ASTR178Other Worlds

A/Prof. Orsola De Marco9850 4241

orsola.demarco@mq.edu.au

• 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.

Announcements

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 HST2 additional from Voyager.

Dark and narrow and flat.

Uranus’ andNeptune’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 tohave changed a lot in the past, unlikely to have been around since the beginning4.5 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 XDiscovered 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 radiation-darkened 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.