Transcript Comets - Earth & Planetary Sciences

EART160 Planetary Sciences
Francis Nimmo
Last Week – Icy Satellites
• For icy satellites, main source of energy is tides – link
between orbital and geological evolution
• Some show present-day geological activity
(Enceladus, Europa, Io, Triton)
• Many show ancient geological activity
• Oceans are quite common – habitability
• Titan is unusual because it has an atmosphere and an
active “hydrosphere” (liquid methane)
• Likely to be targets for future spacecraft missions
This week – Comets and the
Kuiper Belt
• Where do they come from?
• What are they made of?
• What do they tell us about the early Solar
System?
“A star with hair”
Ion tail pointing directly
away from the Sun. Note
the slightly bluish color.
Dust tail slightly curved, brighter
Why do we care about comets?
• Pristine (or nearly pristine) samples of high
volatile components of original nebula
• Important source of volatiles and organic
matter to inner solar system (astrobiology,
atmospheres)
• Orbits tell us about how the early solar system
was assembled
Comet diagram
Not to scale!
Sun
coma (a cloud of gas)
(~104 km)
tail (dust)
nucleus
(~10 km)
tail (ions)
~107 km
Note the two tails
Comet Nucleus and Coma
• Composition: Water ice is the dominant
constituent. There are also methane and ammonia
ices (CH4 and NH3) embedded in a rocky matrix.
• The model is that of a very dirty snowball or dirty
iceberg. However, the outer portion of Halley’s
comet (visited by Giotto and Vega 1 & 2 s/c in
1986) was found to be very, very dark, a shell of
“sludge” left behind as the vapors baked out.
• The coma is a cloud of gas which has evaporated
from the nucleus due to the Sun’s energy
• It may contain nasty substances like cyanide
(HCN) as well as water vapour.
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Formation of the tail
• As comet approaches the Sun, it is warmed by
solar radiation and vapors are released, often
carrying grains of dust with them.
• The comet dimensions increase and it appears to
brighten, develops a tail, and the tail grows longer.
– Comets are seldom seen beyond 3 or 4 AU.
The record is 11.5 AU.
• The maximum diameter of a coma usually occurs
when the comet is between 1.5 and 2.0 AU from
the sun.
• Most apparently have orbital periods of thousands
of years (see later).
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Finding Cometary Matter
• Comets are weakly bound and the matter that produces the
meteor showers doesn’t survive the trip through the
atmosphere.
• Microscopic particles have been collected at high altitudes
with aircraft and rockets. Composition is similar to C1
carbonaceous chondrites (is this a surprise?).
• A spacecraft (Stardust) has returned to Earth dust samples
collected from the coma of comet Wild 2.
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Halley’s comet
• Short period comet (76 years)
• Visited by several non-US spacecraft in 1986-86
• Will next return in 2062
Image taken by Giotto during its
closest approach
Note the dark surface, and the jets
of bright material coming off as the
Sun heats the volatiles.
These jets of gas can perturb the
orbit of the comet and make exact
prediction of its orbit difficult.
5km
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Properties of Halley’s comet
• The nucleus is irregularly shaped, 15 x 7 to 10 km.
The color is dark, fairly neutral, gray.
Its reflectivity is only 4%. Similar to black,
volatile rich, carbonaceous asteroids beyond the
outer asteroid belt.
• Its composition (by number of molecules) is
mainly ice:
water ice
80%
carbon monoxide
10%
carbon dioxide
3.5%
organic compounds
1-2%
• D/H ratio has been used to infer that most of
Earth’s oceans not provided by comets
• It rotates slowly, period of several days, and it
exhibits nodding or nutational motions.
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Comet S-L 9 breaking up into fragments
Comets are apparently quite
weak – perhaps more like an
icy rubble pile than a
snowball?
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Deep Impact (Comet Tempel-1)
1km
• Spectral features (H2O,C-H, CO2) seen
• Impact crater not seen
• Follow-on mission?
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?
Kuiper Belt
Scattered
Disk Objects
ECCENTRICITY
• ~800 objects known so far,
occupying space between Neptune
(30 AU) and ~50 AU
“hot”
• Largest objects are Pluto, Charon,
Quaoar (1250km diameter), 2004
DW (how do we measure their size?)
“cold”
• Two populations – low eccentricity,
low inclination (“cold”) and high
Brown, Phys. Today 2004
eccentricity, high inclination (“hot”)
• 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 and
Alessandro Morbidelli’s review in Science Dec. 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/Scattered
Disk Objects)
“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
• Any interesting consequences for the inner solar system?
Binaries
• A few percent KBO’s are binaries, mostly not tightly
bound (separation >102 radii) – Pluto/Charon an
exception. Why are binaries useful?
• Pluto has two extra satellites (Weaver et al., Nature 2006)
• 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 (close-in
versions are called Scattered Disk Objects)
• 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
2003 VB12 (Sedna) and 2003 UB313
• Sedna 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 a scattered disk object (not a KBO)
• Radius ~ 1000 km (how do we know?)
• 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?)
• 2003 UB313 is another SDO which is interesting mainly because
at ~3000 km it is bigger than Pluto (how do we know?) (Bertoldi
et al. Nature 2006)
Kuiper Belt and SDO’s
Plutinos
Twotinos
SDO’s
Kuiper Belt
Summary
• Comets are dirty snowballs with a dark crust
• They provide samples of (hopefully)
primordial, volatile-rich solar nebula material
• SP comets come from the Kuiper Belt
• LP comets come from the Oort Cloud
• The architecture of the Kuiper Belt is probably
a result of Jupiter, Saturn and Neptune moving
around early in their history!
End of lecture
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NO LECTURE this Weds
Revision lecture on Friday – bring questions
NO LECTURE next Monday
Final 8am next Weds