Solar System
The Solar System
This set of notes by Nick Strobel covers: the solar system--atmospheres of
planets, a comparison of Earth-Venus-Mars, magnetic fields, meteorites,
comets, and the formation of the solar system.
These notes will be in outline form to aid in
distinguishing various concepts. As a way to condense the text a bit, I'll
often use phrases instead of complete sentences. Vocabulary terms are
italicized.
A. Jovian -- Terrestial planets
- Jovian planets (Jupiter-like) have THICK atmospheres with
proportionately small solid cores while terrestrial planets (Earth-like) have
thinner atmospheres with proportionately large solid parts.
-
- Thickness of atmosphere depends on planet's gravity and temperature.
- Escape velocity--speed needed to escape object's gravitational
pull. At the escape speed, an object's Kinetic Energy [0.5
object mass
] = its
Potential Energy [G object mass
planet mass /
(distance to planet's center)]. A more massive planet
will have a higher escape velocity. An object at a larger distance from the
planet's center will have a lower escape velocity.
- Temperature of gas is measure of the average kinetic (motion)
energy of the gas. Higher temperature means faster moving gas particles.
A gas particle's Kinetic Energy [0.5
object (particle) mass
]
= Kinetic Energy [3
k
temperature / 2]. At a given temperature, the more massive gas particle will
have a slower average speed.
- Higher temperature tries to dissipate atmosphere while higher gravity tries to retain atmosphere. To find out which of these competing effects wins,
you must do the calculation comparing a gas particle's average speed and the
planet's escape velocity. If the particle's average speed is close to the
escape velocity, then those type of gas particles will not remain for billions
of years.
- Jovian planets are massive and cold so can retain light gases like Hydrogen and Helium.
A flowchart of this is given on the escaping atmosphere page.
-
- There exists a temperature difference between the
``surface'' of planet and space, and the energy flows from hotter to cooler
areas. There are different ways of transporting energy.
- Radiative--photons (energy packets) leak outward scattering off
gas particles. Nature prefers this way.
- Conduction--fast moving atoms collide with other atoms imparting
some of their motion to them. Not useful in a gas.
- Convection--bulk motions of material. Hot air below expands and
its density decreases so it rises. Cooler, denser air falls and displaces hot
air. As hot bubble rises, it cools; falling cooler air heats up when it comes
into contact with the ``surface''. Energy conveyor belt motion of gas. Nature
does this only if the temperature varies rapidly enough with distance (steep
temperature gradient).
-
- Venus is slightly smaller than Earth, but has a thick
(97%)
and 
(3%) atmosphere.
- The far-IR and radio thermal continuous radiation says Venus has a HOT
surface (750 K = 477 degrees C which is hot enough to melt lead!); over twice
what it would be if the atmosphere was absent. Spacecraft and landers confirm
high temperatures. Spectroscopy says the atmosphere is mostly . Landers
find surface pressure = 90 Earth atmospheres = pressure nearly 1 km below ocean
surface on Earth!
- Greenhouse Effect--see the greenhouse page
for a graphical
picture. Visible light from sun hits the surface to heat it up. The surface
re-radiates the energy in the form infrared radiation. Some of the IR is
absorbed by the atmosphere gases like
and
and radiated back
toward surface. This is like terrestrial greenhouses used to grow plants (hence
the name for the effect). The originally started the
greenhouse;
now keeping it going.
- Where did Venus' water go? Ultraviolet dissociation of water.
Gaseous water rises high enough to to be dissociated (broken apart) by UV
light from the
Sun. Hydrogen escapes and Oxygen combines with other atoms. Water disappears.
- How do we know that Venus originally had more water? Take a look at the
abundances of Hydrogen isotopes. An isotope of a given element will have
the same number of protons in the atomic nucleus as another isotope of that
element but not the same number of neutrons. An isotope with more particles in
the atomic nucleus will be more massive (heavier) than one with less particles
in the nucleus. Ordinary Hydrogen as only one proton in the nucleus, while the
isotope ``Deuterium'' has one proton + one neutron (so it's about twice as
heavy). On Earth the ratio of ordinary Hydrogen to Deuterium is 1000 to 1,
while on Venus the H/D ratio is 100 to 1. We assume that the H/D ratio on Venus
and Earth were originally the same so something caused the very light Hydrogen
isotopes on Venus to disappear (UV disassociation of water!).
- See the Earth-Venus-Mars summary page:
Water starts greenhouse heating. Carbon Dioxide baked out of rocks, further aggravating heating, baking more out of the rocks. Runaway greenhouse. Water dissociates away;
maintains greenhouse.
-
- Mars has 1/10 Earth mass and a thin (95%) and
(3%)
atmosphere.
- Thin atmosphere (1/100 Earth's) means insignificant greenhouse and rapid
cooling between night and day. Night and day temperature differences create
strong winds. Winds whip up dust that can cover entire planet in a few weeks
time and keep it covered for 2-3 months.
- Evidence for running liquid water in past. None now-air pressure too
small for liquid water to exist on Mars (the frozen water turns directly into
a gas without going through a liquid phase).
- Runaway refrigerator--see the Earth-Venus-Mars summary page.
A cooler
Mars enables liquid water to form. Atmospheric dissolves in liquid
water
and is then locked up in the rocks. Temperature drops (less greenhouse heating)
so more water vapor condenses into a liquid making more
locked in the
rocks so the temperature drops even more, etc. Water now frozen in permafrost
below surface.
-
- Earth.
(78%) and
(21%) atmosphere. Nice temperature.
Liquid and some water vapor.
- Bizarre atmosphere! Free Oxygen loves to react with other
atoms/molecules. Life keeps free Oxygen around. The presence of free Oxygen is
one signature of planets with life beyond our solar system.
- Most water is liquid. Some water vapor and gaseous create small
greenhouse, raising temperature (about 30 degrees C above level if the water
wasn't there). There is the equivalent of 70 atmospheres of
locked up in the rocks. See the Earth-Venus-Mars summary page.
- ``Cold Trap'' below the ozone layer. Ozone layer absorbs UV light from
sun so no UV dissociation of water. If water get up too high, it liquifies and
rains back to surface. This height is below the ozone layer--a cold
trap.
-
- Planet magnetic field acts like giant bar magnet in center of planet
that can be aligned differently than rotation axis. Charged particles spiral
around magnetic field lines. Energetic charged particles from sun (solar
wind) are deflected by Earth's magnetic field. Magnetic field acts as a shield.
Charged, spiralling particles can produce non-thermal radiation-Jupiter radio
noise.
-
- Aurorae: some solar wind particles spiral toward magnetic poles,
crashing
into atmospheric molecules, exciting them. Emission lines produced as the
electrons in the atmospheric gas particles drop to lower energy levels. Sky
glows and shimmers. Red light produced by Hydrogen gas and green light produced
by Oxygen gas. The astronomy department at Rice University (Houston, TX) has
a more indepth web page on the interaction of the Earth's magnetic field with
the solar wind called Space Weather.
-
- Ingredients for a magnetic field: 1) liquid conducting interior (metallic);
2) Rapid rotation--conducting material needs to be moving about. Magnetic dynamo theory.
- Venus has liquid conducting interior but spins slowly, therefore it has
no magnetic field.
- Mars spins fast but no liquid interior, therefore it has no magnetic
field.
- Earth spins fast and has a liquid conducting interior, therefore is has a
magnetic field.
- Jupiter has liquid metallic Hydrogen interior and spins very rapidly,
therefore it has a HUGE magnetic field.
- Mercury spins slowly and interior should be solid BUT it has small magnetic field!
-
- Meteorites--small rocks from space that make it to the Earth's
surface. A ``meteoroid'' is a small rock in space and a ``meteor'' is a rock
originally from space that is now in the Earth's atmosphere. There are three
basic types of meteorites.
- STONES 95-97% of the meteorites are these with 85% of the stones being
primitive-unchanged since they first solidified about 4.6 billion years
(4.6 Gyr) ago. Most primitive ones have chondrules--frozen droplets of matter from
early solar nebula. Chondrules are only found in primitive stones!
Carbonaceous meteorites (some have chondrules) are the oldest.
Carbonaceous meteorites contain silicates, carbon compounds, and water (around
22%!). Some even have organic molecules (amino acids).
Other primitives are made of silicates and flecks of metal like Iron.
All primitives are about 4.6 Gyr old.
Differentiated stones (10-12%) are from differentiated parent
objects. Therefore, they are younger, say, 4.4 Gyr. All stones have densities
around 3 g/
. The stoneys look like Earth rocks.
- STONEY-IRONS 1% of the meteorites are these. They come from a
differentiated body at the interface between a metal core and a rock mantle. Varying
mixture of metal (Iron and Nickel) and rock (silicates). Age 4.4 Gyr. Densities
4-6 g/.
- IRONS 2-3% of the meteorites are these, though around 40% of the
finds are these since they are easily distinguished from Earth rocks.
The come from a differentiated body core (iron and nickel). Densities around 7
g/. Irons
sometimes have large, coarse-grained crystalline patterns
(Widmanstatten patterns) that is evidence that they cooled slowly. Age
4.4 Gyr
- Primitive meteorites hold clues to composition and temperatures in early solar nebula.
- Most stoneys look like Earth rocks--hard to spot. The rare irons are
easy to distinguish from Earth rocks. Go to Antarctica where stable white ice
pack makes darker meteorites easy to find. Get an unbiased sample of
meteorites. Most meteorites from asteroids and most asteroids have compositions
(determined by spectroscopy) similar to stones. A few are from the Moon.
A select few may be from Mars (SNC meteorites).
- Differentiation--early planetoid is hot liquid and the heavy
materials sink toward the center while the lighter stuff floats up to the top.
Some meteorites come from larger differentiated asteroids that have been broken
up.
- Radioactive Dating--absolute dating system using atoms that spontaneously break apart into more stable smaller atoms.
- a.
- Isotope--particular form of element. Isotopes of a given
element have the same number of protons BUT different number of neutrons in
nucleus so they have the same chemistry but different nuclear reactions.
- b.
- Parent and daughter isotopes--the parent is the original
isotope (e.g., Uranium-238 or Potassium-40) and the daughter is the end product
(e.g., Lead-204 or Argon-40).
- c.
- Half-life--large number of radioactive isotopes decay in a
regular exponential way such that after one half-life, 1/2 of parent material
has decayed to daughter material. Find solidification age.
-
- Comets are small (few hundred meters to about 20 kilometers)
``potato-shaped'' objects made of dust and gas (``dirty icebergs''). They
are primitive objects--unchanged since they first solidified about 4.6
billion years (4.6 Gyr) ago. Examples: Halley, Shoemaker-Levy 9, and Hale-Bopp.
This picture is courtesy of David Doody at JPL and is part of the
Basics of Space Flight
manual for all operations personnel.
- When it gets close to the Sun, the comet changes and we see these
parts:
- a.
- Nucleus--all the material comes from here. It's 0.5-20 km in size,
potato-shaped conglomerate of dust (silicates and carbonaceous) embedded in ice
(frozen water, carbon dioxide, carbon monoxide, and methane) and has a mass
of only
kg (the Earth is 
kg). When a comet
nears the Sun around the Jupiter-Saturn distance, it warms up. Ices
sublime--abruptly change from solid to gas. Jets
of material can alter orbit (remember Newton's third law of motion?)
We have pictures of one comet's nucleus up close:
Halley's Comet.
- b.
- Coma--gas and dust pouring out from nucleus forms huge envelope
surrounding it (a cometary ``atmosphere'') 100,000's km across. Nucleus' low
gravity (you could jump off it!) cannot hang onto the escaping dust and gas.
- c.
- Tails--sunlight pressure and solar wind form two tails--ion
tail and dust tail around Mars' distance. Solar wind travels along solar
magnetic field lines extending radially outward from the Sun. Ultraviolet light
from the Sun ionizes some gases from coma and these charged particles are forced
along magnetic field lines to form the ion tail millions of km long. Bluish
ion tail acts like a ``solar'' wind sock. Dust grains pushed by solar wind collisions
and collisions with solar photons. Dust forms long curved tail that lies
slightly farther out from the Sun than nucleus' orbit. Millions of km long.
Comet West exhibited these two tails nicely in its
1976 passage.
- d.
- Hydrogen cloud--water vapor from nucleus' jets are dissociated by
solar UV. Tens of millions of km across.
- Comet orbits.
- a.
- Oort Cloud--large spherical cloud 50,000-100,000 A.U.
surrounding the Sun filled with billions to trillions of comets. Has not
been observed.
- i.
- Existence has been inferred from observations of long period
comets. Long period comets have very elliptical orbits and
come into inner solar system from all different random angles (not just along
ecliptic). Use Kepler's 3rd law to find orbital periods of 100,000's to
millions of years. Kepler's 2nd law says they spend 2-4 years in inner part of
solar system where the planets are and most of their time at 50,000-100,000
A.U. We find several long period comets every year. All this implies a large
spherical cloud 50,000-100,000 A.U. surrounding the Sun filled with billions
to trillions of comets.
- ii.
- Passing stars tug on Oort cloud comets, ``perturbing'' their orbits so
some of them go through inner solar system. Long period comets are
sometimes deflected by a jovian planet into an orbit with a shorter period
(decades). Jupiter and Saturn tend to deflect long period comets completely out
of the solar system (or gobble them up--Shoemaker Levy-9) while Uranus and
Neptune tend to deflect the long period comets into orbits that stay within the
solar system. Halley is an example?
- b.
- Kuiper Belt--disk of 100's of millions of comets from 30-100+
A.U. from the Sun orbiting roughly along the ecliptic. First observed 1992.
- i.
- Short period comets (those with orbital period less than 200 years) have smaller orbits (Kepler's 3rd law). Their perihelia are around the
terrestrial planets' distances from the Sun and their aphelia are just beyond
Neptune (at 30 A.U.) and roughly along the ecliptic. Originally from Kuiper
Belt, their orbits were perturbed by Neptune and Uranus and made more
elliptical. Examples: Encke, Giacobini-Zinner, Shoemaker-Levy 9.
- ii.
- Objects observed from ground are 100-300 km in size and orbit between 30
and 60 A.U. from the Sun. Right now there are 28 observed. At least 29
smaller objects (10-20 km diameter) have been observed with the Hubble Space
Telescope. Chiron (170 km diameter) and 5 others orbiting between Saturn (9.5
A.U.) and Uranus (19.2 A.U.) are other members. Pluto (2300 km diameter) and
its moon, Charon (1200 km diameter), may be members.
The currect list of objects of the Kuiper Belt is at the Minor Planets Center.
They keep a list of the tran-Neptunian objects
and a list of the Centaurs
which are small bodies orbiting between Jupiter and Neptune (like Chiron and
5145 Pholus). Select here to bring up a plot of the
positions of the observed Kuiper Belt objects.
-
- a.
- Comets formed 4.6 billion years ago along with the rest of the
planets from the same solar nebula material. Too small and cold to undergo any
geologic activity (they did not differentiate), so they preserve the record of
the early solar nebula composition and physical conditions.
- b.
- What happened to them after they formed depended on where they were.
Comets close to Jupiter and Saturn were ``gravitationally slingshot'' and
ejected from the solar system. Those around Uranus and Neptune were deflected
outward to form the Oort cloud. Those further out never coalesced to form a
planet and now make up the Kuiper Belt.
- c.
- Some comets were deflected to the inner planets and the Sun. Water on
Venus, Earth, and Mars may have come from comets!
- d.
- Short period comets make 100's - 1000's of passes around the Sun spewing
out gas and dust. The dust bits (size of a grain of sand or smaller) hitting
Earth's atmosphere make meteor showers. Perseids in mid-August are due
to Swift-Tuttle and Leonids in mid-November are due to Tempel-Tuttle.
Eventually the nucleus becomes ``dead.''
-
- a.
- Comet Introduction
is a great place to start. It is part of the Views of the Solar System
created by Calvin Hamilton. The comets page talks about several well-known
comets and all the spacecraft that have visited comets.
- b.
- Comet Photo Gallery at NSSDC. Great photos!
- c.
- Comet Hale-Bopp
homepage. This one looks like it could be a spectacular one in early spring
of 1997. Stay tuned!
- d.
- Comets Online
has practically all the links you need for comet information on the net.
- e.
- Bill Arnett's tour of the planets includes a stop at some
comets.
-
- Observables--what any model of the formation of the solar system must
account for:
- a.
- All the planets' orbits lie roughly in same plane.
- b.
- Sun's rotational equator lies nearly in this plane.
- c.
- Planetary orbits slightly elliptical-nearly circular.
- d.
- Planets and Sun revolve in same west-to-east direction.
- e.
- Planets differ in composition. Composition varies roughly with
distance from Sun: dense, metal-rich planets in the inner part; giant,
Hydrogen-rich planets in the outer part.
- f.
- Meteorites differ in chemical and geologic properties from planets
and Moon.
- g.
- Sun and most planets (Uranus & Venus exceptions) rotate in same
west-to-east direction. Obliquity (tilt of rotation axis with respect to orbit)
usually small.
- h.
- Planet and asteroid rotation rate is similar-5-15 hours, unless
tides slow them down.
- i.
- Planet distance obey Bode's law--a descriptive law that has
no theoretical justification. Neptune serious
exception.
- j.
- Planet-satellite systems resemble solar system.
- k.
- Oort cloud of comets.
- l.
- Planets contain about 90% of solar system's angular momentum--see
the angular momentum page.
-
- Condensation Model is the preferred one. Here are its features and how it
explains the observable items above.
- a.
- Gas cloud with dust collapses. As it collapses its slight rotation increases-conservation of angular momentum (see the angular momentum page).
- b.
- Centrifugal effects cause outer parts of nebula to flatten into a
disk, while central part of nebula forms Sun. Planets form in disk (item a) and
the Sun is part of the disk (item b).
- c.
- Gas molecules and dust grains move in circular orbits. Those on
noncircular orbits collide with other particles and eventually dampen
noncircular motion. Large scale motion is parallel, circular orbits (items c
and d).
- d.
- Collapsing gas and dust heats up through collisions among particles.
Heats up to around 3000 K so everything in gaseous form. Hydrogen (about 90%)
and Helium (about 10%) make up most of nebula with silicates and Iron
compounds making up about 1%. Nebula cools with outer parts cooling off more
than inner parts (that are close to hot proto-sun). Metal stuff can condense
(freeze) at high temperatures while volatile stuff can condense only at lower
temperatures. Local temperature and density depend on distance from proto-sun
(item e). Around Jupiter distance the temperature is cool enough to freeze
water (``snow line''). Further out have ammonia and methane material freezing
out. Chondrules of highest freezing temperature material form and become
incorporated in lower freezing temperature material; planets will also
differentiate later on so heavy metals in core and lighter metals nearer
surface. (item f).
- e.
- Gas and dust particles in parallel, circular orbits with small eddies
collide at low velocities. Stick together by gravity and electrostatic forces.
Coalescing particles tend to form bodies rotating in same direction as
revolution with similar rotation rates (items g and h). Gravity tends to divide
nebula into ring-shaped zones (later form planets-item i).
- f.
- More massive planetesimals pull in more of surrounding nebula. Some
can form mini-solar nebulae to form moons (item j). Jupiter and Saturn have a
lot of water ice mass, so can sweep up a lot of Hydrogen and Helium. Uranus and
Neptune less so.
- g.
- Icy planetesimals near Jupiter and Saturn flung out of solar system.
Those near Uranus and Neptune flung to large orbits (Oort cloud-item k).
- h.
- Early Sun has magnetic field and spews out ions. Ions dragged along
by magnetic field rotating with sun. Dragging ions brake the Sun. Also
accretion disks like solar nebula tend to transfer angular momentum outward
(item l).
- j.
- Proto-sun core gets to about 10 million degrees Kelvin and starts
fusion. Sun turns on. T-Tauri winds sweep out rest of nebula that was not
already incorporated into the planets.
Introductory Planets Course
Toby Smith has created an excellent web page for the UW's introductory
planets course, Astronomy 150.
If you need more information about the solar system than what I have in my
notes, then Toby's page is the place to check next.
Nine Planets Tour
Take a look at the Nine Planets Tour site or it's local mirror site.
You get a tour of each of the planets in our solar system with additional stops
at some of the moons.
Views of the Solar System
Calvin J. Hamilton has put together a very nice page called
Views of the Solar System
that discusses all of the objects of the solar system with great pictures and
details about the different spacecraft that have visited the planets.
last updated 08 Nov 95
Nick Strobel --
Email:
strobel@astro.washington.edu
(360) 754-4049
University of Washington
Astronomy
Box 351580
Seattle, WA 98195-1580
