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3 MS 3 – Session 9: New projects and instruments October 11 th 2012 – Moscow, Russia

ROYAL OBSERVATORY OF BELGIUM. ROYAL OBSERVATORY OF BELGIUM. Belgium-Geodesy e xperiment u sing Direct-To- Earth Radio- link : Application to Mars and Phobos Rosenblatt P., Le Maistre S., M. Mitrovic , and Dehant V.

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3 MS 3 – Session 9: New projects and instruments October 11 th 2012 – Moscow, Russia

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  1. ROYAL OBSERVATORYOF BELGIUM ROYAL OBSERVATORYOF BELGIUM Belgium-GeodesyexperimentusingDirect-To-Earth Radio-link: Application to Mars and Phobos Rosenblatt P., Le MaistreS., M. Mitrovic, and Dehant V. 3MS3 – Session 9: New projects and instruments October 11th 2012 – Moscow, Russia

  2. Overview • Why a Geodesy experiment in the Martian system?Scientific rationale:Mars’ deepinterior (size, innercore?) coreevolutionPhobos’ interior (internal mass distribution) origin of the MartianmoonsGoals:Precisemeasurements of the rotational state (Mars’ nutation, Phobos’ librations)Usingdedicatedpayload:X-band coherenttransponder (LaRa, Lander Radioscience, developed by Belgium)

  3. In the absence of seismicdata, geodesybringspreciousinformation on deepinterior of terrestrialplanets Measurements oftides and rotation variations mantle crust outer core (radius 3480 km) inner core (radius 1221 km) ProbingEarth’sinterior

  4. Current knowledge of the Martian core from geodesy Core radius estimates given possible mantle temperature end-members, mantle rheology, and crust density and thickness range (Rivoldini et al., 2010). 250 km ROB/CNESsolution JPL solution k2 tidal Love number determined from orbiters (Yoder et al., 2003; Konoplivet al., 2006; Marty et al., 2009) Tidal Love number • 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).

  5. 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

  6. Free core nutation and transfer function rigid Mars’ nutations Amplitudes 250 days transfer function 250 days non-rigid Mars’ nutations Amplitudes 250 days ROB IMPORTANTFOR: • retrograde ter-annual nutation • retrograde semi-annual nutation • retrograde 1/4 year nutation • prograde semi-annual nutation

  7.  Resonance Large amplification Rigid nutation Free core nutation and transfer function observations Known from theory Transferfunction Core moment of inertia Constraint on core size and shape Rigid nutation amplification → core dimension & moment of inertia

  8. 1.5% to 3% ... > 20% prograde semi-annual nutation Amplification of rigid Mars’ nutation due to a liquid core • Primary effect on retrograde ter-annual and prograde semi-annual nutations Resonance Amplification at ~3% of rigid nutationamplitude of 500 mas  ~15 mas forthe liquid core signature. retrograde ter-annual nutation Amplification at >20% of rigid nutationamplitude of 10 mas  >2 mas forthe liquid core signature.But it can be much more if FCN period ~Ter-annual period 1 mas = 1.6 cm at Mars’ equator

  9. ROB Ter-annual nutation (period of 229 days)amplification depends on liquid core size (i.e. FCN period). Improvement of core size determination.

  10. 1.5% to 3% ... > 20% prograde semi-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 Amplification of rigid Mars’ nutation due to a liquid core • Effect of an inner core on nutation amplification. Resonance retrograde ter-annual nutation

  11. Geodesy experiment to monitor Mars’ spin axis nutation LaRaelectronic box X-band radiolink Uplink in [7.145,7.190] GHz • Coherenttransponder (LaRa) initiallydesigned and constructed by Belgium: TRL-5 • Mass: 850 grams. Power peakconsumption (20 W). • Direct-To-Earth (DTE) radio-linkbetween Mars and tracking stations on Earth • X-band 2-way Doppler shift measurements: Precision0.1 mm/s •  Monitoring of the rotational motion of Mars Coherent transponder Downlink in [8.400,8.450] GHz maser

  12. Direct-to-Earth radio-link (with one Lander)Numerical simulations (1) ! Semi-annual progradenutation amplitude 1/3 annual retrograde nutation amplitude • Predictions of precision and accuracy on the retrieval of nutation amplitude Le Maistre et al., 2012 (Planet. SpaceSci.) Milli-acr seconds (mas) Milli-acr seconds (mas) FCN=230 days Mission duration (days) Mission duration (days) FCN=240 days • Nutation amplitude canberetrievedwithenoughprecision to detectliquidcoreespeciallywhen the FCN periodis close to the ter-annualperiod (229 days).

  13. Direct-to-Earth radio-link (with one Lander)Numerical simulations (2) ! Le Maistre et al., 2012 (Planet. SpaceSci.) • Determiningtransferfunctionparameterswith one Lander at Mars’ surface  Challengingtask ! (because of non-linearity). • Use of more Landers Network

  14. Opportunity of pre-network experimentINSIGHT + ExoMars • NASA-INSIGHT scout mission due to land on Mars in 2016. Radioscienceexperimentwith US instrument. • If Radiosciencetransponder (possiblyLaRa) onboardExoMars (2018) wemayperform Single BeamInterferometry (SBI) experiment.  Lander relative position knownat the sub-cm precisionlevel. • Improvement of the determination of the Mars’ spin axis nutations.

  15. ‘Puzzling’ Phobos (and Deimos) All model of originare flawed Capture scenario:PROS:Shape, ViS/NIR spectraCarbonaceousasteroid. CONS: Ambiguous surface composition fromremotesensing data.Currentorbitrequireshigh tidal dissipation rate insidePhobos. In Situ formation PROS:CurrentmoonorbitsHighlyporous.Additional argument:A silicate composition. CONS: No modellingyet(Rosenblatt and Charnoz, Accepted in Icarus, 2012) Phobos MEX/HRSC image Interiorrelevant to the origin:composition, mass distribution, dissipative properties … Seerecentreview:Rosenblatt P., A&A Rev., vol. 19, 2011.

  16. Which ‘Bulk interior’ for Phobos ? Rock+ice Blocksof rocks No monolithicPhobos !Compositional and/or structural heterogeneitiesinsidePhobos.Principal moments of inertia to constrainit. From Fanaleand Salvail(1989) Murchie et al. (1991) Stickney-inducedfractures Highlyporousrocky body (Rubble Pile) From Rambaux et al., accepted in A&A, 2012See also, PD1 Poster Session FromAndert et al. (2010)

  17. 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.

  18. Mars Express: Libration/gravitymeasurement (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°) • Updatedshape model (Nadezhdina et al., EPSC, 2012): θ = 1.09° +/- 0.1 ° (9%)(Homogeneous= 0.93°) • Homogeneous/Heterogeneous … • GravityfieldC20heterogeneity but error bar ~50% (Andert et al., EPSC, 2011)

  19. Modeling heterogeneity inside Phobos Probability density functions of the quadrupole gravity coefficients C20 and C22 Expected C20value Red linehomogeneous Heterogeneousmodels: rock+ice+porositywhich fit the observedlibration withinitserror bar. Porosity: 10% 30% 40% Water ice: 23% 7% 0% Expected C22value Red linehomogeneous • Geodeticparameters of heterogeneousinteriordepartsby a few percent (<10%) from the homogeneousinterior • Precisemeasurementisrequired (geodeticexperiment) FromRivoldini et al., 2011

  20. Radio-science instrumentation X-band radiolink LaRaelectronic box Uplink in [7.145,7.190] GHz Downlink in [8.400,8.450] GHz Coherent transponder maser • Coherenttransponder (LaRa) initiallydesigned by Belgium for Martian Lander experiment • Direct-To-Earth (DTE) radio-linkbetweenPhobos Lander/Orbiter on Phobos and trackingstations on Earth (DSN, ESTRACK and VLBI) • X-band 2-way Doppler shift measurements: Precision0.1 mm/s •  Monitoring of the rotational and orbital motion of Phobos

  21. Phoboslibration from future Phobos Lander:Numerical simulations (1) ! Uncertainty on C versus uncertainty on C20 (or C22) Relative momentsof inertia • Phobos’ rotational model: richspectrum of libration (Rambaux et al., 2012) • Short periodscontain information on the interior: Relative moments of inertia. • Numerical simulations of geodesyexperimentwith a Lander on Phobos show: •  Short-periodic libration with a precision < 1% after a few weeks of operation Knowledge of quadrupolegravity coefficients isalsorequired

  22. Additional constraint from Tides Amplitude of periodic tidal displacement Expectedconstraint on the interior Predictions of formalerror and accuracy Le Maistre et al., 2012 • Phobos’ surface displacement due to Tidesraised by Mars insidePhobos(up to 5 cm), depending on itsinterior structure (« rubble-pile » vs monolith) • Precise monitoring of Lander (transponder) position  interior

  23. CONCLUSION & PERSPECTIVES • A geodesy (radio-science) with one (or more) Lander willprovideconstraints on the Martiancore,(i.e. light elements content, innercore, …), therewith on itsevolution. • Sameexperiment on Phobos (one Lander) willprovideconstraintson itsbulkinterior structure (i.e. water-ice/porosity content), therewith on itsorigin. • Radioscience instrument: X-band coherenttransponderLaRa (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 bulkinteriorstructure of solar system bodies.

  24. Lander network experiment

  25. Core moment factor Core moment factor Landers (network) orbiter radio-link Numerical simulations ! Coremomentum factor: Free core nutation period: 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).

  26. 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.

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