1445 Introductory Astronomy 8b

7/31/2019 1445 Introductory Astronomy 8b

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1445 Introductory Astronomy I

Chapter 8b

The Gas Giants: Saturn, Uranus and Neptune

R. S. Rubins Fall, 2010


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Key Features of the Gas Giants

Jupitersmost extraordinary feature is its Great Red Spot,agiant storm, over twice the width of the Earth, which has been

seen for more than 300 years.a

Saturnskey features are its spectacular rings, and a hexagonal

cloud pattern at its North pole, large enough to contain twoEarths, which rotates slower than the surrounding zones

Uranus is tilted so much, probably due to a collision with an

Earth-sized planet early in its life, so that its axis of rotation lies

in the plane of its orbit around the Sun.

Neptune has jet-stream winds traveling at more than 1500 mph,

which are the fastest planetary winds seen in the solar system,

despite being so far from the Sun.2


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The Beauty of Saturn and its Rings

An Earth-sized storm may be seen just above the equator.


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Different Views of Saturn from Earth


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Saturn Data Average distance from Sun, 9.6 AU.

Mass, 95.2 ME. Average radius, 9.45 RE.

Average Density, 0.13 dE, making it the least dense planet.

Siderial revolution period, 29.5 Earth years.

Rotation period: varies from 10 hours 14 min at the equator

to 10 hours 47 min at high latitudes.

Tilt of rotation axis: 26.7

o

. Temperature of outer atmosphere, 130 K ( 220oF);

cf. 160 K on Jupiter.

No. of moons, 60 (2007).


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Internal Structure of Saturn


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Atmospheres of Jupiter and Saturn Compared


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Voyager View of Saturn

Less contrast between belts and zones than on Jupiter.


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Dragon-shaped Storm on Saturn


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Hurricane-like Storm at Saturns South Pole

A storm measuring 2/3 the Earths diameter, with speeds of350 mph (about twice the speeds ever found on Earth), was

observed at Saturns south pole in 2006.

Like a hurricane, it has well-developed eye surrounded by

towering clouds.


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Hexagonal Cloud System at Saturns North Pole

This very unusual unexplained hexagonal structure, largeenough to hold four Earths, has remained intact since it was

discovered in the Voyagermissions twenty years ago.

A similar storm to that found on the south pole of Saturn was

recently observed at its north pole.


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Aurora on Saturn

Infrared photo of aurora at Saturns north pole.

The huge aurora appears not to fit the models usedto describe auroras on the Earth or on Jupiter.


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Auroras on Saturn observed in the UV


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Changing Auroras on Saturn in UV


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Saturns Rings

Galileo observed Saturns rings in 1610.

In 1675, Giovanni Cassini discovered a dark division in the

rings.

Saturns rings are less than 100 m thick, but stretch for over

a hundred thousand km, comparable to a sheet of tissuepaper spread across a football field.

The rings extend to the Roche limit, below which Saturnstidal forces overwhelm the tendency for the particles to

conglomerate.

The structure of the rings is produced by subtle gravitational

effects involving the planet and shepherd moonswithin the

rings, which nudge particle into particular orbits.


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Voyager 1 Photo of Saturns Rings


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Shepherd Moons Confine Particles to the F Ring


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Saturn and Mimas


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Saturns Rings and Mimas


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Dione, Saturns 4th Largest Moon from Cassini


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Dione and Titan

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Saturns Rings Bisect Titan

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Saturn in Natural Color

Photo taken by Cassiniin 2003.


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Giant Ring of Saturn in IR

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Titan 1

Saturns largest moon is Titan, the 2nd largest moon in thesolar system, with a radius of 0.40 RE, which is larger thanthat of Mercury, 0.38 RE.

Titan is the only moon in the solar system with a substantialatmosphere, and a surface pressure much larger than Earth.

The Titan atmosphere is roughly 90% nitrogen, 10% argon,with the remainder largely methane (CH4).

A dense orange-colored hydrocarbon smog at high altitudes,containing a rich variety of organic molecules, is caused by

UV light from the Sun breaking up the methane molecules. Experiments on Earth made with the chemicals present in

Titans atmosphere, exposed to a strong UV source, haveindicated that the atmospheric haze could produce aminoacids, proteins and early life-forms.


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Huygens on Titans Surface: Artists Impression

Three weeks after being parachuted from the Cassini

spacecraft in 2005, the Huygensprobe transmittedimages of the Titan surface, which in this region

consists of small pieces of ice on moist sand, for over

an hour.

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Titan 2

First observed in 1980 as a yellow-orange ball by Voyager 1,

details of Titan were revealed by NASAs Cassini spacecraftin 2005, after a 7 year voyage from Earth.

The probe Huygenstransmitted data from Titans surface ,while Cassinihas collected data in over 60 orbits of Titan.

The images from Huygensshow a rugged terrain similar tothat of the Earth, containing dunes, craters, rivers and lakes,

with methaneoccupying the role played on the Earth bywater.

Titan is a very cold, with a surface temperatures of about 94 K

( 290 oF).

If it were not for the greenhouse effectproduced by the

methane, the atmosphere would freeze out entirely.


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Titan Surface: Artists Impression 2

After over 60 flybys of

Titan by Cassiniin the lastfive years, scientists have

pieced together a detailed

picture of the planet.

Bigger than Mercury, and

with an atmosphere

denser than Earth, Titan

reminded observers of the

California coast or theFrench Riviera - quite

unlike other large moons

in the outer solar system.

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First View and Structure of Titan

View from Voyager 1, 1980. Structure 2010

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Titan 3

Variations in the rate at which Titan spins about its axis of

rotation have indicated that an ocean of water lies below athick crust of water ice.

The only other moons thought to contain deep sub-surfaceoceans of water are Jupiters moons Europa, Ganymedeand

Callisto, and Saturns Enceladus. Titans atmosphere, although about 200 oC colder than

Earth, contain equivalent layers to the Earths atmosphere:troposphere, stratosphere, mesosphere and thermosphere.

Because of Titans weaker gravity, these layers are stretchedvertically.

While the temperature of the Earths stratosphere increaseswith height because of UV absorption in the ozone layer, the

increase in Titan is due to absorption of sunlight by the haze.


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Titan and Earths Atmospheres Compared

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Titan: Methane Circulation

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Titan 4

On Titan, methane replaces water as the working fluid of

the climate, existing in the atmosphere, and forming cloudsand rain.

Methane is broken apart by UV radiation from the Sun at a

fast rate, which means that there must be a source of

methane in the deep interior of Titan.

Cassini flybys have shown that the level of Titans largest

lake in its southern hemisphere has fallen by 15 feet in the

last four years, because of enhanced evaporation caused

by Titans very elliptical orbit, which brings it closer to theSun during the southern summer.

If there is some biology on Titans surface, it must exist in

methane (or ethane), and not require water.34


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Sunlight Reflects from a Titan Lake

Sunlight is reflected

from Kraken Mare, alake presumably

containing thehydrocarbons methane

and ethane.

When the photo was

taken, Spring hadarrived after 15 years of

winter.

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Enceladus 1

Only 500 km across, Enceladus,the ice-covered moon of

Saturn, which is over a billion km from the Earth, has become aprime target for observing potential life-forms.

Although very tiny, internal gravitational heating, caused by its

elliptical orbit, makes possible the existence of liquid water.

Enceladus and Saturns ringsMaterial from Enceladus

formed Saturns F-ring.


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Enceladus 2

In September 2008, the space-probe Cassinipassed just 30

miles above Enceladus, traveling at 40,000 mi/h, and furtherpasses occurred in October and November.


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Enceladus 3

A warm area near the south pole of Enceladus shoots

geysers of vapor and ice, from a region containing fractures,known as tiger stripes.

False image

of geysers.

Green ice


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Enceladus 4

A 2009 observations from the Cassiniprobe indicates that

Enceladus has retained liquid water.

The detection of sodium compounds by Cassini suggests that

water has leached out sodium from the rocks, as happened

over time in the Earths oceans.

It appears that liquid water may reside in caverns just below the

surface of the moon, though this hypothesis is not yet proved.

The tiger stripesnear Enceladus south polar region are

fractures running across a warm spot, with V-shaped wallsabout 1000 ft deep, caused by tidal heatingbecause of theplanets proximity to Saturn.

The tiger stripesare the source of immense plumes of water

vapor and ice particles39


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Enceladus 5

The plumes of water vapor and ice result from the fractures

being 100oC warmer than the surrounding ice surface.

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Enceladus Geysers 1

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Enceladus Geysers 2

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Sunset Imagined on Enceladus

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Geysers on Enceladus: Artists Impression

Chosen as a ScientificAmericanWonder of

the Solar System,Enceladus has geysers,

which send ice-crystals

into space at more than

1000 mph.

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Blackened Face of Iapetus

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Iapetus 2

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Saturns Moon Phoebe

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Heschel Crater on Mimas

This crater, 160 km in diameter, is almost big enough tohave destroyed Saturns tiny moon, Mimas.


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Herschel Crater, Mimas: Artists Impression

Chosen as a Scientific AmericanWonder of the SolarSystem, Mimas has a peak at the center of the crater more

than 6 km above the floor (cf. Mt.Everest, height 9 km).49


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Whale-like Prometheus

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Uranus in Visible Light


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Uranus in the InfraredIR photo (in false color) taken by the Hubble telescope

shows moons, rings, belts and clouds.

U D


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Uranus Data

Mass: 14.5 ME.

Equatorial radius: 4.0 RE.

Average distance from Sun: 19.2 AU.

Average density: 23.4 % dE.

Orbital eccentricity 0.047.

Siderial revolution period: 84.0 Earth years.

Equatorial rotation period: 16 hours 30 min (retrograde).

Temperature of outer atmosphere is 80 K ( 320oF).


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About Uranus

Uranus is characterized by its blue-green color andfeatureless surface in visible light, although a series of

belts is observed in the infrared.

The outer layers are composed mainly of hydrogen and

helium. Methane, which absorbs red light, forms the

higher clouds, giving the planet its blue color.

The tilt of the rotation axis is 98o, so that, as Uranus orbits

the Sun, its north and south poles alternately point almost

directly towards or directly away from the Sun, producing

exaggerated seasons.


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Seasons on Uranus


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Edge-on View of Uranus Rings

The edge-on view of Uranus rings, as seen in 2007, happens

only every 42 years. This view allows particular features of the rings, such as the

almost transparent layers of dust, to be seen more clearly,

since the dust merges to a very thin band along the line-of

site.


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Uranian Moon, Miranda

The smallest of Uranuss 5 moons, Mirandais a mixture

of dense rocks and ice. The cliffs at the lower right soarabout 20 km twice as high as Mt. Everest.


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Light Clouds on Neptune


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Interiors of Uranus and Neptune

About Neptune


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About Neptune

Equatorial radius: 3.9 RE.

Mass: 17.1 ME. Average distance from Sun: 30.1 AU.

Average density: 29.7 % Earth density.

Orbital eccentricity 0.009. Siderial revolution period: 165 Earth years.

Equatorial rotation period: 19 hours 6 min.

Temperature of outer atmosphere: 70 K ( 330o

F). Winds blow as fast as 2000 km/h.

Physically similar to Uranus, it appears blue because of theabsorption of red light by methane.


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Neptunes Great Dark Spot


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Neptunes Bands


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Neptunes Rings

Triton 1


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Triton 1

Neptunes largest moon Triton(diam. 2700 km) has a

retrograde orbit, indicating that it was captured about 3-4billion years ago.

Neptunes tidal forces are causing it to spiral inwards

(opposite to our Moon), so that in about 200 million years,

the entire moon will be demolished by tidal forces,creating a new ring system for Neptune.

Tritonhas a very thin nitrogen atmosphere, and a surfacethat consists mainly of water ice. A layer of nitrogen frost

forms and evaporates over the polar caps.

The surface temperature, measured byVoyager 2, was 36K ( 395oF), the coldest yet found.

Triton Showing Nitrogen Ice


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Triton Showing Nitrogen Ice


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Geyser on Triton: Artists Impression

Chosen as a ScientificAmericanWonder of

the Solar System,Triton has many

cryogeysers, probablycomposed of nitrogen

frost and dark organiccompounds.

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1445 Introductory Astronomy 8b