Transcript The Origin of the Solar System

The Origin of the Solar System
Interstellar Cycle
Ultimately,
stars form the
interstellar
medium.
Stars replenish
the interstellar
medium at the
end of their life
cycle.
There is a
balance between
the interstellar
medium and
stars.
The Interstellar Medium
• 99% gas
– Mostly Hydrogen and Helium
– Some volatile molecules
• H20, CO2, CO, CH4, NH3
• 1% dust
– Most common
• Metals (Fe, Al, Mg)
• Graphites (C)
• Silicates (Si)
Molecules Found
Based on spectroscopy, more than 136
molecules have been found in the ISM.
Among them the most common, H2 and the
more exotic like formaldehyde and glycine
and acetic acid.
Interstellar Molecules
Here are some
formaldehyde
(H2CO)
emission
spectra from
different parts
of M20:
How Did the Solar System Form?
Nebular contraction:
Cloud of gas and dust contracts due to
gravity; conservation of angular momentum
means it spins faster and faster as it contracts
How Did the Solar System Form?
Conservation of angular momentum says
that product of radius and rotation rate
must be constant:
Solar System Formation
How Did the Solar System Form?
Condensation
theory:
Interstellar dust grains
help cool cloud, and act
as condensation nuclei
Asteroid Ice
Belt
Line
Kuiper
Belt
T
Terrestrial
planets
Condensation – where things become solid
T
Jovian
planets
Where could the dust be solid?
Where could the ices be solid?
Where would you get terrestrial planets? Jovian planets?
The Condensation of Solids
To compare densities of planets,
compensate for compression due
to the planet’s gravity:
Only condensed materials
could stick together to form
planets
Temperature in the protostellar
cloud decreased outward.
Further out  Protostellar cloud
cooler  metals with lower
melting point condensed 
change of chemical composition
throughout solar system
How Did the Solar System Form?
Temperature in cloud determines where
various materials condense out:
Formation and Growth of Planetesimals
Planet formation
starts with clumping
together of grains of
solid matter:
Planetesimals
Planetesimals (few
cm to km in size)
collide to form
planets.
Planetesimal growth through condensation and accretion.
Gravitational instabilities may have helped in the growth of
planetesimals into protoplanets.
The Story of Planet Building
Planets formed from the same protostellar material
as the sun, still found in the Sun’s atmosphere.
Rocky planet material formed from clumping
together of dust grains in the protostellar cloud.
Mass of less than ~ 15
Earth masses:
Planets can not grow by
gravitational collapse
Earthlike planets
Mass of more than ~ 15
Earth masses:
Planets can grow by
gravitationally attracting
material from the
protostellar cloud
Jovian planets (gas giants)
The Growth of Protoplanets
Simplest form of planet growth:
Unchanged composition of
accreted matter over time
As rocks melted, heavier
elements sink to the center
 differentiation
This also produces a
secondary atmosphere
 outgassing
Improvement of this scenario:
Gradual change of grain
composition due to cooling of
nebula and storing of heat from
potential energy
Primary Atmospheres
• The primary atmosphere for every terrestrial world was composed mostly
of light gases that accreted during initial formation. These gases are
similar to the primordial mixture of gases found in the Sun and Jupiter.
That is 94.2% H, 5.7% He and everything else less that 0.1%.
• This primary atmosphere was lost on the terrestrial planets. Why?
– mass, radius of planet (factors of escape velocity of a planet)
– surface temperature (distance from Sun plus effects of atmosphere
heating)
– Mass of the atoms
• What determines if a particular atom is retained by a planet's gravitational
field? if the atom is moving less than the escape velocity for the planet, it
stays. If it moves faster than escape velocity, it escapes into outer space.
• So note that for the outer Jovian worlds, all the primary, initial atmosphere
is held. But for the inner worlds, most of the original H and He has been
lost. These inner worlds then will form a secondary atmosphere
composed of the outgassing from tectonic activity.
Secondary Atmospheres
• For the warmer terrestrial worlds, the light, gaseous elements (H, He) are
lost.
• The remaining elements are grouped into the rocky materials (iron,
olivine, pyroxene) and the icy materials (H2O, CO2, CH4, NH3, SO2). The
icy materials are more common in the outer Solar System, they are
delivered to the inner Solar System in the form of comets.
• The rocky and icy materials mix in the early crust and mantle. If the planet
cools quickly, there is little to no tectonic activity and the icy materials are
trapped in the mantle (like the Galilean moons). If the planet has a large
mass (which means lots of trapped heat from formation), then there is a
large amount of tectonic activity -> volcanos.
• The icy materials are turned to gases in the warm mantle and returned to
the planet surface in the form of outgassing to produce a secondary
atmosphere. The atmospheres of Venus, Earth and Mars are secondary
atmospheres.
• The composition of outgassing is similar for Venus, Earth and Mars and is
composed of 58% H2O, 23% CO2, 13% SO2, 5% N2 and traces of noble
gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven
primarily by the surface temperature and chemistry of the planet.
The Jovian Problem
Two problems for the theory of planet formation:
1) Observations of extrasolar planets indicate that
Jovian planets are common.
2) Protoplanetary disks tend to be evaporated quickly
(typically within ~ 100,000 years) by the radiation of
nearby massive stars.
 Too
short for Jovian planets to grow!
Solution:
Computer simulations show that Jovian planets can
grow by direct gas accretion without forming rocky
planetesimals.
Our Solar System
The Overall Layout of the Solar System
All orbits but Pluto’s are close to same plane
Orbits generally
inclined by no
more than 3.4o
All planets in almost
Exceptions:
circular (elliptical)
Mercury (7o)
orbits around the
Pluto (17.2o)
sun, in approx. the
same plane
(ecliptic).
Planetary Orbits
Mercury
Venus
Earth
Sense of revolution:
counter-clockwise
Sense of rotation:
counter-clockwise
(with exception of
Venus, Uranus,
and Pluto)
(Distances and times reproduced to scale)
Terrestrial and Jovian Planets
Survey of the Solar System
Relative Sizes
of the Planets
Assume, we reduce all
bodies in the solar system
so that the Earth has
diameter 0.3 mm.
Sun: ~ size of a small plum.
Mercury, Venus, Earth, Mars:
~ size of a grain of salt.
Jupiter: ~ size of an apple seed.
Saturn: ~ slightly smaller than
Jupiter’s “apple seed”.
Pluto: ~ Speck of pepper.
Two Kinds of Planets
Planets of our solar system can be divided into
two very different kinds:
Terrestrial (earthlike)
planets: Mercury,
Venus, Earth, Mars
Jovian (Jupiter-like) planets:
Jupiter, Saturn, Uranus,
Neptune
Terrestrial Planets
Four inner
planets of the
solar system
Relatively
small in size
and mass
(Earth is the
largest and
most massive)
Rocky surface
Surface of Venus can not be seen
directly from Earth because of its
dense cloud cover.
Craters on Planets’ Surfaces
Craters (like on
our Moon’s
surface) are
common
throughout the
Solar System.
Not seen on
Jovian planets
because they
don’t have a
solid surface.
The Jovian Planets
Much lower
average density
All have rings
(not only Saturn!)
Mostly gas; no
solid surface
Clearing the Nebula
Remains of the protostellar nebula were cleared away by:
• Radiation pressure of the
sun
• Solar wind
• Ejection by close encounters with
planets
• Sweeping-up of space debris by planets
Surfaces of the Moon and Mercury show
evidence for heavy bombardment by asteroids.
Evidence for Ongoing Planet Formation
Many young
stars in the Orion
Nebula are
surrounded by
dust disks:
Probably sites of
planet formation
right now!
Dust Disks Around Forming Stars
Dust disks
around
some T
Tauri stars
can be
imaged
directly
(HST).
Quiz Questions
1. How is the solar nebula theory supported by the motion of
Solar System bodies?
a. All of the planets orbit the Sun near the Sun's equatorial
plane.
b. All of the planets orbit in the same direction that the Sun
rotates.
c. Six out of seven planets rotate in the same direction as the
Sun.
d. Most moons orbit their planets in the same direction that the
Sun rotates.
e. All of the above.
Quiz Questions
2. Which of the following is NOT a property associated with
terrestrial planets?
a. They are located close to the Sun.
b. They are small in size.
c. They have low mass.
d. They have low density.
e. They have few moons.
Quiz Questions
3. According to the solar nebula theory, why are Jupiter and
Saturn much more massive than Uranus and Neptune?
a. Jupiter and Saturn formed earlier and captured nebular gas
before it was cleared out.
b. Jupiter and Saturn contain more high-density planet building
materials.
c. Uranus and Neptune have suffered more interstellar wind
erosion.
d. Both a and b above.
e. All of the above.
Quiz Questions
4. How does the solar nebula theory account for the drastic
differences between terrestrial and Jovian planets?
a. The temperature of the accretion disk was high close to the
Sun and low far from the Sun.
b. Terrestrial planets formed closer to the Sun, and are thus
made of high-density rocky materials.
c. Jovian planets are large and have high-mass because they
formed where both rocky and icy materials can condense.
d. Jovian planets captured nebular gas as they had stronger
gravity fields and are located where gases move more slowly.
e. All of the above.
Quiz Questions
5. What is the difference between the processes of
condensation and accretion?
a. Both are processes that collect particles together.
b. Condensation is the building of larger particles one atom (or
molecule) at a time, whereas accretion is the sticking together
of larger particles.
c. Accretion is the building of larger particles one atom (or
molecule) at a time, whereas condensation is the sticking
together of larger particles.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
6. Which of the following is the most likely major heat source
that melted early-formed planetesimals?
a. Tidal flexing.
b. The impact of accreting bodies.
c. The decay of long-lived unstable isotopes.
d. The decay of short-lived unstable isotopes.
e. The transfer of gravitational energy into thermal energy.
Quiz Questions
7. Which of the following accurately describes the
differentiation process?
a. High-density materials sink toward the center and lowdensity materials rise toward the surface of a molten body.
b. Low-density materials sink toward the center and highdensity materials rise toward the surface of a molten body.
c. Only rocky materials can condense close to the Sun,
whereas both rocky and icy materials can condense far from
the Sun.
d. Both rocky and icy materials can condense close to the Sun,
whereas only rocky materials can condense far from the Sun.
e. Small bodies stick together to form larger bodies.
Quiz Questions
8. How did the solar nebula get cleared of material?
a. The radiation pressure of sunlight pushed gas particles
outward.
b. The intense solar wind of the youthful Sun pushed gas and
dust outward.
c. The planets swept up gas, dust, and small particles.
d. Close gravitational encounters with Jovian planets ejected
material outward.
e. All of the above.
Answers
1.
2.
e
d
3.
4.
5.
6.
7.
8.
a
e
b
d
a
e