ESS 298: OUTER SOLAR
SYSTEM
Francis Nimmo
Io against Jupiter,
Hubble image,
July 1997
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?
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
Neptune
Triton (retrograde)
Nereid (small, eccentric,
inclined, long way out)
Where is Triton?
Jupiter
Distance (Rp)
5
15
10
Neptune
25
20
30
Triton
ms
(1020 kg)
Rs
(km)
r
157o
214
1353
2.05
0.28o
1076
2403
1.85
a
P
e
(103 km) (days)
i
Triton
355
5.88R .000016
Callisto
1883
16.69
.007
No information on MoI – single flyby at 40,000 km (Voyager 2, 1989)
(Mg m-3)
Triton’s 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
Neptune
29o
Triton
What’s 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 Titan’s – CN compounds generated by
photolysis)
• Extreme seasonal variations
• Surprisingly geologically interesting for such a small
and cold body
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-> 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
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
Triton’s orbit precesses with a period of 688 yrs
So the angle between Triton’s pole and the Sun varies very
widely (see diagram below)
29o
Neptune
21o
Triton
21o
164 yrs
From Cruikshank, Solar System Encyclopedia
Voyager
observations
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
Triton’s peculiar surface
• Very few impact craters ->
young (~100 Myrs)
• “Cantaloupe terrain”
• Plains suggestive of
cryovolcanism
• Tectonic features
• Geologically active!
500 km
Cantaloupe terrain
• Possible cryovolcanic region
• Smooth plains indicate low viscosity
• Ammonia-water melt has viscosity
comparable to basalt
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
8 km
Particles falling out
Dark streak developing
Activity of this kind is unlikely
to be able to explain the
absence of big impact craters,
again indicating that Triton’s
surface is very young
Tectonic features
ridge
Cantaloupe
terrain
220 km
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.
Scale bars are
2 km for
Europa and 40
km for Triton
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?
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
Several puzzles and a solution
• 1) Why is Triton’s 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) Triton’s 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)
Hypothetical scenario
Triton
Triton’s orbit
circularizes due
to tides
Triton on
heliocentric
orbit
Neptune
Inoffensive
prograde
satellites
Other satellites
scattered outwards by
close encounters as
Triton’s orbit evolves
Inoffensive
prograde
satellite
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?
So what?
• Gigantic tidal dissipation (see next slide)
• Circularization explains absence of other bodies
• Collision explains Nereid’s orbit (small, very far
out, high e and i – due to perturbations as
Triton’s orbit circularized)
• Young surface age suggests (relatively) recent
collision – how likely is this?
• Improbable events can happen – what’s another
example of an improbable event?
• Where did Triton come from? (see later)
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 ! (Where’s 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
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
Hypothetical internal
structure of Triton (see e.g.
McKinnon et al., Triton,
Ariz. Univ. Press, 1995)
ice
1352 km
rock
950 km, 0.4 GPa
iron
3.0 GPa
600 km , 1.5 GPa
Comets and the Kuiper Belt
Comets and their Origins
• Two kinds of comets
– Short period (<200 yrs) and long period (>200 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)
Short-period comets
From Weissmann, New Solar System
From Zahnle et al. Icarus 2003
• Period < 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?
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:
Slope -3.5
Freq.
Critical
size
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
Size
• ~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”)
ECCENTRICITY
Kuiper Belt
“hot”
“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 Brown’s article in Physics Today Apr. 2004
Building the Kuiper Belt
Growth time
• 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
From Stern A.J. 1996
Different lines
are for different
mean random
eccentricities
Solar system age
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.
Kuiper Belt Formation
Early in solar system
Ejected planetesimals (Oort cloud)
“Hot” population
J
S
U
Initial edge of
planetesimal
swarm
N
18 AU
Present day
J
30 AU
“Hot” population
Planetesimals transiently pushed
out by Neptune 2:1 resonance
S
U
48 AU
N
Neptune
3:2 Neptune
stops at
original edge resonance
(Pluto)
See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003
“Cold”
population
2:1 Neptune
resonance
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
Binaries
• A few percent KBO’s are binaries, mostly not tightly
bound (separation >102 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 (<~1 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?
Long-period comets
• Periods > 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(?)
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 10102 Earths
• Why spherical? Combination of initial random
scattering from Jupiter, plus passages from nearby stars
• Forms the reservoir for long period comets
Earth
1 AU
Saturn
10 AU
After Stern, Nature 2003
Pluto
Oort cloud
(spherical after ~5000 AU)
Kuiper Belt
100 AU
1,000 AU
10,000 AU
100,000 AU
Sedna (2003 VB12)
• Discovered in March 2004, most distant solar system
object ever discovered
• a=480 AU, e=0.84, period 10,500 years
• Perihelion=76 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?)
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 3:2 resonance with Neptune, so that the two
never closely approach (stable over 4 Gyr)
• Charon is a large fraction (12%) of Pluto’s mass and
orbits at a distance of 17 Pluto radii
• Charon’s orbit is almost perpendicular to the ecliptic;
Pluto’s 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
Discoveries
• Neptune’s 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 Neptune’s orbit
• Pluto was discovered at Lowell’s
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 . . .
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
Pluto’s
rotation
pole
View from Earth. Note that
Charon’s orbit is inclined to Pluto’s
(and to the ecliptic). From Binzel
and Hubbard, in Pluto and Charon,
Univ. Ariz. Press, 1997
Pluto and Charon
• Pluto’s orbit: a=39.5 AU, orbital period 248
years, e=0.25, i=17o , rotation period 6.4 days
• Charon’s orbit: a=19,600km (17 Rp),
period=6.4 days, e=?, i=0o
Pluto
Charon
Triton
Mass (kg)
1.3x1022
1.6x1021
2.2x1022
Radius (km)
~1150
~625
1353
Density (g/cc)
~2.0
~1.7
2.05
Rotation (days)
6.4
6.4
5.9
Obliquity
120o
-
157o
Compositions
• Pluto’s surface
composition very similar to
Triton: CH4 (more than
Triton), N2, CO, water ice,
no CO2 detected as yet
• Charon’s 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
Pluto’s 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-dependent)
• 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
Charon’s Eccentricity (?)
• Difficult to observe, but HST gives value of 0.003-0.008
• Why is this important?
• What is its source?
–
–
–
–
Can’t be primordial (circularization timescale ~107 yrs)
Can’t 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
Pluto/Charon Origins
• Compositional similarities to Triton suggest same
ultimate source – Kuiper Belt
• Pluto’s current orbit is probably due to perturbations by
Neptune as N moved outwards (recall the 3:2 resonance)
• Charon is most likely the result of a collision. Clues:
– Its orbital inclination (and Pluto’s 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. EarthMoon 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)
Angular Momentum
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:
m0  m1  m2
C0w0  C1w  m2 a 2w
w
r1
Charon
Pluto
m1
w
a
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.:
Gm0
-2
=
0.67
ms
r02
m2
Centripetal acc.:
r0w02 =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.
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)
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!)
How do we detect them?
• The key to most methods is that the star will move
(around the system’s centre of mass) in a detectable
fashion if the planet is big and close enough
• 1) Pulsar Timing
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
pulsar
planet
Earth
• 2) Radial Velocity
Spectral lines in star will be Doppler-shifted by
component of velocity of star which is in
Earth’s line-of-sight. This is easily the most
common way of detecting ESP’s.
Earth
star
planet
How do we detect them? (2)
• 2) Radial Velocity (cont’d)
The radial velocity amplitude is given by
Kepler’s laws and is
1/ 3
 2G 

v  
 Porb 
Earth
i
Does this make sense?
M p sin i
Ms
1
(M s  M p )2 / 3 1  e2
Mp
Note that the planet’s mass is
uncertain by a factor of sin i. The
Ms+Mp term arises because the star
is orbiting the centre of mass of the
system. Present-day instrumental
sensitivity is about 3 m/s; Jupiter’s
effect on the Sun is to perturb it by
about 12 m/s.
From Lissauer and Depater, Planetary Sciences, 2001
How do we detect them? (3)
• 3) Occultation
Planet passes directly in front of star.
Very rare, but very useful
because we can:
1) Obtain M (not M sin i)
2) Obtain the planetary radius
3) Obtain the planet’s 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.
What are they like?
• Big, close, and often highly eccentric – “hot Jupiters”
• What are the observational biases?
Note the absence of high
eccentricities at close
distances – what is causing
this effect?
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
What are they like (2)?
• Several pairs of planets have been observed, often in
2:1 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 . . .
From Lissauer and Depater,
Planetary Sciences, 2001
Sun
Mean local value of
metallicity
Simulations of solar system
accretion
• Computer simulations can be a valuable tool
eccentricity
distance
star
giant planet
(observed)
Laughlin, Chambers and Fischer
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.
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 didn’t just fall into the star is because the
disk is absent very close in, probably because it gets
cleared away by the star’s 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
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!
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
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)
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 Jupiter’s 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