comparative planetology a travel guide to the outer planets of...

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Comparative Planetology of the Outer Planets

Chapter 18 (150+ slides)

A Travel Guide to the Outer Planets

• Hydrogen-rich atmospheres• belt-zone circulation

• shallow atmospheres• interiors mostly liquid hydrogen• Large satellite systems

Jupiter

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Physical Properties of Jupiter

• Equatorial Radius:– 71,494 km– 11.20 times the earth’s radius

• Mass– 317.83 times the earth’s mass

• Distance from Sun– 5.2028 AU

• Orbital Period– 11.867 years

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The scale of the Solar System

Planet Mass (Earth)

Mercury 0.055

Venus 0.815

Earth 1.000

Mars 0.107

Jupiter 317.938

Saturn 95.181

Uranus 14.535

Neptune 17.135

Pluto 0.000

Object Distance (million km) AU Light

Minute Light Hour Light Day Light Year

Mercury 58 0 3 0 0 0

Venus 108 1 6 0 0 0

Earth 150 1 8 0 0 0

Mars 228 2 13 0 0 0Ceres (asteroid) 419 3 23 0 0 0

Jupiter 778 5 43 1 0 0

Saturn 1,427 10 79 1 0 0

Uranus 2,870 19 159 3 0 0

Neptune 4,500 30 250 4 0 0

Pluto 5,900 40 328 5 0 0

Farthest comets 14,958,302 99,722 831,017 13,850 577 2

Proxima Centauri (nearest star) 41,041,264 274,340 2,280,070 38,001 1,583 4

Orion Nebula 14,200,920,000 94,672,800 788,940,000 13,149,000 547,875 1,500

Center of the Milky Way 246,149,280,000 1,640,995,20

013,674,960,0

00 227,916,000 9,496,500 26,000

Andromeda Galaxy (farthest naked-eye

object)22,342,780,800,000,000 148,951,872,000,0

001,241,265,600,000

,00020,687,760,000,00

0 861,990,000,000 2,360,000,000

JupiterLargest and most

massive planet in the solar system:

Contains almost 3/4 of all planetary matter in the

solar system.

Most striking features visible from Earth: Multi-colored

cloud belts

Visual image

Infrared false-color image

Jupiter’s InteriorFrom radius and mass Average density of Jupiter ≈ 1.34 g/cm3

=> Jupiter can not be made mostly of rock, like earthlike planets.

Jupiter consists mostly of hydrogen and helium.

Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.

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Aurorae on Jupiter

Just like on Earth, Jupiter’s

magnetosphere produces aurorae concentrated in rings around the magnetic poles.

~ 1000 times more powerful

than aurorae on Earth.

Jupiter’s Magnetic FieldDiscovered through observations of decimeter (radio) radiation

Magnetic field at least 10 times stronger than Earth’s magnetic field.

Magnetosphere over 100 times larger than

Earth’s magnetosphere.

Extremely intense radiation belts:

Very high energy particles can be

trapped; radiation doses corresponding to ~ 100 times lethal doses for humans!

Comet Impact on Jupiter

Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb: ~ 0.15 megaton)!

Visual: Impacts seen for many days

as dark spots

Impact of 21 fragments of

comet Shoemaker-

Levy 9 in 1994

Impacts occurred

just behind the horizon

as seen from Earth, but came into view about 15 min. later.

Impact sites appeared

very bright in the infrared.

Jupiter’s AtmosphereJupiter’s liquid hydrogen ocean has no surface:

Gradual transition from gaseous to liquid phases as temperature and pressure combine to exceed

the critical point.

Jupiter shows limb darkening hydrogen atmosphere

above cloud layers.

Only very thin atmosphere above cloud layers;

transition to liquid hydrogen zone ~ 1000 km below clouds.

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Clouds in Jupiter’s Atmosphere (II)

Three layers of clouds:

1. Ammonia (NH3)

crystals

2. Ammonia hydrosulfide

(NH4SH)

3. Water crystals

The Cloud Belts of JupiterDark belts and bright zones.

Zones higher and cooler than belts; high-pressure regions of rising gas.

The Cloud Belts on Jupiter (II)Just like on Earth, high-and low-pressure zones

are bounded by high-pressure winds.

Jupiter’s cloud belt structure has remained unchanged since humans began mapping them.

Jupiter

Coriolis

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Jupiter’s RingNot only Saturn, but all four

gas giants have rings.

Jupiter’s ring: dark and reddish; only discovered by

Voyager 1 spacecraft.

Galileo spacecraft image of Jupiter’s ring, illuminated from behind

Composed of microscopic particles of rocky material

Location: Inside Roche limit, where larger bodies (moons)

would be destroyed by tidal forces.

Ring material can’t be old because radiation pressure and Jupiter’s

magnetic field force dust particles to spiral down into the planet. Rings must be constantly

re-supplied with new dust.

Jupiter

Neptune

Uranus

Saturn

Rings made of Ice

Rings made of Dust Rings made of Dust

Rings made of Dust

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Jupiter’s Moons

• Io• Europa• Callisto• Ganymede

Jupiter

1. Io 2. Europa 3. Ganymede 4. Callisto5. Amalthea6. Himalia7. Elara8. Pasiphae 9. Sinope10. Lysithea11. Carme12. Ananke13. Leda 14. Thebe 15. Adrastea16. Metis 17. Callirrhoe

18. Themisto19. Megaclite20. Taygete21. Chaldene22. Harpalyke23. Kalyke24. Iocaste25. Erinome26. Isonoe27. Praxidike28. Autonoe29. Thyone30. Hermippe31. Aitne32. Eurydome33. Euanthe34. Euporie

35. Orthosie36. Sponde37. Kale 38. Pasithee39. Hegemone40. Mneme41. Aoede42. Thelxinoe43. Arche44. Kallichore45. Helike46. Carpo47. Eukelade48. Cyllene49. Kore50. Herse

1. S/2003 J2 2. S/2003 J3 3. S/2003 J4 4. S/2003 J5 5. S/2003 J9

6. S/2003 J10 7. S/2003 J12 8. S/2003 J15 9. S/2003 J16

10. S/2003 J18

11. S/2003 J19 12. S/2003 J23 13. S/2010 J 1 14. S/2010 J 2

Jupiter’s MoonsJupiter

50 moons & 14 provisional moons are known of for Jupiter

Four largest moons discovered by Galileo are know as The Galilean Moons

Io Europa Ganymede Callisto

Jupiter and two moons

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Callisto: The Ancient FaceTidally locked to Jupiter, like all of Jupiter’s moons.

Av. density: 1.79 g/cm3

composition: mixture of ice and rocks

Dark surface, heavily pocked with craters.

No metallic core: Callisto never differentiated to form core and mantle.

No magnetic field.

Layer of liquid water, ~ 10 km thick, ~ 100 km below surface, probably heated by radioactive decay.

Jupiter CallistoJupiter

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen. Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometers. The likely presence of an ocean within Callistoindicates that it can or could harbor life. However, this is less likely than on nearby Europa. Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter.

Ganymede: A Hidden PastLargest of the 4 Galilean moons. Av. density = 1.9 g/cm3

• Rocky core • Ice-rich mantle • Crust of ice

1/3 of surface old, dark, cratered; rest: bright, young, grooved terrain

Bright terrain probably formed through flooding

when surface broke

Jupiter

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GanymedeJupiter Jupiter

Europa: A Hidden OceanAv. density: 3 g/cm3

composition: mostly rock and metal; icy surface.

Close to Jupiter should be hit by many meteoroid impacts;

but few craters visible.

Active surface; impact craters rapidly erased.

Jupiter

The Surface of Europa

Cracked surface and high albedo (reflectivity) provide further evidence for geological activity.

Jupiter

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The Interior of EuropaEuropa is too small to retain its internal heat

Heating mostly from tidal interaction with Jupiter.Core not molten No magnetic field.

Europa has a liquid water ocean ~ 15 km below the icy surface.

Jupiter EuropaJupiter

Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating.

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Io: Bursting EnergyMost active of all Galilean moons;

no impact craters visible at all.

Over 100 active volcanoes!

Av. density = 3.55 g/cm3

Interior is mostly rock.

Activity powered by tidal interactions

with Jupiter.

Jupiter Jupiter - Io

Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.

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With over 400 active volcanoes, Io is the most geologically active object in the Solar System. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Io's surface is composed of extensive plains coated with sulfurand sulfur-dioxide frost.

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• Formed from cold gas in the

outer solar nebula, where

ices were able to condense.

• Rapid growth

• Soon able to trap gas directly through gravity

• Heavy materials sink to the center

• In the interior, hydrogen becomes metallic (very good

electrical conductor)

• Rapid rotation strong

magnetic field

• Rapid rotation and large size belt-zone cloud pattern

• Dust from meteorite impacts onto inner moons trapped to form ring

The History of Jupiter

Moons(For Test – Including Terrestrial Planets)

• Deimos (Mars) • Io (Jupiter)• Europa (Jupiter)• Callisto (Jupiter)• Ganymede (Jupiter)• Moon (Earth)• Phobos (Mars)

Saturn

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Physical Properties of Saturn

• Equatorial Radius:– 60,330 km– 94.2 times Earth’s radius– 0.84 times Jupiter’s radius

• Mass– 95.147 times Earth’s mass– 0.300 times Jupiter’s mass

• Distance from Sun– 9.5388 AU (Jupiter is 5.2 AU)

• Orbital Period– 29.461 years (Jupiter is 11.867 years)

Saturn SaturnMass: ~ 1/3 of mass of Jupiter

Radius: ~ 16% smaller than Jupiter

Av. density: 0.69 g/cm3 Would float in water!

Rotates about as fast as Jupiter, but is twice as oblate No large core of heavy elements.

Mostly hydrogen and helium; liquid hydrogen core.Saturn radiates ~ 1.8 times the energy received from the sun.

Probably heated by liquid helium droplets falling towards center.

Saturn

Saturn’s Atmosphere

Three-layered cloud structure,

just like on Jupiter

Main difference to Jupiter:

Fewer wind zones, but much stronger winds than on Jupiter:

Winds up to ~ 500 m/s near the equator!

Saturn Saturn

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Saturn’s RingsRing consists of 3 main segments: A, B, and C Ring

A Ring

B Ring

C Ring

separated by empty regions:

divisions Cassini DivisionRings can’t have been

formed together with Saturn because

material would have been blown away by particle stream from hot Saturn at time of

formation.

Rings must be replenished by fragments of

passing comets / meteoroids.

The full set of rings, as photographed by the Cassini spacecraft on September 15, 2006

(brightness has been exaggerated in this image).

Saturn

Saturn Saturn

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SaturnComposition of Saturn’s Rings

Rings are composed of ice particles

moving at large velocities around Saturn, but small relative velocities (all moving in the same direction).

Saturn’s Moons

• Titan

Saturn

1. Mimas2. Enceladus3. Tethys 4. Dione5. Rhea 6. Titan 7. Hyperion 8. Iapetus9. Erriapus10. Phoebe 11. Janus 12. Epimetheus13. Helene 14. Telesto15. Calypso 16. Kiviuq17. Atlas 18. Prometheus

19. Pandora 20. Pan 21. Ymir22. Paaliaq23. Tarvos24. Ijiraq25. Suttungr26. Mundilfari27. Albiorix28. Skathi29. Siarnaq30. Thrymr31. Narvi32. Methone33. Pallene34. Polydeuces35. Daphnis 36. Aegir

37. Bebhionn38. Bergelmir39. Bestla40. Farbauti41. Fenrir42. Fornjot43. Hati44. Hyrrokkin45. Kari 46. Loge 47. Skoll48. Surtur49. Greip50. Jarnsaxa51. Tarqeq52. Anthe 53. Aegaeon

1. S/2004 S7 2. S/2004 S12 3. S/2004 S13 4. S/2004 S17 5. S/2006 S1

6. S/2006 S3 7. S/2007 S2 8. S/2007 S3 9. S/2009 S1

Saturn’s MoonsSaturn

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A Shepherd, his Staff and the Sheep

Shepherd Moons

Some moons on orbits close to the

rings focus the ring material, keeping the rings confined.

Saturn

Shepherd Moons(Saturn)

SaturnDivisions and Resonances

Moons do not only serve as “shepherds”.

Where the orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material

in the disk (“resonance”), the material is cleared out Divisions

Saturn

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Electromagnetic Phenomena in Saturn’s Rings

Radial spokes in the rings rotate with the rotation period

of Saturn:

Magnetized ring particles lifted out of the ring plane

and rotating along with the magnetic-field

structure

Titan: Saturn’s Largest Moon

Thick atmosphere, hiding the surface from

direct view.

About the size of Jupiter’s moon

Ganymede.

Rocky core, but also large amount of ice.

Saturn

Titan’s AtmosphereBecause of the thick, hazy

atmosphere, surface features are only visible in infrared images.

Many of the organic compounds in Titan’s atmosphere may have

been precursors of life on Earth.

Surface pressure: 50% greater than air pressure on Earth

Surface temperature: 94 K (-290 oF)

Methane and ethane are liquid!Methane is gradually converted to

ethane in the Atmosphere Methane must be constantly replenished, probably through breakdown of ammonia (NH3).

Saturn Liquid Methane on the surface of TitanSaturn

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Saturn Saturn

Saturn’s Smaller MoonsSaturn’s smaller moons formed of rock and ice.

Tethys:

Heavily cratered; marked by 3 km deep, 1500 km

long crack.


Iapetus:

Two-toned moon with larger crater

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Enceladus:

Possibly active; regions with fewer craters, containing parallel

grooves, believed to be filled with frozen water.


• Deimos (Mars) • Titan (Saturn)• Io (Jupiter)• Europa (Jupiter)• Callisto (Jupiter)• Ganymede (Jupiter)• Moon (Earth)• Phobos (Mars)

Uranus

What you expectThe way it is

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The Motion of UranusOrbit slightly elliptical; orbital

period ≈ 84 years.

19.18 AU

Very unusual orientation of rotation

axis: Almost in the orbital plane.

97.9o

Large portions of the planet exposed to

“eternal” sunlight for many years, then

complete darkness for many years!

Possibly result of impact of a large

planetesimal during the phase of planet

formation.

Physical Properties of Uranus

• Equatorial Radius:– 25,559 km– 4.01 times Earth’s radius– 0.36 times Jupiter’s radius



• Orbital Period– 84.013 years (Jupiter is 11.867years)

Uranus

• 1/3 the diameter of Jupiter

• 1/20 the mass of Jupiter

• no liquid metallic hydrogen

• Deep hydrogen + helium

atmosphere

The Atmosphere of UranusLike other gas giants: No surface.

Gradual transition from gas phase to fluid interior.

Mostly H; 15% He, a few % methane, ammonia and

water vapor.

Optical 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.

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Cloud Structures of

UranusHubble Space

Telescope image of Uranus shows cloud

structures not present during Voyager’s passage in 1986.

Possibly due to seasonal changes of the cloud structures.

The Rings of UranusRings of Uranus and Neptune are similar to Jupiter’s rings.

Confined by shepherd moons; consist of dark material.

Rings of Uranus were discovered through

occultations of a background star

Apparent motion of star behind Uranus

and rings

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Uranus’ Moons

• Oberon• Titania• Umbriel• Ariel• Miranda

Uranus

1. Cordelia2. Ophelia 3. Bianca 4. Cressida 5. Desdemona 6. Juliet 7. Portia 8. Rosalind 9. Mab

10. Belinda 11. Perdita12. Puck 13. Cupid 14. Miranda 15. Francisco 16. Ariel 17. Umbriel18. Titania

19. Oberon 20. Caliban21. Stephano22. Trinculo23. Sycorax24. Margaret 25. Prospero 26. Setebos27. Ferdinand

Uranus’ MoonsUranus

The Moons of Uranus

5 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.

Uranus’s Moons

Moon Location Size %Moon• Oberon outermost 775km 44%

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Uranus’s Moons

Moon Location Size %Moon• Titania … 805km 44%

Uranus’s Moons

Moon Location Size %Moon• Umbriel … 595km 35%

Uranus’s Moons

Moon Location Size %Moon• Ariel … 580km 35%

Uranus’s Moons

Moon Location Size %Moon• Miranda innermost 242km 14%

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Uranus’s Moons

Moon Location Size %Moon• Oberon outermost 775km 44%• Titania … 805km 44%• Umbriel … 595km 35%• Ariel … 580km 35%• Miranda innermost 242km 14%

They all have active surfaces


• Oberon (Uranus)• Titania (Uranus)• Umbriel (Uranus) • Ariel (Uranus)• Deimos (Mars)

• Miranda (Uranus)• Titan (Saturn)• Io (Jupiter)• Europa (Jupiter)• Callisto (Jupiter)• Ganymede (Jupiter)• Moon (Earth)• Phobos (Mars)

Neptune

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Physical Properties of Neptune

• Equatorial Radius:– 24.750 km– 39.3 times Earth’s radius– 0.35 times Jupiter’s radius



• Orbital Period– 164.793 years (Jupiter is 11.867 years)

The Rings of Neptune

Made of dark material, visible in forward-

scattered light.

Interrupted between denser segments (arcs)

Focused by small shepherd moons embedded in the

ring structure.

Ring material must be regularly re-supplied

by dust from meteorite impacts on the moons.

NeptuneDiscovered in

1846 at position predicted from gravitational

disturbances on Uranus’ orbit by J. C. Adams and U. J. Leverrier.

Blue-green color from methane in the atmosphere4 times Earth’s diameter; 4 %

smaller than Uranus

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The Atmosphere of Neptune

Cloud-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

Neptune’s Moons

• Triton• Nereid

Neptune

1. Naiad2. Thalassa3. Despina4. Galatea5. Larissa6. Proteus7. Triton8. Nereid9. Psamathe

1. S/2002 N 12. S/2002 N 23. S/2002 N 3

4. S/2002 N 4

The Moons of NeptuneTwo moons (Triton and Nereid) visible from Earth;

6 more discovered by Voyager 2

Unusual orbits:

Triton: Only satellite in the solar system

orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit;

very long orbital period (359.4 d).

The Surface of Triton

Triton can hold a tenuous atmosphere of

nitrogen and some methane; 105 times less

dense than Earth’s atmosphere.

Very low temperature (34.5 K)

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 CO2ice polar cap cycles on Mars.

Dark smudges on the nitrogen ice surface, probably due to methane rising from below surface, forming carbon-rich deposits when exposed to sunlight.

Neptune

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The Surface of Triton (II)Ongoing surface activity:

Surface features probably not more than 100 million years old.

Large basins might have been flooded multiple

times by liquids from the interior.

Ice equivalent of greenhouse effect may

be one of the heat sources for Triton’s geological activity.

Neptune


• Triton (Neptune)• Nereid (Neptune)• Oberon (Uranus)• Titania (Uranus)• Umbriel (Uranus) • Ariel (Uranus)• Deimos (Mars)


Jupiter

Saturn

Uranus

Neptune

The Jovian Planets Name Distance from sun (AU) Size (km)1 Ceres 3 9502 2004TY364 39 5403 2002KX14 39 5604 2002XV93 39 4305 2003VS2 39 6106 1999TC36 39 4407 2001QF298 39 4908 Orcus 39 11009 2003AZ84 39 710

10 Pluto 40 230011 Ixion 40 98012 Huya 40 48013 2005RN43 42 74014 1995SM55 42 47015 2002MS4 42 74016 2004SB60 42 56017 2004GV9 42 68018 2002UX25 43 81019 Varuna 43 78020 2002TX300 43 80021 1996TO66 43 54022 2003OP32 43 65023 2003EL61 43 200024 Quaoar 44 129025 2003QW90 44 56026 1999CD158 44 41027 1997CS29 44 41028 2000CN105 45 43029 1998WH24 46 45030 2005FY9 46 160031 2004PR107 46 52032 2003MW12 46 74033 2002CY248 46 41034 2002KW14 47 51035 2002AW197 47 94036 2002WC19 48 41037 2003QX113 50 45038 2003FY128 50 43039 2001UR163 51 62040 2002TC302 55 71041 1999DE9 56 49042 2004XR190 57 54043 2000YW134 58 43044 Eris 68 240045 2005RM43 90 56046 Sedna 486 1800

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Physical Properties ofPluto

• Equatorial Radius:– 1185 km– 0.18 times Earth’s radius– 0.02 times Jupiter’s radius



• Rotation Period– 247.7 years (Jupiter is 11.867 years)

Pluto is NOT a Planet• Virtually 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.

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Definition of a Planet• Planets: The eight worlds starting with Mercury

and moving out to Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

• Dwarf planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."

• Small solar system bodies: All other objects orbiting the sun.

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.

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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:

Pluto and Charon members of Kuiper belt of small, icy objects. Collision between Pluto and Charon may have caused the peculiar orbital patterns and large inclination of Pluto’s rotation axis.

Pluto and Charon formed as moons of Neptune, ejected by interaction with massive planetesimal. Mostly abandoned today since such interactions are unlikely.

Modern theory:




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October 7, 2002NASA's Hubble Space Telescope has measured the largest object discovered in the solar system since the discovery of Pluto 72 years ago. Approximately half the size of Pluto, the icy world 2002 LM60, dubbed "Quaoar" (pronounced kwa-whar) by its discoverers, is the farthest object in the solar system ever to be resolved by a telescope. It was initially detected by a ground-based telescope, as simply a dot of light, until astronomers aimed the powerful Hubble telescope at it.

Sedna

This artist's concept shows the planet catalogued as 2003UB313 at the lonely outer fringes of our solar system. Our Sun can be seen in the distance. The new planet, which is yet to be formally named, is at least as big as Pluto and about three times farther away from the Sun than Pluto. It is very cold and dark. The planet was discovered by the Samuel Oschin Telescope at the Palomar Observatory near San Diego, Calif., on Jan. 8, 2005. Image credit: NASA/JPL-Caltech

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