The Outer Worlds

Draft 12/03/2006Updated May 05, 2011

(accidentally!) by William Herschel using a 6 inch telescope[he thought it was a comet!]

1781: Uranus Discovered2

• Le Verrier : proposed that another planet was creating deviations in orbit of Uranus. Predicts position.

• John Couch Adams: independently does same

• Johann Gottfried Galle: (student of Encke) using Le Verrier’s prediction, observes planet.

1846: Neptune Discovered3

Distance Comparison

1 AU 5.2 AU 9.6 AU 19.2 AU 30.1 AU

3.7% as much sunlight as Earth

1.1% as much sunlight as Earth

0.3% as much sunlight as Earth

0.1% as much sunlight as Earth

Sun

Uranus86 year orbit

Neptune165 year orbit

Saturn29 year orbit

Jupiter12 year orbit

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Exaggerated Seasons On Uranus

• 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 planetlike object early in the history of our solar system. Such a collision could have knocked Uranus on its side

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Seasons are 42 years long!

Uranus Rotates Sideways 6

Size Comparison

12,757 km(1 DE)

142,984 km(11 DE)

120,536 km(9.4 DE)

51,118 km(4.0 DE)

49,528 km(3.8 DE)

Uranus

Neptune

7 Density Comparison

• Masses determined from orbits of their moons

• Uranus & Neptune are much higher density than expected, implies more heavier elements (INCONSISTENT with theories of planet formation which predict outer planets would be lighter in density)

• Temperatures of Uranus & Neptune are nearly the same!

Item Jupiter Saturn Uranus Neptune

Mass (earth masses)

318 95 14.5 17.2

Density (kg/m3) 1326 687 1318 1638

Albedo .44 .46 .56 .51

Temp 165 93 55 55

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1977 Rings Discovered 9 1977 Voyager 2 Probe

• 1986 Uranus Encounter• 1989 Neptune Encounter

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Sees thin, dark rings

NeptuneUranus

11Voyager at Uranus and Neptune

• Significant differences from Jupiter and Saturn– bluish color– Uranus very bland– Neptune faint belts and zones and also a few storms– much smaller (and less massive) than Jupiter and Saturn

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Uranus: Atmosphere

• Nearly featureless, though recent Hubble images show some small storms

• 82.4%H, 15.2% He, 2.3 % CH4 (methane)

• The blue tint is caused by methane

• Too cold for ammonia or water ice clouds, so any clouds are also methane

• Does not emit excess heat

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As the vernal equinox approaches, Uranus is becoming stormy:

Heating of northern hemisphere

produces storms, made visible by

clouds of methane.

Recent Hubble Pictures

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Neptune: Atmosphere• Similar composition as

Uranus (80% H, 19% He, 2% CH4)

• More storms and clouds

• Also belts and zones– why? less sunlight

should mean less active atmosphere

=> Neptune, like Jup and Sat, emits more heat than it receives. This internal heat must drive atmospheric activity

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Neptune’s Clouds

• Much more cloud activity is seen on Neptune than on Uranus

• This is because Uranus lacks a substantial internal heat source

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Neptune’s atmosphere is being warmed from below by internal heat.

This extra heat drives large circular storms, like the “Great Dark Spot” seen above.

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Uranus & Neptune: Winds• Both Uranus and Neptune winds blow west at equator

and east at N and S poles and mid-latitudes• Not understood why Neptune’s winds do not follow belts

and zones like on Jupiter and Saturn• Rotation: 17 Hours Uranus, 16 Hours Neptune

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Uranus & Neptune: Magnetic Fields• No metallic layer in interior

– fields generated by ionized liquid ammonia and water

• Rotated large amounts from spin vectors and offset from center– perhaps currently undergoing reversal, as Earth does– perhaps very large impact knocked them off line

19Uranus & Neptune: Interior Structure• “Rocky” core ~ size of Earth• Liquid mantle – probably NH3 and H2O or related material• Liquid molecular H and He with some liquid methane• Thin atmosphere

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Uranus+Rings+Moons21

Some of Uranus’s satellites show evidence of past tidal heating

Uranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites. Biggest is 1600 km (i.e. half our moon’s)

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• Old, heavily cratered, and icy surface.

• little evidence of internal activity other than some unknown dark material that apparently covers the floors of many craters.

• image shows several large impact craters towards the center of the image.

• On the limb, a high mountain rises 6 kilometers (4 miles) above its surroundings.

• There are bright rays similar to those seen on Jupiter's moon Callisto.

Oberon (outermost moon)23

Titania (next closer)

• The LARGEST moon

• Has a few large impact basins, but is generally covered with small craters and very rough rocks.

• Long 1,600 km trench

• large double walled crater can be seen towards the top of the image.

• many faults indicating there has been internal forces molding its surface.

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Umbriel

• Darkest of the Uranian moons

• It is about the same size as Ariel and has about the same density.

• The surface appears to be old with large craters and does not change much from one location to another.

• Near the top is a puzzling bright ring called the fluorescent cheerio. It is probably the floor of a crater.

25Ariel

• Brightest of the Uranian moons

• It is about the same size as Umbriel and has about the same density.

• The surface is pock-marked with craters

• long rift valleys stretching across the entire surface. Canyons much like the ones on Mars appear in the pictures. The canyon floors appear as though they have been smoothed by a fluid. The fluid could not have been water because water acts like steel at these temperatures. The flow marks might have been made by ammonia, methane or even carbon monoxide

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Miranda

• huge fault canyons as deep as 20 kilometers

• Such a small moon should not have had any significant internal heating or evolution, so the features were quite a surprise.

• scientists believe that Miranda may have been shattered as many as five times during its evolution

27Miranda• Gigantic Scarp!

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Neptune has 13 known moons.12 are small (< 500 km across) and irregular.One is the giant moon Triton.

Proteus Triton Ner eid

29 30

Triton is a frigid, icy world with a young surface and a tenuous atmosphere

• 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

31 Triton: 38K, frozen methane & nitrogen32

Triton has crater-free plains; perhaps the frozen calderas of extinct ice volcanoes?

Density of Triton = 2100 km/m3; ice mantle over rocky core.

33 34

35Triton has geysers 5 miles high driven by

vaporized nitrogen.

Plumes are swept downwind in the very tenuous nitrogen atmosphere.

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• Triton is on a tilted retrograde orbit around Neptune.

• It has most likely been captured by Neptune.

• Tidal forces are moving Triton toward Neptune. In 100 million years, Triton will enter the Roche limit.

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A farewell to the Solar System:

Voyager 2 looks back at Neptune and Triton.

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Notes

• Polar caps are frozen nitrogen