Post on 05-Aug-2020
Lecture 5: Overview of the Icy Moons, Comets, & Kuiper Belt Objects
Astro 202Prof. Jim Bell (jfb8@cornell.edu)
Spring 2008
But first...
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Graded Paper 1 will be handed back Thursday
Astro 202 3
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
Astro 202 5
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
Astro 202 6
Relative sizes of
the major satellites
and small planets
Sizes (not distances) to scale
Astro 202 7
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!
Astro 202 10
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
Astro 202 17
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/
Astro 202 20
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
Astro 202
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
Astro 202 24
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
Astro 202
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
Astro 202 35
Small satellites
of Saturn
Proteus (Neptune)
Outer Uranian moon
Inner satellites of Jupiter
Minor Satellites
Astro 202 36
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...?
Astro 202 37
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
Astro 202 38
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
Astro 202 39
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
Astro 202 41
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)
Astro 202 43
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+
Astro 202 44
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
Astro 202 45
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)
Astro 202 46
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??
Astro 202 47
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!)
Astro 202 49
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?
Astro 202 51
Spectacular
Comet Deaths!!Comets smashing into Jupiter
– Shoemaker-Levy 9, July 1994
Earth for scale
Comet impact site
Astro 202 52
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
Astro 202 53
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!
Astro 202
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”
Astro 202 55
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
Astro 202 56
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!)
Astro 202 57
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
Astro 202 58
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!
Astro 202 59
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...
Astro 202 60
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
Astro 202 61
What will Pluto really be like?
Astro 202 62
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
Astro 202 63
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...
Astro 202 64
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