Astr 1050    Fri., Dec. 13, 2002
   Today: Astronomy Articles
Homework #11
                        Finish Ch. 18, Chapter 19, Pluto and “Debris”

Homework #11
Q1:  If you were an astronomer in the Alpha Centauri system (assume 4.2 light years from Earth) looking toward the solar system, what would be the maximum angular separation between Jupiter and the sun you could ever see? (Hint: 1 ly equals 63,000 AU).
Small Angle formula, Jupter 5.2 AU from sun, yields 4 arcseconds
Q2:  If the Atlantic Seafloor is spreading at 30 mm/year and is now 6400 km wide, how long ago were the continents in contact?
6400 km/30 mm/year = 210 million years.
Q3:  Which effect is the most important for clearing the remaining gas and dust in the solar nebula following planet formation?
Radiation pressure.
Q4:  Europa has few craters because
It has erased craters nearly as fast as they form.
Q5:  A meteor shower is produced when
Earth passes through the orbital path of a comet.
Q6:  An asteroid has an orbital period around the sun of 5.2 years. How far from the sun is this asteroid?
Kepler’s Law.  P2 = a3.  P = 5.2 years, so a in AU is the cube root of 5.2 squared, which is 27, making a = 3 AU.

Comparison of Jovian Planets
Variation in distance presumably ultimate causes other effects
P:      Kepler’s third law
T:      Falloff mostly just result of falling solar energy
But Neptune hotter because more internal heat
M:     Clue to details of solar nebula mode
Less material in outer solar system – or perhaps less efficient capture
r:      Should drop with mass because less compression
Works for Saturn vs. Jupiter
Increase for Uranus, Neptune indicates less H, He and more heavy material

Effects of T (and E) on Atmospheres
Saturn’s bands much less distinct than Jupiter’s
Temp. lower on Saturn Þ cloud condense lower
Deeper clouds Þ markings less visible
Differences at Uranus and Neptune
Even colder Þ clouds even deeper
So cold CH4 can condense
Little solar energy to drive weather
Uranus has strange seasons – tipped on its side
Neptune has strong internal heat source,
so it still can have weather
Large amounts of heavy elements compared to amount of H, He on Jupiter, Saturn
Large amounts of CH4 gas absorb red,
 make planets appear blue

Implications of M, r for nebula
Relative amount of H, He (compared to heavy elements) drop for Saturn
then drop dramatically at Uranus and Neptune
Why were these outer planets so less efficient at capturing H, He?
Their mass is still great enough to do this, especially given low temperatures
 in the outer solar system
May be a problem of timing
Accretion takes longer in the outer solar system because
The velocities of all objects there are much less
The distances between objects are greater
This is the same reason the periods of the orbits are so long
Uranus and Neptune may have only started to grow to critical size by the time the H, He gas was being driven out of the solar system

The Jovian planets are miniature solar systems
Because planets captured gas as they were forming
they had small “solar” nebulae
Þ tests of nebula theory
Regular satellite systems
Large moons in direct (i.e. counterclockwise), equatorial orbits
Preserve solid material from time of formation
Jupiter shows solar-system like density gradients
Rings
Moons unusual and interesting objects in their own right
Io – most volcanically active body in solar system
Europa – also active, may have liquid water ocean (and life!)
Titan – only satellite with a thick atmosphere
Many properties are due to tidal forces and resonances

The Roche Limit
When can tides tear a moon apart?
As a planetary body get close to another object, tidal forces distort the body more and more.
Remember, Earth raises tides on the Moon
just like it raises tides on the Earth
If the distortion gets large enough, the moon will be pulled apart
Happens at “Roche Limit” when moon is
~2.44
´ radius of planet away
At that point, tidal force pulling up on surface of moon is greater than moon’s gravity pulling down
Only matters for objects held together by gravity
Astronaut in orbit will not be pulled apart
Is held together by much stronger chemical forces
Astronaut standing on the outside of the shuttle, hoping the shuttle’s gravity would hold her there, will be pulled away from the shuttle

Saturn as seen by the Hubble Space Telescope

Rings are individual particles all orbiting separately
Each particle – dust to golf ball to boulder size –
is really a separate moon on its own orbit
Orbit with Keplerian velocities:  high in close, slow farther out
Nearby relative velocities are low – so particles just gently bump into each other – slowly grinding themselves up
Structure in rings largely caused by gravity of moons

Resonances:  Properly timed gravitational “pushes”
Like someone pushing kid on a swing
Timing of pushes just as important as force used
Pushing at random times has little effect
Pushing at just right point in each cycle can produce big effect

Cassini division at 1:2 resonance with Mimas

Comparison of Rings
All within Roche limit
Details controlled by Resonances and Shepard Satellites

Jupiter as a miniature solar system
Four large moons (Io, Europa, Ganymede, Callisto)
Regular (equatorial, circular) orbits
Pattern of changing density and composition with distance
Inner two (Io, Europa) mostly rocky
Outer two (Ganymede, Callisto) more icy

Io, Europa break rules about activity
Io most volcanically active body in solar system
Europa shows new icy surface with few craters

Tidal heating explains activity
Large tides from Jupiter flex satellites
Friction from flexing heats interiors
Important for Io, Europa, some other outer solar system satellites

Probable H2O ocean on Europa
Tidal heating may keep H2O liquid under ice cover
Perhaps a location where life could evolve
“Europa Orbiter” Mission being planned to determine if ocean exists

Callisto not active

Comparison of Satellites

Titan
Largest moon of Saturn
Has thick atmosphere
Pressure ~ 1 earth atmosphere
Mostly N2, some CH4
Gas held because of low T
UV acting on CH4 Þ smog
Ethane produced – Lakes?
Can “see” surface only in IR
Cassini will drop probe in Fall 2004
“Code of the Lifemaker” by James P. Hogan, good sf

Triton
Largest moon of Neptune
In unusual retrograde orbit
Probably captured after it formed
Tides during capture may have caused heating
Does have thin atmosphere
Shows recent “activity”
Not volcanic – rather volatile related
Ices migrate with seasons
“Geysers” caused by heated ices

Chapter 19:  Meteorites, Asteroids, Comets
Small bodies are not geologically active
They provide “fossil” record of early solar system
Asteroids
Mostly from region between Mars and Jupiter
Left over small debris from accretion, never assembled into a large planet
Meteorites come mostly from asteroids
Comets
“Stored” on large elliptical orbits beyond planets
Thought to be “planetesimals” from Jovian planet region, almost ejected from solar system in its early history
Meteorites provide only samples besides Apollo
With sample in hand, can perform very detailed analysis:  detailed chemistry; radioisotope age; other isotope info

Asteroids
Most located between Mars and Jupiter
Largest is Ceres
1/3 diameter of moon
Most much smaller
>8,000 known
Total mass << Earth
A few make it to earth
source of the meteorites

Meteorites from Asteroids
If meteorite speed and direction is observed as it enters Earth’s atmosphere, you can work backwards to find its orbit.
Almost all of the meteorites with well determined orbits have most distant part of orbit ellipse within the asteroid belt.

The larger asteroids

Asteroid Belt Structure
As Jupiter formed it stirred up velocities in what would become the asteroid belt
Higher velocities meant planetesimals destroying each other rather than accreting
Gaps and concentrations occur at resonances with Jupiter

Are Asteroids Primitive?
Ida (56 km diam.) and its moon Dactyl (1.5 km diam.)
Colors have been “stretched” to show subtle differences
Imaged by Galileo on its way out to Jupiter

Another Galileo Asteroid:  Gaspra

Phobos & Deimos:  Two “misplaced” asteroids?
Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars
Look like captured asteroids rather than moons formed in place
Are “C” class – i.e. dark “Carbonaceous” type “asteroids”

Clues from Meteorites
Three main kinds of meteorites
Carbonaceous chondrites: Most primitive material – dark because of C
Stones Similar to igneous rocks
Irons Metallic iron – with peculiarities
Why do we have different kinds?
How are the main types of meteorites related to the asteroids?

Types of asteroids observed
Simple classification by albedo and color
Three main types
C  (carbonaceous?)
S   (stones?)
M (metals?)
Finer classification by spectra

Origin of different asteroid types
Carbonaceous = undifferentiated?
Stones and Metals from differentiated planetesimals?
S = mantles
M = cores
Try to sort out using meteorite samples

Meteors vs. Meteorites
Meteor is seen as streak in sky
Meteorite is a rock on the ground
Meteoroid is a rock in space
Meteor showers (related to comet orbits) rarely produce meteorites
Apparently most comet debris is small and doesn’t survive reentry
Meteorites can be “finds” or “falls”
For a fall – descent actually observed and sometimes orbit computed
Most have orbits with aphelion in asteroid belt

Large Meteor over the Tetons (1972)

The Leonids  2001
APOD site:  Picture by Chen Huang-Ming

Meteor Showers and Comets
Meteor showers caused by large amount of small debris spread out along comet orbits
Almost none makes it to the ground – no meteorites
Occur each year as earth passes through orbit of comet
Appears to come from “radiant point” in sky
Leonids:  Mid November

Comets:         Hale-Bopp in April 1997

Comet characteristics
Most on long elliptical orbits
Short period comets – go to outer solar system
“Jupiter family” still ~ in plane of ecliptic
“Halley family” are highly inclined to ecliptic
Longer period ones go out thousands of AU
Most of these are highly inclined to ecliptic
Become active only in inner solar system
Made of volatile ices and dust
Sun heats and vaporizes ice, releasing dust
“Dirty snowball” model

Comet structure
Gas sublimates from nucleus
Dense coma surrounds nucleus
Ion tail is ionized gas points directly away from sun
shows emission spectrum
ions swept up in solar wind
Dust tail curves slightly outward from orbit
shows reflected sunlight
solar radiation pressure gently pushes dust out of orbit

Hale-Bopp clearly shows components

Where do comets come from?
Long period comets:  The Oort Cloud
Most (original) orbits have aphelions of  >1000 AU
Need ~6 trillion comets out there to produce number seen in here
Total mass of 38 MEarth
Passing stars deflect comets in from the cloud

Formation of Oort cloud comets
Composition indicates formation in region between Jupiter and Neptune
Ejected to the Oort cloud by near collisions as Jovian planets formed
Most probably lost from solar system – a few have just barely closed orbits
Occasional passing stars perturb more comets into orbits passing in close to sun

Where do the Jupiter family comets come from?:
  The recently discovered Kuiper Belt
Material beyond Neptune never ejected into the Oort cloud
Pluto and Charon the biggest members – now also Quarar
Very hard to detect because very faint
far from the sun so little illumination
comets not active at that distance
Hubble and new large telescopes have recently detected ~100

Importance of comets
Evidence of solar nebula
Source of H2O and CO2 for earth
Impacts continue
Impacts on Earth
Extinction of the dinosaurs
SL-9 impact on Jupiter

Chapter 16-19 Review
Solar Nebula
Terrestrial Planets
Properties of Earth
Greenhouse Effect (cf. Venus, Mars)
Cratering, origin of moon
Jovian Planets
Properties of Jupiter, composition, atmosphere
Rings
“Debris”
Asteroids and Comets

Chapter 16-19 Review
We’ve covered this material fast – exam will not cover subtle concepts or obscure facts.  Very basic information and only the most fundamental ideas.
Things you should know include:
Order of planets in solar system, general sizes of orbits, sizes and compositions of the planets (also asteroids and comets in general, notable moons).
How these items fit into the solar nebula picture.

Chapter 16-19 Review
Example questions:
True/False:
Jupiter was probably influential in preventing the formation of a planet at the location of the asteroid belt.
The dirty snowball theory suggests that the head of a comet is composed of ices.
Jupiter radiates more heat than it absorbs from the sun.
Venus is very hot because its atmosphere is rich in CO2.
The Greenhouse effect occurs because gases like carbon monoxide are opaque to IR radiation.
The Jovian planets have lower densities than the terrestrial planets.

Chapter 16-19 Review
Example questions:
True/False:
Meteorites appear to be composed of material similar to that found in comets.
Jupiter’s interior is mostly liquid helium.
Saturn’s rings are composed of metallic dust grains.
Flow channels on Venus suggest it was once rich in water.
The oxygen in Earth’s atmosphere was outgassed by volcanic explosions.
Mars is the third rock from the sun.

Chapter 16-19 Review
Example questions:
Multiple choice:
On a photograph of the moon, the moon measures 30 cm in diameter and a small crater measures 0.2 cm.  The moon’s physical diameter is 1738 km.  What is the physical diameter of the small crater?
About 1738 km
About 12 km
About 520 km
About 350 km
About 3.5 km

Chapter 16-19 Review
Example questions:
Multiple choice:
Though Titan is small, it is able to retain an atmosphere because?
It is very cold.
It is very dense.
It rotates very slowly.
It attracts gas from the solar wind.
It has a very strong magnetic field.

Final Exam
30 Multiple Choice questions, 15 true/false, 3 essay/written questions, plus 1 follow-up extra credit problem (computational and meant to be challenging).
About 1/3 of the questions the same as or slightly modified from previous exams
About 1/3 of the questions covering the solar system
About 1/3 of the questions completely new but covering old material
Questions mostly cover the basics and are not intended to be subtle or tricky.

Final Exam
List of possible topics for essay questions:
Dark Matter
Cosmic Microwave Background Radiation
The Evolution of the Sun on the H-R diagram
The Solar Nebula
Extrasolar Planets
Comparative Planetology of Venus, Earth, and Mars
The Seasons
Phases of the Moon
The Distances to Astronomical Objects (Distance Ladder)