Next: Are all of the
Up: PART 5 - INTERESTING
Previous: PART 5 - INTERESTING
- Planets of the solar system
- How do we know about planetary compositions?
- measurement of densities. How? Masses and radii.
- absorption in atmospheres of planets (for those that have
atmospheres)
- What do we know about planetary compositions?
- When we measure mean planetary densities, we find that planets do
not all have the same density. The inner four planets have densities
of about 5gm/cm3 , while the outer planets have densities of about
1gm/cm3 . Pluto doesn't quite fit with the other; it has a density of
about 2gm/cm3 . To get a feel for what these numbers mean, note that
water has a density of 1gm/cm3 . The units, gm/cm3 , can be understood
by noting that the density in gm/cm3 just gives the mass of a small
cube of the material which is 1cm on each side.
- We see that planets are split into 3 classes: the inner planets
(terrestrial) which are more dense, the outer planets (Jovian) which
are less dense, and Pluto (and other Kuiper belt objects),
which is intermediate.
- Since the densities of the planets are different, we infer that the compositions of planets differ. The inner planets are made of rocky
material, which has higher density; these are often known as terrestrial
(earth-like) planets. The outer planets are made mostly
of low density gases; these are often known as Jovian (Jupiter-like) planets.
Pluto may be made of some combination of rocks and ice, and may represent
one of the largest of a third category of objects, called Kuiper belt objects,
that we discussed a bit in the first unit of the class.
- Why are the compositions of planets different from each other?
Why are the compositions different from those of stars?
What determines the composition of planets?
- The reason for different composition of planets has to do with how
the solar system formed. In the early solar system, there was a disk
of material rotating around the Sun, from which the planets eventually
formed. We've used this model to understand:
- how the planets get the transverse velocity needed to keep them
orbiting around Sun
- why all the planets orbit in the same direction (and in roughly
circular orbits)
- why all the planets orbit in the same plane
- To form planets required some kind of initial clump in
the protoplanetary disk. These clumps were probably formed from collisions
of solid particles. Different elements become solid at different temperatures:
the most common elements, hydrogen and helium, never became solid, but
hydrogen compounds (like water) can solidfy at colder temperatures, like
those in the outer solar system. In the inner solar system, it was too
hot for these compounds to solify; only rocks and metals can solidify at
these temperatures. Hence, only small planetessimals formed in the inner
solar system.
- In the atmospheres of planets, atoms are held to planets by gravity.
However, if an atom can move fast enough, it can escape the gravitational
pull of the planet, in the same way that we can launch spacecraft which
can escape the gravitational pull of the Earth by shooting them off
fast enough.
- In fact, atoms do move around. The amount they move is related to
their temperature. In a hotter environment, atoms move faster. However,
the speed that atoms move also depend on the type of element; at the same
temperature, heavier atoms move slower than lighter atoms.
- These properties can be used to generally understand why the
compositions of the inner planets differ from those of the outer planets.
- The outer planets are sufficiently massive and sufficiently cool that
no elements can escape their gravitational attraction, so their composition
is similar now as to what it was when they originally formed. This is also true
of the Sun. This primordial composition is the same as that of most of the
Universe: about 90% hydrogen, 9% helium, and only 1% for everything
else combined. The normal matter in the Universe is mostly hydrogen.
- The inner planets are less massive and hotter than the outer
planets, so many elements can escape their gravity. On Earth, hydrogen
and helium escape so they are not an important constituent of our
atmosphere. Instead, our atmosphere is mostly nitrogen and oxygen.
- On the smallest objects, e.g., Mercury and the moons of most
planets, gravity isn't sufficiently strong to retain any atoms -
so these objects don't have any atmospheres at all.
- Although these principles are the most important in determining the
composition of planetary atmospheres, other effects can also be important,
such as the presence of volcanism, life, or manmade pollution.
Next: Are all of the
Up: PART 5 - INTERESTING
Previous: PART 5 - INTERESTING
Jon Holtzman
2009-11-09