Transcript Today`s Powerpoint

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Two Kinds of “Classical” Planets
"Terrestrial"
"Jovian"
Mercury, Venus,
Earth, Mars
Jupiter, Saturn,
Uranus, Neptune
Close to the Sun
Small
Mostly Rocky
High Density (3.3 -5.3 g/cm3)
reminder: liquid water is 1 g/cm3
Slow Rotation (1 - 243 days)
Few Moons
No Rings
Main Elements Fe, Si, C, O, N:
we learn that from the spectra
Far from the Sun
Large
Mostly Gaseous
Low Density (0.7 -1.6 g/cm3)
Fast Rotation (0.41 - 0.72 days)
Many Moons
Rings
Main Elements H, He
Mars' Moons Phobos and Deimos
Phobos: 28 x 20 km
Deimos: 16 x 10 km
Properties similar to asteroids. They are probably asteroids captured
into orbit by Mars' gravity.
Clicker Question:
From Mars, Deimos has an angular diameter
of 140 arcseconds. Would colonists on Mars
ever see Deimos produce a total solar
eclipse?
A: Yes, every day on Mars
B: Yes, every new moon
C: Yes, but rarely
D: Never
Clicker Question thoughts:
From Mars, Deimos has an angular diameter
of 140 arcseconds.
The Sun has an angular diameter of 1300
arcseconds
Deimos is the smaller of the Mar’s two
moons. Even the larger, Phobos, will never
produce a solar eclipse. Phobos’ angular
diameter is about 40% of the sun’s as seen
from Mars
The Jovian Planets
Saturn (from Cassini probe)
Jupiter
Uranus
Neptune
(roughly to scale)
Clicker Question:
Which gas giant has the lowest average
density:
A: Jupiter
B: Saturn
C: Uranus
D: Neptune
Discoveries
Jupiter and Saturn known to
ancient astronomers.
Uranus discovered in 1781 by
William Herschel.
Neptune discovered in 1845 by
Johann Galle. Predicted to exist by
John Adams and Urbain Leverrier
because of irregularities in Uranus'
orbit.
Basic Properties
Mass
(MEarth)
Jupiter
Radius
(REarth)
Orbit semi-major axis Orbital Period
(AU)
(years)
318
11
5.2
11.9
Saturn
95
9.5
9.5
29.4
Uranus
15
4
19.2
84
Neptune
17
3.9
30.1
164
(0.001 MSun)
Major Missions
Launch
Voyager 1
1977
Planets visited
Jupiter, Saturn
Voyager 2
1979
Jupiter, Saturn, Uranus, Neptune
Galileo
1989
Jupiter
Cassini
1997
Jupiter, Saturn
Jupiter's Atmosphere and Bands
Whiteish "zones" and brownish "belts".
Optical – colors dictated by how
molecules reflect sunlight
Infrared - traces heat in
atmosphere, therefore depth
So white colors from cooler, higher clouds, brown from warmer, lower
clouds. Great Red Spot – highest.
Composition: mostly H, some He, traces of other elements
(true for all Jovians). Gravity strong enough to retain even
light elements. Mostly molecular. Spectroscopy of
reflected sunlight reveals which molecules present.
Altitude 0 km defined as
top of troposphere (cloud
layer)
NH4SH
(NH3)
These molecules
should all give white
clouds. Molecules
responsible for colors
actually not clear!
Other Jovian planets: banded structure and colors
More uniform haze layer makes
bands less visible. Reason:
weaker gravity allows clouds to
rise higher and spread out to
create more uniform layer
Blue/green of Uranus and blue
of Neptune due to methane.
Colder than Jupiter and Saturn,
their ammonia has frozen and
sunk lower. Methane still in
gas form. It absorbs red light
and reflects blue.
- Zones and belts mark a convection cycle. Zones higher up than belts.
- Zones were thought to be where warm gas rises, belts where cooled gas sinks.
Now less clear after Cassini, which found numerous upwelling white clouds in
the dark belts.
-
- Jupiter's rapid rotation stretches them horizontally around the entire planet.
- Winds flow in opposite
directions in zones vs.
belts. Differences are
hundreds of km/hr.
Clicker Question:
Jupiter’s atmosphere is primarily made up
of:
A: hydrogen
B: helium
C: carbon dioxide
D: ammonia
Clicker Question:
It takes 8 minutes for light to travel 1 AU,
how long does it take for light to travel from
Earth to Jupiter at its closest point to Earth
in its orbit?
A: 1 minute
B: 5 minutes
C: 30 minutes
D: 2 days
E: 1 year
Storms on Jovian Planets
Jupiter's Great Red Spot: A hurricane
twice the size of Earth. Has persisted for at
least 340 years. Reaches highest altitudes.
New storm “Oval BA”
"white ovals" - may last decades
"brown ovals" - only seen near 20°
N latitude. Not known why. May
last years or decades
Neptune's Great Dark Spot: Discovered by
Voyager 2 in 1989. But had disappeared by
1994 Hubble observations. About Earthsized.
Why do these storms last so long?
Jupiter's Internal Structure
Can't observe directly. No seismic information. Must rely on physical
reasoning and connection to observable phenomena.
(acts like a liquid metal,
conducts electricity)
Core thought to be molten or partially molten rock,
maybe 25 g/cm3, and of mass about 10-15 MEarth .
Other Jovians similar. Interior temperatures, pressures and densities less
extreme.
Rapid rotation causes Jupiter and Saturn to bulge:
Gravity
Gravity
without rotation
with rotation
Jupiter and Saturn rotate every ~10 hours.
Radius at equator several % larger due to bulge.
Differential Rotation
Rotation period is shorter closer to the equator:
Near poles
At equator
Jupiter
9h 56m
9h 50m
Saturn
10h 40m
10h 14m
Uranus
16h 30m
14h 12m
How do we know?
Differential Rotation
Rotation period is shorter closer to the equator:
Near poles
At equator
Jupiter
9h 56m
9h 50m
Saturn
10h 40m
10h 14m
Uranus
16h 30m
14h 12m
How do we know? Tracking storms at various
latitudes, or using Spectroscopy and Doppler shift.
Uranus' rotation axis is tilted by 98o
Why? Unknown. Perhaps an early, grazing collision
with another large body.
Clicker Question:
The Great Red Spot is:
A: A large basin on Mars
B: A long-lived high-pressure storm in Jupiter’s
atmosphere.
C: The colored polar cap of Jupiter
D: Clouds of dust swirling around Jupiter’s largest volcano
Clicker Question:
Saturn is less massive than Jupiter but
almost the same size. Why is this?
A: Saturn’s interior is hotter than that of Jupiter’s.
B: Saturn is composed of lighter material than Jupiter.
C: Saturn is rotating faster than Jupiter so the increased
centrifugal force results in a larger size
D: Saturn’s smaller mass provides less gravitational force
to compress it.
The Solar System
●
Gas Giants
–
Massive: MJ = 318 Mearth ≈ 0.001 MSun
–
Strongly influence dynamics/evolution of solar system
●
Terrestrial Planets – (land/water/air interface)
●
Moons and Rings
●
Comets & Kuiper Belt Objects – water and other materials
●
Asteroids – metals, water, other materials
●
Zodiacal Dust — eroding asteroids & KBOs (comets)
–
Small in size, but large in surface area
–
Intercepts sunlight – observable scattered and thermal signatures
●
Tdust ≈ 30K - 1500K (evaporation)
●
Tdust (Asteroid) ~ 160K - 200K
●
Tdust (KBO) ~ 30 - 80K
Moons of Jovian Planets
The Galilean Moons of Jupiter
(sizes to scale)
Io
Closest to Jupiter
Europa
Ganymede
Callisto
Furthest from Jupiter
Radii range from 1570 km (Europa, slightly smaller than our Moon), to 2630
km (Ganymede - largest moon in Solar System).
Orbital periods range from 1.77 days (Io) to 16.7 days (Callisto).
The closer to Jupiter, the higher the moon density: from 3.5 g/cm3 (Io) to 1.8
g/cm3 (Callisto). Higher density indicates higher rock/ice fraction.
Io's Volcanism
More than 80 have been observed. Can last months or years.
Ejecta speeds up to 1000 m/s. Each volcano ejects about 10,000 tons/s
Rich in S, SO2. S can be orange, red, black depending on temperature.
Frozen SO2 snowflakes are white.
Activity causes surface to slowly change over the years:
Voyager 2 (1979)
Galileo (1996)
Volcanic activity requires internal heat. Io is a small body. Should be
cold and geologically dead by now. What is source of heat?
First, Io and Europa are in a "resonance orbit":
Jupiter
Day 0
Europa
Io
Day 1.77
Jupiter
Europa
Io
Day 3.55
Europa
Io
Jupiter
The periodic pull on Io
by Europa makes Io's
orbit elliptical.
orbital
speed
slower
Io
orbital
speed
faster
(exaggerated ellipse)
- Tidal bulge always points to Jupiter. So the angle of the bulge changes faster
when Io is closer to Jupiter.
- But Io rotates on its axis at a constant rate.
- So bulge moves back and forth across surface => stresses => heat => volcanoes
Europa may have Warm Ocean beneath Icy Surface
Fissures suggest large moving ice sheets.
Hardly any impact craters.
860 km
42 km
Icebergs or "ice rafts" suggest broken
and reassembled chunks.
Dark deposits along cracks suggest
eruptions of water with dust/rock
mixed in (Europa’s density => 90%
rock, 10% ice).
What is source of heat? Similar to Io: resonant orbits
with Ganymede and Io make Europa's orbit elliptical =>
varying tidal stresses from Jupiter => heat.
Warm ocean => life?
Europa
Io Jupiter
Jupiter
Ganymede
Europa
(exaggerated ellipses)
Saturn's Titan: A Moon with a Thick Atmosphere
Taken during
Huygens’ descent
From CassiniHuygens mission
Surface from
Huygens probe
Surface pressure is 1.6 atmospheres, T=94 K. Atmosphere 90% Nitrogen.
Evidence for methane rain, a few possible slushy lakes of methane/ethane,
drainage channels, liquid-eroded rocks, icy volcanoes (replenishing the
methane?), complex hydrocarbons in atmosphere (e.g. benzene C6H6).
Mostly dry now - liquid flow may be episodic.
Origin of atmosphere: probably gases trapped in water ice at formation,
released by heat from natural radioactivity and volcanos into atmosphere.
Trapped by Titan’s cold temperature and relatively high gravity.
Saturn's Rings (all Jovians have ring systems)
- Inner radius 60,000 km, outer radius
300,000 km. Thickness ~100 m!
- Composition: icy particles, <1 mm to
>10m in diameter. Most a few cm.
- A few rings and divisions distinguishable
from Earth.
Origin of Saturn's Rings:
If a large moon, held together by gravity, gets too close to Saturn, the
tidal force breaks it apart into small pieces. The radius where this
happens is called the Roche Limit.
Total mass of ring particles equivalent to 250 km moon.
Perhaps a collision between moons sent one inwards this way, or a
captured stray body.
Rings expected to survive only 50-100 million years.
Voyager probes found that rings divide into 10,000's of ringlets.
Structure at this level keeps changing. Waves of matter move like
ripples on a pond.
Origin of Cassini Division:
another resonance orbit
Approximate radius of Mimas' orbit
Mimas' orbital period is twice that
of particles in Cassini division.
Makes their orbits elliptical. They
collide with other particles and
end up in new circular orbits at
other radii. Cassini division
nearly swept clean.
Other gaps have similar origins.
Rings of other Jovian Planets
The rings of Uranus.
Discovered by
"stellar occultation".
Jupiter, Uranus, Neptune rings much thinner, much less material. Formed
by breakup of smaller bodies? Also maybe "sandblasting" of material off
moon surfaces by impacts.
Given rings have short lifetime and all Jovian planets have them, their
formation must be common.
Neptune's moon Triton is spiraling in to the planet and should produce
spectacular ring system in 100 million years.
Clicker Question:
The only Jovian planet without a large moon
is:
A: Jupiter
B: Saturn
C: Uranus
D: Neptune
Clicker Question:
Jupiter’s moon Europa is thought to have a
large ocean of liquid water under a frozen
surface. What is the heat source that keeps it
from freezing?
A: Heat trapped inside the moon since formation.
B: A strong greenhouse effect from a dense atmosphere.
C: Tidal forces exerted by Jupiter, Io and Ganymede.
D: Radioactive decay of heavy elements in the mantle.
Clicker Question:
Saturn’s rings are not perfectly uniform.
What causes the observed gaps?
A: The gravitational influence of Saturn.
B: The gravitational influence of Saturn’s moons.
C: Radiation pressure from Saturn.
D: The gravitational influence of the Sun and Jupiter.
Zodiacal Dust (looking out)
View from the Earth
View in Galactic
Coordinates
Pluto
Predicted to exist by remaining irregularities in Uranus' orbit.
Discovered in 1930 by Clyde Tombaugh (1905-1997).
Irregularities later found to be incorrect!
Model created from HST images.
This is the most detail we have.
Pluto may have two more
moons, found in 2005
Discovery image of Pluto's
moon Charon (1978)
Basic Properties of Pluto
Mass 0.0025 MEarth or 0.2 x mass of Moon
Radius 1150 km or 0.2 REarth
Density 2.0 g/cm3 (between Terrestrial and Jovian densities. More like
a Jovian moon)
Icy/rocky composition
Eccentric, tilted orbit
Moons: Charon: radius about 590 km or 0.1 REarth . Pluto and Charon
tidally locked. S/2005 P1 and S/2005 P2: about 30-100 km.
The New “Dwarf Planet” (2003 UB313 = Eris)
It too has a moon (Keck telescope)
orbit
Very eccentric orbit. Aphelion 98 AU, perihelion 38 AU. Period 557
years. Orbit tilt 44°.
Radius 1200 ± 50 km so bigger than Pluto. Icy/rocky composition,
like Pluto. More massive than Pluto.
Origin of Pluto and Eris
Now known to be just the largest known of a class of objects in the
outer reaches of the Solar System. These objects are:
The Kuiper Belt Objects
100's found since 1992. Probably
10,000's exist.
Icy/rocky.
Orbits tend to be more tilted, like Pluto's.
Leftover planetesimals from Solar
System formation?
Bizarre Orbits of some of Saturn's Moons
Tethys
Telesto and Calypso share orbit
with Tethys, and are always 60
deg. ahead and behind it! They
stay there because of combined
gravity of Saturn and Tethys.
Janus and
Epimethius
Janus and Epimethius are in close
orbits. When the approach each
other, they switch orbits!
Shoemaker-Levy Impact
More Solar System Debris
Comets
Comet Halley (1986)
Short Period Comets
Comet Hale-Bopp (1997)
Long Period Comets
50-200 year orbits
Few times 105 or 106 year orbits
Orbits prograde, close to plane of
Solar System
Orbits have random orientations
and ellipticities
Originate in Kuiper Belt
Originate in Oort Cloud
Oort Cloud is a postulated huge, roughly spherical reservoir of comets
surrounding the Solar System. ~108 objects? Ejected planetesimals.
A passing star may dislodge Oort cloud objects, plunging them into
Solar System, where they become comets.
If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its
orbit may be changed and it may plunge towards the inner Solar
System and become a comet.
Comet Structure
Nucleus: ~10 km ball of ice, dust
Coma: cloud of gas and dust
around nucleus (~106 km across)
Tail: can have both gas (blue) and
dust tails (~108 km long). Always
points away from Sun.
Coma and tail due to gas and dust
removed from nucleus by Solar
radiation and wind.
Far from Sun, comet is a nucleus
only.
Meteor Showers
Comets slowly break up when
near Sun, due to Solar radiation,
wind and tidal force.
e.g. Halley loses 10 tons/sec
when near Sun. Will be
destroyed in 40,000 years.
Debris spreads out along comet
orbit.
IF Earth's orbit crosses comet
orbit, get meteor shower, as
fragments burn up in
atmosphere.
Meteoroids
Even smaller rocky pieces left over from Solar System formation.
If one lands on Earth, called a Meteorite.
Note: Meteor is only the name of the visible streak as the rock burns
in atmosphere.
Clicker Question:
The Oort Cloud is:
A: a spherical solar system halo of icy objects far beyond
the orbit of Pluto.
B: a flat region just outside the orbit of Neptune in which
icy and rocky objects circle the Sun.
C: the collection of rocky objects orbiting the Sun between
the orbits of Mars and Jupiter.
D: a swarm of small satellites around Jupiter.
Clicker Question:
The Perseids meteor shower happens every
year when:
A: the Earth passes through the constellation Perseus.
B: the Earth passes through the remnants of comet SwiftTuttle.
C: the Oort cloud emits a burst of comets.
D: the Earth comes within closest approach to the asteroid
belt.
Asteroids
Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000
known.
Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The
Trojan asteroids orbit 60 o ahead of and behind Jupiter. Some asteroids
cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity.
Asteroids and Kuiper Belt Objects
L3
L5
L4
~ 5 AU
~ 45 AU
Gaspra
Ida and Dactyl
Total mass of Asteroid Belt only 0.0008 MEarth or 0.07 Mmoon. So it is not
debris of a planet.
Probably a planet was trying to form there, but almost all of the
planetesimals were ejected from Solar System due to encounters with
Jupiter. Giant planets may be effective vacuum cleaners for Solar Systems.
Lagrange Points