Uranus & Neptune

Lecture 25

Featureless Atmosphere of Uranus

Uranus from Voyager 2 This image looks nearly straight down onto Uranus’s south pole.

None of the Voyager 2 images of Uranus show any pronounced cloud patterns. The color is due to methane in the planet’s atmosphere, which absorbs red light but reflects green and blue.

No feature (storm) lack of energy

Uranus from the Hubble Space Telescope Images made at ultraviolet, visible, and infrared wavelengths werecombined and enhanced to give this false-color view of cloud features on Uranus.

82.5% H, 15% He, 2.3% CH4, very small amount of NH3 (because it got frozen out).

Uranus in UV, Visible, and IR

Extreme seasons in UranusFor Uranus the rotation axis is tilted by 98° from the perpendicular. This causes severely exaggerated

seasons. For example, during midsummer at Uranus’s south pole, the Sun appears nearly overhead for many

Earth years, while the planet’s northern regions are in continuous darkness. Half an orbit later, the seasons are reversed.

Wind flows to the rotation direction in northern/southern hemispheres excepting for the equatorial region (backward).Strong seasonal mix + lack of strong zonal winds featureless Uranus

Nearly identical atmospheric composition to Uranus and the same temperature (55K0 at top of atmo.

The white clouds are thought to be composed of crystals of methane ice.

Great Dark Spot = measured about 12,000 by 8000 km, comparable in size to the Earth similar to the Jupiter’s Great Red Spot (but has shorter timescale).

NeptuneNeptune from Voyager 2

Cirrus Clouds over Neptune Voyager 2 recorded this image of clouds near Neptune’s terminator.

Like wispy, high-altitude cirrus clouds in the Earth’s atmosphere, these clouds are thought to be made of ice crystals. The difference isthat Neptune’s cirrus clouds are probably methane ice, not water ice as on Earth (because NH3 and H2O got frozen out long before).

Neptune : Methane world

Neptune further away than Uranus (and same size), yet Neptune shows weather pattern!

White areas denote high-altitude clouds; the very highest clouds (near the top of the image) are shown in yellow-red. The green belt near the south pole is a region where the atmosphere absorbs blue light, perhapsindicating a different chemical composition there.

Neptune’s Banded Structure

Internal Structures Both Uranus and Neptune have a rocky core, resembling a terrestrial planet; a

mantle of liquid water with ammonia dissolved in it; and an outer layer of liquid molecular hydrogen and liquid helium

Origin of Uranus and Neptune

1. Formed closer : Uranus and Neptune were formed at 4-10AU (i.e., same region as Jupiter and Saturn), then they were “ejected” to the outer part stopped growing further due to lacking material

2. Disk instability model : They were formed in situ directly from gas and later accreted icy material. Nearby passing star “evaporate” away H+He and prevented them from growing further.

? at these large distances, the solar nebula was too sparse to form these massive planets. Takes long, long time to grow from low density material before gas is being blown away by the young Sun.

Enigmatic magnetic fields of Uranus and Neptune lower gravity no metallic hydrogen no magnetic field. But, they have magnetic field and they are off centered!!

Rings of UranusUranus’s Rings viewed from Voyager

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Discovery of Uranus’ rings from Earth

During 1977 occultation of Uranus, astronomers try to determine the precise radius of Uranus and Atomsphere the star “blinked” several times!Neptune’s ring was discovered from stellar occultation also.

They all lie within 86,000 km of the planet’s center (only about one-fifth of the distance from the Earth to our Moon) and all are less than 160 km (100 mi) in diameter. The arcs show how far each satellite moves around its orbit in 90 minutes

Uranus’s Rings and Small Satellites

This composite of Voyager 2 images shows that part of Miranda’s surface is ancient and heavily cratered, whileother parts are dominated by parallel networks of valleys and ridges.

At the very bottom of the image—where a “bite” seems to have been taken out of Miranda—is a range of enormous cliffs that jut upward to an elevation of 20 km, twice as high as Mount Everest.

One idea is that the entire moon was broken apart by a massive impact, only to have the fragments reassemble themselves.

Miranda : Moon of Uranus

Triton: Surprising possibility of potential habitability

Retrograde orbit captured satellite 23° tilted from the Netune’s equator

largest moon among 13 Neptune’s moons

Triton’s cantalope skinPossibly formed by diapirism (i.e., slow boiling pattern)

Soon after the capture, its orbit was quite elliptical. Overtime, it got circularized through tidal interaction (ongoing heating!)

Continuing tidal heating : because of retrograde motion (spiraling inward). In ~100 Myr, Triton will be inside of the Roche limit.

Crater count Triton’s surface is 10-100 million years old.

Active ice geysers!!

Remnant internal heat from the capture may drive the geological activity…possible subsurface liquid oceaneven at -230°C, possible habitable world!

Triton: Surprising possibility of potential habitability

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

Jovian Planets in SummaryUranus and Neptune are not simply smaller versions of Jupiter and Saturn.

In summary…

Important ConceptsUranus and Neptune are not the

smaller version of Jupiter and Saturn

Captured satellites.Energy source of Triton

Important TermsOccultationCryovolcanism

Chapter/sections covered in this lecture : sections 14-1 through 14-8