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Tilman Spohn

German Aerospace Center, Berlin, Germany

The Terrestrial Planets Alpbach Summer School 2014: Geophysics of the Terrestrial Planets

2014-2015 The Rosetta Year

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Mission confirmation Nov. 1993

Launch March 2004

1st Earth gravity assist March 2005

Deep Impact observations Jume/July 2005

Mars gravity assist Febr. 2007

2nd Earth gravity assist Nov. 2007

Steins flyby Sept. 2008

3rd Earth gravity assist Nov. 2009

Lutetia flyby July 2010

Hibernation July 2011- Jan. 2014

Pre landing comet

characterization phase

June – Nov. 2014

Philae landing Nov. 2014

Escort Phase - Dec. 2015

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What is a Terrestrial Planet?

To the atmosphere scientist, a planet with an Earth-like atmosphere. Venus, Mars, and Mercury will

NOT qualify. Titan may!

To the geoscientist, a planet composed mostly of rock and iron. Venus, Mars and Mercury qualify. But

so do other planetary objects such as the Moon, Io, Pluto, and Ganymede. However, these are NOT

planets according to the IAU definition

According to IAU definition passed in 2006, a planet of the Solar System must have three

qualities:

it must be round, indicating its interior is in hydrostatic equilibrium;

it must orbit the Sun;

it must have gravitationally cleared its zone of other debris.

The discovery of Exoplanets has opened new samples of planets and planetary systems, most of the

discovered (presumably) rocky objects being sigifcantly more massive than Earth: Super Eartths and

Mega Earths

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Mercury Venus Earth Mars

Four Planets, four individuals! Many more Moons and „earthlike“ Exoplanets (tbc)

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Exoplanet Summary

Modified from Rauer et al. 2012

Still need more data, in particular mass AND radius

Mercury Venus Earth Mars Moon Ganymede Io

Radius 0.38 0.95 1.0 0.54 0.27 0.41 0.28

Mass 0.055 0.815 1.0 0.107 0.012 0.018 0.015

Density [kg/m3] 5430. 5250. 5515. 3940. 3340. 1940. 3554.

r0 [kg/m3] 5300. 4000. 4100. 3800. 3400. 1800. 3600.

MoI 0.34 ? 0.3355 0.3662 0.3905 0.3105 0.378

Rc/Rp 0.82 0.55 0.546 0.5 0.20 0.3 0.5

Dipole Moment

[1019 A m2]

4.9 <0.4 7980. <2.5 <4x10-9 14 ?

Data, Terrestrial Planets and Moons

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Mercury

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Venus

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Atmosphere

Composition

CO² 96,5%, N² 3,5%

Minor elements H²O, SO2, Ar, CO, He, Ne

Mass: 4.1019 kg

Pressure: 92 atm

av Temperature: 737 K

Wind speed: 1 – 4 km/h

Acid Rain!

0

20

40

60

80

100

120

Mar

s

Venus

Ear

th

Ar

H2O

O2

N2

CO2

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...and its Moon

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Mars

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Mars: Major Regional Features

MOLA global topography map (Mars Global Surveyor)

northern lowlands

southern highlands

Tharsis

volcanic province

Hellas

impact basin

Valles Marineris

Outflow

channels

seasonal ice cap

(both N and S pole)

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Atmosphere

Composition CO² 95%, N² 2,7%, Ar 1.6%, O²

0.13%

Mass: 2.17.1016 kg

Pressure: 0,006 atm

Temperature: 145 – 300 K (225 K average)

Wind speed: 10-30 km/h (Summer), 20 – 40 km/h (Fall), 60 - 100 km/h (Dust storms)

0

20

40

60

80

100

120

Mar

s

Venus

Ear

th

Ar

H2O

O2

N2

CO2

Chemical Components:

Gas (H, He), Ice (NH3, CH4, H2O),

Rock/Iron

Mars

Ganymede

Jupiter

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What is Planetary Geophysics?

Geophysics is the physics of the Earth and its space environment.

Earth's shape; its gravitational and magnetic fields; its internal structure and composition; its dynamics and their tectonic surface expressions, the generation of magmas, volcanism and rock formation.

A broader definition includes the hydrological cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial relations; and analogous problems associated with the Moon and other planets.

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Geophysical and Geodetical Methods (in a stricter sense)

Altimetry (Radar, Laser)

Gravimetry

Global field, local field

Magnetometry

Global magnetic field, electro-magnetic induction methods, rock and

paleo- magnetism

Seismology

Passive, active

Heat Flow

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Theory of Terrestrial Planets (Thermodynamics)

Interior Structure

Iron-rich core, rocky mantle and crust, phase transitions and chemical layerings,

variations with depth of thermodynamic and transport variables

Interior Dynamics

Core and mantle convection (heat transfer), volcanism, tectonism, magnetic field

generation

Rotation and Tides

Tidal dissipation, Seasons

Evolution

Accretion, differentiation (core formation, outgassing), cooling

Habitability and Life

Feedback to Planetary Evolution?

Important Elements of the Theory

Thermodynamic Properties, State variables

Density, Temperature, Pressure, Composition

Chemical Reactions, Phase Transitions

Energy Sources

Accretion, Differentiation, Radioactive Decay

(235, 238U, 232Th, 40K, 26Al, 60Fe), Tidal Heating

Transport Properties

Viscosity (strongly temperature and pressure

dependent), thermal conductivity, electrical

conductivity

Courtesy A. Plesa

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Technology Challenges for Planetary Exploration

Very limited resources

Power

Mass

Challenging environment

Temperature (Pressure)

Radiation

Communication

Autonomy

High risks – including financial

High demands – quality etc

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Mission Classes

Orbiters, (Fly-bys)

Geodesy, Geophyics packages

Laser altimeters, radio science package, stereo cameras

Goal: Geodetic network, figure of planet, gravity field and in general, explore what is out

there

Magnetometers and plasma packages

Magnetic field, Ionosphere, Magnetosphere

Landers, Networks, Rovers

Seismology, Heat Flow, Rock Magnetism

Sample Return

Rock physical properties (in addition to chemistry, mineralogy)

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Bepi Colombo

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Bepi Colombo

ESA Cornerstone Mission to Mercury

Launch 2016, Arrival 2020

Geodesy, Geophysics package (Laser

Altimeter, Radio Science, Stereo Imaging)

Magnetospheric Orbiter + Magnetometer on

Remote Sensing Orbiter

No lander unfortunately

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InSight

Geophyscial Observatory

Seismology, Heat Flow, Rotation

paramters, Magnetism

Mars Interior Structure

Interior, Evolution and Energy

Balance

NASA Discovery Mission with

European Payload

To be launched 2016

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Netlander

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Page 41 Perspektiven Prof. Dr.Tilman Spohn

Page 42 Perspektiven Prof. Dr.Tilman Spohn

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Mercury Venus Earth Mars

Terrestrial Planets vs Earth! Most important differences: Plate Tectonics and Life

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Earth

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Forms of Mantle Convection

Stein and Hansen 2008

Courtesy of Kai Stemmer+

Magnetosphere

Subduction,

regassing, and

enhanced cooling

Atmosphere

Biosphere

Hydrosphere

Crust

Core

Convective Cooling

Dynamo Action

Mantle

Volcanism Degassing

Space

Erosion by

solar wind;

Impacts

Shielding

Planets are Heat Engines

..that convert thermal into

gravitational, deformational

and magnetic field energy.

But the engine is an integral

part of a complex system!

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Habitability and Plate Tectonics

Many believe that (complex, evolved) life requires plate tectonics to operate

Plate tectonics recycles near surface rock and volatiles with the planet’s

interior through subduction. This helps

to cool the deep interior and to generate a magnetic field in the core

to create geologic diversity, e.g., granitic cratons that will form

continents and continental shelfs

to replenish depleted surface rock as the base for the nutrition chain

to help stabilize the atmosphere temperature in the Carbon Silcate and

other cycles

help generate a magnetic field

Water Cycle

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Planets and Life

Water consumption upon melting

Mantle regassing

Continental Crust

(& lithosphere)

Continental crust production

Dewatering

Free water (pores, cracks)

Water in stable minerals

Höning et al., 2013

Ocean-Continent Subduction Zone

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The model is gauged to the present Earth. Parameters are chosen such that with the present weathering rate, the present mantle water content and continental surface area is recovered

Results

0dt

dAcont

0dt

d

Continental

Surface

Area

Mantle

water

Reducing the weathering rate brings about two more equilibrium points, one unstable and a second stable point. The area of attraction of the dry stable point increases with further decreasing weathering rate .

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Bringing in time

biotic

abiotic abiotic

biotic

Perspectives in Planetary Science

Study the earthlike planets and satellites in the solar system: How do planets work?

Study asteroids and comets and exoplanet systems: How do solar systems form?

Adress the habitability of planets: What is the chance for (primitive) extraterrestrial life? Is life an universal phenomenon or is ours a “Rare Earth”?

How unique is the Earth, the solar system?

Planetary Science, Solar System Science, Astrobiology

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Acknowledgements

I profited from input from Doris Breuer,

Lena Noack, Tina Rückriemen, Wladimir

Neumann, Dennis Höning, Hendrik

Hansen-Goos, Allessandro Airo, Heike

Rauer, and Vlada Stamenkovic

I have used material from various sources

including the new Encyclopedia of the Solar

System ed. Spohn, Breuer, Johnson and

Vol. 10 of the Treatise on Geophysics, a

new edition getting to the market later this

year

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Rosetta

Thank You