Transcript Key Ideas

ASTR178
Other Worlds
A/Prof. Orsola De Marco
9850 4241
[email protected]
Announcements
• If you do not make your talk day you risk not getting
that side of the assessment.
• Observing is on October 6th 7:15PM and 13th 8:15PM.
(Sign up in class or on my door: E7A-316).
• Assignment 2 posted, due in 17th September.
• Moon practical due in 17th September.
Sample questions for this
lesson
• 1-6, 8-11, 13-36,38,41, pages 349-351
In last class:
gas giants part II
•Jupiter and Saturn.
• Atmospheres
• Inner composition
• Magnetic fields
• The rings
• Introduction to the moons of Jupiter
In this class:
giants moons
• Introduction to the moons of Jupiter
• Io
• Europa
• Ganymede
• Callisto
• Saturn’s Titan
• … and all the other moons …
Giant Moons
The Galilean satellites
•Noticed by Galileo ~1610
•Could be resolved by naked eye were it not for Jupiter’s glare.
•Synchronous rotation.
•Innermost 3 have orbital periods in a 1:2:4 resonance. Why only 3 not all 4?
•Transits and occultations used to measure diameters
•The Pioneers and the Voyagers satellites measured the moons’ masses
– how?
• From radii and masses one could infer densities – how?
The composition of the satellites
• Low density Callisto and Ganymede are likely made of water ice (low
density).
• Io and Europa have densities similar to rock on Earth.
• All but Io have spectra indicative of some water ice on the surface.
The formation of the satellites
• Density decreases as you go away from Jupiter, as is the case for the Solar
System.
• Jupiter formed as a miniature Solar System, where Jupiter itself was a
source of heat: rocky materials condense close where it is warmer while rock
+ ice coalesce further out, where ice exists because it is colder.
Io
Not quite like the Moon!!!
Io’s heat source
• Tidal forces from Jupiter + Io’s elliptical orbit.
• How much heat is generated? 24 tonnes of TNT exploding every second!
• How much heat goes through the surface? 2.5 Watt/square meter – Earth’s:
0.06 Watt/square meter!
Volcanos
• Plumes 70-280 km high.
• Ejected at ~1100-3600 km/h. Compare Earth’s 360 km/h!
• More like geysers on Earth – but propelled by Sulfur dioxide.
• Rapidly cooled sulfur will lead to all the colours
• Sulfur dioxide leads to white flakes
• Aside from geysers there are explosive lava flows.
• Hotter lava (from IR observations).
• Contain magnesium – hotter interior.
• 100 km crust floating on a fully magma
interior – explain widespread volcanism
• 300 active volcanos at any
one time.
• Enough material to cover Io
in 1 m of ejecta in 100 years.
• Jupiter’s field generates a current across Io
(400,000 V!)
• This in turn generates a magnetic field on Io.
• moving magnetic fields induce currents
• Magnetised strip in your credit card induces a current in
the card reader.
• Measured field on Io is larger than predicted from above
argument - hence Io has additional magnetic field from
molten interior.
• Smallest body in Solar System with its own field.
• Differentiation caused a solid iron and iron sulfide core
• Core ½ the size of Io – how do we know?
Europa
• The smoothest world
• The density indicates rock
composition…
• … but the spectrum is pure ice. – what
is a spectrum?
• Almost no craters – what does it mean?
• Cracks probably from geologic activity – stress.
• Heat source as for Io, just less so since Europa
is further from Jupiter.
• Cracks may be the result of tidal flexing
• Many young areas (smooth) indicate much activity
Frozen sea near
Greenland, photo
Taken by Francisco
Diego from a plane
(10,000 m high)
Ice on Europa
taken from similar
Altitude
A liquid ocean under the crust?
• Evidence from rafts and cracks.
• Galileo measured magnetic field induced from Jupiter –
variable, needs conducting medium – water + minerals – colour
on cracks.
• 100 km ocean on rocky core on metallic core?
• Water + heat = life?
• (Galileo was deorbited into Jupiter not to collide with
Europa)
And the atmosphere?
• O2 atmosphere (very very tenuous) from collisions of
particles with ice, liberating O (H escapes into space)
• Ganymede
young icy
craters …
• Older darker
• Younger
lighter (unlike
the Moon!)
• Old cratered terrain.
• Younger (but still 1 billion yr old) lighter terrain.
• Fractures – tectonic activity. Heat in this moon till recently.
• Ice on the surface, come through cracks?
• Strange since Ganymede is very small to have retained heat.
• Magnetic field twice as strong as Mercury. Molten interior!
• Highly differentiated metallic core – how do we know?
• Changes in magnetic field: Layer of liquid water – why?
• Source of heat???
Callisto
• Craters on ice
• What is all the “muck”?
• Lots of large craters, where are the small ones?
• Changing magnetic field – hence liquid water – who keeps it liquid?
Ammonia as antifreeze. Still….
• Not differentiated – hence cold.
• This CO2 atmosphere
Cassini-Huygens explores Saturn and Titan
Short animation
Saturn’s Titan
• Discovered by Huygens in 1665
• Suspected to have an atmosphere in 1900
• Found to have one in 1944 – Kuiper found to have NH3 atmosphere
Saturn’s Titan
• NH3 in atmosphere broken down such that 95% atmosphere is nitrogen
• Surface pressure 1.5 times Earth’s. 10 times more atmosphere.
•Temperature 95 K – methane and ethane liquid.
• Cassini’s flybys mapped Titan in Infrered
• Sand dunes parallel to equator made of ice and polymers: wind action
• Methane from recent (100 million year) volcanos – also white spots.
• Where does the heat come from? Possible periodic reheating of the core
due to radioactive decay.
• The Huygens probe was launched from
Cassini in 2005.
• It landed safely and broadcast images for
70 minutes.
• CH4 chemistry leads to hydrocarbons and
the reaction of those with N2 leads to
polymers which have a reddish colour – they
can be in the air as aerosols or on the
ground.
Movie
• Possible liquid methane lakes only detected near the north pole –
might be seasonal.
• Huygens probe vaporised methane.
• Only 5 cm “rainfall” per year
• Few craters - erosion
Saturn and Titan in the news
10 August 2009
Titan was observed to have
tropical clouds forming in
this infrared image taken
with the 8 m Gemini telescope
Titan’s clouds are likely to
make liquid methane rain.
Star Trek
Titan’s geologic history
A proposal
• Solid core, liquid water mantle icy crust formed 4.5 billion years ago.
• Methane from volcanos
• Methane destroyed between 3.5 and 2 billion years ago.
• Radioactive isotopes – reheating of interior, convection transports the
heat to the surface, new outgassing of methane (2 billion years ago)
• Methane destroyed once again.
• 500 million years ago one more reheating episode. Once more outgassing.
• In the future methane will go. We are just observing Titan at a special
time…
• The 4 innermost
satellites of Jupiter.
• All within Io’s orbit.
• All prograde motion:
they formed with the
Galilean moons and J.
• There are 55 outer
ones, all outside the
orbit of Callisto.
• These are likely
captured (inclined
orbits and 48 have
retrograde motion).
• 23 discovered in 2003
alone!
• All non spherical –
why?
• Saturn has Titan and 6 larger
satellites (spherical).
• They orbit on the Saturn’s
equatorial plane, have prograde
orbits and are phase locked.
• Orbit between 3 and 59 Saturn
radii.
• They all look different!
• It also has 54 small satellites,
some of which might be captured
others are likely fragments of
impacts.
• Low average density
• Mimas is old and
cratered and has a HUGE
impact crater.
• Enceladus has a new
surface.
• Most reflective object
in the solar system
• Ice eruptions come from
the cracks.
• Eruptions from Enceladus spew particles out that form a thin ring.
• Heat source? Dione 2:1 resonance… possible. (Mimas and Tethys
are also in a 2:1 ratio, but no heating…)
Key Ideas
• Nature of the Galilean Satellites: The four Galilean satellites orbit
Jupiter in the plane of its equator. All are in synchronous rotation.
• The orbital periods of the three innermost Galilean satellites, Io,
Europa, and Ganymede, are in the ratio 1:2:4.
• The two innermost Galilean satellites, Io and Europa, have roughly
the same size and density as our Moon. They are composed
principally of rocky material. The two outermost Galilean
satellites, Ganymede and Callisto, are roughly the size of
Mercury. Lower in density than either the Moon or Mercury, they
are made of roughly equal parts ice and rock.
• The Galilean satellites probably formed in a similar fashion to our
solar system but on a smaller scale.
Key Ideas
• Io: Io is covered with a colorful layer of sulfur compounds
deposited by frequent explosive eruptions from volcanic vents.
These eruptions resemble terrestrial geysers.
• The energy to heat Io’s interior and produce the satellite’s
volcanic activity comes from tidal forces that flex the satellite.
This tidal flexing is aided by the 1:2:4 ratio of orbital periods
among the inner three Galilean satellites.
• The Io torus is a ring of electrically charged particles circling
Jupiter at the distance of Io’s orbit. Interactions between this
ring and Jupiter’s magnetic field produce strong radio emissions.
Io may also have a magnetic field of its own.
Key Ideas
• Europa: While composed primarily of rock, Europa is covered
with a smooth layer of water ice.
• The surface has hardly any craters, indicating a geologically
active history. Other indications are a worldwide network of
long cracks and ice rafts that indicate a subsurface layer of
liquid water or soft ice. As for Io, tidal heating is responsible
for Europa’s internal heat.
• An ocean may lie beneath Europa’s frozen surface. Minerals
dissolved in this ocean may explain Europa’s induced
magnetic field.
Key Ideas
• Ganymede: Two types of terrain are found on the icy surface
of Ganymede: areas of dark, ancient, heavily cratered surface
and regions of heavily grooved, lighter-colored, younger
terrain.
• Ganymede is highly differentiated, and probably has a metallic
core. It has a surprisingly strong magnetic field and a
magnetosphere of its own.
• While there is at present little tidal heating of Ganymede, it
may have been heated in this fashion in the past. An induced
magnetic field suggests that it, too, has a layer of liquid water
beneath the surface.
Key Ideas
• Callisto: Callisto has a heavily cratered crust of water ice. The
surface shows little sign of geologic activity, because there
was never any significant tidal heating of Callisto. However,
some unknown processes have erased the smallest craters
and blanketed the surface with a dark, dusty substance.
• Magnetic field data seem to suggest that Callisto has a
shallow subsurface ocean.
Key Ideas
• Titan: The largest Saturnian satellite, Titan, is a terrestrial
world with a dense nitrogen atmosphere. A variety of
hydrocarbons are produced there by the interaction of
sunlight with methane. These compounds form an aerosol
layer in Titan’s atmosphere and fall as a gentle rain on the
surface.
• Titan’s surface shows that liquid hydrocarbons have flowed
over its surface, forming streams, rivers, and outflow
channels. Very little of this liquid appears to be present on
Titan’s surface today.
Key Ideas
• Other Satellites: As of 2006, Jupiter has a total of 63 known
satellites and Saturn has a total of 56.
• In addition to the Galilean satellites, Jupiter has four small
inner satellites that lie inside Io’s orbit. Like the Galilean
satellites, these orbit in the plane of Jupiter’s equator. The
remaining satellites are small and move in much larger orbits
that are noticeably inclined to the plane of Jupiter’s equator.
Many of these orbit in the direction opposite to Jupiter’s
rotation.
Key Ideas
• In addition to Titan, six moderate-sized moons circle Saturn in
regular orbits: Mimas, Enceladus, Tethys, Dione, Rhea, and
Iapetus. They are probably composed largely of ice, but their
surface features and histories vary significantly. The other,
smaller moons include shepherd satellites that control the
shapes of Saturn’s rings and captured asteroids in large
retrograde orbits.