Download Tides, Moons, Rings, and Pluto

Tides, Moons,
Rings, and Pluto
[Oct 27, 2016]
As with all course material (including homework, exams),
these lecture notes are not be reproduced, redistributed,
or sold in any form. Ocean Tides
high
low
Ocean Tides
water closer to Moon feels stronger gravitational pull.
Moon
Earth
Gm1 m2
F =
d2
Ocean Tides
similarly the Earth is pulled by gravity, but less so than
the ocean water closest to the Moon
Moon
Gm1 m2
F =
d2
Ocean Tides
and finally, what about the rest of the water?...
Moon
Gm1 m2
F =
d2
Ocean Tides
what would this look like from the
perspective of the Earth?
Moon
Gm1 m2
F =
d2
Ocean Tides
whatwill
would
ocean
be this look like from the
perspective
of
the
Earth?
shallower
ocean will be
deeper
Gm1 m2
F =
d2
Moon
Ocean Tides
ocean will be
shallower
(low tide)
ocean
will be
deeper
(high
tide)
Earth
ocean will be
shallower
(low tide)
ocean
will be
deeper
(high
tide)
Moon
Tides are the result of the gravitational force
from the Moon and the Sun
Effect of the Moon on high/low tides
High tide
Low tide
Solar System
more than just the planets
Image not to scale — solar system is much less dense than this!
The Galileo Mission
joint US + Germany
mission
Launched on Oct 18,
1989 from the Space
Shuttle Atlantis
Goal: study Jupiter
The Galileo Mission
Flight path included
gravity assists from
Venus and Earth
Passed by two asteroids
on its way to Jupiter
Went into orbit around
Jupiter in 1995; spent 8
years (35 orbits) there
The Galileo Mission
In July, 1995, Galileo
dropped this probe into
Jupiter’s atmosphere.
After jettisoning its heat
shield and deploying a
parachute, it sent back
~1 hour of data before
succumbing to the
pressure and heat of
Jupiter’s atmosphere.
probe is ~1.3 meters in size
Why name this mission after Galileo?
Galileo Galilei
(1564-1642)
Jupiter’s moons
4 largest: the Galilean
satellites
Io, Europa, Ganymede, Callisto
67 moons now known: some
are only a few km across, and
are probably captured
asteroids
Jupiter’s Moons
1. Metis
2. Adrastea
3. Amalthea
4. Thebe
5. Io
6. Europa
7. Ganymede
8. Callisto
9. Themisto
10. Leda
11. Himalia
12. Lysithea
13. Elara
14. S/2000 J11
15. Iocaste
16. Praxidike
17. Harpalyke
18. Ananke
19. Isonoe
20. Erinome
21. Taygete
22. Chaldene
23. Carme
24. Pasiphae
25. S/2002 J1
26. Kalyke
27. Magaclite
28. Sinope
29. Callirrhoe
30. Euporie
31. Kale
32. Orthosie
33. Thyone
34. Euanthe
35. Hermippe
36. Pasithee
37. Eurydome
38. Aitne
39. Sponde
40. Autonoe
41. S/2003 J1
42. S/2003 J2
43. S/2003 J3
44. S/2003 J4
45. S/2003 J5
46. S/2003 J6
47. S/2003 J7
48. S/2003 J8
49. S/2003 J9
50. S/2003 J10
51. S/2003 J11
52. S/2003 J12
53. S/2003 J13
54. S/2003 J14
55. S/2003 J15
56. S/2003 J16
57. S/2003 J17
58. S/2003 J18
59. S/2003 J19
60. S/2003 J20
61. S/2003 J21
62. S/2003 J22
63. S/2003 J23
The Galilean Moons of Jupiter
Io
Europa
Ganymede
Callisto
Semimajor axis of orbit around Jupiter:
421,000 km
671,000 km
1,070,400 km
1,882,700 km
Io
Io’s volcanos
Voyager
Galileo
Io’s surface is very volcanically active- it’s the youngest surface
of any solar system object
Numerous volcanos erupting silicate lava, with geysers
erupting sulfur and sulfur dioxide
Galileo orbiter found more than 100 volcanoes erupting
simultaneously
Changes in Io’s surface
Voyager, 1979
Galileo, 1996
Interior heating in Io
Io’s orbit is slightly elliptical, due to
gravitational pull from Europa
Jupiter
1/2 degree
wobble
Io
Note- diagram not to scale!
The orbital ellipticity is greatly
exaggerated here.
Tidal forces from Jupiter heat Io’s
interior
Io radiates away 100 trillion watts
of power from this tidal heating
Europa’s structure
Ganymede
The largest moon in the
solar system: radius 2634 km
(larger than Mercury)
Differentiated structure: iron
& rocky core, mantle of ice
and silicates, and crust of
mostly water ice
Possible liquid ocean about
200 km below the surface?
Complex terrain with
mountains, valleys, craters,
lava flows
Differentiated structure of Ganymede
Callisto
Almost as large as Mercury:
radius 2403 km
Very heavily cratered: one of
the oldest surfaces in the
Solar System
possible ocean of liquid water
about 100 km below the
surface?
The Galileo Mission
Mission finished in 2003, spacecraft was
crashed into Jupiter’s atmosphere at a
speed of ~50 km/s.
Cassini-Huygens
— launched in 1997
— mission: to study Saturn and its moons
— flyby of Earth, Venus, & Jupiter
— 4th probe to visit Saturn, 1st to orbit it
— has been orbiting since 2004
— Huygens landed on Titan in 2005
— mission will end in 2017
Saturn’s Moons
Titan
The 2nd-largest moon in the
solar system, with radius 2575
km
Thick, hazy atmosphere: about
1.5 times Earth’s atmospheric
pressure
Atmosphere is primarily
nitrogen, with some methane,
ethane, other compounds
liquid hydrocarbon lakes on
the surface (methane, ethane),
and liquid methane rain in the
atmosphere
The Huygens Probe
Cassini images of Saturn’s moons: Tethys
Iapetus
Dione
Rhea
Mimas
Epimetheus
ra
Telesto
Hyperion
Enceladus
Recent discovery from Cassini: geysers of water ice erupting
from Enceladus
Probably from sub-surface pockets of liquid water
These plumes are most prominent at
the southern pole of Enceladus.
Likely physical
origin of plumes on
Enceladus
Cassini in the news…
On Oct 28, 2015, Cassini passed by Enceladus
at a distance of ~50 km (or 30 miles), which
allowed it to directly study (i.e. sample) the
material in the plume.
One of the goals of this close flyby is to determine
what molecules may be in the plume. Enceladus is
particularly interesting as it may be one of the best
places to search for life — e.g. if it has water,
energy, and complex molecules.
Close-up of Enceladus
1 km
Planetary rings
Gm1 m2
F =
d2
1
r2
log(r)
Planetary Rings and the Roche Limit
Consider a small moon made of dust
and pebbles that’s held together by
its own self-gravity: a “rubble pile”
rather than a solid boulder
Ro
ch
e
Lim
it
If the moon is far away from the
planet, it remains stable.
What happens if we move the
moon closer to the planet?
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
As the moon gets closer to
the Roche limit, it becomes
distorted by the tidal pull of
the planet
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
As the moon gets closer to
the Roche limit, it becomes
distorted by the tidal pull of
the planet
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
At the Roche limit, the tidal
forces on the moon are so
strong that it begins to break up
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
Material closer in to the planet
will orbit faster than material
farther out
This makes the material
gradually spread out into a ring
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
Material closer in to the planet
will orbit faster than material
farther out
This makes the material
gradually spread out into a ring
as it orbits the planet
Planetary Rings and the Roche Limit
Ro
ch
e
Lim
it
Eventually, we’re left with a ring
of small particles orbiting the
planet
The Roche Limit
Inside the Roche limit, material in a ring or disk can’t clump
together to form a single satellite, because the planet’s tidal
forces are too strong
In this region, material ejected from impacts or collisions on
moons can settle into rings orbiting the planet
Rings are temporary features: over time rings will dissipate
due to collisions, and particles will drift away from the rings
So why haven’t Saturn’s rings dissipated?
[rings are fed by material ejected from moons
— e.g. plumes on Enceladus]
Saturn’s rings
Cassini’s
division
Rings are only about 20 meters thick
Composition: mostly water ice, some organic compounds and
carbon
Sizes from small pebbles up to large boulders
Saturn’s rings
Cassini’s division is produced by the gravitational force of the
moon Mimas
This is due to an orbital resonance: a particle in Cassini’s
division would orbit twice for each one orbit of Mimas
Saturn’s rings
Huge numbers of gaps and ringlets- the rings aren’t smooth!
These are caused by the gravitational interaction of the ring
particles with Saturn’s moons and with many tiny moonlets
within the ring system
Some very thin rings are kept in place by “shepherd” satellites
that prevent the ring from spreading out further
Shepherd moons
Pandora
Prometheus
Jupiter’s ring
Discovered by Voyager
Ring radius is about 1.8 times Jupiter’s radius
Not very reflective- composted of sooty dust particles
Main ring is made of dust originating from moons Adrastea
and Metis; gossamer rings are dust that originated from
Amalthea and Thebe
Uranus
Uranus’s rings were originally
discovered because they blocked
the light of background stars as
Uranus moved across the sky
Shepherd
moons and
Uranus’s
rings
Neptune’s rings
Solar System
more than just the planets
What about poor Pluto?
It’s very small, its moon is very large, it has a
highly elliptical and inclined orbit
Definition of a planet
Then in 2005, an object orbiting beyond Pluto but 27% more massive
than Pluto was discovered: this object was named Eris
So should we make Eris the “tenth planet”?
In 2006, the International Astronomical Union convened in Prague to set
the definition of a planet and settle the question
A planet within the solar system is a celestial body that
1. Orbits the Sun
2. Is massive enough for its self-
gravity to give it a nearly round shape
3. Has cleared the neighborhood around its orbit
Objects that meet the first two criteria but not the third are dwarf planets – so Pluto and Eris are both dwarf planets
Deciding Pluto’s fate