Transcript Chapter 7: Comets Comets • Coma and tail form at a distance of ~2.5-3 AU, where ice can sublimate • The sublimation consumes a lot.

Chapter 7: Comets
Comets
• Coma and tail form at a
distance of ~2.5-3 AU,
where ice can sublimate
• The sublimation consumes a
lot of energy, providing an
additional, effective
cooling source.
Comet composition
• Comets become visible as such at a distance of about 2.5-3 AU.
What temperature does this correspond to?
1/ 4
 1  AV 

T  
 1  AIR 
• At this temperature,
ice can sublimate to
form water vapour
280K
280

 177
1/ 2
d / AU 
2.5
Sublimation
• The vapour pressure of a given substance at temperature T is
given by :
H
H 
pv  p0 exp L  L 
 kT0 kT 
where HL is the latent heat of
vaporization, and p0 is the vapour
pressure at some temperature T0.
• The sublimation rate (number of molecules per unit time per unit
area) depends on the vapour pressure and temperature:
Z
pv
12kTmH
Energy Balance
1. Heating: radiation absorbed from the Sun, with efficiency
(1-Av)
2. Cooling:
a) Reradiation in the thermal infrared, with efficiency (1-AIR)
b) Sublimation carries off an energy 4pR2ZHL
To calculate the temperature at radius r, and the
sublimation rate Z, you have to solve the energy balance
equation by setting the heating rate equal to the cooling
rate.
Sublimation
CH4
H2O
CO2
NH3
Equilibrium T without
sublimation
T
280K
r / AU 1/ 2
• Calculations of the gas
outflow rate as a function
of heliocentric distance,
for different ices.
• Water begins to sublimate
at about 3 AU.
Sublimation
CH4
H2O
CO2
NH3
H2O
NH3
CO2
CH4
• Calculations of the gas
outflow rate as a function
of heliocentric distance,
for different ices.
• Water begins to sublimate
at about 3 AU.
• Sublimation requires a lot
of energy, effectively
cooling the surface of the
comet
Orbits
• Most comets have orbital periods >200 year
 A 1997 database for 937 comets lists only 191 short-period (P<200
yr) comets
 From Kepler’s third law, the semimajor axis of these long-period
comets must be >34 AU: halfway between Neptune and Pluto
Kuiper Belt
• Small objects detected in the region of Neptune, in 1992
 Currently several hundred are known
 Expect there are at least ~70,000 objects with diameters of 100km
or more.
• Kuiper belt believed to extend from 40-400 AU
 Flattened, in the plane of the rest of the solar system
Comet Orbits
• Distribution of semi-major axes has a peak at a~104
AU
 Orbits are highly eccentric, so aphelion is ~2a.
 Originate in the very distant solar system
 Very high orbital energy. Bound to the solar system… but
just.
500 AU
40 AU
Oort cloud
• Long-period comets come from all directions: not confined to the
ecliptic
• Therefore it was postulated that a huge, spherical shell of
cometary material surrounds the solar system. This is the Oort
cloud.
• Outer edge expected to be at about 105 AU, where gravitational
influence of Alpha Centauri will begin to dominate.
Meteor showers
• Meteor showers appear at predictable
times of year
 meteors from a given shower all radiate
from the same region of space and
move with similar velocities
• These are due to the Earth passing
through debris from cometary tails.
Cometary meteors
•
•
•
From measurements of deceleration, we can
tell that these meteors are tiny, low density
dust particles
No meteor from a shower has ever been
known to make it to Earth
Rockets and high-alititude aircraft have
collected examples of this dust
Orbit changes
• Cometary orbits can be perturbed by
gravitational interactions (somewhat
predictable)
• However, mass loss can also change
the orbit in unpredictable ways.
 Mass ejected from the tail gives rise
to a rocket effect that can change
the orbit.
• Calculate the change in period caused
by a small change in velocity as a
comet approaches the Sun.
Orbit changes
• Cometary orbits can be perturbed by
gravitational interactions (somewhat
predictable)
• However, mass loss can also change
the orbit in unpredictable ways.
 Mass ejected from the tail gives rise
to a rocket effect that can change
the orbit.
• E.g. the comet Swift-Tuttle (P=120 y)
was predicted to appear in 1982, but
did not appear until 1992.
 Comet is associated with the Perseid
meteor shower, and therefore losing
mass
Break
Coma composition
• Spectrum of the coma shows bright emission lines due to small
molecules (2-3 atoms).
 These emisison lines dominate the light
 Atoms in the coma absorb solar photons, then re-emit them in all
directions.
Coma
• Coma can begin to appear at distances as great as 5 AU
• Indicates significant fractions of volatiles: methane, ammonia,
carbon dioxide, nitrogen
• From the heating rate and the chemical composition, we can
calculate the amount of mass lost to sublimation.
Sublimation of comets
• Consider a hypothetic comet, with a pure water-ice nucleus 1 km in
radius. If the sublimation rate is ~1022 molecules/m2/s, how many
passages will the comet be able to make through the inner solar
system?
Tails
• Tails extend for millions of kilometers
• Always point away from the Sun
• Two types (often both are visible at once)
 Ion tail: straight, bluish-coloured tail
 Dust tail: broad, curved, and yellowish
Plasma (ion) tail
• Straight, but complex: with rays, streamers and knots
• Spectra dominated by ionized molecular emission lines
• Pushed away from the sun by the solar wind
Dust tail
• Smooth,
featureless
• Spectrum nearly
identical to the
solar, absorption
spectrum
Made up of dust
particles less
than about 1
micron in size
• Radiation pressure
forces the dust
particles steadily
farther from the
Sun
Comet Nuclei
Halley (1986)
Borrelly (2001)
Wild (2004)
Deep Impact
(2005)
Visiting comets
• Need to know orbit accurately
• Comets have large velocities relative to Earth (10-70 km/s)
 Thus visiting spacecraft launched from Earth will face debris of small
particles flying at very high velocities
• E.g. Halley’s comet has a retrograde orbit, so the relative velocity is
about 70 km/s
 European Giotto probe passed within 600 km of Halley’s nucleus
• Discoveries:
 Comet abundances are very near solar
 Very low albedo, only 4% (darker than a
lump of coal).
 Most of the surface is covered with a
thick dust crust, through which gas cannot
escape.
 Gas evaporating from the comet comes
from vents or jets, on only about 10% of
the surface
 Density is low, only 300 kg/m3, indicating
that it is loosely bound icy material.
Wild
• The spacecraft
Stardust visited
comet Wild2 in 2004
• Collected samples of
dust, which were
jettisoned back to
Earth in Jan 2006
• Nucleus is covered
with numerous
craters and hills
• At least 10 active
gas vents
Tempel-1
• Impacted by Deep Impact probe in 2005
• Impact created a crater no more than about 50 m deep – only
scratched the surface
• Demonstrates that nucleus is not a loose agglomeration of material
• Surface is more dusty than icy: and finer than normal sand.
Collisions
Sun
• This “Sun-grazing” comet was
observed by the SOHO
spacecraft a few hours
before it passed just 50,000
km above the Sun's surface.
• The comet did not survive its
passage, due to the intense
solar heating and tidal
forces.
• Shoemaker-Levy collided
with Jupiter in 1994
• Was previously tidally
disrupted into a string of
fragments
• Each fragment hit Jupiter
with the energy of a 10
megaton nuclear bomb
explosion
Summary
• As expected, comets
are warmer on their
sun-facing side, as this
temperature map from
the Deep Impact
mission shows (comet
Tempel 1)
• Sublimation occurs
more rapidly on one side
than the other.
Asteroid and comet sources
Short-period comets
• Jupiter-type comets are those
with P<20 yr
 Small inclinations, relatively
small eccentricities
 E.g. Encke, Tempel2
 Likely originate in the Kuiper
belt. Perturbed by Neptune or
Uranus?
• Halley-type comets have
20<P<200 yr
 More eccentric, and higher
inclinations
 E.g. Halley has P=76 yr but
e=0.97, and a retrograde orbit
with i=162 deg
 These probably originate from
the Oort cloud, but have had
their orbit perturbed.