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ROYAL OBSERVATORY OF BELGIUM ROYAL OBSERVATORY OF BELGIUM Belgium-Geodesy experiment using Direct-To-Earth Radio-link: Application to Mars and Phobos Rosenblatt P., Le Maistre S., M. Mitrovic, and Dehant V. 3MS3 – Session 9: New projects and instruments October 11th 2012 – Moscow, Russia Overview Why a Geodesy experiment in the Martian system? Scientific rationale: Mars’ deep interior (size, inner core?) core evolution Phobos’ interior (internal mass distribution) origin of the Martian moons Goals: Precise measurements of the rotational state (Mars’ nutation, Phobos’ librations) Using dedicated payload: X-band coherent transponder (LaRa, Lander Radioscience, developed by Belgium) crust In the absence of seismic data, geodesy brings precious information on deep interior mantle of terrestrial planets outer core (radius 3480 km) Measurements of tides and rotation variations inner core (radius 1221 km) Probing Earth’s interior Current knowledge of the Martian core from geodesy 250 km Core radius estimates given possible mantle temperature end-members, mantle rheology, and crust density and thickness range (Rivoldini et al., 2010). ROB/CNES solution JPL solution Tidal Love number k2 tidal Love number determined from orbiters (Yoder et al., 2003; Konopliv et al., 2006; Marty et al., 2009) Liquid core inside Mars (k2 > 0.08), but large discrepancies (+/- 250 km). Better core radius estimate is required to better constrain other core parameters (sulfur content, solid inner core…), which drive its thermal evolution. More data are needed. Space geodesy can play an important role by measuring nutations of the rotation axis of Mars ( Lander(s) on Mars). Nutations of the planet Mars liquid core solid core Measured nutation rigid nutation = Constraint on deep interior Mars’ nutation have not been measured so far, but they can be precisely computed considering Mars’ interior is rigid. If the core is liquid, nutation amplitudes can be amplified w.r.t. “rigid nutations”. Precise measurements of nutations Information on the deep interior structure Amplitudes Free core nutation and transfer function rigid Mars’ nutations IMPORTANT FOR: 250 days transfer function 250 days Amplitudes non-rigid Mars’ nutations • retrograde terannual nutation • retrograde semiannual nutation • retrograde 1/4 year nutation • prograde semiannual nutation 250 days ROB Free core nutation and transfer function • Rigid nutation amplification → core dimension & moment of inertia observations FFCN Anonrigid 1 FCN Known from theory Arigid Transfer function Core moment of inertia Constraint on core size and shape FCN Resonance Large amplification Rigid nutation FFCN Cf C Cf FCN (1 ef ) C (e f ) C Cf Amplification of rigid Mars’ nutation due to a liquid core Primary effect on retrograde ter-annual and prograde semi-annual nutations > 20% ... 1.5% to 3% Resonance prograde semi-annual nutation retrograde ter-annual Amplification at >20% of rigid nutation nutation amplitude of 10 mas >2 mas for the liquid core signature. But it can be much more if FCN period ~Ter-annual period Amplification at ~3% of rigid nutation amplitude of 500 mas ~15 mas for the liquid core signature. 1 mas = 1.6 cm at Mars’ equator Ter-annual nutation (period of 229 days) amplification depends on liquid core size (i.e. FCN period). Improvement of core size determination. ROB Amplification of rigid Mars’ nutation due to a liquid core Effect of an inner core on nutation amplification. > 20% ... 1.5% to 3% Resonance prograde semi-annual nutation retrograde ter-annual nutation The existence of an inner core is expected to remove FCN semi-annual prograde amplification detection of inner core if it does exist Geodesy experiment to monitor Mars’ spin axis nutation X-band radiolink LaRa electronic box maser Coherent transponder Coherent transponder (LaRa) initially designed and constructed by Belgium: TRL-5 Mass: 850 grams. Power peak consumption (20 W). Direct-To-Earth (DTE) radio-link between Mars and tracking stations on Earth X-band 2-way Doppler shift measurements: Precision 0.1 mm/s Monitoring of the rotational motion of Mars Direct-to-Earth radio-link (with one Lander) Numerical simulations (1) ! Predictions of precision and accuracy on the retrieval of nutation amplitude Le Maistre et al., 2012 (Planet. Space Sci.) 1/3 annual retrograde nutation amplitude Milli-acr seconds (mas) Milli-acr seconds (mas) Semi-annual prograde nutation amplitude Mission duration (days) FCN=230 days FCN=240 days Mission duration (days) Nutation amplitude can be retrieved with enough precision to detect liquid core especially when the FCN period is close to the ter-annual period (229 days). Direct-to-Earth radio-link (with one Lander) Numerical simulations (2) ! Le Maistre et al., 2012 (Planet. Space Sci.) Determining transfer function parameters with one Lander at Mars’ surface Challenging task ! (because of non-linearity). Use of more Landers Network Opportunity of pre-network experiment INSIGHT + ExoMars NASA-INSIGHT scout mission due to land on Mars in 2016. Radioscience experiment with US instrument. If Radioscience transponder (possibly LaRa) onboard ExoMars (2018) we may perform Single Beam Interferometry (SBI) experiment. Lander relative position known at the sub-cm precision level. Improvement of the determination of the Mars’ spin axis nutations. ‘Puzzling’ Phobos (and Deimos) Capture scenario: All model of origin are flawed In Situ formation PROS: Shape, ViS/NIR spectra Carbonaceous asteroid. PROS: Current moon orbits Highly porous. CONS: Ambiguous surface composition from remote sensing data. Current orbit requires high tidal dissipation rate inside Phobos. Additional argument: A silicate composition. See recent review: Rosenblatt P., A&A Rev., vol. 19, 2011. CONS: No modelling yet Phobos MEX/HRSC image Interior relevant to the origin: composition, mass distribution, dissipative properties … (Rosenblatt and Charnoz, Accepted in Icarus, 2012) Which ‘Bulk interior’ for Phobos ? Blocks of rocks Rock+ice No monolithic Phobos ! Murchie et al. (1991) Highly porous rocky body (Rubble Pile) Compositional and/or structural heterogeneities inside Phobos. From Fanale and Salvail (1989) Stickney-induced fractures Principal moments of inertia to constrain it. From Andert et al. (2010) From Rambaux et al., accepted in A&A, 2012 See also, PD1 Poster Session Internal mass distribution through geodetic parameters Internal mass distribution related to principal moments of inertia (A<B<C). Principal moments of inertia also related to quadrupole gravity coefficients C20 and C22 and the libration amplitudes θ Modeling internal mass distribution Constraining those models by measurements: Geodetic experiment Where M is the mass of Phobos, r0 is the mean radius of Phobos and e is the ellipticity of its orbit around Mars. Mars Express: Libration/gravity measurement (Willner et al., 2010) Shape model Monitoring of control points network (Willner et al., 2010) θ = 1.2° +/- 0.15 ° (12.5%) (Homogeneous value from the shape = 1.1°) Updated shape model (Nadezhdina et al., EPSC, 2012): θ = 1.09° +/- 0.1 ° (9%) (Homogeneous = 0.93°) Homogeneous/Heterogeneous … Gravity field C20 heterogeneity but error bar ~50% (Andert et al., EPSC, 2011) Modeling heterogeneity inside Phobos Probability density functions of the quadrupole gravity coefficients C20 and C22 Expected C20 value Red line homogeneous Porosity: Water ice: 10% 23% 30% 7% 40% 0% Heterogeneous models: rock+ice+porosity which fit the observed libration within its error bar. Expected C22 value Red line homogeneous Geodetic parameters of heterogeneous interior departs by a few percent (<10%) from the homogeneous interior Precise measurement is required (geodetic experiment) From Rivoldini et al., 2011 Radio-science instrumentation X-band radiolink LaRa electronic box maser Coherent transponder Coherent transponder (LaRa) initially designed by Belgium for Martian Lander experiment Direct-To-Earth (DTE) radio-link between Phobos Lander/Orbiter on Phobos and tracking stations on Earth (DSN, ESTRACK and VLBI) X-band 2-way Doppler shift measurements: Precision 0.1 mm/s Monitoring of the rotational and orbital motion of Phobos Phobos libration from future Phobos Lander: Numerical simulations (1) ! Relative moments of inertia 𝐶−𝐵 𝛼= 𝐴 𝐶−𝐴 𝛽= 𝐵 𝛾= Uncertainty on C versus uncertainty on C20 (or C22 ) 𝐵−𝐴 𝐶 Phobos’ rotational model: rich spectrum of libration (Rambaux et al., 2012) Short periods contain information on the interior: Relative moments of inertia. Numerical simulations of geodesy experiment with a Lander on Phobos show: Short-periodic libration with a precision < 1% after a few weeks of operation Knowledge of quadrupole gravity coefficients is also required Additional constraint from Tides Amplitude of periodic tidal displacement Expected constraint on the interior Predictions of formal error and accuracy Le Maistre et al., 2012 Phobos’ surface displacement due to Tides raised by Mars inside Phobos (up to 5 cm), depending on its interior structure (« rubble-pile » vs monolith) Precise monitoring of Lander (transponder) position interior CONCLUSION & PERSPECTIVES A geodesy (radio-science) with one (or more) Lander will provide constraints on the Martian core, (i.e. light elements content, inner core, …), therewith on its evolution. Same experiment on Phobos (one Lander) will provide constraints on its bulk interior structure (i.e. water-ice/porosity content), therewith on its origin. Radioscience instrument: X-band coherent transponder LaRa (TRL 5) easy to implement on Landing platform of future missions to Mars, Phobos, the Moon, Ganymede, … (ExoMars, INSPIRE, PHOOTPRINT, GETEMME, Phobos-Soil-2, JUICE …) Radio-science instrument part of the ‘core package’ to probe in-situ the bulk interior structure of solar system bodies. Lander network experiment Landers (network) orbiter radio-link Numerical simulations ! Core momentum factor: FFCN FFCN Core moment factor Arigid Core moment factor F Anonrigid 1 FCN FCN FCN Free core nutation period: FCN Nutation parameters are recovered (case where a liquid core is considered). Same results for Polar Motion and Lentgh-Of-Day variations. The effect of desaturation on the orbiter motion have been taken into account and the tracking is assumed to be as continuous as possible (from Rosenblatt et al., Planet. Space Sci., 2004). Acknowledgements This work was financially supported by the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office.