruption of Captured CometsCollisional Disruption of Captured Comets
Impact into “Rubble-Pile” Object Impact into “Rubble-Pile” Object
Durda, Bottke et al. (2006)Durda, Bottke et al. (2006)
Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)
We need to model how We need to model how KBOs disrupt. KBOs disrupt.
–Comets are likely weak. Comets are likely weak.
–No adequate collisonal No adequate collisonal disruption models yet disruption models yet exist that account for all exist that account for all relevant physics. relevant physics.
Collisional Disruption of Captured CometsCollisional Disruption of Captured Comets
We tested disruption We tested disruption laws between strong laws between strong and weak ice. and weak ice.
Weak iceWeak ice
Strong iceStrong iceAsteroidsAsteroids
Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)
Collisional Disruption of Captured CometsCollisional Disruption of Captured Comets
We tested disruption We tested disruption laws between strong laws between strong and weak ice. and weak ice.
Weak iceWeak ice
Strong iceStrong iceAsteroidsAsteroids
Model Model CometsComets
Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)Reference for weak comets: Leinhardt and Stewart-Mukhopadhyay (2008)
Size Distributions of Primordial DiskSize Distributions of Primordial Disk
Nix and Hydra-sized Nix and Hydra-sized objects decrease objects decrease over 600 Myover 600 My
– 15% of inner disk 15% of inner disk objects make it 600 Myobjects make it 600 My
– 30% of outer disk 30% of outer disk objects make it 600 objects make it 600 My. My.
Size Distributions of Primordial DiskSize Distributions of Primordial Disk
Extreme case with Extreme case with no D < 80 km no D < 80 km objects. objects.
Size Distributions of Primordial DiskSize Distributions of Primordial Disk
Extreme case with Extreme case with no D < 80 km no D < 80 km objects. objects.
Nix and Hydra-sized Nix and Hydra-sized objects still disrupt, objects still disrupt, but more slowly in but more slowly in outer disk.outer disk.
– 15% of inner disk 15% of inner disk objects make it to endobjects make it to end
– 40% of outer disk 40% of outer disk objects make it to end. objects make it to end.
Problems Making Pluto System Problems Making Pluto System Early!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 We assume that most Pluto-size bodies experience a Pluto-system class impact eventclass impact event
– Pluto system formed by oblique impact from object 30-50% mass of Pluto system formed by oblique impact from object 30-50% mass of Pluto at V < 0.8 km/s (Canup 2005).Pluto at V < 0.8 km/s (Canup 2005).
– But…Nix and Hydra each have ~15% probability of survival against But…Nix and Hydra each have ~15% probability of survival against collisions over 600 My.collisions over 600 My.
T ~ Few My after CAIsT ~ Few My after CAIs T ~ Few My after T ~ Few My after CAIsCAIs
T ~ 600 My after CAIsT ~ 600 My after CAIs
Survival Probability of Nix/Hydra is only ~2%Survival Probability of Nix/Hydra is only ~2%
Impact “Jolting” of Nix and HydraImpact “Jolting” of Nix and Hydra
Key factorKey factor: Nix/Hydra have orbital velocities around Pluto : Nix/Hydra have orbital velocities around Pluto of ~ 100 m/s. Impact velocity with Nix/Hydra is ~ 800 m/s!of ~ 100 m/s. Impact velocity with Nix/Hydra is ~ 800 m/s!
Projectile ~0.01 × mass of Nix/Hydra leads to Projectile ~0.01 × mass of Nix/Hydra leads to ΔΔaa / / aa ~ 0.1. ~ 0.1.
Nix/Hydra can only tolerate Nix/Hydra can only tolerate ΔΔaa / / aa < 0.01 if they are to stay < 0.01 if they are to stay close to 4:1 and 6:1 resonances with Charon.close to 4:1 and 6:1 resonances with Charon.
PlutoPluto
Nix (100 m/s)Nix (100 m/s)
Impactor Impactor (800 m/s)(800 m/s)
PlutoPluto
NixNixNixNix
PlutoPluto
(1)(1) (2)(2) (3)(3)
Problems Making Pluto System Problems Making Pluto System Late!Late!
Assume the Pluto system formed in the Assume the Pluto system formed in the excited inner disk over first 600 My:excited inner disk over first 600 My:
– Roughly ~1000 Plutos and standard inner disk Roughly ~1000 Plutos and standard inner disk collision probabilities. collision probabilities.
– If Pluto system formation event happens too If Pluto system formation event happens too early, Nix and Hydra can be destroyed or jolted. early, Nix and Hydra can be destroyed or jolted.
–Only a small fraction (< few %) of the Plutos Only a small fraction (< few %) of the Plutos experience Pluto system formation event over experience Pluto system formation event over 600 My. 600 My.
–Getting our Pluto system appears to be an Getting our Pluto system appears to be an amazing fluke!amazing fluke!
T ~ 100-600 My T ~ 100-600 My after CAIsafter CAIs
Late Pluto System Impact Events Unlikely!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 Hypothesis For Creating the Pluto System
PlutoPluto
CompanionCompanion
(1) Pluto Forms in Distant Binary (1) Pluto Forms in Distant Binary with Pluto-Sized Companion with Pluto-Sized Companion
PlutoPluto
CompanionCompanion
(2) After 600 My, Distant Encounter (2) After 600 My, Distant Encounter with Uranus/Neptune in “Nice Model”with Uranus/Neptune in “Nice Model”
(3) Companion Put Into (3) Companion Put Into Kozai Resonance; Kozai Resonance;
Eventually Hits Pluto.Eventually Hits Pluto.
(4) Pluto System Created (4) Pluto System Created When Primordial Disk When Primordial Disk
Depleted!Depleted!
Largest Craters Expected on Nix and HydraLargest Craters Expected on Nix and Hydra
Assuming scaling law with crater/projectile diameter is ~10, the objects Assuming scaling law with crater/projectile diameter is ~10, the objects making craters on Nix/Hydra are making craters on Nix/Hydra are DD < 3-4 km. < 3-4 km.
Burchell and Leliwa-Kopystynski (2009)
Rocky Bodies
Icy Bodies
Nix and Hydra
Crater Populations on Nix and HydraCrater Populations on Nix and Hydra
If largest crater on If largest crater on Nix/Hydra is < 40 km, Nix/Hydra is < 40 km, the crater SFD should the crater SFD should resemble disk objects resemble disk objects with with DD < 4 km. < 4 km.
– A partial “wave” seen A partial “wave” seen near 2-3 km. near 2-3 km.
– Shallow SFD for Shallow SFD for DD < 4 < 4 km.km.
Crater Populations on Nix and HydraCrater Populations on Nix and Hydra
The Nice model erases The Nice model erases the primordial disk and the primordial disk and send comets into send comets into current reservoirs.current reservoirs.
Depletion factor of Depletion factor of ~1000.~1000.
Not enough mass to Not enough mass to change SFD, so we change SFD, so we expect same SFD now expect same SFD now as we had during first as we had during first 600 My for 600 My for DD < 4 km. < 4 km.
Nix/Hydra Crater Production PopulationNix/Hydra Crater Production Populationand Cratering Ratesand Cratering Rates
Present-day impact population based on fresh craters/observations. For Present-day impact population based on fresh craters/observations. For Nix and Hydra, use Nix and Hydra, use DDprojectileprojectile < 4 km and < 4 km and DDcratercrater < 40 km. < 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)Zahnle et al. (2003)
It is difficult to construct a scenario where the It is difficult to construct a scenario where the Pluto system was created over first ~600 My.Pluto system was created over first ~600 My.
If it was formed during the Nice model via a If it was formed during the Nice model via a binary impact, several constraints make sense:binary impact, several constraints make sense:– Nix and Hydra look surprisingly undisturbed.Nix and Hydra look surprisingly undisturbed.–We can explain how the Pluto impact occurred in the first place.We can explain how the Pluto impact occurred in the first place.
Early Nix/Hydra craters may come from “system Early Nix/Hydra craters may come from “system debris” striking at ~50 m/s.debris” striking at ~50 m/s.
Other Nix/Hydra craters come from Other Nix/Hydra craters come from DD < 4 km < 4 km comets from Kuiper belt/scattered disk.comets from Kuiper belt/scattered disk.
ConclusionsConclusions