Francis Nimmo

1
ESS 298 OUTER SOLAR SYSTEM

• Francis Nimmo

Io against Jupiter, Hubble image, July 1997
2
In this lecture

• Triton (largest moon of Neptune)
• Pluto/Charon
• Kuiper Belt
• Oort Cloud
• Extra-solar planets
• Where do we go from here?

Reminder computer writeup due this
Thursday! ESS250?
3
Neptune system unusual

• Uranus and Saturn both have interesting and
  diverse collections of moons
• But the Neptune system is almost empty apart from
  . . .
• Triton, which is retrograde (unique)

Neptune system (schematic)
Small, close moons
Triton (retrograde)
Nereid (small, eccentric, inclined, long way out)
Neptune
4
Where is Triton?
Jupiter
Neptune
Triton
No information on MoI single flyby at 40,000 km
(Voyager 2, 1989)
5
Tritons peculiar orbit

• It is retrograde almost unique, especially
  amongst large bodies. Why?
• Rotation is also synchronous (and retrograde)
• There are no other sizeable bodies in the system

159o
Neptune
29o
Triton
6
Whats it like?

• High albedo (0.85) so 38 K at the surface
  coldest place in the solar system
• Surface (based on terrestrial spectroscopy)
  consists of frozen N2 (at the polar cap), H2O,
  CO2, CO,CH4
• Thin (14 mbar) N2 atmosphere, hazes (presumably
  similar to Titans CN compounds generated by
  photolysis)
• Extreme seasonal variations
• Surprisingly geologically interesting for such a
  small and cold body

7
Chemistry and Composition

• At low temperatures characteristic of outer solar
  system, kinetics may mean C remains as CO not CH4
  (see Week 1) means less oxygen available to
  form water ice
• Predicted rock/ice mass ratio in this case is
  70/30 which gives a density of 2000 kg m-3,
  similar to that observed for both Triton and
  Pluto
• In hotter nebula, CO-gt CH4, oxygen then available
  to form water ice, rock/ice mass ratio 50/50,
  giving a density of 1500 kg m-3
• Detection of CO is also consistent with low
  temperatures during formation of Triton (and
  Pluto)
• Gives a clue as to where Triton formed

8
Seasonal cycles (1)

• Neptune has a period of 164 yrs and an obliquity
  of 29o
• Triton has an inclination of 21o and a period of
  0.016 yrs
• Tritons orbit precesses with a period of 688 yrs
• So the angle between Tritons pole and the Sun
  varies very widely (see diagram below)

29o
Neptune
21o
Triton
21o
Voyager observations
164 yrs
From Cruikshank, Solar System Encyclopedia
9
Seasonal cycles (2)

• At the time of the Voyager encounter, Triton was
  in a maximum southern summer
• Models suggest that N2 was subliming from the S
  pole and accumulating to the N
• These models also predicted winds flowing N from
  the S pole (observationally confirmed)
• Over 688 years, more energy is deposited at the
  equator than either pole
• So the existence of high albedo, presumably
  volatile deposits, covering most of the S
  hemisphere is embarrassing to the modellers

10
Tritons peculiar surface

• Very few impact craters -gt young (100 Myrs)
• Cantaloupe terrain
• Plains suggestive of cryovolcanism
• Tectonic features
• Geologically active!

Cantaloupe terrain
11
Active Geysers (!)

• Only recognized after the event
• Presumably powered by N2 (sublimates at 2o above
  mean surface temperature)
• Directions of dark streaks suggest winds blowing
  away from the pole (as expected)

100 km
Activity of this kind is unlikely to be able to
explain the absence of big impact craters, again
indicating that Tritons surface is very young
Particles falling out
8 km
Dark streak developing
12
Tectonic features
Scale bars are 2 km for Europa and 40 km for
Triton
The characteristic spacing of cantaloupe terrain
must be telling us something. Is it the signature
of thermal convection or is it some kind of
Rayleigh-Taylor instability? Salt domes on Earth
are examples of the latter, and generate similar
features.
Ridge morphology on Triton resembles that on
Europa (though widths are very different). Is a
similar kind of process at work on the two bodies?
13
Cratering Statistics

• Strong apex-antapex asymmetry
• Larger than predicted by models of NSR (!)
• May be partly caused by partial resurfacing (e.g.
  cantaloupe terrain)
• Not well understood

From Zahnle et al., Icarus 2001
14
Several puzzles and a solution

• 1) Why is Tritons orbit retrograde?
• 2) Why are there so few satellites in the system?
• 3) Why is the surface so young?

TRITON WAS CAPTURED

• 1) Collision and capture of an initially
  heliocentric body is essentially the only way to
  explain retrograde orbit
• 2) Tritons orbit will have adjusted following
  capture, sweeping up any pre-existing moons
• 3) Capture can occur at any time (and releases
  enormous amounts of energy when it occurs)

15
Hypothetical scenario
See e.g. Stern and McKinnon, A.J., 2001
An alternative is that capture occurred due to
gas drag. Why is this scenario less likely?
16
So what?

• Gigantic tidal dissipation (see next slide)
• Circularization explains absence of other bodies
• Collision explains Nereids orbit (small, very
  far out, high e and i due to perturbations as
  Tritons orbit circularized)
• Young surface age suggests (relatively) recent
  collision how likely is this?
• Improbable events can happen whats another
  example of an improbable event?
• Where did Triton come from? (see later)

17
Tidal Heating

• Orbit was initially very eccentric and with a
  large semi-major axis
• Tidal dissipation within Triton will have reduced
  both e and a and generated heat
• DT GMp/aCp 105 K ! (Wheres this from?)
• Capture resulted in massive melting
• Perhaps this melting caused compositional
  variations which allowed the cantaloupe terrain
  to form?
• Heating means differentiation almost inevitable

18
Internal Structure

• Density 2050 kg m-3, MoI unknown
• Chemical arguments suggest 70/30 rock/ice ratio
  (see earlier slide)
• Volatiles except H2O are assumed to be minor
  constituents of interior
• Assume differentiated due to tidal heating

ice
Hypothetical internal structure of Triton (see
e.g. McKinnon et al., Triton, Ariz. Univ. Press,
1995)
rock
iron
19
Comets and the Kuiper Belt
20
Comets and their Origins

• Two kinds of comets
• Short period (lt200 yrs) and long period (gt200
  yrs)
• Different orbital characteristics

ecliptic
Short period prograde, low inclination
Long period isotropic orbital distribution

• This distribution allows us to infer the orbital
  characteristics of the source bodies
• S.P. relatively close (50 AU), low inclination
  (Kuiper Belt)
• L.P. further away (104 AU), isotropic (Oort
  Cloud)

21
Short-period comets

• Period lt 200 yrs. Mostly close to the ecliptic
  plane (Jupiter-family or ecliptic, e.g. Encke)
  some much higher inclinations (e.g. Halley)
• Most are thought to come from the Kuiper Belt,
  due to collisions or planetary perturbations
• Form the dominant source of impacts in the outer
  solar system
• Is there a shortage of small comets/KBOs? Why?

From Weissmann, New Solar System
From Zahnle et al. Icarus 2003
22
Missing small comets(?)

• Effects of an impact depend on size of body being
  impacted
• Small bodies are more likely to fragment (why?)
• For Kuiper Belt objects, critical size above
  which fragmentation ceases is 100 km (Stern,
  A.J. 1995)
• This critical size will be apparent in
  size-frequency plots

Objects just smaller than the critical size will
not be replenished by fragmentation of larger
objects Objects larger than the critical size
will not be fragmented (and may even continue to
accrete slowly) Fragmented populations have slope
typically -3.5
Critical size
Slope -3.5
Freq.
Size
23
Kuiper Belt

• 800 objects known so far, occupying space
  between Neptune (30 AU) and 50 AU
• Largest objects are Pluto, Charon, Quaoar (1250km
  diameter), 2004 DW (how do we measure their
  size?)
• Two populations low eccentricity, low
  inclination (cold) and high eccentricity, high
  inclination (hot)

hot
ECCENTRICITY
cold
Brown, Phys. Today 2004

• Total mass small, 0.1 Earth masses
• Difficult to form bodies as large as 1000 km when
  so little total mass is available (see next
  slide)
• A surprisingly large number (few percent)
  binaries
• See Mike Browns article in Physics Today Apr.
  2004

24
Building the Kuiper Belt
From Stern A.J. 1996

• Planetesimal growth is slower in outer solar
  system (why?)
• Calculations suggests that it is not possible to
  grow 1000km size objects in the Kuiper belt with
  current mass distribution

Different lines are for different mean random
eccentricities
Solar system age
Growth time
Disk mass (ME)

• How might we avoid this paradox (see next slide)?
• 1) Kuiper Belt originally closer to Sun
• 2) We are not seeing the primordial K.B.

25
Kuiper Belt Formation
Early in solar system
Ejected planetesimals (Oort cloud)
Hot population
Initial edge of planetesimal swarm
J
N
S
U
18 AU
30 AU
48 AU
26
What does this explain?

• Two populations (hot and cold)
• Transported by different mechanisms (scattering
  vs. resonance with Neptune)
• Cold objects are red and (?) smaller hot
  objects are grey and (?) larger
• Hot population formed (or migrated) closer to Sun
• Formation and (current) position of Neptune
• Easier to form it closer in current position
  determined by edge of initial planetesimal swarm
  (why should it have an edge?)
• Small present-day total mass of Kuiper Belt for
  the size of objects seen there
• It was initially empty planetesimals were
  transported outwards

27
Binaries

• A few percent KBOs are binaries, mostly not
  tightly bound (separation gt102 radii)
  Pluto/Charon an exception. Why are binaries
  useful?
• How did these binaries form?
• Collisions not a good explanation low
  probability, and orbits end up tightly bound
  (e.g. Earth/Moon)
• A more likely explanation is close passage (lt1
  Hill sphere), with orbital energy subsequently
  reduced by interaction with swarm of smaller
  bodies (Goldreich et al. Nature 2002). Implies
  that most binaries are ancient (close passage
  more probable)
• Any interesting consequences of capture?

28
Long-period comets

• Periods gt 200 yrs (most only seen once) e.g.
  Hale-Bopp
• Source is the Oort Cloud, perturbations due to
  nearby stars (one star passes within 3 L.Y. every
  105 years). Such passages also randomize the
  inclination/eccentricity
• Distances are 104 A.U. and greater
• Maybe 10-102 Earth masses
• Sourced from originally scattered planetesimals
• Objects closer than 20,000 AU are bound tightly
  to the Sun and are not perturbed by passing stars
• Periodicity in extinctions(?)

29
Oort Cloud

• What happens to all the planetesimals scattered
  out by Jupiter? They end up in the Oort cloud
• This is a spherical array of planetesimals at
  distances out to 200,000 AU (3 LY), with a
  total mass of 10-102 Earths
• Why spherical? Combination of initial random
  scattering from Jupiter, plus passages from
  nearby stars
• Forms the reservoir for long period comets

Oort cloud (spherical after 5000 AU)
Earth
Saturn
Pluto
Kuiper Belt
100,000 AU
100 AU
1,000 AU
10,000 AU
1 AU
10 AU
After Stern, Nature 2003
30
Sedna (2003 VB12)

• Discovered in March 2004, most distant solar
  system object ever discovered
• a480 AU, e0.84, period 10,500 years
• Perihelion76 AU so it is probably not a KBO, and
  may be the first member of the Oort cloud
  detected
• Radius 1000 km
• Light curve suggests a rotation rate of 20 days
  (slow)
• This suggests the presence of a satellite (why?),
  but to date no satellite has been imaged (why
  not?)

31
Pluto and Charon

• Pluto discovered in 1930, Charon not until 1978
  (indirectly can now be imaged directly with HST)
• Orbit is highly eccentric sometimes closer than
  Neptune (perihelion in 1989)
• Orbit is in 32 resonance with Neptune, so that
  the two never closely approach (stable over 4
  Gyr)
• Charon is a large fraction (12) of Plutos mass
  and orbits at a distance of 17 Pluto radii
• Charons orbit is almost perpendicular to the
  ecliptic Plutos rotation pole presumably also
  tilted with respect to its orbit (i.e. it has a
  high obliquity)
• Pluto-Charon is (probably) a doubly synchronous
  system

32
Discoveries

• Neptunes existence was predicted on the basis of
  observations of Uranus orbit (by Adams and
  LeVerrier)
• Percival Lowell (of Mars canals infamy)
  predicted the existence of Pluto based on
  Neptunes orbit
• Pluto was discovered at Lowells observatory in
  1930 by Clyde Tombaugh (who looked at 90 million
  star images, over 14 years)

Blink-test discovery of Pluto

• Charon was discovered by James Christy at the US
  Naval Observatory in 1978. This was good timing .
  . .

33
A lucky coincidence

• Once every 124 years, Pluto and Charon mutually
  occult each other. Why is this important?
• Charon discovered in 1978 mutual occultation
  occurred in 1988
• This event allowed much more precise
  determinations of the sizes of both bodies

Plutos rotation pole
Plutos orbital path
View from Earth. Note that Charons orbit is
inclined to Plutos (and to the ecliptic). From
Binzel and Hubbard, in Pluto and Charon, Univ.
Ariz. Press, 1997
34
Pluto and Charon

• Plutos orbit a39.5 AU, orbital period 248
  years, e0.25, i17o , rotation period 6.4 days
• Charons orbit a19,600km (17 Rp), period6.4
  days, e?, i0o

35
Compositions

• Plutos surface composition very similar to
  Triton CH4 (more than Triton), N2, CO, water
  ice, no CO2 detected as yet
• Charons surface consists of mostly water ice
• Charon is significantly darker than Pluto,
  suggesting the presence of other (undetected)
  species

From Cruikshank, in New Solar System
CH4
CO
36
Plutos atmosphere

• It has one! 10 microbars, presumably N2
  (volatile at surface temp. of 40 K)
• First detected by occultation in 1988
  (perihelion)
• Atmospheric pressure is determined by vapour
  pressure of nitrogen (strongly temperature-depende
  nt)
• More recent detection (Elliot et al. Nature 2003)
  shows that the atmosphere has expanded (pressure
  has doubled) despite the fact that Pluto is now
  moving away from the Sun. Why?
• Possibly because thermal inertia of near-surface
  layers means there is a time-lag in response to
  insolation changes

37
Charons Eccentricity (?)

• Difficult to observe, but HST gives value of
  0.003-0.008
• Why is this important?
• What is its source?
• Cant be primordial (circularization timescale
  107 yrs)
• Cant be planetary perturbations (too small)
• Could be an as-yet unidentified companion
• Could be due to recent close encounter/collision
  with another KBO (probabilities are small)
• See Stern et al., A.J., 2003

38
Pluto/Charon Origins

• Compositional similarities to Triton suggest same
  ultimate source Kuiper Belt
• Plutos current orbit is probably due to
  perturbations by Neptune as N moved outwards
  (recall the 32 resonance)
• Charon is most likely the result of a collision.
  Clues
• Its orbital inclination (and Plutos rotation)
  strongly suggest an impact (c.f. Neptune)
• The angular momentum of the system (see next
  slide)
• Comparable size of two bodies also suggestive
  (c.f. Earth-Moon system)
• Are the compositional differences between Pluto
  and Charon the result of the impact?
• If correct, then neither Pluto nor Charon are
  pristine Kuiper Belt objects (e.g. tidally heated)

39
Angular Momentum
w
If Pluto and Charon were originally a single
object, we can calculate the initial mass m0 and
rotation rate w0 of this object by conservation
of mass and angular momentum
r1
Charon
Pluto
a
w
m2
m1
Here C0 and C1 are the moments of inertia C1
0.4 m1 r12 etc.
If we do this, we get an initial rotational
period of 2.1 hours. Is this reasonable? We can
compare the centripetal acceleration with the
gravitational acceleration
Grav. Acc. 0.67 ms-2
Centripetal acc. 0.85 ms-2
So the hypothetical initial object would have
been unable to hold itself together (it was
rotating too fast). This strongly suggests that
Pluto and Charon were never a single object the
large angular momentum is much more likely the
result of an impact.
40
New Horizons

• An ambitious mission to fly-by Pluto/Charon and
  investigate one or more KBOs (PI Alan Stern,
  managed by APL)

• Launch date Jan 2006, arrives Pluto 2015
• Powered by RTG (politically problematic . . . )
• Very risk-averse (almost every system is
  duplicated)
• Science limited by high fly-by speed (but we know
  very little about Pluto/Charon right now)

41
Extra-Solar Planets

• A very fast-moving topic
• How do we detect them?
• What are they like?
• Are they what we would have expected? (No!)

42
How do we detect them?

• The key to most methods is that the star will
  move (around the systems centre of mass) in a
  detectable fashion if the planet is big and close
  enough
• 1) Pulsar Timing
• 2) Radial Velocity

pulsar
A pulsar is a very accurate clock but there will
be a variable time-delay introduced by the motion
of the pulsar, which will be detected as a
variation in the pulse rate at Earth
planet
Earth
star
Spectral lines in star will be Doppler-shifted by
component of velocity of star which is in Earths
line-of-sight. This is easily the most common way
of detecting ESPs.
planet
Earth
43
How do we detect them? (2)

• 2) Radial Velocity (contd)

The radial velocity amplitude is given by
Keplers laws and is
Earth
i
Does this make sense?
Ms
Mp
Note that the planets mass is uncertain by a
factor of sin i. The MsMp term arises because
the star is orbiting the centre of mass of the
system. Present-day instrumental sensitivity is
about 3 m/s Jupiters effect on the Sun is to
perturb it by about 12 m/s.
From Lissauer and Depater, Planetary Sciences,
2001
44
How do we detect them? (3)

• 3) Occultation

• Planet passes directly in front of star. Very
  rare, but very useful because we can
• Obtain M (not M sin i)
• Obtain the planetary radius
• Obtain the planets spectrum (!)
• Only one example known to date.

Light curve during occultation of HD209458. From
Lissauer and Depater, Planetary Sciences, 2001

• 4) Astrometry Not yet demonstrated.
• 5) Microlensing Ditto.
• 6) Direct Imaging Brown dwarfs detected.

45
What are they like?

• Big, close, and often highly eccentric hot
  Jupiters
• What are the observational biases?

HD209458b is at 0.045 AU from its star and seems
to have a radius which is too large for its mass
(0.7 Mj). Why?
Jupiter
Saturn
From Guillot, Physics Today, 2004
46
What are they like (2)?

• Several pairs of planets have been observed,
  often in 21 resonances
• (Detectable) planets seem to be more common in
  stars which have higher proportions of metals
  (i.e. everything except H and He)

There are also claims that HD179949 has a planet
with a magnetic field which is dragging a sunspot
around the surface of the star . . .
Sun
Mean local value of metallicity
From Lissauer and Depater, Planetary Sciences,
2001
47
Simulations of solar system accretion

• Computer simulations can be a valuable tool

This is one of an extra-solar system (47 UMa). It
turns out that the giant planet b makes it hard
to form a terrestrial planet at 1 AU.
48
Puzzles

• 1) Why so close?
• Most likely explanation seems to be inwards
  migration due to presence of nebular gas disk
  (which then dissipated)
• The reason they didnt just fall into the star is
  because the disk is absent very close in,
  probably because it gets cleared away by the
  stars magnetic field. An alternative is that
  tidal torques from the star (just like the
  Earth-Moon system) counteract the inwards motion
• 2) Why the high eccentricities?
• No-one seems to know. Maybe a consequence of
  scattering off other planets during inwards
  migration?
• 3) How typical is our own solar system?
• Not very, on current evidence

49
Consequences

• What are the consequences of a Jupiter-size
  planet migrating inwards? (c.f. Triton)
• Systems with hot Jupiters are likely to be
  lacking any other large bodies
• So the timing of gas dissipation is crucial to
  the eventual appearance of the planetary system
  (and the possibility of habitable planets . . .)
• What controls the timing?
• Gas dissipation is caused when the star enters
  the energetic T-Tauri phase not well understood
  (?)
• So the evolution (and habitability) of planetary
  systems is controlled by stellar evolution
  timescales hooray for astrobiology!

50
Where do we go from here?

• Ground-based observations are amazingly good, and
  will only get better
• Next generation of space-based telescopes SIRTF
  already in place, Terrestrial Planet Finders are
  on the drawing boards
• Missions? Depends on the vagaries of NASA, but
  New Horizons is probably secure, and maybe one
  (several?) JIMO-class missions will fly . . .
• Outer solar system has 3 disadvantages
• Long transit timescales (ion drives?)
• Some kind of nuclear power-source required
• Prospects for life are dim

51
Summing Up - Themes

• Accretion (timescales, energy deposition, gas
  accumulation . . .)
• Volatiles (gas giants, antifreeze effect,
  atmospheres etc.)
• Energy transfer (insolation, convection,
  radioactive heating, tidal dissipation . . .)
• Tides (satellite evolution, disk clearing,
  geological features . . .)
• Diversity no-one would have predicted such
  variability (and this solar system may not even
  be typical)

52
Summing Up - Lessons

• Timescales and lengthscales both longer than
  inner solar system (accretion period, Hill sphere
  etc.)
• The early outer system was very different from
  today
• Giant planets were in a different place
• Satellite orbits have evolved
• Large population of planetesimals (now scattered)
  
• Single most important event was Jupiters
  formation
• Scattering of planetesimals asteroid gaps etc.
• Earlier formation would have increased inwards
  migration (why?)
• Other solar systems look very different to our
  own
• What is typical, and why?
• Extra-solar planets will continue to be a major
  focus of research