Chapter 24: Modified

UranusChance discovery

by William Herschel in 1781.

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 motionB.A straight line from the Earth to a starC.A straight line from the Earth to a planetD.The backwards motion of a planet as it appears from EarthE.The gravitational pull an object has on light

The Motion of UranusVery 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 UranusLike other gas giants:

No surface.

Gradual transition from gas phase to fluid interior.

Mostly Hydrogen15 % HeMethane, ammonia and

water vapor in smaller percents.

iClicker Question

What is unique about the rotation of Uranus?

A.It does not rotateB.It is tidally locked to the SunC.It rotates nearly in its orbital planeD.Its core rotates but its cloud cover does notE.None of the above

The Atmosphere of UranusOptical 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 AtmosphereOnly 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.HydrogenB.HeliumC.Water VaporD.MethaneE.Ammonia

iClicker Question

Why does Uranus appear blue?

A.It’s sadB.It’s covered in water vaporC.It’s covered in MethaneD.It’s surface is covered in iceE.It only appears blue in the UV spectrum

Cloud Structure of UranusKeck

Telescope images of Uranus show clear variability of the cloud structures

The Interior of Uranus

Average density ≈ 1.29 g/cm3 larger portion of rock and ice than Jupiter and Saturn.

The Magnetic Field of UranusNo metallic core no magnetic field was expected.

A magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered

The Magnetic Field of UranusOffset from center

30 % of planet’s radius

Inclined by 60-degrees against axis of rotation.

Magnetosphere with weak radiation belts

iClicker QuestionWhat is the most important thing contributing to a planet’s magnetic field?

A.Atmospheric conditionsB.GravityC.Moons / RingsD.CoreE.The Sun

iClicker QuestionWhy is the magnetic field of Uranus so off-center?

A.Collision with a planetesimalB.Effects of Neptune’s magnetic fieldC.Interaction with the solar windD.Effects of the ring/moon systemE.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 QuestionWhat caused the large period of no light from the occulted star?

A.The magnetic fieldB.The planetC.The moonsD.The largest ringE.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 Uranus5 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 QuestionThe center of Uranus’ moons consists mostly

of

A.Metallic hydrogrenB.Liquid hydrogenC.IceD.RockE.Gas

iClicker Question

The albedo of Uranus’ moons is

A.high because of iceB.high because of dustC.low because of iceD.low because of dustE.high because of the atmospheres

Interiors of Uranus’s Moons

Large rock cores surrounded by icy mantles.

OberonOld, inactive, cratered

surface, but probably active past.

Long fault across the surface.

Dirty water may have flooded floors of some craters.Oberon

TitaniaLargest moon

Heavily cratered surface, but no very large craters.

Active phase with internal melting might have flooded craters.

Titania

UmbrielDark, cratered

surface

No faults or other signs of surface activity

Umbriel

Ariel

ArielBrightest 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

MirandaMost 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 SizeB.Earth SizeC.Mars SizeD.Earth’s Moon SizeE.Smaller than Earth’s Moon

NeptuneDiscovered 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.AccidentallyB.By studying gravitational effects on UranusC.By studying gravitational effects on PlutoD.By Voyager IIE.Looking at the gap between Uranus and Pluto

The Atmosphere of NeptuneCloud-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 NeptuneRing 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 MoonsB.Ring MoonsC.Regulatory MoonsD.Channeling MoonsE.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 NeptuneTwo 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 LowellB.William HerschelC.Clyde TomboughD.J.C. AdamsE.Galileo Galilee

iClicker Question

Which objects orbit backwards?

A.VenusB.NereidC.TritonD.Objects outside the Earth’s orbitE.All of the above

The Surface of TritonVery 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.

PlutoDiscovered 1930 by C. Tombaugh.

Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances.

Pluto as a PlanetVirtually 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 (17o) 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 CharonOrbit highly inclined against orbital plane.

From separation and orbital period:

Mpluto ~ 0.2 Earth masses.

Density ≈ 2 g/cm3

(both Pluto and Charon)

~ 35 % ice and 65 % rock.

Large orbital inclinations Large seasonal changes on Pluto and Charon.

The Origin of Pluto and CharonProbably 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.