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www.DLR.de • Chart 1
Mercury Geodesy and Geophysics
J. Oberst & T. Spohn
DLR Institute of Planetary Research, Berlin, Germany
Planet Mercury: Basic Features
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First planet, closest to the Sun
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Small elongation from the Sun == >
difficult to observe from Earth
High solar radiation, 10 x stronger
than near Earth and high surface
temperatures: max. 430°C
Small (4878 km Ø)Strindberg
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Only slightly larger than Moon
But e.g. smaller than Ganymede
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High in density (5.4 gr/cm 3)
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Magnetic field
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Large iron-rich core?
Dipole
Fluid outer core
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Mean Density Constraint
Planet Mercury: Orbit and Rotation
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High-eccentricity orbit (e=0.205)
 Solar irradiation varies over the year
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Mercury in spin/orbital 3:2 resonance
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planet makes 3 full rotations while it
moves about the Sun 2 times
Small librations (oscillations of mean
Strindberg
rotational rate)
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amplitude / phase of librations linked
to interior structure (molten core?)
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Mercury’s Dynamical State
Mercury Precession
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In 1859, French mathematician
and astronomer Urbain Le Verrier:
slow precession of Mercury’s orbit
around the Sun could not be
completely explained by
Newtonian mechanics and
perturbations by the known
planets.
Search for hypothetical planet,
named Vulcan, but no such planet
ever found
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Mercury Precession
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Newtonian mechanics, including
perturbations from other planets,
predicts a precession of 5557
arcsec (1.5436°) per century.
Albert Einstein’s General Theory of
Relativity provided explanation for
observed precession: relativistic
perihelion advance excess, 42.98
arcsec per century
(much smaller effects for other
planets: 8.62 arcseconds per
century for Venus, 3.84 for Earth,
1.35 for Mars)
www.DLR.de • Chart 8
> Vortrag > Autor • Dokumentname > Datum
Peale‘s Experiment
- Since Mercury is in a bound rotation state (Cassini state) some of the
ambiguity in determining the moment of inertia from the low order terms of
the gravity field can be removed.
- In particular, if J2 and J22 are known, together with the libration amplitude
and the planet‘s obliquity, then
- C/MR2 and Cm/C can be calculated, where C is the moment of inertia of
the planet about the rotational axis, M ist mass and R its equatorial radius.
Cm/C is the ratio between the moment of inertia of the solid part of the
planet to that of the entire planet.
- From Cm and C, Cc can be calculated
- These features make Mercury unique targets of applying
geophysical/geodetic tools of interior structure modeling
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Early Measurements of Librations
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Measurements of librations using radar-reflecting surface
disparities and „Disco ball effect“
Arecibo and other radar systems operating jointly
Measurements possible today using MESSENGER data from
orbit
MESSENGER Spacecraft
MESSENGER = Mercury Surface, Space Environment, Geochemistry and Ranging
•Mission in the NASA Discovery program
•Launch: August 3, 2004
•Mercury Orbit Insertion: March 18, 2011
•Jürgen Oberst and Jörn Helbert, European CoI‘s
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MESSENGER Camera and Laser Altimeter
Mercury Dual Imaging System (MDIS)
MESSENGER Laser Altimeter (MLA)
Polar elliptic orbit
Implications for instrument
operations
Laser altimeter coverage
only for Northern
hemisphere
Northern hemisphere: mostly
wide-angle imaging
Southern hemisphere:
mostly narrow-angle imaging
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Measurement of Mercury Rotation Parameters:
Rotation pole, rotation rate, libration amplitude
Measurement idea:
- Stereo DTMs geometrically comparably rigid
- Measure offsets of multitemporal MLA profiles with respect to rigid DTM
- Use co-registrations of stereo DTMs and MLA profiles for precise measurements
Rotation Pole Position
all ellipses are 1σ
Margot’ 12
Margot’ 07
Mazarico’ 14
H03
H05
equal obliquity line
(Margot et al., 2012)
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Libration Amplitude (arc sec)
(B – A)/Cm
[10^(-4)]
JLM’ 07
2.03 ± 0.12
JLM’ 12
2.18 ± 0.09
H03
2.07 ± 0.22
H03*
2.03 ± 0.22
H03**
2.17 ± 0.18
H05
2.30 ± 0.30
* MLA profiles at merging times of SPKs (Wednesdays ± 1 day) were excluded
from estimation (we are currently making tests using new SPK kernel, May 2…)
** fit to longitudinal shifts of individual profiles, corrected for spin rate trend,
best fit shown in red
Mercury’s Internal Structure
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www.DLR.de • Chart 17
MESSENGER Constraints
Improved geodetic parameters provided by
MESSENGER
Value
Radius
1σ
2439.1km
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Mass
3.3012x1023kg
0.0004x1023kg
C/MR2
0.353
±0.017
Cmantle/C
0.452
±0.035
Smith, D., and 16 co-authors: Gravity Field and Internal Structure of Mercury
from MESSENGER, Science, vol. 336, pp. 214-217, 2012.
www.DLR.de • Chart 18
Four-Layer-Structural Model
Constraints:
 Mean Density
 MoI
 Cm/C
Model Assumptions:
 Hydrostatic Equilibrium
 Adiabatic temperature profile, 50 K nonadiabatic temperature jumps at CMB and ICB
 2nd order Birch-Murnaghan EOS
 Fe - inner core, Fe-FeS outer core
Parameters varied:
 Mantle Density between 3200 and 4300 kg/m3
 Crust Thickness between 50 and 150 km at
2700 kg/m3
 Core radius
 Inner Core Radius
Results:
 Outer Core Sulfur Content
 Density Profile
 Elastic Parameters
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www.DLR.de • Chart 19
Distribution of Models: Geodetic Constraints
Cm/C = 0.452
C/MR2=0.353
Cm/C
x
Margot et al. 2012
MoI
www.DLR.de • Chart 20
Distribution of Models: Geodetic Constraints
±1σ
x
±1σ
Mantle MoI
Cm/C = 0.452
C/MR2=0.353
MoI
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www.DLR.de • Chart 21
Variation of Mantle Density
mantle density:
4300 kg/m3
mantle density:
3700 kg/m3
x
mantle density:
3200 kg/m3
www.DLR.de • Chart 22
Distribution of Models
x
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www.DLR.de • Chart 23
Distribution of Models
x
Tidal Love Numbers h2 und k2
Body tide Love numbers are useful to set further constraints
on Mercury‘s interior structure.
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Model Range
Radius: 2439.1 km
Pressure: 0 GPa
Crustal thickness: 25...75 km
Pressure: 0.3...0.7 GPa
Radius: 1960...2060 km
Pressure: 4.5...6.5 GPa
Radius: 1...1000 km
Pressure: 20...40 GPa
Radius: 0 km
Pressure: 32...41 GPa
Geodetic constraints are satisfied by an olivine mantle and
a plagioclase-rich crust.
Mercury Surface
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First visual appearance: surface heavily cratered, very Lunar-like
Extensive volcanism
– Plains: km-thick deposits
sequentially emplaced
(alternative: impact ejecta
ponding?)
– Evidence for existence of
volcanic vents
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Evidence for high tectonic
activity in the past
– rapid cooling and contraction
– „Discovery Rupes“, thrust fault
system, 400 km long
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www.DLR.de • Chart 27
Seismological Observables Inferred from Structural Models of Mercury’s Interior > T. Steinke • > 14.06.2012
Volatiles and Crust Rock Chemistry
www.DLR.de • Chart 28
> Vortrag > Autor • Dokumentname > Datum
Magnetic Field
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Ice on Mercury?
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Rotational axis of Mercury
almost perpendicular to orbit
plane == >
– No seasons (!)
– Always low sun, long shadows
near poles
– Some crater floors near pole in
permanent shadow
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Arecibo radar discovered highly
reflective material in polar
craters, 1999
Ice? Sulfur?
Deposits in craters confirmed by
MESSENGER, 2011
450 km
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Lobate scarps and global contraction
• Mercury’s surface is dominated by
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Watters et al. (2009)
contractional features
“Lobate scarps” are the most
prominent tectonic landform
Surface-breaking thrust faults
resulting from global planetary
contraction due to secular cooling
and, possibly, core freezing
Measurements of scarps length
and elevation yield fault
displacement and, in turn, global
reduction of planetary radius
Global contraction from MESSENGER
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Byrne et al. (2014)
Early estimates of contraction based on Mariner 10
images between 1 and 2 km (e.g. Watters et al.,
2009)
Latest MESSENGER observations yield a significantly
larger contraction between 4 and 6 km due to global
coverage and better illumination conditions
(Di
Achille et al., 2012; Byrne et al. 2014)
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Global contraction and interior evolution
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Global contraction poses a tight
constraint on interior dynamics and
evolution
• Planetary radius changes can be
calculated from thermo-chemical
evolution models of mantle and core:
- interior heating / cooling ⟹
expansion / contraction
- partial melting and crustal production
⟹ expansion
- core solidification ⟹ contraction
Tosi et al. (2013)
Evolution scenario
Tosi et al. (2013)
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Initial heating phase accompanied by crustal production and expansion
Typical crustal thicknesses between 20 and 40 km
After ~1.5 Gyr: mantle and core cooling (~50 K/Gyr) and global contraction (~2
km/Gyr)
Mantle convection ceases after ~4 Gyr: Mercury may be dynamically inactive at
present
Models favor late core freezing with a small contribution to global contraction
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MESSENGER
Geodesy and Mapping Results
Topographic modeling using stereo images:
Quadrangle Scheme
H01
H05
H04
H09
H10
H14
H03
H08
H13
H02
H07
H12
H06
H11
H15
Map tile sheet, Mollweide projection
• For practical reasons the Mercury surface is separated into 15 tiles
• Northern hemisphere quads are used for MLA co-registration and comparisons of both
topographic products
• After completion of each tile all tiles will be combined to a homogenous global DTM
representation
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Northern high latitude and polar quads
• ~3,000 MDIS-NAC and ~17,000
MDIS-WAC G images used
• ~50,000 stereo pairs
• ~10 billion object points
• Mean intersection error ~40 m
H03
H04
H01
-5.4
Elevation [km]
3.0
-5.0
Elevation [km]
4.3
H05
H02
-4.9
-5.3
Elevation [km]
4.0
Elevation [km]
2.4
Elevation [km]
-5.6
3.6
H03 – Search for New Impact Basins
3.0
Elevation [km]
-5.4
DLR Stereo DTM
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H03 – Search for New Impact Basins
3.0
Elevation [km]
-5.4
Sobkou basin
(~ 800 km diameter)
DLR Stereo DTM
Global Topography
6.0
-6.0
Elevation [km]
Hill-Shaded Color-Coded DTM, 192 pixel/degree grid (222 m/pixel), Equidistant projection centered @ 0°
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Mercury Ellipsoid Parameters
MLA + Occultation DTM
Ellipsoid
a
2440.50 km
b
2439.37 km
c
2438.29 km
phi
(ellipsoid orientation)
-10.7°
Mean radius
2439.39 km
DLR Stereo DTM
Ellipsoid
a
2440.83 km
b
2439.36 km
CoF/CoM
offsets [m]
c
2438.24 km
dx
22
phi
(ellipsoid orientation)
-6.7°
dy
241
dz
-35
Mean radius
2439.48 km
Mercury: Pronounced oblateness and equatorial ellipticity!
Future Bepi Colombo Mission
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The BepiColombo Mission: Overview
Joint mission between ESA (European Space Agency) and JAXA (Japan Aerospace Exploration
Agency)
Target: Mercury, launch: 2016 , arrival: 2024, 1 year + 1 year extension
MCS (Mercury Composite Spacecraft): MPO (Mercury Planetary Orbiter, ESA), MMO (Mercury
Magnetic Orbiter, JAXA) which are carrying 16 instruments (11 MPO, 5 MMO), MTM (Mercury
Transfer Module), MOSIF (MMO sunshield and interface structure)
Total wet start mass of about 4100 kg
Orbit MPO: 1508x400, polar, orbital period 2.3 h, velocity between 2.2 and 3.0 km/s
MMO
MPO
The BepiColombo Mission: Objectives
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Investigate the origin and evolution of a planet close to the parent star
Study Mercury as a planet: its form, interior structure, geology, composition and craters
Examine Mercury's vestigial atmosphere (exosphere): its composition and dynamics
Probe Mercury's magnetized envelope (magnetosphere): its structure and dynamics
Determine the origin of Mercury's magnetic field Investigate polar deposits: their
composition and origin
Perform a test of Einstein's theory of general relativity
Current Status:
• FM (flight model) assembly, preparation SFT
(system functional test) under TV (thermal
vacuum) and TB (thermal balance) conditions
(summer/autumn 2014)
• Start of the assembly of the MCS 2015
Assembly MPO at TAS-I (Thales Alenia Italy)
> Lecture > Author • Document > Date
DLR.de • Chart 44
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BELA instrument: Overview
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The BepiColombo Laser Altimeter BELA is an instrument aboard the MPO
It will measure the distance between the MPO and the Mercury surface
Range
: 400 km to 1000 km
Laser type
: Nd:YAG
Frequency
: 1064 nm
Energy
: 50 mJ (BOL), 40 mJ (EOL)
Repetition rate
: 10 Hz
Receiver sensor
: Silicon APD
Mass
: 15,5 kg
Power consumption
: 44 W
Data rate
: 10 K bit/s
Measurement principle: Delay between the emission of a pulse and the receipt of the
reflected pulse is measured. This is converted to a distance using speed of light (z=c*t/2
with z:distance BELA - Mercury surface, c: speed of light in vacuo, t: time of flight of the
photons).
DLR: Tx, baffles, DPM, operating system, operations,
data processing and analysis
Measurements will be used to create a topographical
map of Mercury, graphic shows Mars (MOLA)
> Lecture > Author • Document > Date
DLR.de • Chart 45
BELA instrument: Overview
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www.DLR.de • Chart 47
Conclusions, Next Steps
- Messenger gravitational field data suggest that Mercury‘s core radius is
between 1980 km and 2060 km.
- Geodetic constraints can be satisfied by wide range of models.
- The solid inner core radius is less than half the core radius for a Fe-FeS
model; that is, it is within a range where geodetic parameters are not very
sensitive to the inner core radius
- The logical next step would be a lander mission although this will be a
difficult one from the point of
- Orbital dynamics
- Environment
- An interesting orbiter mission could try to map the gravity of the Hermean
core, do a better magnetic survey and try to get the water budget
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