Lecture 5: Overview of the Icy Moons, Comets, & Kuiper Belt Objects

Astro 202Prof. Jim Bell (jfb8@cornell.edu)

Spring 2008

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Icy Moons, Comets, and KBOs!

! Io, Europa, Ganymede, & Callisto

! Titan and Triton

! ...and many smaller worlds

! Comets

! Pluto and the Kuiper Belt

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Moons!

!Jupiter– 4 large "Galilean satellites"

• Io, Europa, Ganymede, Callisto

– 59 other smaller satellites

!Saturn– Titan: large moon with a thick atmosphere

– 6 medium, and 53 smaller satellites

!Uranus– 5 medium, 22 smaller satellites

!Neptune– Triton: large moon with a thin atmosphere

– 12 other smaller satellites

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Major Properties

• Distinct and important

differences in density,

composition, and

orbital properties

• Io has volcanoes!

• Europa has an ocean?

• Titan has a thick

atmosphere! Surface

details unknown...

• Triton has a thin

atmosphere (very

similar to Pluto?)

• Many other small icy

satellites

59

53

22

12

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Relative sizes of

the major satellites

and small planets

Sizes (not distances) to scale

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Major Satellites

of Jupiter! New worlds discovered

by Galileo in 1610

! A "mini solar system"... by Jove!

! Io

– Active volcanoes!

! Europa– Subsurface ocean?

! Ganymede– Large-scale tectonism

! Callisto

– Battered surface

Io

Europa

Ganymede

Callisto

Galilean Satellite Interior Models

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Io ("EYE-oh")

! Inner Galilean moon

!Most volcanically-active world in the solar system!

! Interior melted by tides from Jupiter & other moons

Density:

3.57 g/cm3

Eccentricity

not exactly 0

Tidal energy

heats the

interior!

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Io's Amazing Landforms!• Volcanic calderas several km deep• Lakes of molten sulfur• Some mountains that are not volcanoes • Lava flows hundreds of km long

Low viscosity fluid (sulfur rich?)

Flow temperatures > 1000°C to 1500°C

• Volcanic vents

• Io's color caused by sulfur compounds

Volcanism on Io:

Extensive lava flows

No impact craters

Colors consistent

with molten sufur

at different Temps.

(up to ~2000 K) Astro 202 12

Europa

• Ice-covered Moon

• flat flat flat!

• Crust broken up into

moving plates!

• "salty" deposits well

up between plates

• Subsurface ocean??

Map of salt-rich "contaminants" in Europa's crust

• Ridges show different

compositions:

hydrated sulfate salts

that appear to have

“erupted” from the

interior (ocean?)...

Ganymede• Largest moon in solar system

• Complex grooved, cratered

surface

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Ganymede

• Icy surface with

bright & dark areas

• Many more craters

than Europa (older)

• But craters degraded

or "softened"

• Extensive network

of tectonic ridges

“Palimpsests”

(ghost craters)

• Evidence for

icy/viscous

relaxation?

Cross-cutting grooves,

furrows; evidence of

tectonic folding,

stretching

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Callisto• Ancient, heavily

cratered surface

• Bright icy deposits

• Dark "powdery"

coating of unknown

composition

Callisto• Why no active geology?

• Why is surface “powdery”?

Smaller Jovian Moons...

From left to right, Galileo spacecraft images of Metis (longest

dimension is approximately 60 kilometers or 37 miles across),

Adrastea (20 kilometers or 12 miles across), Amalthea (247

kilometers or 154 miles across), and Thebe (116 kilometers or 72

miles across).

Distance Radius

Satellite (000 km) (km) Disc.

--------- -------- ------ -----

Metis 128 20 1979

Adrastea 129 10 1979

Amalthea 181 98 1892

Thebe 222 50 1979

Io 422 1815 1610

Europa 671 1569 1610

Ganymede 1070 2631 1610

Callisto 1883 2400 1610

Leda 11094 8 1974

Himalia 11480 93 1904

Lysithea 11720 18 1938

Elara 11737 38 1905

Ananke 21200 15 1951

Carme 22600 20 1938

Pasiphae 23500 25 1908

Sinope 23700 18 1914

47 tiny outer moons found in the last decade...

More probably still to be found!

Are these captured asteroids?

http://www.dtm.ciw.edu/sheppard/satellites/

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Major Satellites of Saturn

!Titan

– Larger than

Mercury!

– Thick atmosphere!

!6 mid-size moons

– All icy

– All heavily cratered

– But different crater

densities, implying

unique resurfacing

histories

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Titan

• Mercury-sized “planet”!

• Thick, hazy atmosphere, with lots

of hydrocarbons

! - formed by sunlight breaking

up CH4 molecules

- similar to early Earth?

• Surface pressure: 1.5 bars!

• Surface temp.: 95K (-290°F)

• Cassini/Huygens lander: 2005

HST

Infrared

Voyager 2

- Infrared, Radar Mapping

- Huygens descent

imaging and lander

View during descent,and from the surface

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Mid-sized icy Saturnian moons...

Enceladus• Only 500 km diameter, yet internally active!

• Subsurface liquid water? Newest astrobiology “hotspot” in the solar system...

?

Satellites of Uranus

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Major Satellites

of Uranus!5 mid-size moons

!All icy

!All fairly dark [why?]

!All heavily cratered

– But: Evidence for resurfacing, tectonism, and other processes

Miranda Ariel Umbriel Titania Oberon

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Miranda• Tiny moon (< 500 km diameter)

• Yet some of the strangest and least

understood geology in the solar system!

• Recall that out here, ice acts like rock

20 km cliff!

Ariel (above),

Titania (left):

Extensional

Graben?

Heavily cratered

Umbriel (top right)

and Oberon (right)

Neptune’s large moon Triton:

N2 ice, few craters, exotic terrains,

active geology

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Triton: Neptune's

only large satellite

!Nearly same size and density as Pluto– Triton: R = 1350 km; ! = 2.08 g/cm3

– Pluto: R = 1140 km; ! = 2.07 g/cm3

!Bizarre backwards orbit and high tilt

– is Triton a "captured Pluto"?

!Surface is bright: albedo = 0.7 to 0.8

!Surface T only ! 35K (-391°F)

!Thin atmosphere! P = 0.01 mbar

!Few craters: Surface is "young"

!Voyager discovered active geysers!– “volcanoes” of ice and other volatiles “Lakes” “cantaloupe

terrain” and ice volcanoes on Triton

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Small satellites

of Saturn

Proteus (Neptune)

Outer Uranian moon

Inner satellites of Jupiter

Minor Satellites

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Comets!

Comets are small icy objects from the outer solar system that evaporate

spectacularly as they get heated when their orbits carry them closer to the Sun

•Where do they come from?

•Relationship to asteroids and planets...?

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Comet Lingo! "Comet" comes from the Greek word

"kometes", meaning long-haired

! The solid inner part of a comet is called

the nucleus. Most comet nuclei are only

a few km in diameter, and are

composed of rock and ice.

! The "fuzzy" bright region around the

nucleus is called the "coma" ("hair"),

and is composed of gas and dust.

! Solar heating leads to the formation of

an extended region of gas and dust,

called the tail.

! An enormous hydrogen cloud

surrounds the coma of most comets

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Comet Tails

! Dust tails are small pieces (micron to mm sized) of dust, rock, and ice that are shedding off the nucleus and following Keplerian orbits

! Ion tails (or plasma tails) are gases evaporated from the nucleus, ionized, and pushed in the anti-Sun direction by the solar wind

Dust

Gas

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The life

of a Comet

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A Model Comet...

!A comet is essentially a "dirty snowball"

– Model proposed by astronomer Fred Whipple in 1950

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Halley's Comet Up Close!

! Images and other data from the VEGA and Giotto

spacecraft (1986 flybys)

Sketch mapGiotto image

Other comets up close...

Borrellyabout 8 km long

(Deep Space 1 mission, 2001)

Wild-2about 4 km wide

(Stardust mission, 2004)

Tempel-1about 6 km wide

(Deep Impact mission, 2005)

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Cometary Composition

! The best measured is comet Halley, with a composition of nearly 80%

water ice. But Halley is highly evolved, and may not be typical

! Farther from the Sun, at temperatures below the evaporation

temperature of water ice, evidence for CO, CO2, and other ices is seen

! The dark surface material may be complex organic molecules formed

from the residual cometary material after the ices are melted away

(from direct spacecraft measurements, as well as spectroscopy from

telescopes)

Atoms, Molecules, and Ions Identified in Comets

Atoms: H,C,O,S,Na,Fe,K,Ca,V,Cr,Mn,Co,Ni,Cu,Si,Mg,Al,Ti

Molecules:OH,S2,OCS,H2S,H2O,HDO,H2CO,C2,C3,CH,CN,CO,CO2,C

S,CH4,C2H2,C2H6,NH,NH2,HCN,CH3CN,N2,NH3

Ions: H2O+,OH+,H3O+,CO+,CO2+,CH+,CN+,N2

+,C+,Ca+

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Comet Orbits

! There are two populations of comet orbits: – short period comets orbit the Sun in < about

200 years, orbit close to the plane of the rest of the planets, and only travel out as far as Jupiter

• Examples:

– Halley (76 years)

– Encke (3.3 years)

– long period comets take thousands of years to orbit the Sun, and they come from all directions in the sky

• Examples:

– Hyakutake (32,000 years!)!

! These two populations must have different sources

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Short-Period Comets

! Recent discoveries of a disk of small bodies

beyond Neptune, the Kuiper Belt, appears to

provide a source for the short period comets

! They are brought closer to the Sun by

frequent encounters with Jupiter and other

giant planets

– Their orbits can be altered, and some

become short period comets

! Only the largest and brightest Kuiper Belt

Objects (KBOs) have been discovered so far

! There may be 100,000 or more KBOs larger

than a few hundred km in size

! There may be hundreds of millions of KBOs

that are comet-sized (few km)

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Long-Period Comets

! Orbits are traced back in time...

!Most lead to a vast but unobserved spherical cloud of small objects:

– This is the Oort Cloud

– 20,000 to 100,000 AU

! Stars passing "close" to the Sun can jostle these objects and cause them to fall inwards

! There may be 1011 to 1012 comets in the outer Oort cloud, and perhaps ten times that many comets in the cloud as a whole!

! Comet mass > 1000 Earths??

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Meteors: Pieces of Comets?

! The short period comets appear to be the sources of several

meteor showers, as the Earth crosses through their debris

Fireball on home video from Peekskill, NY; Oct. 9, 1992 Astro 202 48

Why are Comets Important?

! Comets can be thought of as big celestial chemistry sets! – Heat, light, water, organic chemicals...

– Aren't these the building blocks of life?

! Comets must occasionally impact the Sun & planets...– They deliver volatiles and organic molecules (Bringers of Life!)

– They can also cause massive ecological destructions (Bringers of Death!)

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A part of the Bayeux Tapestry, commemorating the Norman

conquest of 1066 and also archiving the appearance of a

"portentous star" in the sky. The star is now known to have been

Halley's cometAstro 202 50

Spectacular Comet Deaths!

!Comets hitting the Sun– SOHO satellite data

– "Sun grazers"

– Do these events

influence solar

activity?

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Spectacular

Comet Deaths!!Comets smashing into Jupiter

– Shoemaker-Levy 9, July 1994

Earth for scale

Comet impact site

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Pluto and Beyond

!Pluto is a small icy planet

with a relatively large icy

moon

!It is the first discovered

and (among) the largest

known member of a class

of small bodies known as

Kuiper Belt Objects

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Discovery

! Pluto was discovered in 1930 by a fortunate accident

! Percival Lowell made calculations which later turned out to be in error that predicted a planet beyond Neptune, based on the motions of Uranus and Neptune

! Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey

– For 14 years Tombaugh scanned the skies, and discovered

• 1 globular star cluster

• 1 comet

• 1 supercluster of galaxies

• 5 open star clusters

• 775 asteroids

• and Pluto!

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Pluto & the Kuiper Belt• Pluto is the largest

known member of a class of small bodies known as Kuiper Belt Objects (KBOs)

• First KBO discovered in 1992

• Currently more than 1000 known to exist beyond Neptune

• Hundreds of km sizes; some larger than Pluto!

• Icy compositions

http://cfa-www.harvard.edu/iau/lists/OuterPlot.html

PlutoNeptune

This giant swarm of “trans-Neptunian objects” is why some astronomers don’t think Pluto deserves to be called a “planet”

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Surface Composition

!Spectroscopy

!Lab ice studies

!Computer Modeling

!Methane ice (CH4) discovered in 1976

!Subsequently, N2 and CO ices discovered

!Very low pressures and temperatures

– Comparable to Triton

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Pluto has a thin atmosphere

!Between 1979 and 1999, Pluto

was near perihelion and closer to

the Sun than Neptune

!Surface temperatures warm

enough for some CO, CH4, and

N2 ices to sublimate

!Atmosphere discovered by stellar

occultations by Pluto

!Surface pressure is only a few

microbars (10-6 bar)

!As Pluto recedes from the Sun,

the atmosphere is going to freeze

out back onto the surface! (it's

temporary!)

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Charon: Pluto's Moon

! Discovered in 1978

! Charon is large relative to Pluto

– 1172 km (half Pluto's size!)

– Revolves around Pluto in 6.4 days

– Pluto's spin period is 6.4 days

! Density about 2 g/cm3

! Charon is bright: icy

! But spectra show H2O ice, not

CO, N2, or CH4 like Pluto

! Is it a captured moon?

! Was it formed by a giant impact?

Groundbased, 1978 HST, 1995

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KBOs:

Kuiper Belt Objects

!Primitive objects with 30 to 100 AU away

!Many protected from encounters with the

planets by orbital resonances (like Pluto)

!KBOs can become short period comets

!There may be 35,000 to 100,000 or more

KBOs larger than 100 km

– This is several hundred times the number and

mass of asteroids in the main asteroid belt!

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So then what is Pluto?

Planet? Asteroid? Comet? Dog?

!Pluto is a Planet!

– It is in an independent orbit around the Sun

– It is a spherical body with significant gravity

– It has an atmosphere

!Pluto is not a Planet!

– It is essentially a large, distant comet

– There may be 100,000 more "Plutos" out there

!The debate rages...

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A Mission to Pluto!

• The “New Horizons” mission

will fly by Pluto/Charon and

then go on to a KBO…

Launch! Arrival at Pluto KBO

Jan. 2006 2015 or 2016 < 2026

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What will Pluto really be like?

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Summary

!There are 6 planet-sized moons in the outer solar system, and each has unique and complex traits

– Io: Active volcanism, extremely young surface

– Europa: Active "plate tectonics"; subsurface ocean?

– Ganymede, Callisto: intense tectonism, cratering

– Titan: Thick smoggy atmosphere; early Earth analog?

– Triton: Active geysers; Pluto/Kuiper Belt clone?

!There are about a dozen more mid-sized icy satellites, with evidence for complex surface geology

!There are more than 140 other small, irregular moons that may be captured asteroids or comets

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Summary

! Comets are small, irregular, icy and rocky objects that evaporate spectacularly as they approach the Sun

! Comets fall into two main classes:

– Short period comets (P < 200 yr.); source = Kuiper Belt

– Long period comets (P > 1000 yr.); source = Oort cloud

! The main parts of a comet are the nucleus, coma, tail, and extended hydrogen cloud

! Comets are essentially "dirty snowballs" composed of water and other ices, silicate minerals, and organic compounds

! Comet impacts may have brought substantial quantities of water, other volatiles, and organics to all the planets...

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Summary

! Pluto is a small icy outer solar system world

– Surface composed of CO, N2, CH4 ices

– Thin atmosphere formed by ice sublimation

! It has a uniquely eccentric and inclined orbit

– Pluto is "protected" in a 3:2 resonance with Neptune

! Pluto has a moon (Charon) half its size

! Is Pluto a planet or not?

– It is one of the largest members of a family of

perhaps 100,000 or more Trans-Neptunian objects

Next TimeStarting the “Dating the Universe” unit

Formation of the Solar System

Reading:

Read NSS 2

Read PBD 6