Transcript Lecture 2—Formation of the Earth and Moon

Formation of the Earth and the
Terrestrial Planets
Let’s start with topics that we won’t 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 
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
Eagle
Nebula
(“Pillars of
Creation”)
From Hubble
Space Telescope
Horsehead
Nebula
(also from HST)
http://forums.airbase.ru/cache/sites/a/n/antwrp.gsfc.nasa.gov/apod/image/0310/468x468/horsehead_cfht.jpg
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/solar-system/formation.jpg
Oort
Cloud
&
Kuiper
Belt
http://www.harmsy.freeuk.com/oimages/oort_cloud.jpg
• The Solar System also includes comets, both within the Kuiper Belt
(within the disk) and the Oort Cloud (spherical shell)
Early stages of planet formation
• Dust settles to the midplane of
the solar nebula
• The dust orbits slightly faster
than the gas because it doesn’t
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 kmsize planetesimals
Chambers, EPSL (2004), Fig. 1
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
Runaway growth stage
Chambers, EPSL (2004), Fig. 2
• 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
Inner Solar System Evolution
Morbidelli et al., Meteoritics & Planetary Sci. (2000), Fig. 1
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
b
Range:
0 to 0.06
a
Cycles: 100,000 yrs
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.
Back to generalities. Let’s look at the
results of planetary formation in more
detail…
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.
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
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
• We’ll talk later about how solar composition is obtained
• Different compounds condense out at different temperatures…
Condensation sequence
(high temperatures to low)*
1. Refractory oxides (CaTiO3, Ca2Al2SiO7,
MgAl2O4)
2. Metallic Fe-Ni alloy
3. MgSiO3 (enstatite)
4. Alkali aluminosilicates
5. FeS (troilite)
6. FeO-silicates
7. Hydrated silicates (kinetically inhibited)
*Ref.:
Lewis and Prinn, Planets and their Atmospheres (1984), p. 60
Condensation sequence (cont.)
8.
9.
10.
11.
12.
H2O
NH3
CH4
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?
•
•
Equilibrium
condensation
model
Venus
Earth
Mars
• 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
Ref.: J. S. Lewis and R. G. Prinn, Planets and Their Atmospheres (1984)
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 Earth’s
volatiles