Transcript Uranus

Chapter 13: Uranus, Neptune, and Pluto
The Outer Worlds of the Solar System
Uranus
Neptune
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Pluto and Charon
Discovery
Comparisons
Physical Properties
Moons and Rings
Discovery
• The three outermost planets were
unknown to the ancients.
• All were discovered by telescopic observations:
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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
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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.
• 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.
Interior of
Uranus
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
Ophelia
Bianca
Cressida
Desdemona
Juliet
Portia
Rosalind
Belinda
1986U10
Puck
Miranda
Ariel
Umbriel
Titania
Oberon
Caliban
1999U1
Sycorax
1999U2
50
54
59
62
63
64
66
70
75
75
86
130
191
266
436
583
7100
10000
12200
25000
13
16
22
33
29
42
55
27
34
20
77
236
579
585
789
761
30
20
60
20
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6.30e19
1.27e21
1.27e21
3.49e21
3.03e21
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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
•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.
Oberon
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
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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
• Rings: narrow, dark,
o
Orbit inclination: 1.77
contain concentrations of
Orbit eccentricity: 0.009
particles called ring arcs.
Mean temperature: 48K
• Moons: 8, but they do not
form regular system.
Atmospheric components:
• Magnetic field: ~100 x Earth’s,
74% hydrogen
46o tilt to rotational axis,
25% helium
displaced 0.55 radii,
1% methane (at depth)
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 • No radio signals have been
around the planet’s
detected which relate to aurora
magnetic pole.
or auroral processes.
• Type-B aurora has also been • Scientists are still working to
observed.
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.
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They do not comprise a regular moon system
(no moons in roughly circular, equatorial, pro-grade orbits).
Proteus
Neried
Triton
Moons: Unusual Orbits
• Two of its moons have very peculiar orbits.
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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.
Neried
Triton
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
A variety of terrain in southern
polar region indicates history of
surface activity:
deep ridges, gashes, frozen
water lakes, and ice volcanoes.
This cantaloupe-textured
terrain is ~1000 km across.
Complex tectonic and volcanic
forces involving icy viscous
fluids combined to develop this
deformed landscape pattern.
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
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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
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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
Image courtesy of Marc W. Buie/ Lowell Observatory
• 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.
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
classified as Kupier belt objects.
re-
Pluto: Origins
Facts
1.
2.
Pluto is neither terrestrial nor jovian in its makeup
and it is similar to the ice/rock moons of the outer planets.
Pluto's eccentric, inclined orbit is quite unlike the orbits
of the other known planets.
Speculation
1. 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.
3.
Pluto may, like Triton, be an icy planetesimals left over from
the early times of planet formation in the outer Solar System.
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.