Download PHYS178 Planets

ASTR178
Other Worlds
A/Prof. Orsola De Marco
9850 4241
[email protected]
Announcements
• Observing is on October 6th 7:15PM and 13th 8:15PM.
(Sign up in class or on my door: E7A-316).
• Assignment 2 due in 17th September.
• Moon practical due in 17th September.
In this class:
Uranus and Neptune Part
• Monday: talk by Craig O’Neil
• U. and N.: worlds unseen – the discovery
• Uranus: cloudless and tilted
• Uranus’ atmosphere
• Neptune’s atmosphere and clouds
• Neptune’s bands and internal heat
• The formation of U. and N.
Uranus and Neptune
(Almost) Featureless Uranus
• H + He + CH4:
• Methane: blue colour, forms clouds at higher pressure – no clouds.
• Some banding.
• Storms in spring. Homogenization of equatorial bands.
• William Herschel noticed 2 moon rotating perpendicularly to orbit –
hence spin is perpendicular to orbit (98 degrees).
• 84 year period: seasons last 21 years and are extreme.
Uranus in the news
• In 2007 Uranus went through the equinox – what is it?
• The period of Uranus is 84 years – how often are the equinoxes?
• Storm near South band moved north.
• South band dimmed and north band brightened – bright bands might change
hemisphere.
http://www.news.wisc.edu/newsphotos/uranus2008.html
Neptune in the news
• In 2006 (old news!) Neptune was found to have a warm South Pole.
• These Very Large Telescope images show a hot spot over the pole just above
the troposphere, but a wider ring of warm clouds at higher altitudes.
Uranus’ Rings
1977 Stellar
occultation: 9 rings
spotted.
2 detected from HST
2 additional from
Voyager.
Dark and narrow and
flat.
Uranus’ and
Neptune’s Rings
Uranus’ 5 moderate size satellites
No tidal heating likely – no resonances.
Could have been different in the past….
Uranus’ 22 smaller satellites … some prograde some retrograde orbits – some
might have been captured, others maybe not. Orbits are very chaotic, likely to
have changed a lot in the past, unlikely to have been around since the beginning
4.5 billion years ago.
Uranus’ Miranda (one of the 5) swaths of young surface –
likely when Miranda was in a different orbit (likely in a resonance)
that allowed tidal heating.
•Neptune’s Triton has a circular retrograde orbit – what does it mean?
•Surface is locally new.
•Heat source might have been tidal heating in an elliptical orbit (orbit
was likely very elliptical after capture).
•Plumes of ejected gas seen by Voyager – heat today!
•Everything is frozen on Triton
•(even N2 has solid features in
the spectrum).
•Some parts of Triton warm
enough to free some of the
N2 which forms a thin
•Atmosphere, causing some
weather features on the
surface.
•Triton is falling on Neptune
(it has 100 million years to go!)
•When it enters the Roche
limit…. What will happen?
Pluto
Searched for following calculations on Neptune perturbations: planet X
Discovered serendipitously by Clyde Tombaugh in 1930 after big search.
Which one is Pluto?
Artist rendition from image
Best resolution reconstructed “image”
Best images (HST)
Pluto’s subtends only 0.15’’ (remember the Moon is 30’; The eye can resolve
~1’)
Pluto’s satellites
• Charon found in 1978 by rotating
elongation in image of Pluto
• Very close: 5% of the Earth-Moon
distance.
• Very large for a moon: ½ of Pluto’s size.
• Pluto and Charon are phase locked:
period ~6 days.
• Nix and Hydra discovered in 2005!
• In 1985-1990 Earth views Pluto-Charon
edge on: many measurements:
• Diameters (density only 2000 kg/m3)
what does it mean?
• Bright polar cap on Charon
• Spectrum of Pluto CH4, CO and N2
(likely N2 and CO in volatile form:
T~40K)
• Charon probably no atmosphere
(features of water ice)
The Kuiper belt
• Pluto shares orbit with ~100
stable plutinos:
• 2:3 Pluto: Neptune orbit
resonance.
• Kuiper belt sharp outer edge
50AU - 1:2 resonance with
Neptune.
• (Saturn’s rings gap resonance
2:1 with Mimas.)
• Some objects (Sedna and Eris)
are trans-Neptunian but not
KBOs (their semi-major axes
are: 489 and 67 AU).
• KB leftover from formation of
the Solar System. Some objects
formed there (circular orbits)
some where kicked there
(elliptical orbits).
Key Ideas
• Discovery of the Outer Planets: Uranus was discovered by
chance, while Neptune was discovered at a location predicted by
applying Newtonian mechanics. Pluto was discovered after a
long search.
• Atmospheres of Uranus and Neptune: Both Uranus and
Neptune have atmospheres composed primarily of hydrogen,
helium, and a small percentage of methane.
• Methane absorbs red light, giving Uranus and Neptune their
greenish-blue color.
• No white ammonia clouds are seen on Uranus or Neptune.
Presumably the low temperatures have caused almost all the
ammonia to precipitate into the interiors of the planets. All of
these planets’ clouds are composed of methane.
Key Ideas
• Much more cloud activity is seen on Neptune than on Uranus.
This is because Uranus lacks a substantial internal heat source.
• Interiors and Magnetic Fields of Uranus and Neptune: Both
Uranus and Neptune may have a rocky core surrounded by a
mantle of water and ammonia. Electric currents in these mantles
may generate the magnetic fields of the planets.
• The magnetic axes of both Uranus and Neptune are steeply
inclined from their axes of rotation. The magnetic and rotational
axes of all the other planets are more nearly parallel. The
magnetic fields of Uranus and Neptune are also offset from the
centers of the planets.
Key Ideas
• Uranus’s Unusual Rotation: Uranus’s axis of rotation lies
nearly in the plane of its orbit, producing greatly exaggerated
seasonal changes on the planet.
• This unusual orientation may be the result of a collision with a
planet-like object early in the history of our solar system. Such
a collision could have knocked Uranus on its side.
• Ring Systems of Uranus and Neptune: Uranus and Neptune
are both surrounded by systems of thin, dark rings. The low
reflectivity of the ring particles may be due to radiationdarkened methane ice.
Key Ideas
• Satellites of Uranus and Neptune: Uranus has five satellites
similar to the moderate-sized moons of Saturn, plus at least
22 more small satellites. Neptune has 13 satellites, one of
which (Triton) is comparable in size to our Moon or the
Galilean satellites of Jupiter.
• Triton has a young, icy surface indicative of tectonic activity.
The energy for this activity may have been provided by tidal
heating that occurred when Triton was captured by Neptune’s
gravity into a retrograde orbit.
• Triton has a tenuous nitrogen atmosphere.
Key Ideas
• Worlds Beyond Neptune: Pluto and its moon, Charon, move
together in a highly elliptical orbit steeply inclined to the
plane of the ecliptic.
• More than a thousand icy worlds have been discovered
beyond Neptune. Pluto and Charon are part of this
population.
• Most trans-Neptunian objects lie in a band called the Kuiper
belt that extends from 30 to 50 AU from the Sun. Neptune’s
gravity shapes the orbits of objects within the Kuiper belt.