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

Note the two tails

Not to scale!

tail (ions)

coma (a cloud of gas)(~104 km)

nucleus(~10 km)

Sun

tail (dust)

~107 km

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

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5km

Image taken by Giotto during its closest approachNote 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.

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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Comets are apparently quite weak – perhaps more like an icy rubble pile than a snowball?

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Comet S-L 9 breaking up into fragments

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:

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

ecliptic

• 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• 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?

From Weissmann, New Solar System

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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”)

• 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

Brown, Phys. Today 2004

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“cold”

“hot”

Scattered Disk Objects

Building the Kuiper Belt• 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

Solar system age

Disk mass (ME)G

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From Stern A.J. 1996

• 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.

Different linesare for different mean random eccentricities

Kuiper Belt Formation

Initial edge of planetesimal swarm

30 AU

Ejected planetesimals (Oort cloud/Scattered Disk Objects)

48 AU18 AU

J S U N

“Hot” population

Early in solar system

2:1 Neptune resonance

J S U N

Neptune stops at original edge

Planetesimals transiently pushed out by Neptune 2:1 resonance

“Hot” population

“Cold” population

See Gomes, Icarus 2003 and Levison & Morbidelli Nature 2003

3:2 Neptune resonance(Pluto)

Present day

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 10-102 Earths

• Why spherical? Combination of initial random scattering from Jupiter, plus passages from nearby stars

• Forms the reservoir for long period comets

1 AU 10 AU 100 AU 1,000 AU 10,000 AU 100,000 AU

After Stern, Nature 2003

Earth SaturnPluto Kuiper Belt

Oort cloud (spherical after ~5000 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

Kuiper Belt

SDO’s

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

• NO LECTURE this Weds

• Revision lecture on Friday – bring questions

• NO LECTURE next Monday

• Final 8am next Weds