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Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonancein Planetary Bodies
Harsh Menon
Spin-Orbit Resonance Planetary Physics EP 420
Overview
• An Introduction to Tidal Forces
• Spin-Orbit Resonance
• Resonance Criteria
• Capture into Resonance
• Surface of Section
• Conclusion
Spin-Orbit Resonance Planetary Physics EP 420
Introduction to Tidal Forces
• Force produced due to “gravitational gradient or variation of gravitational force across the body”
• Proportional to radius of larger body
• Inversely proportional to distance between the centers of the bodies
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
• Interaction between a planet’s orbital angular momentum and a satellite’s gravitational permanent quadrupole moment
• Resonance results when there is a simple integer or near integer relationship between the spin period and orbital period of the satellite
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
pM
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
• Result in oscillations called librations• http://aa.usno.navy.mil/graphics/Moon_mo
vie.gifThe equation of motion of the satellite is
given by:
• Analogous to equation of motion for a pendulum
2sin2
1))],(([ 2
0epHsign
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
• Libration Frequency
5.0
0 ),(3
epHC
ABn
43
3423
44
16
533),3(
48
845),
2
5(
16
123
2
7),
2
3(
16
1
2
51),1(
16
1
2
1),
2
1(
0),0(48
1),
2
1(
24
1),1(
eeHeeH
eeeHeeeHeeeH
eHeeHeeH
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
• Physical Meaning of γ
• “Orientation of the long axis of the satellite on the passage of the satellite through the pericenter or that it is a stroboscopic angle that is evaluated when M=0”
Spin-Orbit Resonance Planetary Physics EP 420
Spin-Orbit Resonance
http://cseligman.com/text/planets/DayOnMercury.swf
Spin-Orbit Resonance Planetary Physics EP 420
Resonance Criteria
• Loop is closed
• p is integer multiple of ½
• Torque acting on satellite >Torque due to tidal drag
Spin-Orbit Resonance Planetary Physics EP 420
Capture into Resonance
• Pendulum Analogy• Capture into Resonance • Variation of average tidal torque with dγ/dt
Spin-Orbit Resonance Planetary Physics EP 420
Surface of Section
Spin-Orbit Resonance Planetary Physics EP 420
Surface of Section
Spin-Orbit Resonance Planetary Physics EP 420
Surface of Section
• Chirikov’s Criterion
• Chaotic behavior depends on eccentricity and asphericity parameter
• Linearly on e, exponentially on α
C
AB )(3
Spin-Orbit Resonance Planetary Physics EP 420
Conclusions
• Spin-Orbit Resonance
• Resonance Criteria
• Capture into Resonance
• Surface of Section
Spin-Orbit Resonance Planetary Physics EP 420
Recommendations
• Current theory “naïve”
• More accurate theory - Hamiltonian Theory on Tidal Forces
Spin-Orbit Resonance Planetary Physics EP 420
References
• Murray, C.D. & Dermott, S.F. (1999). Solar System Dynamics. New York: Cambridge University Press.