Francis Nimmo

1
ES 290Q OUTER SOLAR SYSTEM

• Francis Nimmo

Io against Jupiter, Hubble image, July 1997
2
Last Week

• Galilean satellites
• Tidal deformation is dominant process on icy
  satellites
• There may have been initial variations in
  composition and structure due to lateral nebular
  gradients
• Subsequent histories determined by tidal
  evolution
• Oceans are common
• Various sources of stress (tidal, NSR, freezing
  etc.)
• This week Saturnian/Uranian satellites

3
Titan and Other Satellites

• Titan (largest moon of Saturn)
• Atmosphere
• Surface
• Interior
• Other Satellites of Saturn
• Cassini and Huygens
• Uranian and Neptunian satellites

4
Why is Titan important?

• It has a thick atmosphere (unique amongst
  satellites)
• Large (Mercury-size)
• Optically thick haze surface hard to image
• Astrobiologically interesting (hydrocarbons/organi
  cs)
• Methane hydrological cycle predicted
• Current exploration Cassini/Huygens

M E T D R. T H I P
Titan
5
Basic Parameters

• Only about 1 of the surface has been imaged at
  high resolution
• Limited data on magnetic field, MoI etc. as yet!

Rp is planetary radius, 71492 km for Jupiter,
60268 km for Saturn
6
Why an atmosphere?

• Titan is the only satellite to have a significant
  atmosphere. Why?
• Seems to be a combination of three factors
• Local nebular temperatures sufficiently cold that
  primordial atmosphere was able to form (Saturn is
  twice as far from Sun as Jupiter, and is less
  massive)
• Titans mass sufficiently high that it was able
  to retain a large fraction of this original
  atmosphere (and later cometary additions) (Jeans
  escape)
• Surface temperature warm enough to prevent some
  volatiles (e.g. N2) freezing out (c.f. Pluto,
  Triton)

Haze layer
7
Atmospheric Composition

• Surface pressure 1.5 bar, temperature 94 K, total
  atmospheric mass twice that of Earth (where
  does this number come from?)
• Obtained from UV/IR spectra, radio occultation
  data and Huygens
• Various organic molecules at the few ppm level
• Haze consists of 1 mm particles, methane
  condensates plus other hydrocarbons (generated by
  photolysis of methane)
• Solar system CN ratio is 4-201. On Earth, most
  of the C is locked up in carbonates where is the
  C stored on Titan?

8
Jeans Escape

• Escape velocity ve (2 g R)1/2 (wheres this
  from?)
• Mean molecular velocity vm (2kT/m)1/2
• Boltzmann distribution negligible numbers of
  atoms with velocities gt 3 x vm
• Nitrogen N2, 94 K, 3 x vm 0.7 km/s
• Hydrogen H2, 3 x vm 2.6 km/s
• Titan ve2.6 km/s H2 escapes, N2 doesnt (much)
• A consequence of Jeans escape is isotopic
  fractionation heavier isotopes will be
  preferentially enriched (and now we have the
  observations)
• Nitrogen N2, 168 K (Callisto), 3 x vm 0.94 km/s
  so why doesnt Callisto have an atmosphere too?

9
Isotopic Measurements

• Best constraints are from Huygens measurements
  (see Science 308, Nature 438)
• 14N/15N170-210 c.f. 270 for Earth enrichment
  in heavy N2 suggests Titan lost 80 of its
  atmosphere
• 12C/13C 80, similar Earth, Jupiter, Saturn
  outgassing of methane outpaced atmospheric
  methane loss
• 40Ar 7 ppm 1 outgassing efficiency (why?)
• 36Ar not detected suggests N2 arrived as NH3
  (why?)

• Methane DH ratio about 6 times that of Jupiter
  and Saturn. Half of this is due to Jeans
  fractionation the remainder is probably due to
  cometary additions to the atmosphere (comets have
  higher DH than solar)

10
Atmospheric Chemistry

• Methane gets photodissociated and H2 is lost
  (why?)
• Reactants e.g. ethane will condense and fall to
  the surface
• These two effects mean that the lifetime of
  methane in the present atmosphere is 107 yrs
• So there must be something which is continually
  resupplying methane to the atmosphere
• One suggestion was that this source was a
  methane/ethane ocean at the surface (caused by
  rain-out of the condensing species)
• Methane liquefies at 90.6 K, ethane at 101 K,
  c.f. surface temp. 94 K
• There are other possibilities e.g. comet
  delivery, outgassing
• Atmosphere is reducing because of lack of oxygen.
  Where is the oxygen? Its locked up in solid H2O
  at the surface (temperature).
• This is why theres little CO2 but CH4 instead

11
Atmospheric Processes
Photodissociation
Hydrogen escapes
Ethane condenses at 101 K Reactions produce more
complex organics
Clouds plus organic haze
Methane recharge
Organic drizzle
After Coustenis and Taylor, Titan, 1999
Ethane etc. ponds?
Underground aquifer?

• Theories suggested ethane ocean could 0.5-10km
  deep
• But current observations find very little
  evidence for a global ocean (see later)

12
Atmospheric Structure

• At lowest temperature (tropopause) all
  constituents except N2 are condensed (clouds)
• For an adiabatic atmosphere we have dT/dzmg/Cp
  (derived in next weeks lecture)
• For an N2 atmosphere, m0.028 kg, Cp3.5R30 J
  K-1 mol-1
• So the lapse rate is 1 K/km
• Temperature increases above tropopause due to
  incoming solar radiation
• Particulate haze makes direct observations of
  clouds hard

Particulate haze extends to 300 km
Lapse rate 1 K/km
From Owen, New Solar System
13
Clouds

• Clouds lie beneath the haze layer, at 10-20km,
  and are mainly methane crystals (bright)

Predominance of clouds near S pole not yet
explained but may be due to local convective
columns driven by small changes in surface
temperature.
Keck Adaptive Optics images, from Brown et al.
Nature 2002
450km
Cassini image of clouds near South Pole
14
Titans Surface

• Major questions prior to Cassini
• Is there a surface (methane/ethane) ocean?
• Is it geologically active?
• How old is the surface?

15
Cassini

• 6 tonnes, 2 bn, launched in 1997, planned from
  1985
• Note the absence of scan platform (so what?), and
  the reaction wheels
• Trajectory included Venus and Earth flybys, and
  will flyby Titan 44 times

16
Instruments

• Most interesting one is the radar (uses the same
  system as the communications radio whole
  spacecraft has to reorient itself!)
• Is producing images of the surface at 1km
  resolution in 100km wide swaths bright/dark
  corresponds to rough/smooth
• Also does altimetry, 25km spacing, and measures
  backscatter

55km
Approx. altimetry footprint
Schematic of radar coverage. Two side-looking
image swaths and a central altimeter pulse.
Minimum altitude of spacecraft is 1000km. From
Elachi, Proc. IEEE, 1991
Left-hand image is of Ganymede with resolution
(1.3 km/pix) comparable to Cassini radar
resolution. Right hand image is from Galileo,
75m/pix
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Instruments (contd)

• Although Titans atmosphere is opaque at most
  wavelengths, there are transparent windows (see
  arrows) in the near infra-red (NIR) which allow
  the surface to be viewed
• Both ground-based telescopes and the Cassini
  imaging system can see Titans surface using
  these wavelengths

• Reflectance in different wave-bands can be used
  to obtain crude compositional information
  (spectroscopy)
• Earth-based spectroscopy indicates that Titans
  surface is predominantly water ice

18
Huygens

• Probe launched on Dec 25, 2004
• Communication problems!
• Main chute was jettisoned to prevent probe
  falling too slowly (and freezing)
• Survived for gt 1 hr on surface
• Instruments
• Imaging system
• Wind measurements
• Aerosols
• PT sensors
• Surface package
• GCMS

LeBreton et al. Nature 2005
19
Huygens results

• Landed close to bright/dark terrain boundary
• Penetrometer suggested crème brulee surface
• Spectra suggest ice surface
• Descent imager captured dramatic channel features
• Methane(?) hydrological cycle, possibly sapping
  channels similar to Mars
• What is the origin of the bright/dark contrast?

Tomasko et al. Nature 2005
20
Huygens results (contd)

• Surface shows rounded cobbles similar to those
  seen in rivers on Earth
• Cobbles are made of ice
• Illumination is orange (why?) and equivalent to
  10 mins. after sunset on Earth
• Sun is ten times smaller than on Earth, and as
  strong as a car headlight at 150m

Pebbles are 15cm across

• Shadows are subdued because 90 of the light is
  scattered (indirect)

21
Cassini Results

• Two kinds of terrain bright dark (in both
  radar and NIR images)
• Correlation between two datasets not understood

NIR
RADAR
Image appx. 600km across Note presence of
channels and cat scratches (dunes?)
22
Cassini Results (contd)
Few impact craters imaged so far (why? So what?)
Radar
400 km, 8S, 215 W
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Is there any evidence for surface liquid?

• Channels are clear evidence for hydrological
  cycle
• No evidence so far from Cassini for specular
  reflection (glints) at either radar or optical
  wavelengths
• Some evidence for glints from Earth-based radar
  (Campbell et al. Science 2003) but could be due
  to smooth ice
• Some dark features could be lakes (?)
• Not yet clear what the origin of the bright
  dark material is possibly clean ice overlain by
  organics

diffuse
specular
Surface roughness radar wavelength (13 cm)
24
Cassini Results - Summary

• Very little is yet understood!
• Surface is predominantly icy with organic coating
• Definite evidence of liquid flow on surface,
  little evidence of long-live bodies of surface
  liquid
• Geologically active surface dunes, fluvial
  erosion which has presumably removed impact
  craters
• Little good evidence for tectonic features to
  date
• Little convincing evidence for cryovolcanism or
  volcanism (despite JPL claims to the contrary)
• Nature of bright / dark terrain dichotomy not
  well understood (though bright terrain appears to
  be high maybe it has been washed clean of dark
  organic contaminants by methane rain?)

25
Interior Structure

• Essentially unknown right now density
  constraint
• Cassini will help things
• Main questions
• 1) Is it differentiated? (Ganymede vs. Callisto)
• 2) Is there an ocean? (Why will detecting an
  ocean be much harder at Saturn than at Jupiter?)
• 3) Are there volatiles (other than water) present
  at depth?

• Volatiles?
• Two main ones are CH4 and NH3
• Clearly present in the atmosphere, but may also
  be present at depth other Saturnian satellites
  are inferred to have them on the basis of recent
  geological activity
• Were they stable during Titans formation? Quite
  likely, but depends on poorly known details of
  nebula

26
Volatile Effects

• Ammonia has a dramatic effect on the melting
  temperature of water ice much easier to get
  oceans
• Ammoniamethane ratio 11 in solar nebula

0.2
ice
0.4
0.6
Ice 5 NH3
Pressure, GPa
0.8
1.0
180
230
280
Temperature, K
After Grasset et al., Planet Space Sci., 2000

• Methane will form clathrate structures with water
  of the form NH3.6H2O. These structures are stable
  up to at least 10 GPa (Loveday et al., Nature
  2001) and provide an efficient way of storing
  large volumes of NH3 in the subsurface. Similar
  clathrates are found on Earth.

27
Possible Structures

• Undifferentiated
• Pro distant from Sun and Saturn, no likelihood
  of tidal heating
• Con incorporation of volatiles makes melting
  easier
• Differentiated but no ocean
• Pro hard to avoid differentiation (Callisto?)
• Con hard to freeze ocean completely if NH3
  present
• Differentiated with ocean
• Pro likely end state if NH3 present
• Con no tidal heating (c.f. Ganymede),
  dissipation may create problems
• How might we test these models?

28
Two Afterthoughts

• Why is Titan so exciting?
• One reason is that it may in some respects
  resemble the earliest Earth, before life was
  established. Obviously there are differences
  (e.g. temperature) but Titan may be the best
  example of what the primordial soup (more
  accurately, gazpacho) which gave rise to
  terrestrial life looked like
• We are seeing its surface for the first time!

29
Mid-sized satellites
30
Rogues Gallery
31
Common Themes

• Tidal heating and orbital evolution (Peale Annu.
  Rev. Astron. Astrophys. 1999 is a good reference)
• Role of volatiles (ammonia, methane)
• Size-related effects
• Impact crater populations and effects
• Effect of distance from primary
• Lack of simple explanations . . .

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Things to Notice

• Jupiter has 4 large (gt1500 km) moons, Saturn 1,
  and Uranus and Neptune none. Why?
• Neptune appears to be moon-poor in general. Why?
• All are synchronous, except Hyperion (chaotic)
• Densities are all close to 1 g/cc, suggesting
  mainly volatile ices (see next slide). Uranian
  satellites are denser.
• Uranus satellite densities increase (roughly)
  with distance. Why?
• Several of the periods are close to (or actually
  in) resonance e.g. Mimas-Tethys, Iapetus-Titan.
  May have had significant effects earlier in
  history.
• Uranian system has no resonances (at present day)

34
Eccentricity Damping

• Several of the satellites have eccentricity
  damping timescales t much less than the age of
  the solar system

Here we are assuming Q100 figures from Dermott
et al. Icarus 1988

• This is problematic either their eccentricities
  were recently excited, or the damping timescales
  (and thus the assumed interior structures) are
  incorrect. See later.

35
Densities/Radii
From Morrison et al., in Satellites, 1986

• Model density increases with increasing radius
  (why?)
• Saturnian satellites are probably gt60 ice
• Uranian satellites are denser on average, and
  Triton,Pluto and Charon are denser again (why?)

Theoretical lines
Condensation sequence (Week 1) favours CO, N2
(volatile) at high temps, CH4, NH3 (ice-forming)
at lower temps. But if cooling is too rapid, CH4
and NH3 may not have time to form (kinetics). So
where cooling is slower, more ices form,
resulting in lower overall density.
36
Albedos

• Callisto and Uranian satellites are dark,
  Saturnian satellites bright (except parts of
  Iapetus)
• If anything, albedo decreases with radial
  distance (why?)
• Uranian satellites are denser on average than
  Saturnian

37
Cratering and Ages

• Cratering rate increases with decreasing distance
  to primary (grav. focusing), e.g. x2 at Rhea, x20
  at Mimas compared with Iapetus (Smith et al.
  Science 1982)
• Size of crater caused by particular object
  increases with decreasing distance to primary
• So observed crater density is a strong function
  of distance to primary as well as surface age
• This makes even relative cratering ages hard to
  determine and model-dependent, never mind
  absolute cratering ages (see Zahnle et al. Icarus
  2003)
• A consequence of gravitational focusing is that
  objects near the primary may have been disrupted
  once or several times by impacts (Mimas,
  Enceladus, Miranda, Ariel)

38
Data and Models
Model impact rate

• Note that model impact rate decreases with
  increasing distance high crater density can
  still mean young surface if the satellite is
  close to the primary
• Considerable scatter in observed crater densities

Observed crater density
Only highest densities are plotted here
Open circles denote extrapolations
39
Non-synchronous rotation (?)

• The satellites of Uranus and Neptune are expected
  to show large (6-35 times) variations in crater
  density from leading to trailing hemisphere if
  they have rotated synchronously for 4 Gyrs
• None of them show such a signature. Why not?
• Possible that large impacts (20 km diameter
  crater) are sufficient to break the synchronous
  lock (see Chapman and McKinnon, in Satellites,
  1986)
• There should still be an asymmetry in recent
  (small) impacts, but these are not visible with
  Voyager images
• Note that Iapetus does show a leading/trailing
  hemisphere asymmetry in albedo, suggesting that
  it is synchronously locked at the present day

40
Absolute Ages (?)

• Uncertainties in absolute fluxes mean surface
  ages are very uncertain.
• Iapetus, Oberon, Titania and Umbriel are
  undoubtedly very old
• Mimas and Enceladus are at least slightly, and
  perhaps much, younger
• Parts of Miranda are very young
• Several satellites show a wide spectrum of ages
  (Enceladus, Rhea, Ariel)

From Zahnle et al. Icarus 2003
41
Activity (?)

• Tectonic activity is relatively easy to infer
• Cryo-volcanic activity is much less easy to
  identify (e.g. Galilean satellites post-Galileo)
• Crater counts provide relative levels of activity
• Crater relaxation is an indication of increased
  heat flux

670 km
scarp
Crater counts showing surface age
diversity Kargel and Pozio Icarus 1996
Close-up of Miranda rift, showing large fault
scarp (5km high)
42
Expansion(?)

• As with Galilean satellies, almost all tectonic
  activity appears to be extensional why?
• If satellites started cold (slow accretion) then
  release of radiogenic heat could generate heating
  and expansion (1)
• Tidal heating could also similarly generate
  extension
• Alternatively, as an ocean freezes and converts
  to less dense ice I it will generate extension
  (NB this does not work if it forms higher density
  ice phases, so only applicable to small
  satellites (Plt200 MPa))

43
Volatiles

• There is currently no direct evidence of ices
  such as ammonia or methane on the satellites of
  S,U,N
• But there are reasonable theoretical grounds for
  expecting them to be there
• Likely nebular temperatures consistent with their
  formation
• Presence of ammonia (especially) helps explain
  observed geological activity
• Titans atmosphere does have N2 and methane
• Methane forms a clathrate structure with H2O when
  the latter is present at the correct P,T
  conditions. Such clathrates may form a reservoir
  e.g. for Titans atmosphere.

44
Ammonia
From Kargel, in Solar System Ices, 1998

• A mixture of ammonia and water doesnt completely
  freeze until 178 K
• As freezing continues, the remaining liquid
  becomes more ammonia-rich
• The low temperature of this liquid may prevent
  convection (DT small)

This ammonia-rich liquid is usually denser than
pure ice, but less dense than NH3.2H2O, so that
it is likely to be able to ascend and erupt -
cryovolcanism
45
Saturn Observations
80 km diameter
Small (lt500 km), inactive
Small, active
Herschel
Mimas R196 km
Medium, inactive
Medium, active
46
Uranus Observations
Small (lt500 km), inactive
Small, active
Medium, inactive
Medium, active
47
Activity Summary
I
I
II
II
I
III
H
E
I
M
R
P
D
T
T
I
II
I
II
II/III
M
U
A
T
O

• Metamorphic grade of planet, based on cratering
  observations and tectonic history (Johnson, in
  Solar System Ices, 1998)
• IUnmodified, IIIntermediate, IIIHeavily
  modified

III
I
P
N
T
48
Tidal Heating (1)

• Enceladus is small but active, and currently in a
  resonance with Dione differential orbital
  expansion similar to Io (?)
• So likely that tidal heating is responsible, but
  details are unclear (Squyres et al. Icarus 1983).
  In particular why did Enceladus melt if Mimas
  didnt? (Mimas is in a 21 resonance with Tethys)
• Mimas is also puzzling because its eccentricity
  is high (how?) while at the same time it shows no
  sign of tidal deformation
• Ariel (also small and active) is not in a
  resonance now, but may have been (e.g. with
  Umbriel) in the past. How?
• The same also goes for Miranda (tiny and active).
  The fact that Mirandas orbit is inclined at 4o
  is also suggestive of an ancient resonant episode
  (Tittemore and Wisdom, Icarus 1989)
• As with Ganymede, orbital evolution may explain
  present-day features . . .

49
Tidal Heating (2)
Ariels orbit expands faster than Mirandas
because Ariel is so much more massive
31 resonance responsible for Mirandas
present-day inclination (?)

• Theoretical evolution of orbits (from Murray and
  Dermott c.f. Dermott et al. Icarus 1988)
• Note that various resonances may have been
  encountered on the way to the present-day
  configuration (e.g. MirandaUmbriel 31)
• Passage through resonance will have led to
  transient eccentricities and heating
• Note that diverging paths do not allow capture
  into resonance (though they allow passage through
  it), while converging paths do. This may help to
  explain why there are no examples of resonance in
  the Uranian system.

50
Other effects?

• Tides cant be the only answer e.g.
• Umbriel not resurfaced, though it likely went
  through resonances
• Titania is resurfaced, but no resonance has ever
  been identified
• Some of the resonances do not generate much tidal
  dissipation e.g. ArielUmbriel 21 resonance
• One suggestion is that inner bodies were
  catastrophically disrupted by impacts, and then
  reaccreted. The energy of this reaccretion might
  help to explain early activity.
• How then do we explain Mimas? Close in, but not
  active.
• What about gradients in the initial nebula? Might
  expect more geological activity at smaller
  distances, where more volatiles had time to
  condense.
• Not really borne out by observations (e.g.Umbriel
  v. Titania)

51
Enceladus

• Suspected to be active, because of lightly
  cratered terrain and proximity of Saturns E ring
  (lifetime 104 yrs)
• Suspicion abundantly confirmed by Cassini

• Hot-spot and outgassing centred on S pole why?

52
Enceladus (contd)

• S pole is tectonically deformed (tiger stripes)
  and young
• Could the tiger stripes be similar to double
  ridges on Europa? They appear to be hotter than
  the surroundings

• Heat output is 9 GW where from?

53
Other Oddities 1 Mirandas Coronae

• Roughly circular, large tectonic features with
  extensional faulting on their margins
• Topographic profiles suggest flexure and Te2 km
• What are they? Maybe upwellings, but no-one
  really knows . . .

480km
Whats the naming theme?
From Pappalardo et al., JGR 1997
54
Other Oddities 2 Iapetus Dichotomy

• Albedo varies from 0.5 (ice) to 0.05 from one
  hemisphere to the other
• The dark side is centred on and symmetrical about
  the leading hemisphere. Why? Two explanations
• 1) Impacts on Phoebe generate dust which
  eventually spirals in and impacts on the leading
  hemisphere
• 2) Dark material is produced internally and then
  concentrated on the leading hemisphere e.g. by
  impacts removing a bright frost covering
• What is the equatorial ridge?

Cassini, 2004
1400km
bright
dark
Voyager, 1981
55
Other Oddities 3 - Phoebe

• Small (D200km), dark, retrograde, eccentric
  (0.16) and far (215 Rp)
• Most likely a captured object (from where?)
• Albedo (0.05) comparable to dark side of Iapetus
• Where does the dark material come from?
• High-res images suggest dark and light layering

Cassini images
56
Conclusions

• There is a surprising amount of activity for such
  small satellites
• The energy source for this activity must be tidal
  heating (though the details are usually obscure)
• The presence of low-melting temperature species
  like ammonia is almost certainly required to
  allow the activity to happen, though there is
  little evidence of cryovolcanism
• Impacts have had significant effects in
  disrupting, spinning and eroding satellites
• Distance from primary seems to be a secondary
  control on satellite characteristics
• Extension is dominant

57
End of Lecture
Supplementary material follows
58
This graphic shows Cassini's path, or ground
track, as it crossed over the surface of
Enceladus near the time of closest approach
during the flyby on July 14, 2005. The ground
track is indicated by a yellow line, marked by
increments of 10 seconds before and after closest
approach. The spacecraft came within 175
kilometers (109 miles) from the surface of
Enceladus at closest approach. The red contour
encloses the region on Enceladus around the south
pole that is the approximate boundary of the warm
region, as measured by the composite infrared
spectrometer instrument on Cassini. As previously
announced, temperatures observed within this
region reached as high as 110 Kelvin (-260
Fahrenheit). As Cassini passed over the southern
polar terrain, its ion neutral mass spectrometer
and cosmic dust analyzer instruments detected
material coming from the surface of the moon. The
ion neutral mass spectrometer measured a large
peak in the abundance of water vapor at
approximately 35 seconds before closest approach
to Enceladus, as it flew over the south polar
region at an altitude of 270 kilometers (168
miles). The high rate detector of the cosmic
dust analyzer observed a peak in the number of
fine, powder-sized icy particles coming from the
surface approximately a minute before reaching
closest approach at an altitude of 460 kilometers
(286 miles). Analysis of this detection points to
the south polar region as the source of the
material. Results like these, pouring in from
various Cassini instruments, indicate the warm
south polar region and, in particular, the 'tiger
stripe' fractures straddling the south pole, as
the sources of heat, water vapor and small, icy
particles. Enceladus is a surprisingly active
moon. Why its south pole is the site of its
activity is a mystery. The Cassini-Huygens
mission is a cooperative project of NASA, the
European Space Agency and the Italian Space
Agency. The Jet Propulsion Laboratory, a division
of the California Institute of Technology in
Pasadena, manages the mission for NASA's Science
Mission Directorate, Washington, D.C. The Cassini
orbiter was designed, developed and assembled at
JPL. For more information about the
Cassini-Huygens mission visit http//saturn.jpl.na
sa.gov. For additional images visit the Cassini
imaging team homepage http//ciclops.org.
Image Credit NASA/JPL/Space Science
Institute

59
Keeler gap
70km across, Enceladus
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61
Measuring the Winds (from Earth!)

• This is a very clever technique, which involves
  serendipity (an unexpected binary star), adaptive
  optics, and some (fairly) simple theory
• The basic technique is to use an occultation
  (i.e. when a star (or a spacecraft) passes behind
  a planet as viewed from the Earth)

star

• The occultation has three effects
• The intensity of the light decreases (this is
  easy to measure)
• The angular deflection of the star increases
  (this is usually impossible to measure)
• The refraction leads to a central flash as the
  star passes behind the centre of the planet. If
  the atmosphere is not perfectly axisymmetric,
  other refraction effects are observed and can be
  used . . .

atmosphere
planet
Earth
62
Binary Occultation

• When the 20 Dec 2001 occultation of Titan was
  observed (using adaptive optics), it was realized
  the star was binary. Why was this helpful?

• 1) It allowed the angular deflection to be
  measured, and the location of the refracted
  starlight to be tracked
• 2) It allowed the changes in intensity to be
  measured very accurately (by referencing to the
  unobscured star)

Figures from Antonin Bouchez thesis, Caltech
63
Modelling
Atmosphere (schematic)

• The intensity and angular deflection of the
  refracted starlight allow the shape of a surface
  of constact refractivity (pressure) in Titans
  atmosphere to be established the atmosphere
  acts as a lens
• Assuming an isothermal atmosphere, variations in
  the height of a constant pressure surface have to
  be balanced by zonal (horizontal) winds
• The inferred winds are not symmetric about the
  equator this is probably due to seasonal
  variations in heating as a result of Titans
  obliquity (27o). Wind velocities are also high
  (100 ms-1)

Winds
Atmos. surface altitude varying with latitude
Resulting winds
64
. . . A reddish colour dominated everything,
although swathes of darker, older material
streaked the landscape. Towards the horizon,
beyond the slushy plain below, there were rolling
hills with peaks stained red and yellow, with
slashes of ochre on their flanks. But they were
mountains of ice, not rock . . . Stephen Baxter,
Titan
65
Size effects

• Radiogenic heat flux goes as R
• Cooling rate also decreases as R increases
• But tidal heating is more affected by e than R
• Central pressure 2GpR2r2/3 (why?)
• Ice converts from I-II at 200 MPa so critical
  radius for this conversion to occur is 800 km

Cooling of large satellite will lead to ice I-II
transformation, which causes large change
decrease in radius and thus global compression
cooling
66
Size Effects - Examples

• Compression effect of Ice I-II transformation may
  explain why Iapetus and Rhea (slightly larger)
  are less active than Dione and Tethys
  compression suppresses volcanism and extension.
  Some evidence for compressive features on Rhea.
• More difficult to explain the difference between
  Umbriel (inactive) and Titania and Ariel (active)
  
• Titania is dense, so less ice and more rock means
• May have evaded Ice I-II phase transformation
• More radiogenic heating, so more likely to be
  active
• Ariels activity requires a different explanation
  . . .

67
Radiogenic elements

• Chondritic heat production (present day) Hr3.5
  pW/kg
• Over 4.5 Gyr, this generates 1.8 MJ/kg
• C.f. water latent heat of fusion 0.33 MJ/kg
• In conductive equilibrium the temperature
  difference DT required to get rid of the
  radiogenic heat scales as

Where from?
Here k is the thermal conductivity (3 W/mK).
Note that DT scales as radius2.

• E.g. for a 500 km radius satellite at the present
  day, DT100 K borderline for melting water in
  interior
• Thermal expansion strain aDT 1 - quite a lot
• What complications affect this (simplified)
  analysis?

68
Ammonia (contd)

• Viscosity of erupted material likely to be
  comparable to basaltic-intermediate lavas
• Evidence for such cryovolcanic lavas is currently
  not very strong

1200km
Close-up view of Ariel showing flat-floored
graben. It has been suggested the flat floors are
due to cryovolcanic flooding.
From Kargel, in Solar System Ices, 1998
69
Two populations (?)

• Population I consists of largest craters, is
  associated with the heavy bombardment period, and
  has a slope ( -2) similar to populations on
  Ganymede and the Moon
• Population II consists of smaller craters, with a
  steeper slope, and post-dates the heavy
  bombardment
• Why the difference? Possibly II is debris from a
  disrupted satellite, which might explain the
  unusually steep slope

Cratering on Tethys
Plescia and Boyce Nature 1983
70
Example - Enceladus

• Looks like a miniature Ganymede, including
  relaxed craters and extensional faulting
• Wide variety of surface ages, some lt107 yrs
• May be the source of the E ring, which has a
  lifetime of only 104 yrs
• High albedo, perhaps suggestive of recent
  activity and frosting?

Tectonized crater
Extensional faulting
50km
Cassini, July 2004
71
Enceladus contd

• Early deformation could be due to initial
  freezing and expansion, but there has been much
  more recent activity
• Current eccentricity generates 0.1mWm-2,
  comparable to radiogenic, insufficient to account
  for activity
• Increase in e by 10 times would be sufficient to
  explain activity. What could have caused such an
  increase? Not clear current resonance with
  Dione insufficient
• Increase in eccentricity must be relatively
  recent eccentricity damping timescale 108 yrs.
• Mimas also presents a problem why does it show
  no signs of activity when its closer to Saturn?
• See Squyres et al., Icarus 1983 for a lucid
  discussion