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Admin. 9/29/16
1. Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/
2. Optional Discussion sections: Tue. ~11.30am (period 5), Bryant 3; Thur.
~12.35pm (end of period 5 and period 6), start in Pugh 170, then
Bryant 3
3. Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm, Bryant 302 (but
email me if coming on Wed.).
4. Homework 5: is due Mon. Oct 3rd 11.59pm via Canvas e-learning
under “Quizzes”
5. Reading this week: Chapters 0, 1, 2.1-2.4, 4.1, 5, 6, 7, 8, 4.2, 4.3
6. Midterm 1: was on Tue. Sept. 27th: results via Canvas e-learning soon.
7. Observing project deadline: Thursday Oct. 27th 2016, however, you
are strongly advised to complete observing by Fri. Oct. 7th.
8. Email me Astro-news, jokes, tunes, images: ast1002_tan-l@lists.ufl.edu
9. Printed class notes? Name tags?
Key Concepts: Lecture 16: Jovian Planets: Rings and Moons
Rings and the Roche Limit
Galilean Moons of Jupiter
Titan and other moons of Saturn
Overview
• All Jovian Planets have
–Rings
–many satellites
• Rings & satellites orbit planet like
mini-solar system
• Composition of rings & satellites
differs from objects in inner solar
system
–because temperatures in outer solar system
are much colder
Orbits
• Most satellites of Jovian planets
follow “direct” or “regular” orbits:
counter-clockwise if viewed from
above the north pole.
• Ring systems are in regular orbits
• There are some “irregular” satellites
– retrograde orbits (clockwise)
– or have highly elliptical orbits
– or have highly inclined orbits
Orbits of Jupiter’s moons
• Orbits give us information about the formation of the rings & satellites
Moons and Rings of Uranus and Neptune
• Objects in regular orbits
– probably formed at the same time as the planet
– probably formed by processes similar to those that formed the planets
• Object in irregular orbits
– probably formed far away and were captured later
Rings
• Rings consist of billions of small
particles or moonlets orbiting close to
their planet
–size of particles range from grain of
sand to house-sized boulders
Roche Limit – Ring Formation
A moon is pulled apart by TIDAL FORCES (i.e. gravity stronger on
near side than far side) if it approaches inside about 2.5 planetary radii.
• Particles follow Kepler’s laws
– inner particles orbit faster than those farther
out
– ring not rotating as a solid body, rather
individual moonlets are orbiting
• if ring particles widely spaced - move
independently
• if particles are close - interact
gravitationally
• moons clear gaps in rings
Origin of Rings
• Breakup of shattered satellite
• Or, remains of particles that
were unable to come together
and form satellite
• Gravity plays important role
Saturn’s rings
–differential force of gravity -- i.e.
tidal forces
• tear bodies apart
• inhibit loose particles from coming
together
Much larger ring system - maybe formed quite recently?
Rings of Saturn
The Galilean Moons
• Rings are thin - only about 20
m thick!
• 3 broad, bright rings
– A, B, C
• handful of narrow rings of which F
is most substantial
• Gaps between rings
• Ring particles made up of ices
and rock
• Small “shepherd” moons can
create very narrow rings
Io
• Innermost Galilean moon
The Jupiter System
[You do not need to remember precise number of moons around the giant planets]
• 16+ moons
–4 large satellites : Galilean satellites
Io
Europa
• Most volcanically active place in
solar system
– Voyager I - 8 volcanoes
erupting
– Voyager II (4 months later) - 6
active
• Io - size of Earth’s Moon
• Europa - size of Earth’s Moon
• Ganymede - bigger that Mercury
• Callisto - bigger than Mercury
–12+ much smaller moons
• 3 groups of 4 moons
– 4 orbiting inside the orbit of Io - regular orbits
– 4 outer moons in direct, but inclined orbits
– 4 outermost moons in retrograde orbits
• Faint ring
• Similar size & density as our
Moon
• Surface is constantly changing
Ganymede Callisto
• Volcanic eruptions mainly
composed of sulfur & sulfur
dioxide
• Lava lakes made of liquid sulfur
• Why so much volcanic activity ?
• Tidal heating by Jupiter. Other satellites disturb Io’s orbit,
preventing it from becoming tidally locked to Jupiter. Thus Io is
continually stretched and pulled: leads to heating.
Volcanoes on Io
Europa
Europa
• Size & density similar to our Moon
• mainly rocky composition
– heat from Jupiter during early
evolution evaporated much of the
ice
• smooth surface
• few impact craters
• Surface is ice covered
• extensive & complex network of
cracks in icy crust
– internal geological activity
• Liquid water is rare in the Solar
system, but likely present on Europa
• Europa is good place to look for extraterrestrial life
Ganymede &
Callisto
Ganymede:
- largest satellite in solar system
- third of surface heavily cratered & dark - older
- tectonic activity & volcanism: created mountains, cracked crust, ice-mountain
ridges, cracks & craters flooded with water
- dark material is organic (carbon based)
Callisto:
- Outermost Galilean satellite
- orbital period = 17 days = period of rotation (tidal lock)
- large icy moon size of Mercury, but 1/3 of mass: less dense, less rocky,
mostly ice
Titan
Galilean moons
• As we move away from
Jupiter, the composition
of the moons become
more icy, less rocky, less
dense. (Similar to planets
as we move away from
Sun).
The Saturn System
• Largest satellite of Saturn
• similar size, mass &
density as Callisto
• composition - rock & ice
• Substantial atmosphere
• Active atmospheric chemistry
• Mostly nitrogen
– Also some carbon monoxide, methane,
ethane, hydrogen cyanide, cyanogen
• Thick - atmospheric surface pressure
higher than Earth
• multiple cloud layers
– methane
• due to low temperatures - may be liquid
methane oceans.
HST -IR
Huygens Probe Visits Titan
Jan 2005
• 19+ satellites
–7 satellites in regular orbits
• Titan is the largest
• Titan only one with atmosphere
–Several small moons orbit near rings
–2 distant irregular satellites in retrograde orbit
• Beautiful set of rings
–broad & flat
–huge collection of icy pieces in orbit around
equator
River channels
Surface view
Water ice & Methane springs
Titan
Uranus: moons and rings
• Earth-like geology
• chemistry involved is quite
different
• liquid methane instead of water
• frozen water ice instead of silicate
rock
• hydrocarbon particles instead of
dirt
• volcanoes spew very cold ice &
ammonia instead of lava
• Ring & satellite system are
tilted at 98o
• 15 regular satellites
–5 largest are comparable in size
to moons of Saturn
• 11 rings
–narrow ribbons of material with
broad gaps
Some other moons of Saturn
Neptune:
moons and rings
• 8 satellites
– 6 regular satellites close to planet
– 2 irregular satellites farther out
– Triton
Mimas
• large
• retrograde orbit - probably captured
• has an atmosphere
• active volcanic eruptions
• Rings
Tethys
– narrow & faint