ASTR 330: The Solar System
Announcements
• Make-up exam #2 scheduled for Thursday.
• Extra credit term paper: choose subjects and
register with me by today!
• Topics must be science-focused.
• Papers due December 5th class.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 22:
Large Satellites II:
Io, Titan and Triton
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Io: Jupiter’s Volcanic Moon
• Io is the most volcanically active object in the solar system, showing
extensive re-surfacing between the Voyager images of 1979 and the arrival
of Galileo in 1995. The bright orange oval surrounds the volcano Pele.
Picture credit: NASA/JPL - Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Io’s bulk density and composition
• Io is the innermost of the four Galilean moons of Jupiter.
• Io’s density of 3.3 g/cm3, higher even than Europa, tells us that it has
little or no water. It is a very similar size and density to the Moon.
• The surface is quite reflective, but completely unlike the pale Europa,
being covered instead by brightly colored patches, caused by constant
volcanic activity.
• Io is small enough that it should have cooled long ago: what energy
source could be powering the volcanism today?
• The answer of course is the huge tidal force of Jupiter. Io is about the
same distance from Jupiter as the Moon from the Earth, however Jupiter
is 300 times more massive!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tides on Io
• The tides of Jupiter cause Io to be pulled into a elongated shape
(similar to an egg), with a tidal bulge several km high.
• If Io was able to keep the same face towards Jupiter (like the Moon
does to the Earth), then no heating would occur.
• However, Io’s fate is not so simple. The next outer moons, Europa and
Ganymede exert enough gravitational force on Io to perturb its orbit from
a purely circular one.
• Following an elliptical orbit, like Mercury, Io travels at different speeds
as it gets closer to and further from Jupiter.
• Hence, it is unable to ‘choose’ a rotational state which matches its
orbital speed at all times, and so it twists and turns as Jupiter pulls on
Io’s tidal bulge from different directions. This causes enormous heating.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Energy of Tidal Heating
• We can estimate a lower limit on the amount of energy produced by the
tidal heating, by adding up the heat emitted by its volcanoes.
• We calculate by this method that at least 100 million megawatts (1014
watts) is generated: 10 times the present energy consumption of all
human activity!
• The energy source is the spin of Jupiter, which therefore must be
gradually slowing down very slightly over time.
• This continuous heating has driven off many volatiles over geologic
time: no water, and most carbon and nitrogen compounds are gone.
• Sulfur and its compounds are the most volatile substances remaining,
and which are the source of the many colored surface terrains.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Io Interior
• The silicate
interior of Io is
largely liquid,
with a solid
crust of about
25 km thick.
• The surface is
constantly
changing, and
any geologic
features will not
long survive.
Picture credit: Calvin Hamilton/solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Io Surface
• Io shows no surface craters at all. This is of course due to
the constant volcanic activity, resurfacing the crust at a rate
of 100s of meters per million years.
• Notice the trend toward fewer and fewer craters as we
have traversed the Galilean moons inwards: the opposite to
what we originally expected!
• What are we to make of the dramatic colors of Io’s
surface?
• The only compound that has been unambiguously detected
to date is sulfur dioxide (SO2), a gas on Earth, but which
may freeze as a white frost on Io.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Surface Colors
• The most likely
culprit for the colorful
yellows, oranges,
browns and blacks is
elemental sulfur,
which may form
different colored solid
states depending on
how it has cooled.
• This Galileo image
shows the colorful
deposits around the
Pele volcano.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanic Features
• Instead of craters, Io exhibits a full range of
volcanic features including:
• layered lava plains with flow fronts
• volcanic mountains and calderas
• volcanic vents and lava flows.
• Io has several low shield volcanoes and
mountain ranges taller than those on Earth (the
triangular peak, right-top is 11 km high).
• The surface has collapsed in several places,
apparently due to magma being removed from
beneath. The dark grey region Hi-Iaka Patera
(right) is probably caused by rifting, not collapse.
• Tectonic pressures have produced tilt blocks and
even upthrust mountains.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanic
Eruptions
• Io’s volcanoes are named for fire
gods in various cultures: e.g. Pele
(Hawaiian) and Loki (Norse).
• Plume eruptions, first observed
by the Voyager twins, can reach
300 km above the surface.
• The Galileo image (right) shows
two plumes: one on the limb
above Pillan Patera (130 km
high), and one erupting from
Prometheus in the center of the
image.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Resurfacing
• These 2 Galileo images
show the Pillan Patera before
and after the eruption of the
previous slide: the new dark
spot is 400 km across. The
red circle surrounds the Pele
volcano.
• Plume eruptions occur when hot liquid rushes to the surface, becomes
gaseous as the pressure drops, and then solidifies when it reaches the cold
of space, becoming ‘snow’.
• These eruptions resemble geysers on the Earth such as Old Faithful.
However, the liquid is not water, it is sulfur or sulfur dioxide.
• Io’s plume eruptions release 100,000 tons of material every second:
enough to re-cover 1000s of km2 in a matter of weeks.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eruptions and Jovian Magnetosphere
• Most of the material from the eruptions just rains back down on Io, but
about 0.01%, 10 tons per second, actually escapes into space.
• SO2 is quickly broken up into S and O atoms and ions by the action of
UV light and impacts from other magnetospheric particles.
• Hence part of Jupiter’s magnetosphere is due to eruptions from Io.
• This Galileo image shows
sunlight being scattered from
the plume of the volcano
Prometheus (the bright spot).
• The diffuse yellow glow is due
to scattering of sunlight by
sodium atoms from eruptions
which form a vast cloud around
Io.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Are the plumes from volcanic vents?
• Galileo images of Io showed familiar volcanic eruptions to the Earth:
fire fountains, lava flows and lakes.
• But these were all silicate lava (normal molten rock) events: not molten
sulfur or SO2. What about the plumes then?
• We believe that the plumes occur when a lava flow meets a deposit of
frozen SO2 on the surface. The SO2 explosively heats up and vaporizes.
• We see some similar events when lava flows meet ice or water on
Earth.
• Io’s gravity is so low however that the plumes can reach great heights.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Sulfur Cycle
•
This figures illustrates the role of sulfur in Io’s surface coloration.
1. Sulfur gas (S2) is ejected from surface deposits.
2. Sulfur gas lands on
the surface, solidifies,
and re-arranges its
chemical structure
into red S3 and S4
molecules.
3. Much later, the atoms
further re-arrange into
the most stable
configuration, yellow
S8.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tvashtar Volcanism
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Titan
“It is a riddle wrapped in a mystery inside an enigma”
- Winston Churchill, 1939
(on the subject of Soviet intentions in WW II)
Until January 2005, the surface of Titan was arguably the most enigmatic
‘planetary’ surface in the entire solar system, representing the largest
unexplored, unmapped area. Cassini/Huygens has now changed all that!
Picture credit: NASA/JPL Voyager
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Titan Overview
• We now leave the Jovian system and come to Saturn’s giant moon
Titan.
• Titan has a density of 1.9 g/cm3, much like Ganymede and Callisto.
• Titan was originally believed to be the biggest moon in the solar
system, (hence the name) but we later discovered that the visible disk is
just the orange-colored atmosphere, not the surface.
• The atmosphere was later found to be immense: much bigger than the
Earth’s, which caused the misleading size estimate.
• The surface of Titan is completely obscured at most visible
wavelengths by the ubiquitous orange-brown haze, however we shall
see that there are tricks we can use to see in from outside.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Titan Atmosphere: Background
• In 1944 Gerard Kuiper discovered emission features of methane gas
from the atmosphere in the infrared spectrum.
• Close analysis of the methane lines showed that the gas was in contact
with another gas which was much more prevalent.
• At the time, nitrogen (N2) was guessed to be the mysterious unseen
gas, but little further progress was made until the 1970s.
• By the time of the Voyager Saturn encounter, several hydrocarbon
gases had been discovered, but the major component of the atmosphere
was still unknown.
• Even worse, the surface pressure was unknown: estimates ranged
widely from a few millibars to 10s of bars.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Voyager 1 Titan Encounter
• Voyager 1 made a close fly-by of Titan in November 1980, the only
spacecraft visitor prior to Cassini arrival in mid-2004.
• Voyager 2 did not pass close to Titan, as it was deflected to Uranus and
Neptune.
• The Voyager 1 images were disappointing: they were not able to reveal
much in the way of atmospheric features.
• However, the technique of radio occultation was used to greatly
improve our estimate of the atmospheric temperatures and pressures.
• In this technique, the spacecraft flies behind the moon as seen from the
Earth, and scientists ‘listen’ to the radio signal as it slowly fades through
the atmosphere, then is blocked by the surface, before re-emerging on
the other side. In this way, the atmospheric density profile is measured.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Size of Titan’s Atmosphere
• By assuming a mainly nitrogen composition and using the density
profile measured by radio occultation, a surface pressure of 1.5 bar was
determined.
• Titan therefore has a more massive atmosphere than any terrestrial
planet except Venus.
• The surface pressure of 1.5 bar is not much higher than the Earth’s;
however, the gravity is much less. So, it takes much more atmosphere to
exert the same surface pressure.
• Calculations show that there is ten times as much atmosphere above a
square meter on Titan as above a square meter on the Earth.
• The atmosphere also stretches ten times further into space.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Titan Infrared Spectrum
• The infrared spectrometer on Voyager 1 was able to provide detailed
information the composition of gases in the atmosphere.
• Many of the gases are hydrocarbons (C and H) species; some are
nitriles (C, H and N) but there are few oxygen species.
Figure credit: Andrew Fortes/UCL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Table of Titan
gases
• Is this atmosphere like the
Earth’s in terms of
composition?
• Could we breathe on Titan?
• What gases are found in the
Earth’s atmosphere but not on
Titan.
• Why?
Table credit: NSS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric Composition and Chemistry
• Titan’s atmosphere is now known to be largely nitrogen (like the Earth),
with methane and possibly argon as the most significant minor gases
(unlike the Earth).
• The nitrogen and methane are broken up in the upper atmosphere into
C, H and N atoms and ions, by the action of:
• solar UV radiation (‘photodissociation’)
• bombardment by charged particles from Saturn’s magnetosphere
• cosmic rays
• These fragments re-combine to form the hydrocarbons and nitriles
detected in the infrared.
• Oxygen must come from water, probably supplied by comets. With some
O atoms present in the mix, formation of amino acids (precursor to life)
becomes possible.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Smog On Titan
• This image shows a Voyager image of the ‘limb’ of Titan: the edge of the
planet where we see the atmosphere highlighted against space. This
image has been greatly enhanced to bring out the faint contrast detail.
• The ‘detached’ blue haze
layer lies several hundred km
above the main haze layer.
• HCN molecules can link
together in a chain called a
polymer.
• The reddish brown haze we
see lower down is probably a
mixture of HCN polymer, and
other condensed organic
compounds.
Picture credit: NASA/JPL Voyager
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
CassiniHuygens
• Cassini-Huygens was
launched in 1997 and
arrived at Saturn in 2004,
after a flybys of Venus,
Earth and Jupiter.
• Cassini has since
performed over 30 orbits of
Saturn, photographing all
the major moons and
making many discoveries.
• Cassini carries 12
scientific instruments.
Figure credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Multiple haze layers
• This image dramatically
shows the improvement in
detail seen by Cassini in
2004, compared to Voyager
24 years earlier (previous
slide).
• Many individual haze
layers are now seen,
showing a complex
situation which must involve
not just chemistry, but also
vertical and horizontal wind
movements to cause layers
to separate.
Picture credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Chemical Evolution of Titan’s
Atmosphere
• Titan’s atmosphere is constantly evolving, yet stays the same! How?
• Methane is constantly being destroyed by the photochemical reactions
in the upper atmosphere. Once CH4 is broken into pieces, some of the
hydrogen escapes to space.
• The remaining pieces now have proportionately more carbon compared
to hydrogen than before, and molecules such as C2H2 and C3H8 result.
• The atmosphere is cold enough that these molecules can condense
and form liquid hydrocarbon ‘seas’ or ‘lakes’ on the surface.
• Other atmospheric compounds such as CO2 and the other nitriles can
actually solidify into ices.
Dr Conor Nixon Fall 2004
ASTR 330: The Solar System
Atmospheric Evolution (contd)
• The solid ices would sink rather than float on the hydrocarbon sea,
because their densities are greater than the liquids.
• Titan is evolving, but very slowly due to the extreme cold. Very likely,
we are seeing the same conditions as 4 billion years ago.
• We have evidence that this evolution is occurring. The form of methane
containing the heavy isotope of hydrogen (CH3D) is enhanced over the
normal form of methane (CH4): exactly as we would expect if the lighter,
normal hydrogen isotope (‘H’) is gradually escaping, allowing the heavier
isotope (‘D’) to accumulate.
• Note that Titan’s surface is only 94 K - far too cold for liquid water.
Water will be almost totally lacking from the atmosphere, excepting any
that drifts in from cometary debris.
• However, scientists now know that methane can condense into clouds
and ‘rain’ out under certain conditions.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
South Polar Clouds
• The sequence of three Cassini images below show how an initial image
can be improved by the application of specialist techniques such as
‘stacking images’ (center) and edge-sharpening (right).
• Broad geographic differences can be seen near the top: light and dark
terrains, and right over the south pole, clouds which change on
timescales of just a few hours.
Picture credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric
Structure
• This figure summarizes
all the processes we
have discussed in
Titan’s atmosphere:
• photodissociation
of molecules
• haze formation
• chemical reactions
• methane clouds
and rain.
Figure credit: Andrew Fortes/UCL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Impact Craters
• Several impact craters have now been confirmed on Titan, by using
both special camera filters, and also radar to peer through the hazy ‘veil’.
• The view (right) is a radar
reflection map revealing a
large (60 km) impact crater.
• Brighter areas indicate
rougher terrain: note how
the ejecta blanket is rougher
than the surroundings.
• The darker ‘plains’ also
seem to show linear ‘dune’
features…
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Titan Dunes and Channels
• Here is another radar view of Titan, showing much different terrain.
• The dark linear markings have been dubbed ‘cat scratches’ and
appear to be wind-blown dunes, perhaps of dirty ice crystals.
• We also see an apparent
drainage system in the
lower right, with channels 12 km wide and up to 200 km
long.
• These seem to be
associated with a circular
basin feature 450 km in
diameter and have partly
eroded the basin rim.
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanoes?
• The riddle of Titan’s
methane supply (to
make all that haze) may
finally have been solved
by Cassini’s VIMS
instrument.
• This feature, dubbed the ‘snail’ may
be a cryovolcano (a volcano of ice
lava).
• This may be how fresh methane
escapes from inside the moon to resupply the atmosphere,
compensating for destruction by
sunlight.
Picture credit: NASA/JPL/University of Arrizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Huygens
• The Huygens probe was carried on Cassini for three orbits of Saturn,
before being released on a collision trajectory with Titan.
• The probe coasted in sleep mode
for 22 days, and then woke up 15
minutes prior to entering Titan’s
atmosphere.
• Huygens was slowed by a heat
shield and then two chutes, and
descended through the
atmosphere for 2.5 hrs, finally
landing on the surface.
• The science data was relayed to
the orbiter, and thence back to
Earth.
Picture credit: NASA/ESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Huygens Descent and Landing
• The Huygens landing of
January 2005 was the first ever
landing in the outer solar system,
and the first landing on the Moon
of another planet (other than
Earth).
• Despite some technical glitches,
Huygens was a great scientific
success, and made many
important in situ measurements
including:
• winds
• gas composition and isotopes
• aerosol measurements
• surface characteristics
Picture credit: NASA/ESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Huygens Descent Images
• This image
mosaic has been
reconstructed
from 30 individual
snapshots taken
by the Huygens
cameras looking
in different
directions during
the descent;
ranging from 13 to
8 kilometers in
altitude and
showing a 30 km wide area.
Picture credit: NASA/ESA/University of Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Reaching the surface
• Left images from Huygens show the view from 16 km (top) and 8 km
(bottom). The upper image shows apparent drainage channels reaching a
shoreline of some type.
• The right-hand image
shows the view from
the surface.
• Icy pebbles are seen
in the foreground, which
are 10-15 cm across.
• Work continues to
combine Huygens data
with Cassini results to
assemble the ‘big
picture’ of Titan.
Picture credit: NASA/ESA/University of Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Triton
• This image of
Neptune’s moon
Triton (not to be
confused with
Titan!) must
have been
taken by which
spacecraft?
• The mottled
surface in the
upper part of the
image is called
‘cantaloupe
terrain’ due to its
appearance.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Triton - Overview
• Triton at 2705 km diameter is just over half the size of Titan.
• Triton orbits Neptune the ‘wrong way’ - in the opposite sense to the
motion of the planets around the Sun, and in which most moons around
their parent planets.
• Hence, we suspect that Triton was captured by Neptune’s gravity,
rather than forming with the planet. Triton must have formed as an
independent planetesimal, much like Pluto.
• However, Triton has been able to synchronize its rotation by tidal forces
so that the same side always faces Neptune.
• Triton’s density of 2.1 g/cm3 indicates an ice-rock mixture, but with
more rock than ice. In both density and diameter, Triton is very similar to
Pluto.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Triton’s Surface
• Spectroscopy from the Earth has revealed the signature of methane,
carbon dioxide, carbon monoxide, and nitrogen ices on the surface.
• Triton has no large impact craters and few small ones. Its surface
therefore appears to be very young, perhaps as young as Europa.
• Triton is quite reflective, so that it absorbs little of the faint solar
radiation reaching its surface. It is therefore extremely cold: 37 K.
• The volatile ices we have mentioned (CO, CO2, CH4, N2) overlie a
frozen water ice crust.
• The surface seen by Voyager is mostly a flat plain, with some dark
streaks. It appears to be a polar cap around the south pole.
• The dark material seems to be blown around by a thin wind.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Triton - Atmosphere
• Triton’s thin atmosphere has a pressure of just 16 microbar (16x10-6
bar). However, the atmosphere was not devoid of activity.
• The Voyager cameras photographed haze and clouds. The haze is
probably produced in a similar way to the Titan haze, and much is
already condensed out on the surface.
• The most interesting features were dark plumes of material rising from
the surface and flowing downwind.
• These ‘cryo-geysers’ show a similar
appearance to campfires on the Earth.
• We do not yet know what causes
them.
Picture credit: NASA/JPL
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Largest Moons, Smallest Planets
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary: Galilean Satellite Interiors
Picture credit: NASA/JPL Galileo SSI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary Galilean Satellite Surfaces
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. Compare Io to Europa. Which is (a) denser (b) brighter?
2. What causes the massive volcanic activity on Io?
3. Why can Io not synchronize its rotation to minimize tidal forces?
4. What happened to most of Io’s volatiles?
5. Which gases and compounds have been detected on Io? What causes
the different hues?
6. Why do we see fewer craters as we traverse the Galilean moons from
Callisto to Io?
7. What causes an Ionian plume eruption?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. What are the two major components of Titan’s atmosphere?
9. How was the surface pressure measured?
10. How are hydrocarbons and nitriles formed in Titan’s atmosphere?
11. What might we expect to find on the surface of Titan?
12. Which space probe is going to land on Titan? When?
13. What similarities are there between Triton and Pluto?
14. What do the orbital characteristics of Triton lead us to believe about its
origin?
15. What ices do we see on Triton’s surface?
Dr Conor Nixon Fall 2006