Download The Jovian Planets

Introduction to Astronomy
• Announcements
– Midterm Exam on Thursday
• Closed-book/notes/etc.
This is what happens when a massive star dies.
About 1400 ly distant, it covers an area of sky = 5 full Moons!
The Jovian Planets
Jupiter, Saturn, Uranus, Neptune
The Jovian
planets to
correct scale
Also known
as the outer
planets or the
gas giants
Commonalities
• All are much larger than Earth
• Densities are all lower than the terrestrial
planets
• All have small ice/rock cores surrounded
by liquid layer surrounded by thick gas
atmosphere (which contains optical
surface of planet)
• Rings
• Belts and Zones (high & low pressure)
Jupiter
The King of the Planets
Jupiter
• Name from Roman God of Gods
• Largest of the planets
– 11x diameter of Earth, 318x mass of Earth
• Dense, brightly-colored atmospheric bands
– Fast-moving H, He, NH3, CH4, H2O
• Fast rotation period (10 Earth hours)
– Results in significant equatorial bulge
Interior
• Avg. density ~ 1.3 g/cm3 (slightly higher
than water)
– Much less than Earth density, so must be
made of lighter elements
• 1/6 of the way to the core, gaseous H
turns to liquid H
– Compression by overlying layers
• Even deeper, liquid turns to liquid metallic
H
• Rocky, Iron core
Divers know this phenomenon
well…
As depth increases, pressure
increases.
Need for decompression stops
to avoid Nitrogen Narcosis
(“The Bends”)…pressure on body
drops too rapidly, allows bubbles
of N2 to enter blood stream and
dissolve into brain tissue,
which is highly toxic.
Atmosphere
• Coriolis effect & cloud bands
• Heat in interior drives convection currents
upward
– Stirs up atmosphere, brings hot gases to
surface while cooler gases fall back down
below surface
– Coriolis effect
• Gases moving toward equator get pushed to the
west, gases moving away from equator get pushed
to the east
Here on Earth, we have localized
high- and low-pressure systems
On Jupiter, those same high- and
low-pressure systems are stretched
into globe-circling bands
How to stretch a weather cell
If no rotation, hot gases in the
interior convectively rise.
In pot of boiling water, convection
carries bubbles of WATER VAPOR
to surface, where it is released to
condense into STEAM
When these hot gases reach the surface, they spread
out horizontally. Some travel toward the equator, some
travel toward the poles
• Just in case you don’t believe me, here’s
an example of the Coriolis deflection
– Note I did not say Coriolis “force”
• Movie
Speed of cloud bands varies widely from place to place
• As opposite-moving winds blow, other
uprising material may get caught between
them and get “twisted” up into Jovian
storms
– Like ball rolling between two conveyor belts
– Some transient (short-lived)
– GREAT RED SPOT permanent (as far as we
know)
– Brownish-red bands from Sulfur and/or
Phosphorous tinting
HUGE
“Jet Streams”
Smaller storms
The Kelvin-Helmholtz Instability
• Convection & rotation in liquid Hydrogen
sustains DYNAMO
– BJupiter ~ 20 – 20,000 x stronger than Earth’s
magnetic field
• Largest magnetosphere of all the planets
• Intense auroral displays observed by Hubble and
by Voyager fly-bys
• Radiation belts
• Thunderstorms & lightning
Jupiter has Aurorae just
like on Earth!
Image of Jupiter & aurorae
in UV wavelengths (Hubble/STIS)
March 2007:
Auroral observations
by Chandra X-ray
Telescope overlaid
w/ HST image
Jovian Moon “footprints”
Rings
• Thin rings confirmed by Voyager I fly-by in
1979
This is a false-color
image from Voyager 2
looking back as it
passed Jupiter
Why is the planet’s
surface dark?
Cassini confirmed
rings are small, irregular
“chips” from nearby moons
Moons
• Galilean moons
– 4 largest of Jupiter’s moons
– Mini model of solar system
• In both appearance and formation
• Denser, rockier moons closer in
• Icier moons further out
This is why objects cannot be classified as planets based only on size
Io, Europa, and Ganymede
have a synchronous orbital
resonance: because the moons
are closely-spaced, they create
a rhythmic gravitational pull
on each other…
For every orbit of Ganymede,
Europa orbits twice and Io orbits
four times
Io
• Closest moon, tidally-locked
– Gravitational disturbances prevent closed
orbit…makes ROSETTE pattern
• Most spectacular volcanic activity in solar
system
– Caused by gravitational/tidal forces that
constantly stretch and pull on interior, heating
it up (like a RUBBER BAND)
– Sulfuric volcanoes: “lava” is not molten rock,
but molten Sulfur
• But occasionally see evidence of silicate lava
Visible image of Io, showing a HUGE
volcanic eruption of Sulfur
This plume rose about 65 miles above
the surface of the moon!
This puts any volcanic eruptions on
Earth to shame…
Molten sulfur lava flows
sown volcanic peak before
solidifying…
IR view: simultaneous, surface-wide eruptions
Europa
• Perhaps most interesting moon in the
solar system
• Icy moon
– Surface layer of ice ~ 6 miles thick
– LIQUID WATER underneath!
– White areas are frozen H2O, red areas from
mineral-rich water that oozed through surface
cracks
• Perhaps organic? Red algae?
Size comparison of Europa’s
ice fields
• Tidal heating by Jupiter’s gravity keeps
subsurface H2O in LIQUID form
– Explains lack of cratering…liquid water would
quickly smooth over any surface depressions
– Also has glacier-like surface flows of ice
– LIFE?
• Extremophiles: life-forms that survive in frigid
temps, high temps, high acidity, low light levels,
etc…
Saturn
Saturn
• Avg. density ~ 0.7 g/cm3
– This is less than density of water, so if dropped into
our oceans, SATURN WOULD FLOAT!
– Mostly H & He
• Internal heat source: friction/drag on falling,
condensed liquid Helium droplets
• Similar to Jupiter, but colder, so that ammonia
(NH3) freezes into tiny cloud particles
– Dense cloud cover masks details of surface
Saturn’s weather bands as imaged
by Cassini
Rings
• Originally thought to be solid
• Closer examination shows inner rings
rotate faster than outer rings
– Kepler’s 3rd law again!
– Therefore, rings must be made of a swarm of
small, separate bodies a few cm to a few
meters in diameter
– Mapped via radar echo techniques
• Determined size and composition (Voyager I)
• Primarily H2O ice, but see signs of carbon
compounds
Substructure of Saturn’s rings: composition & speed differences, many gaps
• Ring gaps
– Cassini’s Division: large gap caused by orbit
of moon Mimas periodically pushing particles
out of that particular area
– Shepherding Satellites: small moon-like
objects (larger than average) that direct ring
material into narrow lanes, creating small
gaps between them
These are rings around
Uranus, but principle
is the same…
F-ring
Potato-shaped
moon,
Prometheus
Distortion of Saturn’s F-ring by interacting moonlets
(smaller than moons, but larger than ring particles)
Cassini Division
Saturnian Shadow
Filtered Sunlight
Cassini image from
9 Sep 2006
Shows new ringlets
not seen in Voyager
fly-bys 25 years ago
Mimas in white-light
and IR
a.k.a. The Death Star
Ring Origins
• Destruction of moons & asteroids by tidal
forces
• Roche Limit (RRoche = 2.44 Rplanet)
– Gravity pulls harder on near-side of orbiting
object than on far-side
– If this differential gravitational force is stronger
than the object’s self-gravitation (what holds
the object together), it gets torn apart
– Gravity pulls pieces into orbit
– Side-by-side differences in rings suggest
whole ring system formed from MANY
destroyed bodies
Saturn’s B & C rings in the infrared, highlighting not only
temperature differences, but compositional differences as well
Moons
• Mostly ice, but less dense than Jupiter’s
moons
• Most interesting moon:
– Titan
• D = 3000 miles > DMercury
• So cold, N2 moves slowly, doesn’t reach escape
velocity
– Titan has Nitrogen atmosphere!
• Spectroscopy indicates presence of liquid methane
oceans & ethane-methane “rainclouds”
Filtered sunlight
Introduction to Astronomy
• Announcements
– Midterm Exam tomorrow
– HW #4 due Monday
Uranus
Uranus
• Named for Ouranos
• Pretty much featureless except for faint cloud
bands and faint ring system
• Rich in H (in the form of H2O and CH4)
• Methane causes blue color
– Absorbs red-end of solar spectrum, scatters blue-end
off clouds of frozen methane
• Avg. density ~ 1.2 g/cm3
– Mostly light elements probably with rocky/icy core
Rings & Moons
• Rings similar to
Jupiter & Saturn but
darker and narrower
– Probably more carbonbased molecules
– Thin rings held by
shepherding satellites
• 5 large moons & 20
small ones
As a side note, all Uranus’ moons
are named for Shakespearean characters
• Miranda (large moon)
– Very puzzling appearance
– Large impact, destruction & reformation?
– Rising & sinking motions in interior?
NOT to scale…
Tilt of Uranus
• Equator nearly perpendicular to orbit (spins on
its side)
• Rings & moons are similarly tilted
– Offers clues about formation
– Giant impact knocked the planet over, spewed out
material that then formed rings & moons
• otherwise, wouldn’t expect rings and moons to have tilted
orbits…
– One pole in perpetual day, one in perpetual night (for
half of the Uranian year, 42 Earth-years)
• odd rotation + odd heating = lack of atmospheric
phenomena?
Which is really the north pole?
Neptune
Neptune
• Similar to Uranus
– Cloud bands, deep blue color
– Had a “Great Dark Spot” similar to Jupiter’s Great
Red Spot
• Discovered through orbital anomaly
– Uranus’ position did not quite agree with predictions
– The gravity of another planet located past Uranus
explained disagreement
• Leverrier & John Adams (1843)
• Confirmed when Neptune first discovered (Johanne Galle,
1846)
• Discovered by
Voyager 2 in
August 1989
• Thought to be a
“hole” in the
methane
atmosphere
– Much like our
ozone hole
18 hours from first to
last frame
Waves in overlying clouds
Atmosphere
• Cloud bands generated by Coriolis effect
– 1300 mph winds!
• Methane tinting
– Like Uranus
• Great Dark Spot disappeared (sometime <
1997)…why?!
– not well-understood
Rings & Moons
• Very narrow, tenuous (faint) rings
• 6 moons close-in, 7 further out
• Triton
– Orbits opposite to Neptune’s rotation
– Highly-tilted orbit
• left-over or captured planetesimal?
– Has retained an acidic atmosphere
– “wrinkly”, volcanic soot-laden surface
Outer ring is more clumpy or braided
Triton is actually spiraling-in towards Neptune…
…the Solar System will have a new ring system to rival Saturn’s!
Pluto
• Roman god of the underworld
• Is it a planet?
– Highly-elliptical orbit out of the plane of the rest of the
solar system
• Avg. density ~ 2.1 g/cm3
• Composed of H2O, Methane, Nitrogen & CO2
ices
– Polar caps of frozen Methane
• Thin atmosphere of N2 & CO
• Surface temperature a constant -380 °F
Discovered by Clyde Tombaugh
in 1930, working at Lowell Observatory
in Flagstaff, AZ
Compared (by eye!) an enormous
amount of pairs of photographic
images, looking for faint objects
whose positions slowly changed…
…this is NO small task.
Even the smallest
bodies in the solar
system can have
multiple moon
systems
In fact, Pluto and
Charon are actually
a binary system!
The center-of-mass
of the two objects
is above the surface
of Pluto
Where did this name come from?
New Horizons, scheduled
for Pluto-rendezvous in 2015
HST, 1996
NEXT TIME
• Meteors, Asteroids, and Comets
If Pluto were any closer to the Sun, it would be a comet!