ISP205 Lecture #16, Feburary 29, 2001
- Homework Set 6: Due March 20 !
Pick up today
- Review: Rings of the Giant planets
- All Giant Planets have rings; only Saturns are easily
visible
- Rings are numerous pieces of ice, ranging in size from
dust grains to meter sized boulders
- Mostly water ice (white) but organic compounds on
surface can lead to dark rings
- Each particle orbits the planet like a little moon in a
well
confined plane (only 10-100 m thick)
- Rings are kept in place and structured by "shepards"
- Shepard moons within the ring systems sweep gaps
into the rings
- Gravity from all moons confine little pieces to
their
orbits (and sweep them out of certain unstable orbits)
keeping the rings in shape over time.
- Jupiters rings: mainly smooth dust (dark),
probably from inner moons
- Saturns rings: Main rings A,B,C with Cassini Gap
(4600 km)
between A and B. Plus thin outer F ring.
The A,B,C rings are very broad, very thin (~20 m)
quite bright and easily
visible from earth
Closeup: really >1000 individual rings (picture)
- Uranus rings (9 main): narrow, many meter sized
boulders, hardly dust
and very dark (among darkest objects in solar system)
- Neptune rings (4): very inhomogeneous (look like
thin arcs)
- Rings are probably very young (~50 Million years old)
- Rings form if a massive objects forms/enters the zone
within the tidal stability limit of a massive planet.
The object is disrupted and the pieces form a ring
- Triton is on the way and might be disrupted in ~100 Mio
years
- Titan (Saturn) (picture)
- Size: Slightly larger than Mercury
- A moon with a fuzzy, orange atmosphere that blocks
the view on the surface.
- Atmospheric pressure at surface: 1.6 bar (similar to
earth)
- Atmospheric composition: mainly Nitrogen (similar to
earth)
plus lots of compounds: CO, Hydrocarbons, Hydrogen
cyanide ...
Complex molecules form at high altitudes under impact
of UV light and block view
- Surface temperature: constant 90 K
(picture of
surface layers)
- Surface images in infrared show inhomogeneous surface
(picture)
- Ethane and Methane could form lakes, rivers, on the
surface (maybe an ethane wheather cycle, including
ethane rain).
- Visibility at surface might be good, surface
illumination
in a dark orange.
- The Cassini space craft is on the way to Saturn and
will drop a probe (Huygens) to land on Titan.
(see picture)
- Asteroids
- Asteroids are smaller objects (<1000 km) orbiting
the sun
- First Asteroid discovered 1801 by Giovanni Piazzi
(Ceres - biggest asteroid 933 km diameter)
- More than 10000 Asteroids known (known orbits)
(overview picture)
- Well known examples: (picture)
Ceres: biggest (900 km)
Vesta: brightest (Basalt - known from meteors)
(visible
with naked eye)
1998 KY26: 30 m large, but fastest spinning object in solar
system: 1 day on
1998KY26 lasts only 10.7 minutes.
Eros: First asteroid visited by spacecraft
(Redshift demo: vesta and eros today)
- Asteroids are concentrated:
(Redshift demo: Planet
orbits, picture asteroid distribution)
- In Asteroid belt between Mars and Jupiter
- The Trojans (2 groups of asteroids moving along
with Jupiter in its orbit)
Maybe other planets have Trojans too (Some indication
that Mars has Trojans)
- Centaurs (orbiting in the Uranus/Neptune region and
beyond)
- Some get captured by Planets and become moons:
Like the Mars moons Deimos and Phobos (pictures)
- NEO's: Near Earth Objects (Asteroids)
- estimated ~2000 > 1 km and potentially
dangerous
- Orbits unstable - either ejected or crashed
into
terrestrial planet after ~ 100 Mio years
- 1/4 crashes on earth - every 200000 years on
average
none of the known ones expected to crash soon !
- Eros is the biggest near earth asteroid.
- Asteroid orbits are more eccentric and inclined than
planets
orbits (Redhift Demo, include Eros)
- Different composition types (main types):
- C: Carbon Compounds with rock mixtures,
extremely dark, 75% of all asteroids
mostly in outer part of asteroid belt
- S: Silicates (Rock), mixed with metals; bright
mostly in inner part of asteroid belt
- M: Metals, bright
- D: dark, reddish - unknown composition
mostly at outer edge of asteroid belt and beyond
- There are some basalt asteroids (Vesta)
- Astronomers no 14 types !!!!
- Asteroids can have moons and partners
(Example pictures)
- Eros: (pictures)
- S-type
- Questions: are S-type asteroids fractionated in
metal and silicates?
Eros is not - metals are mixed in.
- Are they a solid piece of rock or a mix of loosley
bound rubble ? Eros is a solid (cracked) piece.
But: Mathilde is a mix of loosley bound rubble
- Is eros a piece of a crushed planet or
leftover
"planetesimals" that never formed a planet
Eros is a leftover planetesimal.
- Comets (pictures, Redshift Demo:
orbits)
- Halley proposed 1705 that comets are members of the
solar system, reappearing in intervals
- Orbits more inclined and eccentric than Asteroids
- 2 groups of comets:
- Short period (< 200 years)
example: Halley - 76 years
- Long period
example: Hale Bopp - 2380 years
- Comet periods change slightly because of influence of
giant planets gravity
- Comet components: (picture,
Redshift demo)
- Nucleus
- in 1986 3 spacecraft had close encounters with
Comet Halley
Giotto took image of nucleus ~1000 km away (picture)
- size: 6x10 km
- Mix of (silicate) rock dust and water ice (dirty snowball
model)
- Coma
- Heat from sun evaporates water and releases
dust
forming a huge cloud around the comet
- size: ~100,000 km
- Sunlight disintegrates water into H and O
H-cloud can be larger than the sun (Halley)
- Dust tail
- Solar wind pushes dust away from the comet
producing
a dust tail
- Dust gets on individual orbit around the sun
because dust is further away it has longer period than comet
and lags behind (bend)
- In 1910 the earth passed through the tail of
Halleys
comet - lots of excitement but nothing happened (too thin)
- Ion tail
- Radiation from the sun ionizes some material
- Ions are pushed away as well, but follow the
magnetic
field lines of the solar system (straight line
away from the sun)
- Fate of comets
- Comets typically survive only a few 1000 orbits
because they lose material each time
- Some comets crash into planets
- Shoemaker-Levy 9 crashed into Jupiter in 1994 (pictures)
- Comets might be origin of water on earth;
maybe also of organic compounds critical
for the formation of life
- Origin of Comets
- Because of their "short" lifetime there
must be a
continuous supply of comets
- Short period comets come from the Kuiper belt and
get
kicked towards the sun by impacts or gravity of the
giant planets. Contains ~200 Mio Comets
(picture)
Some larger Kuiper belt objects have been found:
(picture)
(Plutinos)
- Long period comets come from Oort Cloud, ~1ly away
from sun (Contains ~10 Trillion Comets) (picture)
- Formed from icy objects around giant planets
that
were ejected
- "Comets" orbit Oort cloud since 4.5
billion years,
but get kicked towards the sun by the gravity
influence of nearby stars