Institute for Geophysics, Astrophysics, and Meteorology (IGAM)

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Introduction to Geophysics and Planetary Physics

Lecture, winter term 2015

Ulrich Foelsche & Günter Kargl

ulrich.foelsche@uni-graz.at

http://www.uni-graz.at/~foelsche/

The object of interest

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What‘s „Earth“? The Earth reaches up to the Magnetopause.

Source: MPI Aeronomie

Introduction to Geophysics and Planetary Physics

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(1) Origin of the Earth and of the Solar System

The Sombrero Galaxy seen from the edge – showing that spiral galaxies contain lots of dust – but in very small concentrations (Source: R. Colombari).

Star Formation (1) – Eagle Nebula

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Star Formation (1) – Eagle Nebula

Dark clouds of dust and hydrogen gas in the Eagle nebula M16 (~7 000 light years away, in the “Serpent” constellation), surrounded by young, luminous stars (Source: HST).

Protostellar clouds at the edge of the dust and gas pillars (each larger than our solar system) are places of star formation – lust like our sun, ~4.6 billion years (Gyr) ago.

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The life of a star is a constant battle between radiation and gravitation. In the early phase of a star’s live there are phases characterized by gravitational collapse (with a rapid increase in density), and equilibrium phases, when a strong temperature-induced pressure increase counteracts gravitation (Ralf Launhardt, SdW 08/2013).

Star Formation (2) – Collapse

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The Orion Nebula M42, 1 500 light years away, contains about 700 young stars (IR inset) and at least 150 protostellar clouds. Several of the them evaporate due to the intense UV radiation of four

young stars, building the „Trapeze“. Nr. 5 shows the side view of an Accretion Disc. 1 AU (Astronomical Unit) is 149.6 Mio. km, the mean Earth – Sun distance. Source: HST

IR

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Star Formation (3) – Orion Nebula

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Open Cluster of young stars: The Pleiades (Picture: R. Gendler)

Star Formation (4) – Star Clusters

Open Cluster in NGC 602 (Source: HST)

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Star Formation (6) – Star Clusters

In the center of a star energy is released by nuclear fusion. In sun-like stars the proton-proton chain reaction is dominant: four hydrogen nuclei (protons) ultimately yield a Helium nucleus. Only when the central temperature further increase, Helium nuclei can be fused to Carbon. In Red Supergiants, fusion processes in concentric shells produce heavy elements – but only up to Iron.

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Star Formation (7) – Nuclear Fusion

For another ~5 billion years our sun will remain a comparatively well-behaved main-sequence star. Then hydrogen shell burning around a helium core will lead to the inflation to a red giant. But habitability will end much earlier – the temperature increase in the sun’s center yields a (moderate) luminosity increase of about 0.7 % in 100 million years. This is, however, enough create uncomfortable conditions (for humans) about 500 million years from now (Ralf Launhardt, SdW 08/2013).

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Star Formation (8) – Evolution

Planet Formation (1) – Dying Stars

Planetary Nebula NGC6543 „Egg Nebula“ CRL2688 „Eskimo Nebula“ NGC6392

Planetary Nebulae (1)

At the end of the life of a red giant its outer layers are expelled, thereby enriching the interstellar medium with heavy elements – a prerequisite for the formation of terrestrial planets. UV radiation from the star “remainder” stimulates light emission in the expanding shell – which looks a bit like a planet in small telescopes (therefore the name).

All pictures: HST

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„Helix Nebula“ NGC7293

„Ring Nebula“ M47 Planetary Nebula IC418

Planetary Nebula NGC6751

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Planet Formation (2) – Dying Stars

Planetary Nebulae (1) All pictures: HST

Supernova remnants in the Cygnus constellation. Crab Nebula M1 Supernova 1987A in the LMC

Supernovae

A Supernova is a gigantic explosion of a massive star after gravitational collapse (if no more energy can be gained by nuclear fusion). At maximum a Supernova can be brighter than a whole galaxy. The outer layers are expelled, while the center collapses to a Neutron Star or even to a Black Hole. All (naturally occurring) elements heavier than iron have been formed during supernova eruptions (the heaviest ones like Gold or Uranium maybe even during the collision of binary neutron stars) .

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Planet Formation (3) – Dying Stars

All pictures: HST

The formation of Earth-like planets could only happen after the interstellar medium had been enriched by heavy elements from dying stars.Rotating gas and dust clouds contract to accretion discs, since only in the equatorial plane gravitation and centrifugal force ban be in equilibrium. The picture sequence (Source: Nature) shows the (successful) simulation of the formation of protoplanets in an accretion disc.

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Planet Formation (4) – Accretion Discs

Left: Accretion disc around HL Tauri (ESO), above: Planetesimals in an accretion disk (Artist impression, source: GEO). The collision of planetesimals forms protoplanets.

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Planet Formation (5) – Accretion Discs

Fomalhaut is a bright, young star in the constellation Piscis Australis (“Southern Fish”), 25 light years away. Fomalhaut is surrounded by a debris disk (accretion disk). The prominent ring is shaped by “Fomalhaut b”, the fist extrasolar planet detected at visible wavelengths (Source: HST).

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Planet Formation (6) – Discworld

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Planet Formation (7) – Synopsis

Credit: Nature

www.solarviews.com

The Solar System

NeptuneUranus

Saturn

Jupiter

Gas planets, note different scale

Terrestrial Planets Mars

Moon

Earth

Venus

Mercury

MarsVenusErde

Ganymede Titan Mercury

Io

Callisto

Moon Europa Triton Pluto

Earth

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Moon Formation – Giant Impact

Giant impact (Source: GEO). For about one hour the Earth was brighter than the Sun.

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Simulation of the moon formation. Two (already differentiated) protoplanets collide tangentially. During the second collision (9-16) the iron core of the impactor (blue) merges with the larger body (the Earth). Ejected mantle material (red) forms an accretion disk around the Earth – and later the moon.

The composition of the Moon’s is very similar the Earth’s mantle, suggesting that both formed in the same region of the accretion disc. The Moon’s Iron Core is, however very small, and the Moon is depleted in volatile elements. Only the “Giant Impact Hypotheses” can (largely) explain all findings.

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Moon Formation – Giant Impact

Age of Formation

Our actual stat of knowledge is, that the first planetesimals in our Solar system formed 4 567 million years ago within the protosolar cloud (with an uncertainty of just 2 million years). Right: Ca/Al-rich inclusion within Allende-Meteorite with a diameter of ~1 cm (the oldest material, which could be dated so far). The first „Planetary-Embryos“ formed within just about 100 000 years. Roughly 10 million years later the Proto-Earth was 2/3 „finished“. After another 20 million years. accretion was practically over, Proto-Earth war completely differentiated. The collision with a Mar-size body (Theia – the mythical mother of Selene, the Greek goddess of the moon) created the binary system Earth-Moon. All dates are base on radiometric dating methods (see next chapter).

Source: Nature

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Formation of the first Crust

The lava lake on Erta Ale (Ethiopia) as a „model“ for the formation of the primordial Earth’s crust. Plate tectonics only developed on Earth.

The “Giant impact” resulted in melting of the outer layers of the Earth. The surface then cooled slowly and crystallization resulted in the first crust – which was, however perforated by later impacts (Source: GEO).

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Crater with ~ 80 km diameter on the far side of the Moon (Apollo 11).

Short after their formation, the young planets suffered a heavy bombardment by remaining planetesimals (asteroids, meteorites and comets). Planets and moons without atmospheres (like the Earth’s Moon – left, or Mercury – right) still show their scars. But these impacts also delivered the water on Earth and organic compounds. Most Lunar Maria (“seas”) are remnants of the heaviest impacts (Pictures: NASA).

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Scars from Planet Formation

Scars from Planet Formation

The South Pole Aitken Basin on the far side of the Moon is invisible for observers on Earth. It has a diameter of 2 500 km.Source: GEO

Credit: NASA

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Vesta (573 km × 557 km × 446 km) is the second largest asteroid in the asteroid belt between Mars and Jupiter (Pallas is slightly larger but less dense (and massive). Vesta is differentiated and holds an iron core. The south pole regions features a giant crater – Rheasilvia – with 505 km diameter (!) and a 23 km high central peak (!).

Credit: NASA

Scars from Planet Formation

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Remnants of Planet Formation

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Credit: NASA

Ceres is even larger – but it counts as dwarf planet (the only one in the asteroid belt). Ceres is roughly spherical with an equatorial diameter of 963 km). The nature of the bright spots in the Occator Crater is still completely unknown and also Ahuna Mons looks very strange.

Asteroids and comets (above, composite: E. Lakdawalla) look indeed similar to the ones is „Star Wars“, but they are way further apart. Itokawa (right, length: ~500 m, credit: JAXA) seems to be a combination of a compact (upper) part and a “pile of rubbish”.

Quelle: NASA

Remnants of Planet Formation

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Churyumov–Gerasimenko (~4 km x 4.5 km, also known as “Chury” or (German) “Tschuri”) is an (awaking) comet nucleus with similar appearance as Itokawa. It is currently visited by the European Rosetta mission and its lander Philae O Günter can (will) tell you more (credit: ESA).

Remnants of Planet Formation

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On February 15, 2013 a mere 17 m diameter meteoroid exploded over the Ural mountains – with about 30 times the energy of Hiroshima atomic bomb (Credit: Alex Alishevskikh, Velentin Kazako, RMES).

Planetesimals on Collision Course

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