Figure 1: 93KB Text
Credit: Hui Yang (University
of Illinois) and NASA
Figure 2: GIF Text
Credit: Robert P. Kirshner/ Harvard-Smithsonian Center for Astrophysics,
NASA
Figure 3: 124 KB
Text
Credit: Credit: Mark McCaughrean (Max-Planck-Institute for Astronomy),
C. Robert O'Dell (Rice University), and NASA
Figure 4: 80KB Text
Credit: Mark McCaughrean (Max-Planck-Institute for Astronomy),
C. Robert O'Dell (Rice University), and NASA
Figure 5: 760KB
Credit: NASA
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 Read
These Notes:
The solar nebula
hypothesis is in one form or another the most widely accepted theory of
how our solar system formed approximately 4.6 billion years ago. Although
details may vary, the general picture is widely accepted.
-
Originally
a large cloud of dust and gas (75% H and 24% He) become unstable.
The most dense part of the cloud started to collapse under the force
of gravity (figure 1). The causes of the instability can vary. One
possibility may have been a supernova explosion (def.)
(figure 2).
-
As the
size (radius) of the cloud decreased,, the collapsing cloud increased
its rate of rotation in order to conserve angular momentum (def.)
The effect is similar to that of an ice skater who must pull in his
or her arms in order to increase his or her rate of spin.
-
Just
as the oblateness (def.)
of a planet depends on its rotational velocity, so too, as the rotational
velocity of the cloud increases, it becomes more oblate forming a
disk called a solar nebula (def.)
(figure 3) Most of the matter in the collapsing cloud ended up in
a central bulge
-
As the
cloud collapse, gravitational energy (def.)
is released heating the central portion of the nebula where a protosun
(def.)
forms. (figure 4)
-
Meanwhile,
condensation(def.)
was occurring within the disk surrounding the protosun. Because temperatures
within the disk varied with distance from the center of the nebula,
different materials condensed at different locations within the disk.
Closer to the center, where temperatures were high, high temperature
condensates such as iron and silicates formed. Farther from the center,
where temperatures were low, hydrogen, water and other low temperature
condensates formed.
-
Collisions
between the newly condensed particles cause larger bodies called planetesimals
to accrete. This accretion (def.)
process continued eventually forming the planets and moons. These
violent and cataclysmic process of planetary formation is today evidenced
by the cratered surfaces of Mercury and our Moon (figure 5). The recent
collision between comet Shoemaker-Levy and Jupiter (Mpeg
Animation - 636KB) also gave us a glimpse at what probably
happened on a much grander scale 4.6 billion year ago.
-
The heat
generated by these impacts and by radioactive decay (def.)
of elements resulted in molten planets which subsequently became differentiated
(def.).
-
The evolving
star at the center of the solar nebula becomes a T-Tauri star at which
point it releases burst of energy. These bursts sweep light elements
such as hydrogen out of the outer solar system and into the solar
system where it is swept up by the distant outer planets.
-
The young
protosun gets hot enough to ignite the hydrogen its core. Thermonuclear
reactions in the core is what distinguishes a "sun" from a protosun.
- The terrestrial
planets evolve their secondary (Venus and Mars) and tertiary (Earth)
atmospheres.
Complete
These Self-check
Questions: These questions and their
answers are designed to help you determine how well you understand this objective
and to provide additional instruction.
- How do
the stages described in the solar nebula hypothesis (above) help to
explain the properties of the solar system?. Refer to this
multimedia presentation. This presentation has several
screens to it... you have to be patient because the download is self-timed
(meaning you cannot control it.)
Homework
Questions: Answer
these homework questions after reading the on-line lecture and the readings
in the text. Complete the self-check questions before attempting the homework.
Follow the instructions for submitting the homework on this unit
homework page.
- How does
the solar nebular hypothesis explain the low inclination of the planets'
orbital planes?
- How does
the solar nebular hypothesis explain the common direction in which the
planet's revolve?
- How does
thesolar nebular hypothesis explain the difference between the terrestrials
and the jovians?
- Describe
what a T-Tauri star is and its significance in the evolution of solar
systems.
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