ROYAL OBSERVATORY
OF BELGIUM
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.
Ganymede Lander Colloquium and Workshop.
Session 2. Ganymede: origin, internal structure and geophysics
March 5th 2013 – Moscow, Russia
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
 Internal liquid ocean (Kivelson et al., 2002)
Which thickness? Which ice shell thickness?
 Needs to know Ganymede’s internal structure to reconstruct its
interior evolution, so understanding its surface geological history
crust
In the absence of seismic
data, geodesy brings precious
information on deep interior
mantle of terrestrial planets
and of their moons
outer core
(radius 3480 km)
Measurements of
inner core
(radius
km)rotation variations
tides1221
and
Probing
Earth’s interior
Ganymede: Tidal surface displacements
Latitude from south pole (in radians)
Surface deformation (in meters)
Equatorial
band with
maximum
tidal signal
Longitude (in radians)
 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
Ganymede: Tidal vertical displacements
Maximum surface
displacement (in meters)
h2
Moore and Schubert, 2003
• h2 measurement better
than ~0.01 is required
• 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)
Ganymede: Libration and interior
Baland and Van Hoolst, 2010
 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.
Rotation variations (libration) of Ganymede
Density difference between
Ocean and Ice Shell (in kg/m3)
Libration amplitude
(in meters at equator)
Baland and Van Hoolst, 2010
• 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
Ice shell thickness (in km)
The thinnest the ice shell (the shallowest the
ocean), the greater the libration amplitude
 Assumption: rigid layers.
• Required accuracy:
– 10 meters or better
Probing Ganymede from Geodesy
JUICE
Geodesy from orbit (tides)
Tide vertical Love number: h2
• From Laser altimeter (GaLa):
Cross-over data-points
Vertical precision: 1 meter (Δh2=0.01 )
Lateral precision: (10 meters)
Tidal potential Love number: k2
• Tracking of orbiter (3GM):
Gravity field
Precision: Δk2=0.01
Geodesy from the surface
• Surface tidal vertical displacement: h2 (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).
Geodesy experiment: instrumentation
Ganymede
Lander
JUICE
spacecraft
X-band radio-link
LaRa
electronic box
Coherent
transponder
maser
 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
LaRa: Specially designed for Lander
Electronic box + patch antennas
Main characteristics
LaRa
Electronic box
Total Mass
(box+antennas+
harness+connectors)
850 grams
Dimensions
143.5 mm x 122 mm x 51.5 mm
Frequencies
Reception
Transmission
X-band
7.162 GHz
8.145 GHz
Power consumption
(Tracking mode)
20 W (3 W to the Radio-Wave)
Patch disk antennas
44 mm x 10 mm
 X-band coherent transponder: Allan deviation 10-13 s-1 @ 60sec.
 Designed for Mars, but for Ganymede …
‘Re-sizing’ LaRa for Ganymede
Martian case:
Ganymede case:
Average distance: 1.5 AU
Average distance : 5 AU
Uplink: 34 m. Earth antenna
Uplink: 34 m. Earth antenna
Downlink:
20 W (power to Radio-Freq. 3W)
34 m. Earth’s antenna
Downlink:
25 W (power to Radio-Freq. 5W)
70 m. Earth’s antenna (or 34 m. network)
to get 5dB
received at Earth’s station
to get 5 dB
Received at Earth’s station
Doppler instrumental noise:
0.04 mm/s @ 60sec Doppler count time
Doppler instrumental noise:
0.04 mm/s @ 60sec Doppler count time
 LaRa can provide Doppler signal from Ganymede’s
surface with ‘minor’ technical adjustment.
Simulation
Process
using GINS
software
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 ( 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.
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
Lines: precision
Dots: accuracy
Ocean:
200 km
20 km
No ocean.
Shell rigidity:
109 Pa
1010 Pa
 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.
Simulations: Measurement of
the libration amplitudes
Lines: precision
Dots: accuracy
• 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.
 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.
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.