Ganymede Lander Colloquium and Workshop. Session 2. Ganymede: origin, internal structure and geophysics March 5 th 2013 – Moscow, Russia A Geodesy experiment using a Direct-To-Earth radio-link with a Ganymede Lander: Constraints on Ganymede interior. Rosenblatt P., Le Maistre S., Mitrovic M., Van Hoolst T., Dehant V., Lainey V. Marty J.C. ROYAL OBSERVATORY OF BELGIUM

Overview  Why a Geodesy experiment at the surface of Ganymede? Scientific rationale: Ganymede’ interior issue:  Depth of the liquid water ocean  Thickness of the ice shell Experiment:  Precise measurements of the rotational variations (libration) and tidal vertical displacement Instrument: Designed for Lander  X-band coherent transponder: LaRa (Lander Radioscience) developed by Belgium

Ganymede’s interior issue  Needs to know Ganymede’s internal structure to reconstruct its interior evolution, so understanding its surface geological history  Internal liquid ocean (Kivelson et al., 2002) Which thickness? Which ice shell thickness?

crust mantle outer core (radius 3480 km) inner core (radius 1221 km) Probing Earth’s interior In the absence of seismic data, geodesy brings precious information on deep interior of terrestrial planets and of their moons Measurements of tides and rotation variations

Ganymede: Tidal surface displacements  Pattern of tidal vertical displacements at the surface of Ganymede: up to 2.5 meters in equatorial region in the presence of a internal liquid ocean. Best Signal-To-Noise ratio  near Equatorial Lander Longitude (in radians) Latitude from south pole (in radians) Surface deformation (in meters) Equatorial band with maximum tidal signal

Ganymede: Tidal vertical displacements Moore and Schubert, 2003 Tidal displacements expressed as the tide vertical Love number h 2 It depends on : internal liquid ocean thickness and ice shell thickness, rigidity and viscosity  as small as 0.01 (less than 10 cm of displacement if no ocean and high ice rigidity)  as large as 1.6 (almost 4 meters of displacement if thick ocean and low ice rigidity) h2h2 Maximum surface displacement (in meters) h 2 measurement better than ~0.01 is required

Ganymede: Libration and interior  Layered interior model of Ganymede: Liquid-solid layers.  ‘Decoupling’ between layers: ice shell (surface layer) and liquid ocean  Increase of libration amplitude w.r.t. rigid Ganymede. It depends on thickness and physical properties of layers. Baland and Van Hoolst, 2010

Rotation variations (libration) of Ganymede Amplitudes are about 2 to a few 10 times larger than for models without ocean (10m) Observations of libration amplitude can be used to –confirm the existence of a subsurface ocean –constrain the ice shell: thickness and density Required accuracy: – 10 meters or better The thinnest the ice shell (the shallowest the ocean), the greater the libration amplitude  Assumption: rigid layers. Density difference between Ocean and Ice Shell (in kg/m3) Libration amplitude (in meters at equator) Ice shell thickness (in km) Baland and Van Hoolst, 2010

Geodesy from orbit (tides) Tide vertical Love number: h 2 From Laser altimeter (GaLa): Cross-over data-points Vertical precision: 1 meter (Δh 2 =0.01 ) Lateral precision: (10 meters) Tidal potential Love number: k 2 Tracking of orbiter (3GM): Gravity field Precision: Δk 2 =0.01 Probing Ganymede from Geodesy Geodesy from the surface Surface tidal vertical displacement: h 2 (cross-check with orbiter) Surface lateral displacement: Libration amplitude a precision better than 10 meters (orbiter precision) would bring additional information about the interior (ice shell thickness). JUICE

Geodesy experiment: instrumentation  Direct-To-Earth (DTE) radio-link: Two components 1)Coherent transponder (LaRa) initially designed by Belgium for Martian Lander (> TRL-5) 2)Tracking stations on Earth: (DSN, ESTRACK) and VLBI (like PRIDE experiment on JUICE) X-band 2-way Doppler shift measurements.  Monitoring of the rotational and orbital motion of Ganymede X-band radio-link Uplink in [7.145,7.190] GHz Downlink in [8.400,8.450] GHz Coherent transponder maser LaRa electronic box JUICE spacecraft Ganymede Lander

LaRa: Specially designed for Lander X-band coherent transponder: Allan deviation s 60sec. Designed for Mars, but for Ganymede … Electronic box + patch antennasMain characteristics LaRaElectronic box Total Mass (box+antennas+ harness+connectors) 850 grams Dimensions143.5 mm x 122 mm x 51.5 mm Frequencies Reception Transmission X-band GHz GHz Power consumption (Tracking mode) 20 W (3 W to the Radio-Wave) Patch disk antennas44 mm x 10 mm

Martian case: Average distance: 1.5 AU Uplink: 34 m. Earth antenna Downlink: 20 W (power to Radio-Freq. 3W) 34 m. Earth’s antenna to get 5dB received at Earth’s station Doppler instrumental noise: sec Doppler count time Ganymede case: Average distance : 5 AU Uplink: 34 m. Earth antenna Downlink: 25 W (power to Radio-Freq. 5W) 70 m. Earth’s antenna (or 34 m. network) to get 5 dB Received at Earth’s station Doppler instrumental noise: sec Doppler count time ‘Re-sizing’ LaRa for Ganymede  LaRa can provide Doppler signal from Ganymede’s surface with ‘minor’ technical adjustment.

Simulation of Doppler tracking data: Duration : up to 2 years Ganymede Lander at equatorial area Deep space ground stations: 1 hour per week or 1 hour per day Libration + vertical tides ( h 2 ) Simulated Doppler data (60sec sampling time) with white noise at 0.04 mm/s. Simulation of least-squares fit on the noisy simulated tracking data of: Fitted parameter: Libration amplitude: cosine and sine amplitudes at different periods (among them the orbital period) h 2 vertical tide Love number Quality of the fit: Formal uncertainty (least squares fit quality) and accuracy (discrepancy between retrieved and nominal value) as a function of the mission duration and tracking coverage. Simulation Process using GINS software GINS: Géodésie par Intégrations Numériques Simultanées developed by CNES and further adapated to planetary geodesy appliccations by ROB

Simulations: Measurement of the vertical tide Love number h 2  Case with ocean : Detection after 20 weeks and ~10% of error after 2 years  Case without ocean: Detection after 20 weeks for low ice rigidity only detection after 2 years for high ice rigidity. Lines: precision Dots: accuracy Ocean: 200 km 20 km No ocean. Shell rigidity: 10 9 Pa Pa

Simulations: Measurement of the libration amplitudes Lines: precision Dots: accuracy  But the error on Ganymede’s ephemeris ( km) not taken into account.  LaRa Doppler data to be used for global inversion: libration+tide+ephemeris (part of a tidal instrument suite)  Further simulations are in progress.  Also, spacecraft to Lander radio-link to overcome the ephemeris error problem. Precision: using 1 hour of tracking per week.  degrees (~4.5 meters) after 40 weeks of mission  degrees after 2 years (better than 1 meter !), Precision better than 1 meter after only 20 weeks of mission using 1 hour of tracking per day.

CONCLUSION & PERSPECTIVES  Radio-transponder LaRa designed for Martian Lander can be accomodated to a Ganymede Lander  It allows us to measure libration amplitudes with a sub-meter precision after 20 weeks of mission (1 hour of tracking per day).  It permits to confirm (again) the presence of an internal ocean and to constrain the ice shell thickness, and rheology.  Improvement of Ganymede’s orbit: Using LaRa as a radio-beacon Orbital evolution - Interior structure  Radio-science instrument part of the ‘core package’ to probe in-situ the bulk interior structure of solar system bodies.

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.