Chapter 24: Modified

Uranus Chance discovery by William Herschel in Scanning the sky for nearby objects using parallax

iClicker Question What is parallax? A. The apparent motion of a star due to the Earth’s motion B. A straight line from the Earth to a star C. A straight line from the Earth to a planet D. The backwards motion of a planet as it appears from Earth E. The gravitational pull an object has on light

The Motion of Uranus Very unusual orientation of rotation axis: Almost in the orbital plane. Possibly result of impact of a large planetesimal during the phase of planet formation. Large portions of the planet exposed to “eternal” sunlight for many years, then complete darkness for many years!

The Atmosphere of Uranus Like other gas giants: No surface. Gradual transition from gas phase to fluid interior. Mostly Hydrogen 15 % He Methane, ammonia and water vapor in smaller percents.

iClicker Question What is unique about the rotation of Uranus? A. It does not rotate B. It is tidally locked to the Sun C. It rotates nearly in its orbital plane D. Its core rotates but its cloud cover does not E. None of the above

The Atmosphere of Uranus Optical view from Earth: Blue color due to methane, absorbing longer wavelengths Cloud structures only visible after artificial computer enhancement of optical images taken from Voyager spacecraft.

The Structure of Atmosphere Only one layer of Methane clouds (in contrast to 3 cloud layers on Jupiter and Saturn). Uranus’ cloud layer difficult to see because of thick atmosphere above it.

iClicker Question Uranus’ atmosphere is mostly made of A. Hydrogen B. Helium C. Water Vapor D. Methane E. Ammonia

iClicker Question Why does Uranus appear blue? A. It’s sad B. It’s covered in water vapor C. It’s covered in Methane D. It’s surface is covered in ice E. It only appears blue in the UV spectrum

Cloud Structure of Uranus Keck Telescope images of Uranus show clear variability of the cloud structures

The Interior of Uranus Average density ≈ 1.29 g/cm 3  larger portion of rock and ice than Jupiter and Saturn.

The Magnetic Field of Uranus No metallic core  no magnetic field was expected. A magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered

The Magnetic Field of Uranus Offset from center 30 % of planet’s radius Inclined by 60-degrees against axis of rotation. Magnetosphere with weak radiation belts

iClicker Question What is the most important thing contributing to a planet’s magnetic field? A. Atmospheric conditions B. Gravity C. Moons / Rings D. Core E. The Sun

iClicker Question Why is the magnetic field of Uranus so off-center? A. Collision with a planetesimal B. Effects of Neptune’s magnetic field C. Interaction with the solar wind D. Effects of the ring/moon system E. Interaction with the Kuiper Belt objects

The Magnetosphere of Uranus Direct interaction of solar wind with a pole of the magnetosphere  Bright aurorae UV images

The Rings of Uranus Rings of Uranus were discovered through occultation of a background star

iClicker Question What caused the large period of no light from the occulted star? A. The magnetic field B. The planet C. The moons D. The largest ring E. Transit of the sun

The Rings of Uranus Rings of Uranus and Neptune are similar to Jupiter’s rings. Confined by shepherd moons; consist of dark material.

The Moons of Uranus 5 largest moons visible from Earth. 10 more discovered by Voyager 2; more are still being found. Dark surfaces, probably ice darkened by dust from meteorite impacts. 5 largest moons all tidally locked to Uranus.

iClicker Question The center of Uranus’ moons consists mostly of A. Metallic hydrogren B. Liquid hydrogen C. Ice D. Rock E. Gas

iClicker Question The albedo of Uranus’ moons is A. high because of ice B. high because of dust C. low because of ice D. low because of dust E. high because of the atmospheres

Interiors of Uranus’s Moons Large rock cores surrounded by icy mantles.

Oberon Old, inactive, cratered surface, but probably active past. Long fault across the surface. Dirty water may have flooded floors of some craters. Oberon

Titania Largest moon Heavily cratered surface, but no very large craters. Active phase with internal melting might have flooded craters. Titania

Umbriel Dark, cratered surface No faults or other signs of surface activity Umbriel Ariel

Brightest surface of 5 largest moons Clear signs of geological activity Crossed by faults over 10 km deep Possibly heated by tidal interactions with Miranda and Umbriel. Ariel

Miranda Most unusual of the 5 moons detected from Earth Ovoids: Oval groove patterns, probably associated with convection currents in the mantle, but not with impacts. Surface features are old; Miranda is no longer geologically active.

iClicker Question What is the size of Uranus’s moons? A. Larger than Earth Size B. Earth Size C. Mars Size D. Earth’s Moon Size E. Smaller than Earth’s Moon

Neptune Discovered in 1846 at position predicted from gravitational disturbances on Uranus’s orbit by J. C. Adams and U. J. Leverrier. Blue-green color from methane in the atmosphere 4 times Earth’s diameter; 4 % smaller than Uranus

The Atmosphere of Neptune

Neptune’s Magnetic Field Like Uranus, Neptune’s magnetic field is off center and aligned at an angle to its rotation. The planet itself is tilted off-axis, but not as much as Uranus.

iClicker Question How was Neptune discovered? A. Accidentally B. By studying gravitational effects on Uranus C. By studying gravitational effects on Pluto D. By Voyager II E. Looking at the gap between Uranus and Pluto

The Atmosphere of Neptune Cloud-belt structure with high-velocity winds; origin not well understood. Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived. White cloud features of methane ice crystals

The Rings of Neptune Ring material must be regularly re-supplied by dust from meteorite impacts on the moons. Made of dark material, visible in forward- scattered light.

iClicker Question What is the name of the small moons which keep planetary rings in a thin line? A. Guiding Moons B. Ring Moons C. Regulatory Moons D. Channeling Moons E. Shepard Moons

The Rings of Neptune Focused by small shepherd moons embedded in the ring structure.

The Rings of Neptune Interrupted between denser segments (arcs)

The Moons of Neptune Two moons (Triton and Nereid) visible from Earth; 6 more discovered by Voyager 2 Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”. Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

iClicker Question Which of the following people did not discover a planet or Moon? A. Percival Lowell B. William Herschel C. Clyde Tombough D. J.C. Adams E. Galileo Galilee

iClicker Question Which objects orbit backwards? A. Venus B. Nereid C. Triton D. Objects outside the Earth’s orbit E. All of the above

The Surface of Triton Very low temperature (34.5 K) Triton can hold a atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere. Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide. Possibly cyclic nitrogen ice deposition and re-vaporizing on Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.

The Surface of Triton Ongoing surface activity: Surface features probably not more than 100 million years old.

Pluto Discovered 1930 by C. Tombaugh. Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances.

Pluto as a Planet Virtually no surface features visible from Earth. ~ 65 % of size of Earth’s Moon. Highly elliptical orbit; coming occasionally closer to the sun than Neptune. Orbit highly inclined (17 o ) against other planets’ orbits  Neptune and Pluto will never collide. Surface covered with nitrogen ice; traces of frozen methane and carbon monoxide. Daytime temperature (50 K) enough to vaporize some N and CO to form a very tenuous atmosphere.

Pluto’s Moon Charon Hubble Space Telescope image Discovered in 1978; about half the size and 1/12 the mass of Pluto itself. Tidally locked to Pluto.

Pluto and Charon Orbit highly inclined against orbital plane. From separation and orbital period: M pluto ~ 0.2 Earth masses. Density ≈ 2 g/cm 3 (both Pluto and Charon)  ~ 35 % ice and 65 % rock. Large orbital inclinations  Large seasonal changes on Pluto and Charon.

The Origin of Pluto and Charon Probably very different history than neighboring Jovian planets. Older theory: Modern theory: Pluto and Charon members of Kuiper belt of small, icy objects (see Chapter 25), caught in orbital resonances with Neptune (“Plutinos”). Collision between Pluto and Charon may have caused the peculiar orbital patterns and large inclination of Pluto’s rotation axis. Mostly abandoned today since such interactions are unlikely. Pluto and Charon formed as moons of Neptune, ejected by interaction with massive planetesimal.