Chapter 13: Uranus, Neptune, and PlutoThe Outer Worlds of the Solar System

• Discovery

• Comparisons

• Physical Properties

• Moons and Rings

Uranus Neptune Pluto and Charon

Discovery

• The three outermost planets were unknown to the ancients.

• All were discovered by telescopic observations:– Uranus in 1781– Neptune in 1846– Pluto in 1930

Comparison of the Jovian Planets

Uranus

Seeing Uranus

• Uranus is just barely visible to the naked eye. • The light coming from Uranus during a time

span of 200 years would be equivalent to the light shining from a flashlight in just 1 second!

• Uranus moves so slowly against the background stars, that it went unnoticed as a planet until Herschel's discovery in 1781.

William Herschel

• Called the “greatest observational astronomer ever” and the “Father of Stellar Astronomy.”

• Musician by training; earned his living for a great part of his life teaching, performing, and composing music.

• Built his own 6” reflector.

• Discovered the planet Uranus.

• With help of his sister Caroline, completed first through study of stars and nebulae; discovered existence of double stars; first to correctly describe the form of our Galaxy, The Milky Way.

Caroline Herschel• Caroline was born 16th March,1750.

Her brothers brought her to England as a singer, but she soon followed them into the field of astronomy, initially as William's assistant, noting measurements, grinding telescope mirrors, and carrying out calculations on astronomical data, on top of running the household. In 1782 William gave her a small telescope and she began 'comet hunting'. In her sweeps of the sky she found nebulae, new clusters and a total of eight comets.

• One of the first two women elected to honorary membership in the Royal Astronomical Society.

Uranus: View from Earth

•Discovered in 1781 by William Herschel.•Least massive of the Jovian planets

– 15 x Earth’s mass (~1/20 mass of Jupiter) – 4 x Earth’s radius (~1/3 radius of Jupiter)

•Small angular diameter ( ~4” at opposition) makes few features visible from Earth-based telescopes.

•Discoveries from Earth-based observations: – rotation axis lies nearly in the ecliptic,– five satellites,– system of 9 rings found studying occultation of a star.

Uranus: View from Space•Voyager 2 arrived in 1986 and observed:

–no surface markings,

–little excess energy emitted from the planet,

–a planetary magnetic field • ~100 x Earth’s,

• tilted at 600 to rotation axis, and

• offset from planet center by 1/3 of planet radius,

–rotation period 17.2 hours w/ differential rotation of atmosphere• atmosphere at poles rotates faster than at equator

–ten small moons inside orbit of Miranda,• all in circular, synchronous orbits in equatorial plane

• many related to ring system

–new views of the five known moons,

–confirmed 9 rings and discovered 2 more rings.

Uranus• Mass: 8.68 x 1025 kg (14.5 x Earth’s) (1/20 x Jupiter’s)

• Diameter: 51,118 km (4.0 x Earth’s) (0.36 x Jupiter’s)

• Density: 1.27 g/cm3

• Average distance from Sun: 19.19 AU

• Rotation period: -17.9 hours (retrograde)

• Revolution period: 84 years

• Tilt of axis: 97.9o

• Orbit eccentricity: 0.047

• Atmospheric components:

83% hydrogen,

15% helium,

2% methane (at depth)

• Rings: system of 11 narrow, faint rings; particles are dark, drab gray; may consist of rocky or carbonaceous material.

• Magnetic field strength: 100 x Earth’s (surface strength: 0.7 x Earth’s)

• Moons: 21 known

Orientation and Visibility

• The tilt of Uranus' rotation axis of about 98o means that its rotation is retrograde. – Equatorial plane is nearly perpendicular to its orbital plane.

• This causes it to have the greatest seasonal changes of any planet in the Solar System.

Uranus’ Unusual Seasons

From N-pole summer solstice, see Sun move in circle every 17 hours; eventually begin daily cycle of setting and rising with longer nights until, at autumnal equinox,see 8.5 hour day

and night; day length decreases until reach period of total darkness.

Atmosphere from HST

True color image Color enhanced image

Atmosphere of Uranus•Upper atmosphere like Jupiter

– 84% hydrogen– 14% helium– 2% methane– NO ammonia!!

•Hazes of smog high in atmosphere. •Atmospheric temperature varies

from 120 K to 55 K.•Methane clouds found low in atmosphere are only clouds observed on Uranus.

•Clouds found at height where temperature is 70K, 1 bar pressure

– ammonia gas condenses, falls to interior– little gaseous ammonia in atmosphere

•Blue color from methane gas.

Atmosphere: Comparisons•Uranian atmosphere is similar to Jupiter's and Saturn's.

–Composed mostly of hydrogen and helium. –Methane (CH4) is abundant in the upper clouds. –Ammonia (NH3) has frozen out of the atmosphere or

perhaps dissolved in an internal water layer.

•The blue-green color is caused by methane, which absorbs red light, so that light reflected from the upper layers of the atmosphere is deficient in red. –Jupiter and Saturn have other gases that overly the methane

and produce other colors.–Acetylene and ethane formed in upper atmosphere

•Very few cloud features are seen. –Energy to drive its cloud system must come from the Sun,

because there is very little source of internal heat.

Clouds on Uranus

HST recently found about 20 clouds - nearly as many clouds on Uranus as the previous total in the history of modern observations.

Taken on August 8, 1998, with Hubble's Near Infrared Camera and Multi-Object Spectrometer.

Uranus: Hydrosphere

• Uranus may have an extensive layer of liquid water under the cloud tops.

• The motion of this super dense water might be the source for the planet's strong magnetic field.

Uranus: Biosphere

• No life is expected either in the clouds of Uranus or any of its moons. The lack of liquid water at the necessary temperature and pressure would seem to exclude the formation of life here.

Interior of Uranus

• H + He atmosphere extends ~1/5 of way to planetary center.

• Underlain by ocean of methane, water, ammonia.

• Little difference between atmosphere and ocean.– Methane ice begins forming in

atmosphere.– Continues to increase until slush is

formed, and then solid ice.

• The ice is warm (for Uranus) and can flow – like rocky mantle layers of Earth.

• Center is a small, rocky core– 3.5 times the mass of the Earth

• Probably well differentiated– but may be somewhat more mixed

than either Jupiter or Saturn.

Magnetic Field & Interior Structure•Compared to Jupiter and Saturn, Uranus has very little (or no) metallic hydrogen and much more ice.

•Core composed of heavier, rocky and metal elements.

•IF magnetosphere generated in metallic hydrogen dynamo, Uranus’ magnetosphere

–should be much smaller than Jupiter’s–and aligned with rotation axis.

•Observed field is NOT aligned or centered.•Conducting layer possibly ammonia dissolved in water/methane slush, produce observed magnetic field.

– Some current reports discount likelihood of ammonia source.

•No good theory to explain absence of excess heat.

Magnetic Field: Generation

• One possible explanation for orientation of magnetic field is that Uranus had a gigantic impact while it was forming, knocking the planet on its "side" and disrupting its magnetic field.

• Another idea is that magnetic field is not generated near core, but rather in large volume of liquid water surrounding the core.

Magnetic Fields of Jovian Planets

Rings from HST

Uranus: Rings•The rings are thinner and more widely spaced than Saturn’s. –e.g., if Uranus were the size of a

golf ball, rings would be as thin as a spider web.

•The ring particles are very dark and made of larger particles that Saturn's rings.

•Asymmetric and partial rings have been detected.

•Some shepherd moon were discovered by the Voyager spacecraft flyby mission, but most of the ring structure is still unexplained.

Uranus’ Rings

• 9 main rings are visible (left) as horizontal lines. • Broad lanes of dust surround dark, narrow,

widely-spaced main rings.• The brightest, or epsilon ring, at top is neutral in color.• Fainter 8 remaining rings show slight color differences.

Discovery of Uranus’ Rings

Discovered in 1977 when stellar occultation observed before and after occultation by planet: preceded discovery of rings for Jupiter (1979) and Neptune (1989).

Composition of Rings

• Low reflectivity and lack of color suggest that Uranus’ rings may be mostly carbon.

• Possible origin:– originally methane ice that decomposed to

carbon when exposed to energetic particles in the magnetosphere,

– rings formed from carbon-rich asteroid that broke up under Uranus’ tidal influence

Epsilon-ring and it’s Shepherd Moons

Designated 1986U7 (Cordelia) and 1986U8 (Ophelia),

the two shepherd moons seen here on either side of the bright epsilon ring confine it by their gravitational effects.

Rings and Moons from HST

The Moons of Uranus

• 21 known moons

• Divided into 3 classes

– 11 very small, dark inner

– 5 large

– 4 small, distant

• Most in nearly circular orbits about equator; outer 4 more elliptical.

• Named for characters from Shakespeare, Pope.

Moons of Uranus Distance Radius Mass Satellite (x1000 km) (km) (kg)Cordelia 50 13 ? Ophelia 54 16 ? Bianca 59 22 ? Cressida 62 33 ? Desdemona 63 29 ? Juliet 64 42 ? Portia 66 55 ?Rosalind 70 27 ? Belinda 75 34 ? 1986U10 75 20 ?Puck 86 77 ? Miranda 130 236 6.30e19Ariel 191 579 1.27e21Umbriel 266 585 1.27e21 Titania 436 789 3.49e21 Oberon 583 761 3.03e21 Caliban 7100 30 ?1999U1 10000 20 ?Sycorax 12200 60 ? 1999U2 25000 20 ?

The “Large” Moons of Uranus

• The “large” moons of Uranus are actually moderate to small sized, icy objects.

• They tend to show a surprising amount of geologic activity.

• Seem to be more complex the closer they are to the planet, suggesting that tidal forces may be the cause

of this activity. • They are generally quite similar to

the small, icy moons of Saturn.

5 Largest Moons of Uranus

•Miranda Ariel Umbriel Titania Oberon

•Believed similar to Saturn’s mid-sized moons, but with differences:

•slightly higher density•darker•relatively further from Saturn•show more evidence of internal activity

•Composed of 40-50% water ice, the remainder rocky.

Miranda• Innermost of large satellites.• Some of the most complex

terrain of any moon or planet in the Solar System.

• Surface composed mostly of rolling cratered plains.

• Half of Miranda's surface is younger based on fewer number of craters. – consists of complex sets of parallel

and intersecting scarps and ridges. – bright V-shaped feature in grooved

area is Inverness Corona, nicknamed the "Chevron".

– 5 km cliff on 485 km diameter satellite.

– Huge, jagged canyon on right limb is in direction of Uranus.

Ariel• Complex terrain captured in

this view of Ariel’s southern hemisphere.

• Most of surface is intensely cratered terrain, transected by fault scarps and graben.

• Some of largest graben (seen near the terminator) are partly filled with younger deposits and less heavily cratered.

• Bright spots (near the limb and toward the left) are crater rims.

Umbriel

• Umbriel is an enigma with an observed surface albedo of only 10-15% .

• Uranus’ satellites found inside and outside Umbriel's orbit are much brighter.

• The process by which Umbriel's ancient cratered surface was darkened, leaving only a few bright icy white rings, remains a mystery.

Titania• Obvious abundance of impact

craters and prominent global tectonic features.

• Large fault valleys, 1500 km (930 mi) long, 75 km (47 mi) wide, run approximately perpendicular to ecliptic plane.

• Surface has recorded many types of geologic activity.

–numerous impact scars,–large, trench-like feature near the terminator at middle right suggests at least one episode of tectonic activity.

Oberon• Several large impact

craters, surrounded by bright rays, are visible.

• Near the center of disk is a large crater with bright central peak and a floor partially covered with black material.– May be icy, carbon-rich

material that spilled onto crater floor sometime after crater formed.

• Large mountain, ~6 km high, peeking out on lower left limb.

Neptune

Comparison of the Jovian Planets

Neptune: View from Earth• Discovered in 1845/1846

– Adams and Leverrier independently applied perturbation theory to orbit of Uranus to predict location.

• Too dim to be visible to the naked eye. Small angular size allows little detail from Earth.– Before the Voyager mission, Neptune thought to be very similar

to Uranus, nearly featureless. But Neptune shows light and dark spots, white clouds, and a distinctly banded appearance.

• Smallest size and largest density of jovian planets.– 17 x Earth’s mass– 3.9 x Earth’s radius– 1.64 gm/cm3

• Two satellites known from Earth-based observations– Triton and Neried

• Hints of ring system from Earth-based observations.

Neptune: View from Space•Voyager 2 revealed atmospheric details:

– various sizes of storms and cloud systems

– high-level clouds moved rapidly

– layered cloud structure detected

– emits more excess energy than any other jovian planet

•Magnetic field tilted at 47o to rotation axis and offset by half the planet radius.– Low mass indicates lack of a metallic hydrogen layer.

•Rings are thin and dark; material not uniformly distributed.

•Six new satellites discovered. – Images of large moon Triton revealed complex surface

structure, possible ice geysers, and thin nitrogen atmosphere.

Neptune Vital Statistics• Mass: 1.02 x 1026 kg (17.1 x Earth’s)

• Diameter: 49,528 km (3.9 x Earth’s)

• Density: 1.64 g/cm3

• Ave. distance from Sun: 30.06 AU

• Rotation period: 16.1 hours

• Revolution period: 164.8 years

• Tilt of axis: 29.6o

• Orbit inclination: 1.77o

• Orbit eccentricity: 0.009

• Mean temperature: 48K• Atmospheric components:

74% hydrogen25% helium

1% methane (at depth)

• Rings: narrow, dark, contain concentrations of particles called ring arcs.

• Moons: 8, but they do not form regular system.

• Magnetic field: ~100 x Earth’s, 46o tilt to rotational axis, displaced 0.55 radii, reverse polarity to

Earth’s.

Neptune: Interior • Like the other gas giant planets,

Neptune is dominated by the volatile elements, predominantly hydrogen and helium.

• Descending from the cloud tops toward the core, the gases gradually change to liquid and then solid.

• There is probably a substantial amount of liquid water at some level.

• Neptune's dynamic weather patterns are probably caused by its great amount of internal heat.

• Neptune emits more excess energy relative to its mass than any other planet: 2.7 times more energy into space than it receives from the Sun.

Neptune’s Interior

• Smaller than Saturn, but more massive.

• Voyager 2 data indicates water is a major constituent.

• Presumed to contain – small, rocky core – with icy mantle topped by – deep layer of liquid hydrogen.

• Pressure outside core too low to force hydrogen into metallic state, so hydrogen stays in molecular form.

Neptune: Magnetic Field

•Neptune’s magnetic field is similar to Uranus’: – tilted to the rotation axis (by 55o)– offset from planet center– same strength

•The irregularities in the magnetosphere of Uranus and Neptune are one of the outstanding questions left by the Voyager encounters with the outer planets.

Neptune’s Aurora

• Neptune’s aurora is centered around the planet’s

magnetic pole. • Type-B aurora has also been

observed.

• No radio signals have been detected which relate to aurora

or auroral processes.• Scientists are still working to understand the currents which

cause aurora.

Neptune: Atmosphere•Hydrogen and helium dominant gases.

– Blue appearance (like Uranus) due to methane gas in upper atmosphere.

•Upper level clouds are methane.•Thin, high white clouds are also of methane.

– Look like Earth’s cirrus clouds.– Believed to have been carried 75 km higher in

atmosphere by convection currents carrying excess heat from interior.

•Belts, zones, storms similar to Jupiter & Saturn.– Highest E-W wind speeds at equator ( 2100 km/hr).– Twice speed of peak winds on Saturn .

Neptune: Hydrosphere

• Like Uranus, Neptune probably has a large amount of liquid water in a layer beneath the upper atmosphere and clouds. This may be the source of Neptune's off-centered magnetic field.

Neptune: Biosphere

• None is expected in the atmosphere of Neptune or the surface of Triton.

• It is either too cold or, in the case of Neptune's atmosphere, the water is at too high a pressure.

Neptune’s Dark Spot• Largest storm on Neptune was the

Great Dark Spot.

• This storm exhibits many of the same general features as the Great Red Spot on Jupiter.

– formed at 20o S-latitude

– same shape and relative size

– flow counterclockwise w/ 17 day period (Red Spot = 6 day)

– turbulence evident where associated winds interact with zonal flow to north and south.

• In 1994, the Spot had disappeared or faded in brightness. New dark spot observed in N-hemisphere.

Neptune’s Rings• Four rings: 2 narrow; 2 broad and diffuse• Only two brightest rings show clearly in this image,

but Voyager 2 found additional fainter rings. • Rings rotate

– in same direction as planet and – are nearly in equatorial plane.

• Neptune’s rings are quite narrow and contain more dust-sized grains than the corresponding rings of Saturn and Uranus.

Neptune’s Rings: More Structure

• Rings appear to be clumpy or uneven in the distribution of ring particles.

– Also seen in some of the thin rings of Saturn and Uranus.

• Ring particles seem to be quite small. • Some shepherd satellites have been

found embedded in the rings, helping to create the structure.

The Moons of Neptune• From Earth, only two moons are observed orbiting Neptune, but

six more have been identified by Voyager 2 and HST. • All of its moons but one are small, icy worlds.

• Neptune's moons are considered to be the most “peculiar” in the solar system.

• They do not comprise a regular moon system (no moons in roughly circular, equatorial, pro-grade orbits).

TritonProteus Neried

Moons: Unusual Orbits• Two of its moons have very peculiar orbits.

– Triton orbits backwards (retrograde) in only 6 hours and is tilted 20o with respect to the planet's equator.

– Nereid's orbit is inclined 28o and is highly eccentric.

TritonNeried

Triton: Overview• Triton is smallest of six large moons in Solar System.

– About same size & density as Pluto– 30 times farther from the Sun than Earth.

• It has a thin atmosphere of nitrogen and methane (10-5 Earth's) and may possibly have lakes of liquid nitrogen and nitrogen volcanoes on its surface.

– Its gravity can prevent an atmosphere from completely escaping because its surface is so cold (35-40K).

• Triton's density suggests about half ice and half rock. – Its composition is believed to be similar to that of comets –

although it is much larger than a comet.– Very similar to Pluto, which along with the strange

backward orbit of Triton, imply that it is a captured body.– Pluto and Triton may be left over planetesimals from the

early days of the Solar System.

Neptune’s Largest Moon: Triton• Smallest of solar system’s six large moons–mass = 1/2 x Europa’s

–diameter = 2700 km (3/4 size of Earth’s moon)

–density = 2.1 gm/cm3 • 75% rock

• 25% water ice

–coldest surface of any object visited by spacecraft 35-40 K

–surface composed of frozen water, nitrogen, methane, carbon dioxide

–thin nitrogen atmosphere.

–complex surface

Neptune’s Largest Moon: Triton

•Solar system’s only large moon in retrograde orbit.

•Orbit is –circular, –retrograde, –inclined ~20o to Neptune’s equatorial plane.

•Orbit decaying due to retrograde motion and tidal interaction.

Triton: View from Voyager

• Triton’s surface is very cold and windy, with winds close to the speed of sound, and has a mixed terrain of icy regions and bare spots.

• Evidence for eruptions were seen by Voyager, as well as evidence for crustal movements, perhaps as a result of subsurface melting and collapse.

Triton’s Surface

This cantaloupe-textured terrain is ~1000 km across.

Complex tectonic and volcanic forces involving icy viscous

fluids combined to develop this deformed landscape pattern.

A variety of terrain in southern polar region indicates history of

surface activity: deep ridges, gashes, frozen

water lakes, and ice volcanoes.

Is Triton Warming?• A rise in Triton’s surface

temperature of 2K (3oF) over nine years is based on HST’s detection of an increase in the moon’s atmospheric pressure, which has at least doubled in bulk since the time of the Voyager encounter.

• Any nitrogen ice on Triton that warms up a little results in a considerable leap in atmospheric pressure as the vaporized nitrogen gas joins the atmosphere.

Nereid

• Other moon of Neptune that is visible from Earth.

• 340 km diameter (226 miles, ~distance from LA to San Francisco)

• Most eccentric orbit in solar system.

Proteus: Neptune’s Mid-sized Moon• The irregular shape of Proteus suggests that it has been cold

and rigid throughout its history and subject to significant impact cratering.

• Has an average radius of about 200 km (120 mi) and is uniformly dark with a reflectivity of about 6%.

Neptune’s Five Small Moons• Discovered in 1989 by Voyager 2.

• Tiny, cratered, irregular shaped.

• No signs of geologic activity

• Orbit in same direction as Neptune’s rotation, in equatorial plane.

Thalassic Designate Galatea Larissa

Images smear actual shape to create more elongated appearance.

Pluto

HST view of surface

Pluto in True Color

Credit: Eliot Young (SwRI) et al., NASA

Pluto: Discovery• At a distance of over 39 astronomical units,

Pluto orbits the Sun very slowly, taking almost 250 Earth years to complete the cycle.

• Position prediction errors for Neptune and Uranus suggested an additional planet might lie beyond the orbit of Neptune.

• Pluto was discovered by astronomer Clyde Tombaugh in 1930 after a long search of many years scanning the stars along the ecliptic.

• Now know that Pluto's mass is far too low to have caused perturbations in orbits of outer planets.

• Pluto's diameter (~2,370 km or 1,470 miles) is roughly equivalent to distance from New York to Las Vegas, or about 2/3 the size of Earth’s moon.

Sizes of Pluto and Charon

What’s in a Name?• Pluto, discovered by astronomer Clyde Tombaugh in

1930, was named from a suggestion made by Venetia Burney, an 11-year-old schoolgirl in Oxford, England.

• It’s moon, Charon, was discovered by James Christy in 1978 at the U.S. Naval Observatory.

• In Roman mythology, Pluto is god of the underworld, and Charon is the ferryman across the river Styx, the moat into Pluto's realm.

• Both names have second meanings. –The astronomical symbol for Pluto, "PL," pays homage to

Percival Lowell, who started the search for the ninth planet. –Charon is sometimes pronounced

• "Karen" (like the mythological ferryman) and sometimes • "Sharon," after James Christy’s wife, Charlene (nick-named

"Char").

Pluto’s Vital Statistics

• Mass: 1.29 x 1022 kg (0.0021 x Earth’s) • Diameter: 2300 km (0.18 x Earth’s)• Density: 2.1 g/cm3

• Average distance from Sun: 39.5 AU• Rotation period: 6.39 days (retrograde)• Revolution period: 247.7 years• Tilt of axis: 122.5o

• Orbit inclination: 17.15o

• Orbit eccentricity: 0.249• Mean temperature: 37 K• Atmospheric components: nitrogen and perhaps methane • Surface materials: nitrogen, methane and carbon monoxide ices• Moons: 1

– diameter = 1300 km, – mass =1/6 x mass Pluto, if same density)

• Magnetic field: unknown

Pluto’s Eccentric, Tilted Orbit

• Highly elliptical orbit with average separation of ~ 40 A. U.

• Eccentricity makes orbit lay inside that of Neptune for period from 1979 to 1999.

• Thus, Pluto isn't even the most distant planet during that time.

• Termed “the 9th planet” because its average separation from Sun is greater than that for Neptune.

• Orbital plane tilted 17o to ecliptic.

• 3:2 orbital resonance w/ Neptune.

Pluto and Charon• Unlike seven of the other eight planets (Uranus being the exception),

Pluto rotates on its side, as does Charon in its orbit around Pluto.

• Charon is 19,636 km (12,174 miles) away from Pluto, compared to the Earth-Moon distance of 384,400 km (238,328 miles).

• Charon's orbit inclined at angle of 118° to plane of Pluto's orbit around Sun.

• Pluto is third planet in solar system found to have retrograde rotation.

Pluto: View from Earth

•Appears as a point of light from Earth.– Pluto’s angular diameter is, at best,

only 0.1 arc-second , just about the resolving limit of the Hubble Space Telescope and ground-based observatories with adaptive optics capabilities.

•No spacecraft has ever visited Pluto or Charon.– Consequently, much of what is known about the

Pluto/Charon system has been pieced together from indirect clues.

Earth-based Observations• Surface brightness variation: 6.39 day period.

– Initially assumed to be surface variations.

• Photographs made in 1978 indicated satellite, Charon.– orbital period = 6.39 days

– partially explains brightness variation

– Pluto and Charon both rotate with 6.39 day period.

• In 1980, thin atmosphere detected .– nitrogen and some methane

• 1985-1991, mutual occultation determined– Pluto’s diameter = 0.18 x Earth’s = 0.60 x Moon’s ~ 2300 km

– Pluto’s mass = 0.0024 x Earth’s ~1/6 x Moon’s

Views of Pluto and Charon

Earth-based observation Hubble Space Telescope observation

Atmosphere Discovered by Stellar

Occultation

Pluto: Atmosphere•Pluto's atmosphere was detected in 1988. •Its surface pressure is about 10-5 x Earth’s, but still large enough to expect weather, winds, haze, chemistry, and an ionosphere.

•At the same time, Pluto's weak gravity does not hold the atmosphere very tightly, and the upper portions of Pluto's atmosphere may resemble that of a comet.

•A seasonal atmosphere forms around Pluto near the time of perihelion passage.

– As the planet heats up slightly, nitrogen/methane ice evaporates forming a thin atmosphere.

– By the time the planet reaches aphelion, half a plutonian year later, the atmosphere has disappeared by refreezing.

Pluto: Surface and Interior

• Pluto has some of the most diverse surface markings in the solar system – with areas ranging from “darker than coal” to “brighter than snow”.– bright areas thought to be covered with ices

predominantly nitrogen frost with traces of methane and carbon monoxide ices

– darker regions may be covered with carbon-rich deposits.

• Pluto's composition appears to be similar to the large icy moons in the outer Solar System. 2/3 rock 1/3 water ice

Map of Pluto• The map below covers 85% of the planet's surface and indicates that

Pluto has a dark equatorial belt and bright polar caps.

• The brightness variations may be due to topographic features such as basins and fresh impact craters, but most of the surface features are likely produced by frosts that migrate across Pluto's surface with its

orbital and seasonal cycles.

Pluto: Magnetosphere

Pluto does not appear to have a magnetic field.

Pluto: Biosphere

Pluto is not expected to have a biosphere of any kind.

Pluto: Odd Properties• ORBIT

– Highly eccentric• more like a comet (crosses the orbit of Neptune)

– Highly inclined • more like a comet

• PHYSICAL PROPERTIES – Small size

• like a small moon

– Chemical composition • like an icy moon • not like a gas giant or terrestrial planet

– "Summer" atmosphere of nitrogen/methane • 0.00001 x Earth's• like a comet

• LARGE MOON

Pluto’s Moon: Charon

•Pluto's moon, Charon, is comparatively large. –Charon is about half the size of Pluto.–Some have described them as a "double planet system".

•Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too. –They both keep the same face toward one another. –Makes phases of Charon as seen from Pluto very interesting.

•The motion of Charon has been used to accurately determine the mass of Pluto.

•Mutual occultations between Pluto/Charon (1985-1991) were used to accurately determine the sizes of both.

Composition and

Surfaces

• Despite their proximity, Pluto and Charon are covered with bright frosts of differing compositions:

– Pluto's surface is predominantly nitrogen frost with traces of methane and carbon monoxide ices.

– Charon’s surface is water ice.

• Both objects are about twice as dense as water, implying that, on average, they are made of 2/3 rock and 1/3 water ice.

Image courtesy of Marc W. Buie/ Lowell Observatory

Pluto and Charon

• Unlike other planet-moon systems in our solar system, Pluto and Charon have similar sizes and masses.

• Charon may have – formed with Pluto,

– been captured after the formation of Pluto, or

– been formed when Pluto collided with a similar object, leaving enough mass for the formation of Charon which was then captured by Pluto’s gravity.

• Objects, called Plutinos, found since 1999 with similar characteristics and orbits to Pluto could be debris from such a collision.

• Some suggest that Pluto-Charon system should be re-classified as Kupier belt objects.

Pluto: OriginsFacts

1. Pluto is neither terrestrial nor jovian in its makeup and it is similar to the ice/rock moons of the outer planets.

2. Pluto's eccentric, inclined orbit is quite unlike the orbits of the other known planets.

Speculation1. Catastrophic encounter of some sort might have ejected Pluto

from its original orbit around Neptune; perhaps even simultaneously knocking Triton onto its present retrograde path. This theory is complicated by the presence of Charon.

2. Pluto may, like Triton, be an icy planetesimals left over from the early times of planet formation in the outer Solar System.

3. Pluto may be just what it seems: a planet that formed in its current orbit.

Beyond Pluto: Planet X?

• Many researchers believe that there may have been thousands of Pluto-sized objects initially present in the outer solar system.

• Almost all these Pluto-sized objects have since been "kicked" out to larger distances from the Sun following gravitational interactions with Uranus and Neptune.

• The capture of a few of them by the giant planets could explain some of the strange moons of the outer worlds, especially Triton.

• A survey of the entire sky completed by the Infrared Astronomical Satellite (IRAS) in 1983 revealed no hidden “Planet X” in the outer reaches of our solar system.

• Voyager and Pioneer spacecraft paths not measurably deviated.