19 july 2005ast 2010: chapter 11 1 moons, pluto, & rings
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19 July 2005 AST 2010: Chapter 11 1
Moons, Moons, Pluto, Pluto, & & RingsRings
19 July 2005 AST 2010: Chapter 11 2
Ring and Satellite Systems (1)Ring and Satellite Systems (1)The rings and moons in the outer solar system are different in composition from objects in the inner solar system
Most of these rings and moons contain dark, organic compounds mixed with ice and rockThe presence of dark material implies that the objects reflect very little light
19 July 2005 AST 2010: Chapter 11 3
Ring and Satellite Systems (2)Ring and Satellite Systems (2)Most satellites in the outer solar system are in direct or regular orbits
They revolve about their planet in a west-to-east direction and in the plane of the planet’s equator
A number of them have irregular orbitsThey orbit in a retrograde (east-to-west) direction, or else have orbits with high eccentricity or high inclination
These satellites are usually smaller, located relatively far from their planetThey were probably formed far away and subsequently captured by the planet they now orbit
19 July 2005 AST 2010: Chapter 11 4
Jupiter's MoonsJupiter's MoonsTo date, Jupiter has 63 known satellites
The largest four are Callisto, Ganymede, Europa, and Io
Europa and Io are the size of our MoonGanymede and Callisto are bigger than Mercury
The rest are much smallerMore moons will likely be found in the future
19 July 2005 AST 2010: Chapter 11 5
Galilean MoonsGalilean MoonsThey were first seen by Galileo, using his telescopes
most recently studied by the Galileo spacecraft and by the Hubble Space Telescope
The combined data about the moons have shown important similarities to the terrestrial planets
The differences between the moons seem to be mostly due to distance from Jupiter
IoIo
EuropaEuropa
GanymedeGanymede
CallistoCallisto
19 July 2005 AST 2010: Chapter 11 6
Major Satellites in Solar Major Satellites in Solar SystemSystem
Callisto: Cratered World (1)Callisto: Cratered World (1)This is the outermost of the Galilean satellites
2 million km from JupiterIts noon-time surface temperature is 130 K (140°C below freezing!)Its diameter is 4820 km, about the same as Mercury’sIts mass is about 1/3 of Mercury’s
So it’s 1/3 as dense as MercuryThis means that Callisto has far less rocky and metallic materials than do the terrestrial planetsIt must instead be composed largely of ice
Movie from a mosaic of images taken by the Voyager spacecraft
19 July 2005 AST 2010: Chapter 11 8
Callisto: Cratered World (2)Callisto: Cratered World (2)Callisto seems not to have fully differentiated (separated into layers of different density materials)
The details of gravitational pull on the Galileo spacecraft suggest that Callisto lacks a dense coreThis is surprising to astronomers!
All big icy moons are expected to be differentiated (they should have differentiated more easily than rocky ones) Yet Callisto appears to have been frozen solid before the differentiation was complete
It is covered with impact cratersAlthough they look similar to craters on the Moon, they formed very differently
Callisto lacks the interior forces to drive geological change — it’s geologically dead
19 July 2005 AST 2010: Chapter 11 9
Ganymede: Largest Moon (1)Ganymede: Largest Moon (1)It is the largest satellite in the solar systemIt is cratered, but less so than CallistoCrater counting suggests that ¾ of the surface may have formed more recently than the lunar maria
The lighter areas are thought to be younger than the darker ones
Ganymede is geologically very different from CallistoIt is a differentiated world, like terrestrial planets, with
a core about the size of our Moona mantle and crust of ice “floating” above the corea magnetic field, a signature of a partly molten interior
It is geologically active, powered by internal heat
Ganymede: Largest Ganymede: Largest Moon (2)Moon (2)
It has a diameter of 5262 km Slightly larger than Mercury
Why is Ganymede very different from Callisto? Possible reasons
Their small difference in size and internal heating may have led to the significant differences between the two moons nowThe gravity of Jupiter may be responsible for Ganymede’s continuing geological activity
It is close enough to Jupiter that tidal forces from Jupiter may have occasionally heated its interior This could have triggered major convulsions on its crust
Movie from a mosaic of images taken by the Voyager spacecraft
19 July 2005 AST 2010: Chapter 11 11
Europa: Moon with Ocean (1)Europa: Moon with Ocean (1)Its surface is covered with a thick layer of water ice
For the most part, it is smooth, but is crisscrossed with cracks and low ridges often stretching for thousands of kilometers across icy plains
Under the ice, there may be liquid water or slush If so, tidal heating may be responsible for keeping the water in liquid form
Actual colorActual color Enhanced colorEnhanced color
19 July 2005 AST 2010: Chapter 11 12
Evidence for Warm Oceans on Evidence for Warm Oceans on EuropaEuropa
Galileo images appear to confirm the existence of a global ocean on Europa
It has ridges and multiple-line features that may have formed when thick layers of ice were broken up into giant icebergs and ice flows and then refrozen in place It also has smooth areas where water may have flowed up and refrozen
If it has liquid water, could life exist on Europa?
Enhanced colorEnhanced color
19 July 2005 AST 2010: Chapter 11 13
It is the most volcanically active world in the solar system Io has an elliptical orbit, causing it to twist back and forth relative to Jupiter and experience tidal forces
This twisting and flexing is the likely source of Io’s internal heating that drives its massive volcanism
Io’s interior heat may also have produced a differentiated interior
Io probably has an iron core surrounded by a molten rocky mantle
Io, the movie
Io: Volcanic Moon Io: Volcanic Moon
Jupiter
Io
19 July 2005 AST 2010: Chapter 11 14
Volcanism on IoVolcanism on IoIo has no impact craters
They must have been eradicated by its volcanic activity
Many of its volcanic eruptions produce plumes of material that extend hundreds of km out into space The colors on Io come from sulfur (yellow, black, red) and sulfur dioxide (white) Io’s volcanoes can be very long lived
Some have been observed for 20 years Loki volcano eruptsLoki volcano erupts
Lava fountainsLava fountains
19 July 2005 AST 2010: Chapter 11 15
Infrared Images of IoInfrared Images of IoImages taken by the Keck telescope plus adaptive optics
Bright spots indicate hot lava erupted by volcanoes
Io in infrared, the movie
Galileo imageGalileo image
Without Without adaptive adaptive
opticsoptics
19 July 2005 AST 2010: Chapter 11 16
Saturn’s MoonsSaturn’s MoonsTo date, Saturn has 46 known satellites
The largest is Titanalmost as big as Ganymede
Titan is the only satellite among the jovian moons that has a substantial atmosphere
19 July 2005 AST 2010: Chapter 11 17
Some of Saturn’s MoonsSome of Saturn’s Moons
19 July 2005 AST 2010: Chapter 11 18
Titan: Moon with Atmosphere (1)Titan: Moon with Atmosphere (1)It’s the second largest moon in the solar system It’s the only moon in the solar system with a substantial atmosphere
The thick atmosphere makes its surface impossible to see
Why does Titan have an atmosphere? Possible reasons:
Titan is massive enough to have strong gravity that can retain its atmospheric gases Titan is cold enough so that the gases in the atmosphere are slow moving and hence do not easily escape into space
19 July 2005 AST 2010: Chapter 11 19
Titan: Moon with Titan: Moon with Atmosphere (2)Atmosphere (2)
Titan’s atmosphere has a pressure 1.6 times Earth's is comprised of mostly nitrogen, plus 6% argon and a few percent methanehas trace amounts of organic compounds (such as carbon monoxide, ethane, and hydrogen cyanide) and water has multiple layers of clouds
the bottom layer probably composed of methanethe top layer including a dark reddish haze or smog, which hides Titan’s surface from our view
Its surface has a temperature of about 90 KThis means that on Titan’s surface methane may exist in liquid or solid form, and there may even be seas or lakes of methane, as well as methane ice
19 July 2005 AST 2010: Chapter 11 20
19 July 2005 AST 2010: Chapter 11 21
Cassini SpacecraftCassini Spacecraft
19 July 2005 AST 2010: Chapter 11 22
After After separating separating
from Cassini, from Cassini, the Huygens the Huygens probe landed probe landed
on Titanon Titanin Jan. 2005in Jan. 2005
19 July 2005 AST 2010: Chapter 11 23
Some of Cassini’s Images Some of Cassini’s Images
Cassini-Huygens mission’s website
19 July 2005 AST 2010: Chapter 11 24
Uranus SystemUranus SystemTo date, it has 27 known satellites
none are really large
Its rings and satellites are tilted at 98° just like the planet itselfIt has 11 rings
composed of very dark particlesdiscovered in 1977consisting of narrow ribbons of material with broad gaps
very different from the rings of Saturn
19 July 2005 AST 2010: Chapter 11 25
To date, it has 13 known satellites6 are regular, close to the planet2 are irregular, farther out
Its largest moon is Triton (in mythology, Triton is Neptune’s son)
It is the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit)It may once have been an independent object that Neptune captured
Tritonhas an atmosphere and active volcanismbears some resemblance to Pluto
Neptune’s MoonsNeptune’s Moons
19 July 2005 AST 2010: Chapter 11 26
Triton: Icy Triton: Icy World (1)World (1)It has an atmosphere and
active volcanismIts surface has the coldest temperature (between 35 and 40 K) of any of the worlds our robot spacecraft have visitedIts surface material is made of frozen water, nitrogen, methane, and carbon monoxideIt has a very thin nitrogen atmosphere, with a pressure of only 16 millionths of our atmospheric pressureTriton resembles Pluto in size, composition, and temperature
19 July 2005 AST 2010: Chapter 11 27
On Triton, volcanic eruptions produce plumes of nitrogen gas and are driven by seasonal heating from the Sun
The plumes can rise 10 km above the surface
On Earth, Venus, Mars, rocky magma is driven by internal heatOn Io, sulfur compounds are driven by tidal interactions with JupiterTriton movie
Triton: Icy Triton: Icy World (2)World (2)
19 July 2005 AST 2010: Chapter 11 28
Pluto (1)Pluto (1)Discovered through systematic search
at P. Lowell observatory in 1930
Highest inclination to the ecliptic (17°)Largest eccentricity ~ 0.248Average distance from the Sun ~40 AU
Perihelion closer than Neptune
Orbital period ~248.6 earth yearsRotation: ~6.4 days on its sidePluto's diameter 2240 kmIt has only 1 known satellite, Charon
Charon’s orbit is locked to Pluto, revolving and rotating at the same time as Pluto rotates
Pluto
Charon
19 July 2005 AST 2010: Chapter 11 29
Pluto (2)Pluto (2)It is the only planet not yet visited by spacecraftIt appears very faint from Earth
Its observation requires the best telescopesIts diameter is ~2190 km (60% of the Moon)Its density is ~2.1 g/cm3
It is composed of a mixture of rocky material and water ice
similar to Neptune’s moon TritonIt has a highly reflective surface
frozen methane, carbon monoxide, & nitrogen
Its surface temperature is between 50 and 60 KIt has a tenuous atmosphere
19 July 2005 AST 2010: Chapter 11 30
Pluto (3)Pluto (3)Pluto is not like the terrestrial or jovian planetsPluto, Quaoar, Charon, and Triton are probably examples of Kuiper-belt objects
The Kuiper belt is a disk-shaped region of space beyond Neptune’s orbit
19 July 2005 AST 2010: Chapter 11 31
Quaoar: New planet?Quaoar: New planet?Its orbit is more circular than Pluto'sIt is closer to the ecliptic
7.9° inclination compared to Pluto's 17°
Its diameter is 1280 km
compared to Pluto's 2240 km
Possibly Pluto and Quaoar are both Kuiper-belt objects
19 July 2005 AST 2010: Chapter 11 32
QuaoarQuaoar
19 July 2005 AST 2010: Chapter 11 33
Planetary Rings (1)Planetary Rings (1)All four giant planets have ringsEach ring is a system of billions of small particles (moonlets)Each ring displays complicated structure related to the interaction between the rings and satellitesThe four ring systems are very different from each other in mass, structure, and composition
19 July 2005 AST 2010: Chapter 11 34
Planetary Rings Planetary Rings (2)(2)
Saturn’s ringsmade up of icy particles spread out into several vast, flat rings, with a great deal of fine structure
Neptune’s & Uranus’ ringsmade up of dark particles, confined to a few narrow rings, with broad empty gaps
Jupiter’s ringstransient dust bands, constantly renewed by erosion of dust grains from small satellites
19 July 2005 AST 2010: Chapter 11 35
Planetary Rings (3)Planetary Rings (3)The dotted vertical line is the limit inside which the gravitational (tidal) force of a planet can break up a satelliteIn other words, only really small bodies survive inside the tidal stability limit
19 July 2005 AST 2010: Chapter 11 36
What Causes Rings? What Causes Rings? (1)(1)
Each ring is a collection of vast numbers of objects and particles
Each particle obeys Kepler’s lawsThe inner particles revolve faster than those farther outThe ring does not rotate as a solid bodyParticles within the ring are close to one another The particles exert mutual gravitational influence and occasionally even touch in low speed collisions
Two basic theories on how the rings formed have been proposed
Breakup hypothesis: they are the remains of a shattered satelliteAnother hypothesis: the rings consist of particles that were unable to fuse into a single body
19 July 2005 AST 2010: Chapter 11 37
What Causes Rings? (2)What Causes Rings? (2)In either theory, the gravitation of the planet plays an important role
Tidal forces for orbits close to the planet can tear bodies apart, or inhibit loose particles to come together
The rings of Saturn and Uranus are close to the planetsIn the breakup theory, a satellite, or a passing comet, may have come too close and been torn apart under tidal forces, or through some collision
Some scientists believe that some of the rings are young and must therefore be the result of a breakup
19 July 2005 AST 2010: Chapter 11 38
Rings of Jovian PlanetsRings of Jovian Planets
Jupiter’s ringsJupiter’s rings
Saturn’s ringsSaturn’s rings
Uranus’ ringsUranus’ rings Neptune’s ringsNeptune’s rings
19 July 2005 AST 2010: Chapter 11 39
Satellite-Ring InteractionsSatellite-Ring InteractionsEach ring has an intricate structure, as discovered by VoyagerThe rings’ structures are due mainly to the gravitational effects of the satellites
Without the satellites, the rings would be flat and featureless
There could even be no rings at all…Gaps in Saturn’s A-ring result from gravitational resonances with smaller inner satellites, especially Mimas