Ganymede Lander Colloquium and Workshop. Session 2. Ganymede: origin, internal structure and geophysics

March 5th 2013 – Moscow, Russia

A Geodesy experiment using aDirect-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 OBSERVATORYOF 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 seismicdata, geodesy brings preciousinformation on deep interior

of terrestrial planetsand of their moons

Measurements oftides 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)

Latit

ude

from

sou

th p

ole

(in ra

dian

s)Surface deformation (in meters)

Equatorial band withmaximumtidal signal

Ganymede: Tidal vertical displacements

Moore and Schubert, 2003

• Tidal displacements expressed as the tide vertical Love number h2

• 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)

h2M

aximum

surface displacem

ent (in meters)

• h2 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 betweenOcean and Ice Shell (in kg/m3)

Libr

ation

am

plitu

de

(in m

eter

s at

equ

ator

)

Ice shell thickness (in km)

Baland and Van Hoolst, 2010

Geodesy from orbit (tides)Tide vertical Love number: h2

• From Laser altimeter (GaLa):Cross-over data-pointsVertical precision: 1 meter (Δh2=0.01 )Lateral precision: (10 meters)

Tidal potential Love number: k2

• Tracking of orbiter (3GM):Gravity fieldPrecision: Δk2=0.01

Probing Ganymede from Geodesy

Geodesy from the surface • Surface tidal vertical displacement: h2 (cross-check with orbiter)

• Surface lateral displacement: Libration amplitudea 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] GHzCoherent

transpondermaser

LaRa electronic box

JUICEspacecraft

Ganymede Lander

LaRa: Specially designed for Lander

X-band coherent transponder: Allan deviation 10-13 s-1 @ 60sec.

Designed for Mars, but for Ganymede …

Electronic box + patch antennas Main characteristicsLaRa Electronic box

Total Mass(box+antennas+

harness+connectors)850 grams

Dimensions 143.5 mm x 122 mm x 51.5 mm

FrequenciesReception

Transmission

X-band7.162 GHz8.145 GHz

Power consumption(Tracking mode)

20 W (3 W to the Radio-Wave)

Patch disk antennas 44 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:0.04 mm/s @ 60sec 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 dBReceived at Earth’s station

Doppler instrumental noise: 0.04 mm/s @ 60sec 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 yearsGanymede Lander at equatorial areaDeep space ground stations: 1 hour per week or 1 hour per dayLibration + vertical tides ( h2 )

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)h2 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.

SimulationProcess

using GINSsoftware

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 h2

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: precisionDots: accuracy

Ocean:200 km 20 km

No ocean.Shell rigidity:

109 Pa1010 Pa

Simulations: Measurement of the libration amplitudes

Lines: precisionDots: accuracy

But the error on Ganymede’s ephemeris (50-100 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. 10-4 degrees (~4.5 meters) after 40 weeks of mission 10-5 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 oceanand to constrain the ice shell thickness, and rheology.

Improvement of Ganymede’s orbit: Using LaRa as a radio-beaconOrbital 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.