Lecture 2Formation of the Earth and the Terrestrial Planets

1
Lecture 2Formation of the Earth and the
Terrestrial Planets

• Meteo 466

2
Reading for this week

• Habitable Planet, Chapters 1-2
• In course packet
• Chambers, EPSL (2004)
• On website
• http//www.geosc.psu.edu/kasting/Meteo_466/Meteo_
  466_home.htm

3

• Lets start with topics that we wont talk about
  at any great length in this course
• First, one has to form the universe (the Big
  Bang)
• Then, one needs to form galaxies
• Then, one needs to form stars ?

4
Orion Nebula
Photo from HST

• The Orion nebula is a dense interstellar
• cloud of gas and dust in which stars are
• being formed

http//www.greatdreams.com/cosmic/orion852.jpg
5
Eagle Nebula (Pillars of Creation)
From Hubble Space Telescope
6
Horsehead Nebula (also from HST)
http//forums.airbase.ru/cache/sites/a/n/antwrp.gs
fc.nasa.gov/apod/image/0310/468x468/horsehead_cfht
.jpg
7
Cloud collapse/disk formation

• Then, one needs to form disks (circumstellar
  nebulae)
• This happens quite naturally if the interstellar
  material was spinning ?

http//www.aerospaceweb.org/question/ astronomy/so
lar-system/formation.jpg
8
Beta Pictoris (from HST)

• Beta Pic was the first such interstellar disk to
  be actually observed

9
Early stages of planet formation

• Dust settles to the midplane of the solar nebula
• The dust orbits slightly faster than the gas
  because it doesnt feel the effects of pressure
• Gas drag causes some of the dust to spiral
  inwards
• Turbulence is generated, lifting some of the
  dust out of the midplane
• If the dust density is great enough, then
  gravitational instability sets in, forming
  km-size planetesimals

Chambers, EPSL (2004), Fig. 1
10
Bipolar outflows
From The New Solar System, ed. 4, J.K Beatty et
al., eds., p. 16

• Material falls into the star along the midplane
  of the disk and is
• ejected towards the poles of the star
• Mass flows inward, angular momentum outward

11
Runaway growth stage

• Initially, the planetesimals were small
• Collisions make them grow if the relative
  velocities are small
• Dynamical friction keeps orbits circular and
  relative velocities low
• Gravitational focusing causes the largest bodies
  to grow the fastest
• Runaway growth of planetary embryos

Chambers, EPSL (2004), Fig. 2
12
Inner Solar System Evolution
Morbidelli et al., Meteoritics Planetary Sci.
(2000), Fig. 1
13
Eccentricity e b/a a 1/2 major axis b 1/2
distance between foci Sun-Earth
distances Aphelion 1 e Perihelion 1 - e
Today e 0.017 Range 0 to 0.06 Cycles
100,000 yrs


b
a
14
Final stage of accretion
Chambers, EPSL (2004), Fig. 3

• Results of four different simulations. Segments
  in the pie chart show
• the fraction of material coming from different
  parts of the Solar System.

15

• Back to generalities. Lets look at the results
  of planetary formation in more detail

16
Titius-Bode Law
Ref. J. K. Beatty et al., The New Solar System
(1999), Ch. 2.

• The logarithmic, or geometric, spacing is
  probably not an accident! The Solar
• System is packed, i.e., it holds as many
  planets as it can. If one tries to stick
• even a small planet inside it (except in the
  asteroid belt), it will be ejected.

17
Different planetary types

• There is a pattern to the planets in our Solar
  System
• Small, rocky planets on the inside
• Gas giant planets in the middle
• Ice giant planets on the outside
• Why does this happen this way, and should we
  expect this same pattern to apply elsewhere?

318 ME
95 ME
14.5 ME
17.2 ME
1 ME
18
Solar nebula composition
Ref. J. K. Beatty et al., The New Solar System
(1999), Ch. 14.

• The solar nebula is assumed to have the same
  elemental composition
• as the Sun
• Well talk later about how solar composition is
  obtained
• Different compounds condense out at different
  temperatures

19
Condensation sequence(high temperatures to low)

• Refractory oxides (CaTiO3, Ca2Al2SiO7, MgAl2O4)
• Metallic Fe-Ni alloy
• MgSiO3 (enstatite)
• Alkali aluminosilicates
• FeS (troilite)
• FeO-silicates
• Hydrated silicates (kinetically inhibited)

Ref. Lewis and Prinn, Planets and their
Atmospheres (1984), p. 60
20
Condensation sequence (cont.)

• 8. H2O
• 9. NH3
• 10. CH4
• 11. H2
• He
• Collectively, these last 5 compounds (or
  elements) are referred to as volatiles because
  they are either liquids or gases at room
  temperature
• Volatiles are important, as they are the
  compounds on which life depends most strongly
• So, how did planets acquire them?

21
Equilibrium condensation model

• 1 M? solar nebula (which is
• too high!)
• -- Nebula would be unstable if
• over 0.1 M?
• -- Minimum mass solar nebula
• ? 0.03 M?
• The curve along which the
• planets lie is an adiabat running
• along the midplane of the
• nebula

Venus
Earth
Mars
Ref. J. S. Lewis and R. G. Prinn, Planets and
Their Atmospheres (1984)
22
Problems with the equilibrium condensation model

• Assumed nebular mass (and thus pressure) was too
  high
• Formation of hydrated silicates is kinetically
  inhibited
• Gas-solid reactions are slow
• Actual planetary accretion problem is
  time-dependent
• The equilibrium condensation model applies only
  at a given instant in time
• Planetesimals can move from one part of the solar
  nebula to another
• This will be the key to understanding the origin
  of Earths volatiles