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Planet Mercury: Basic Features

Planet Mercury: Basic Features

29.07.2014

www.DLR.de • Chart 1

Mercury Geodesy and Geophysics

J. Oberst & T. Spohn DLR Institute of Planetary Research, Berlin, Germany

Planet : Basic Features

• First , closest to the • Small elongation from the Sun == > difficult to observe from • High solar radiation, 10 x stronger than near Earth and high surface temperatures: max. 430°C • Small (4878 km Ø)Strindberg • Only slightly larger than • But e.g. smaller than Ganymede • High in density (5.4 gr/cm3) • Large iron-rich core? • Magnetic field • Dipole • Fluid outer core

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Mean Density Constraint

Planet Mercury: and Rotation

• High-eccentricity orbit (e=0.205)  Solar irradiation varies over the year • Mercury in spin/orbital 3:2 resonance  planet makes 3 full rotations while it moves about the Sun 2 times • Small librations (oscillations of mean rotational rate) Strindberg  amplitude / phase of librations linked to interior structure (molten core?)

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Mercury’s Dynamical State

Mercury Precession

• 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 . • Search for hypothetical planet, named Vulcan, but no such planet ever found

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Mercury Precession • 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 , 3.84 for Earth, 1.35 for )

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 2 - C/MR 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

• 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 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

equal obliquity line Mazarico’ 14 (Margot et al., 2012) H03 H05

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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 1σ Radius 2439.1km - Mass 3.3012x1023kg 0.0004x1023kg

2 ± C/MR 0.353 0.017 C /C 0.452 ±0.035 mantle

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 non- adiabatic 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

x

/C m C Margot et al. 2012

MoI

www.DLR.de • Chart 20

Distribution of Models: Geodetic Constraints

±1σ

Cm/C = 0.452

σ

1 C/MR2=0.353

x ±

MoI Mantle Mantle

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

• 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 • 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?

• 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 • 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 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 Watters et al. (2009) reduction of planetary radius

Global contraction from MESSENGER

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

• 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)

• 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 H03 H02

H10 H09 H08 H07 H06

H14 H13 H12 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 H03 MDIS-WAC G images used H04 • ~50,000 stereo pairs • ~10 billion object points • Mean intersection error ~40 m

H01

Elevation [km] Elevation [km] -5.4 3.0 -5.0 4.3

H02 H05

Elevation [km] -4.9 2.4

Elevation [km] Elevation [km] -5.3 4.0 -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 DLR Stereo DTM (~ 800 km diameter)

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 -10.7° (ellipsoid orientation) Mean radius 2439.39 km DLR Stereo DTM Ellipsoid a 2440.83 km CoF/CoM b 2439.36 km offsets [m] c 2438.24 km dx 22 phi -6.7° dy 241 (ellipsoid orientation) 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, 2.3 h, velocity between 2.2 and 3.0 km/s

MMO MPO

The BepiColombo Mission: Objectives • 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

• 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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