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Chapter 6: Planetological foundations for origins of life

Chapter 6: Planetological foundations for origins of life. 2 Planet formation – magic in the residue of stellar formation!. Emmanual Kant and Pierre-Simon Laplace: 18 th century giants. Kant-Laplace hypothesis: planets form in disks… verification 200 years later!

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Chapter 6: Planetological foundations for origins of life

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  1. Chapter 6: Planetological foundations for origins of life

  2. 2 Planet formation – magic in the residue of stellar formation! Emmanual Kant and Pierre-Simon Laplace: 18th century giants Kant-Laplace hypothesis: planets form in disks… verification 200 years later! Two major kinds: terrestrial (rocky) planets: like Earth giants (gaseous) planets: like Jupiter. Formation: terrestrial planets form by collisions of smaller bodies like asteroids? gas giants – gas accreting onto a massive rocky core; or by gravitational instability of disk?

  3. Flared, gaseous, dusty disk HH 30 (from HST) Star formation sets the stage for planet formation Gas Accretion & Gap-formation Protoplanet http://www.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html

  4. Planet formation theories • Giant planet formation; two mechanisms under intense investigation: 1. Core accretion model…. Coagulation of planetesimals that when exceeding 10 Earth masses, gravitationally captures gaseous envelope (eg. Bodenheimer & Pollack 1986) 2. Gravitational instability model …. GI in Toomre unstable disk produces Jovian mass objects in one go (eg. Boss 1998). • For either 1 or 2 – final mass determined by “gap opening” in face of disk “viscosity”. • Terrestrial planet formation; model 1 - do gaps open too?

  5. Core accretion: 3 phases: rapid growth of rocky core, slow accretion of planetesimals and gas, runaway gas accretion after critical mass achieved (near 10 ME) Problem: formation time still uncomfortably long: Jupiter at 5 AU forms in - 1Myr with 10 ME core - 5 Myr with 5 ME core Hubickyj et al 2005, Icarus

  6. GI:rapid formation within few thousand yrs - disk must have Toomre Q < 1 - disk must cool quickly (less than ½ orbital period – Gammie 2001) Problem:latter point not satisfied in detailed simulations (eg. Cai et al 2004) Mayer et al 2002

  7. When do giant planets quit growing? Gap opens in a disk when Tidal Torque ~ Viscous Torque Protoplanet Tidal Torque Disk Viscous Torque Disk

  8. Planetary masses: determined by gap opening Disk pressure scale height h [AU] Disk Radius a [AU] - Gap-opening mass ~ Final mass of a planet - Two competing forces (Tidal vs Viscous) - Smaller gap-opening masses in an inviscid disk Lin & Papaloizou (1993) Depends on disk physics! - disk flaring (h/a) – governed by heating of disk (ie central star - disk viscosity: very low in central region or dead zone

  9. Migration of planets - by tidal interaction with disk: a planet moves in very rapidly (within a million years!) but can be saved by dead zone ( Matsumura, Pudritz, & Thommes 2006) 30 30 20 20 Dead Zone Disk Radius [AU] Disk Radius [AU] 10 10 0 0 0 2×106 4×106 6×106 8×106 107 0 2×106 4×106 6×106 8×106 107 Time [years] Time [years] (w/o Dead Zone) (w/ Dead Zone) =10-3 =10-3 =10-5

  10. Detecting Jovian planets in other disks...close-up view with ALMA Mplanet / Mstar = 0.5 MJup / 1 Msun Orbital radius: 5 AU Disk mass as in the circumstellar disk as around the Butterfly Star in Taurus 50 pc 100 pc Wolf & D’Angelo (2005) astro-ph / 0410064

  11. Birth of a Solar System: what ALMA can do….. ALMA band 7 300 GHz = 1 mm resolution = 1.4” to 0.015” ~ Highest resolution at 300 GHz = 1 mm (0.015”) 100 AU = 0.3” at d=300pc ~ Highest resolution at 850 GHz = 350 mm

  12. Condensation sequence: accounting for compositions of planets • Temperature of disk drops as radius increases. • All materials whose condensation temperatures are higher than disk temperature at that radius can condense out into solids • - so hot innner region of disk has metals – outer cool regions have ices

  13. Biomolecule formation: organic molecules made in protostellar disks • Organic chemistry in “molecular layer” – 3 layer vertical structure at r > 100AU • 2D, stellar ultra-violet irradiation of disks: -molecules dissociated in surface layer, - abundant in gas phase in intermediate layer, - frozen out onto grains in densest layer. (Zadelhoff et al 2003, A&A). • Delivery system of biomolecules to Earth? Water, and biomolecules: by asteroids? comets? • Simulations: Typically find a few Earth ocean’s worth delivered by asteroids from beyond 2.5 AU.

  14. Cometary nucleus – few km in diameter; passage near Sun heats up coma of dust and gas; coma can be 100,000 in size; hydrogen envelope extends millions of km; Comets: Dirty snowballs Halley’s comet as seen in May 1910: May 10 – 30 deg tail; May 12 - 40 deg tail. Period of comet: 76 years

  15. Giotto images of Halley’s comet Evaporating dust and gas from Halley’s nucleus: 30 tons per second for comet inside 1AU – Halley’s comet would evaporate in 5000 orbits In general: density 100 kg/ cubic metre; temperature, few 10s of Kelvins; mass ; composition, dust mixed with methane, ammonia & water ices

  16. Cometary orbits – evidence for two distinct reservoirs of comets Isotropic distribution of comets at 50,000 AU: result of gravitational scattering? Oort cloud Disk-like distribution of comets beyond Neptune: remnant of original disk? Kuiper Belt

  17. Origin of oceans…. delivery of water by comets or asteroids? • Clue to origin of Earth’s water: HDO/H2O = 150 ppm = ½ of cometary value • Asteroids (carbonaceous chondrites) beyond ice line (2.5 AU) can have high water content • No more than 10% of Earth’s water from comets • Perturbations by Jupiter of asteroid system perturbs their orbits into ellipses that cross Earth’s orbit and collide,… bringing in water. • Do amino acids survive during this bombardment? • Evidence for bombardment: craters on Moon and elsewhere… and formation of the Moon itself in late heavy bombardment…

  18. Formation of the Moon – Impact Model 1. Mars – sized object collides with proto-Earth which has already formed iron core: much of impactor and debris encounters Earth a 2nd time. 2. Collision tears off Earth’s mantle material – Moon ends up with composition similar to Earth’s mantle • Debris from collision in orbit around Earth collects together to form the Moon: • < 10% of initial ejected material ends up accreting to form the Moon.

  19. Brief history of the Moon • Just after the end of the major meteoritic bombardment • b) Lunar vulcanism floods maria with lava ending 3 billion years ago • c) Original maria pitted with craters over last 3 billion yr

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