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Planet Mercury: Basic Features

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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 Mercury: Basic Features • First planet, closest to the Sun • 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 • Only slightly larger than Moon • But e.g. smaller than Ganymede • High in density (5.4 gr/cm3) • Large iron-rich core? • Magnetic field • Dipole • Fluid outer core 1 29.07.2014 Mean Density Constraint Planet Mercury: Orbit 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?) 2 29.07.2014 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 planets. • Search for hypothetical planet, named Vulcan, but no such planet ever found 3 29.07.2014 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 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 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 4 29.07.2014 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 5 29.07.2014 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 6 29.07.2014 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 7 29.07.2014 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 8 29.07.2014 www.DLR.de • Chart 17 MESSENGER Constraints Improved geodetic parameters provided by MESSENGER Value 1σ Radius 2439.1km - 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 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 9 29.07.2014 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 10 29.07.2014 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 11 29.07.2014 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. 12 29.07.2014 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 13 29.07.2014 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 14 29.07.2014 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 15 29.07.2014 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) 16 29.07.2014 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 17 29.07.2014 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 18 29.07.2014 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 19 29.07.2014
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