APreliminaryStudyofGiant
ImpactsonUranusandNeptune
C.Reinhardt(UZH),R.Helled (UZH)and J.Stadel(UZH)
Conferenceon«Impactsinplanetary systems»(LundMay2017)
Neptune
Uranus
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Voyager2(NASA/JPL)
Semi-majoraxis:19.1AU
MU=14.5𝑀⊕
Rotationperiod:17.24h
𝜌Uranus=1.27g/cm3
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Voyager2(NASA/JPL)
Semi-majoraxis:30.1AU
MN=17.1𝑀⊕
Rotationperiod:16.11h
𝜌Neptune=1.64g/cm3
The«Twin»Planets?
• Compositionfrom interior models:
• Rock(small core)
• Volatiles(«Ice»mantle)
• H/Heenvelope (~2𝑀⊕ )
e.g.,Nettelmann+(2013)
• Observationaldifferences:
• Uranus(andits satellites!)have alargetilt (~97°)
• Alargedifference inthe observed heat flux
• While Uranushas avery regular set of satellites,
Neptune’s two moons are onirregular orbits
Uranus(HST2005)
GiantImpacts(GI)onUranusandNeptune
Suggested solution for their differences – GIpost-formation:
• Uranus: Oblique impact (tilt;keep the layered structure;eject materialto forma
circum-planetary disk)
• Neptune: Morehead-on collision leading to aconvective (adiabatic)interior,
consistent with thermalevolution models
e.g.:Stevenson(1986),Podolak+Helled(2012)
Previous GISimulations:
• Tilt of Uranus: SPH,5000-8000particles,alayered-structure,differentEOSs(Benz+
1989andSlattery+1991)
• Simulationof planetesimal impacts including gravity andgasdrag (Podolak+Helled2012)
Our Work
Simulate GIusing amodernSPHcode (Reinhardt+Stadel 2017):
• Uranus:concentrate onoblique impacts that could tilt the planet(andmight forma
circum planetary disk).Suchgrazing collisions should leave Uranus'structure intact –
consistent with flux andMoI.
• Neptune: concentrate onhead-oncollisions that can mixthe interior anddeposit
energy (andmass)deep inside of the planetà leading to anadiabatic structure that is
consistent with its observed flux andMoI.
InitialConditions
Target
Impactor
rock
«ice»
rock
or
ice
1-3𝑀⊕
10%rock,90%ice
• V∞ =5km/s,b=0.1– 0.9,Mtot =MUranus or MNeptune,no preimpact rotation
• Tillotson EOS(Tillotson 1962)for both materials
• Code:Gasoline(Wadsley+2004,Reinhardt+Stadel 2017),105 particles for the target
Tilting Uranus' RotationAxis
Animpactofa1-3𝑀⊕ rocky/icybodyonaninitiallynon-rotatingtargetatv∞=5km/s.TosubstantiallychangeUranus‘
spin,thechangeinangularmomentummustbeoftheorderofitscurrentrotationperiodofP=17.24h(Slatteryetal.
1991).
Int.energy (0–1.2e12erg/g)
Originof the material
Density (3.6-12.1g/cm3)
MixingNeptune’s Interior
v∞=5km/s,b=0.1,Mtot=17.1𝑀⊕ ,Mi=2𝑀⊕
Impactor:granite(left),ice (right),26hafterthe
impact.
Preliminary Conclusions
• GiantimpactscouldexplainUranus’spinandNeptune'sconvectiveinterior.
• Theimpactor's composition affects the angularmomentum (andenergy)budget.
• Mixingthe interior is difficult.
FutureWork
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Include H-Heatmospheres
Testthe sensitivity of the results to assumed composition,EOS,resolution,etc.
Accountfor multipleimpacts,pre-rotation
Combinewith N-bodysimulations
Investigate disk formation (moons)​
Thank you.