geodesy with mars lander network
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Geodesy with Mars lander Network. V. Dehant, J.-P. Barriot, and T. Van Hoolst Royal Observatory of Belgium Observatoire Midi-Pyrénées. Precession-Nutation. Z ecliptic. Rotation Axis. I = I 0 + I. Y ecliptic. = 0 + . X ecliptic. Topics. Rotation variations - PowerPoint PPT PresentationTRANSCRIPT
Geodesy with Mars lander Network
V. Dehant, J.-P. Barriot, and T. Van Hoolst
Royal Observatory of BelgiumObservatoire Midi-Pyrénées
Topics• Rotation variations
– Orientation in space: precession/nutation– Orientation in planet:
polar motion– Rotation speed: length-of-day
variations
• Gravity Field
• Modeling of – Interior of planets– Atmosphere dynamics = 0 +
Xecliptic
Yecliptic
Zecliptic
I = I0 + I
Rotation Axis
Precession-Nutation
Overview of the presentation
• Amplitudes of rotation variations of Mars
• Relation to interior structure and atmosphere of Mars
• Simulations of rotation variations and expected precisions (importance of landers)
• Gravity field: seasonal gravity variations
r : 230 m after 700 days Moment of inertia of Mars ( C )
Precession of Mars
r : 15 m (main term: semi-annual)
Precession and nutation of Mars
Nutation and interior structure
• Most important and geophysically interesting influence: existence of a liquid core
• Nutational motion of core differs from that of the mantle (if sphere: no core nutation).
• Core’s nutational effect : amplification of nutation with respect to rigid planet
• Main effect: resonance due to the existence of a free rotational mode related to the core
LdtdH
Free Core NutationRotation axisof the core
Rotation axisof the mantle
close to resonant frequency: large core close to resonant frequency: large core nutational motion in opposite direction of mantle nutational motion in opposite direction of mantle nutation, which can then largely be amplifiednutation, which can then largely be amplified
Restoring forces depend on flattening of coreRestoring forces depend on flattening of core
Flattening mainly depends on core density Flattening mainly depends on core density
FCN Period FCN Period core density, core radius core density, core radius
ROB
Relative rotation of axesRetrograde long period in spaceClose to main nutations
Amplification due to liquid core: 5mas or more
Variations of the rotation speed
(r : 10 m)
Polar motion (r : 10 to over 100 cm)
Measuring Doppler shifts on Lander-Orbiter link
R
RV
Dop.
≈ Projection of relative velocity on line-of-sight Lander-Orbiter
days 7000700 700 700 7000 0 0
Error: centimeter level
Number of landers
Lander-Earth link
Graphes de resultats
Low degree zonal gravity coefficients and rotation rate
• Variations in C20 give information about the CO2
cycle. But strongly linked LOD (mass redistribution is main factor).
• Doppler shifts between landers – orbiter (LOD) and orbiter – Earth (C20)
• Previous results assume a perfectly known orbit• Numerical simulations with GINS (Géodésie par
Intégrations Numériques Simultanées, CNES) software
Time-Varying Gravity Field
C20 C30
The precision of current gravity observations are not sufficient enough to provide additional constrains to C02 cycle
Gravity observation from SC, High Electron detector observation,GCM
Simulations with MGS (I=93°, e=0.01) & MEX (I=86°, e=0.6)
Simulations with two orbiters MGS (I=93°, e=0.01)+MEX (I=86°, e=0.6)
The error is reduced by a factor of about 2
C40 C50
C20 C30
Effect of a Lander Network(single orbiter)
Landers help to resolve the LOD, to determine better the orbit ascending node hence the even coefficients
Conclusions
• An additional lander – orbiter link improves the determination of rotation variations and gravity variations
• and makes it possible to extract information on Mars’ interior and atmosphere/polar caps CO2 cycle
Signature of MOPs
Change in lander velocity due to MOP
Geometric effect= change in direction lander-orbiter due to MOP
Large effect for low altitude satellite
cosMOPV
sinVMOP
|V| : velocity différence between landers and orbiter (~3 km/s), |VMOP| : change in |V| due to (~ mm/s), : angle between |V| and line-of-sight lander-orbiter, MOP : change in due to MOP (~ 10-7 rad).
Effect of the Landers-orbiter Doppler tracking on the J2 determination
: model C20
+: DSN, fixed ΔLOD but modified
----: DSN, fixed ΔLOD
O: DSN + lander data
Landers-Orbiter Doppler tracking and seasonal gravity field
: model Cl0
o: near polar, DSN
x: near polar DSN + lander tracking
+: near polar + Starlette like orbiter, DSN tracking
Landers-Orbiter Doppler tracking and seasonal gravity field : Formal error
o: near polar, DSN
x: near polar DSN + lander tracking
+: near polar + Starlette like orbiter, DSN tracking
Lander-Orbiter Doppler tracking and rotation rate determination
Simulation of Mars’Rotation rate determination From the Landers-orbiter Doppler-link (four landers and one near-polar orbiter).