Comet Activity and
Composition
K. Meech
Ast 734 Seminar
8/30/04
Dirty Snowballs
H2O
100
CH4
0.6
CO
1-20
C2H2
0.1
CO2
3-20
C2H6
0.3
H2CO
0.1-1
NH3
0.6
CH3OH
1-7
HCN
0.2
HCOOH
0.05
CH3CN
0.02
HNCO
0.1
HC3N
0.03
NH2CHO
0.01
H2S
1.5
Inactivity to Activity

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Sublimation of gases
Drags dust from nucleus

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Gravity low
Most dust escapes
Solar radiation pressure
 coma  tail
Photodissociation of gas

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Ionization  gas tail
Activity develops

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Km-scale nucleus
Coma ~ 105 km
Tail ~ 106-107 km
Dust Coma Development
04/06/98 r=4.3 AU
q-620 dy; Afr = 14 cm
08/22/00 r=2.6 AU
q+210 dy; Afr = 87 cm
01/19/99 r=3.1AU
q-350 dy; Afr = 31 cm
09/30/00 r=2.8 AU
q+300 dy; Afr = 83 cm
07/15/99 r=2.2 AU
q-150 dy; Afr = 105 cm
11/12/01 r=4.4 AU
q+590 dy; Afr = 13 cm
Who Cares?
“extragalactic student”
“extragalactic astronomers observing a comet”
Cosmic Solar System History
>4.6 Gy
ISM dark cloud
Earth in the Hadean
Oceans & rocks form
~4.4 Gy ago
Planetesimals condense
Planets accrete
Form few x100 million years
The Archean Epoch
Oldest life on Earth
3.5-3.8 Gy ago
Late planetary bombardment
Comets, asteroids bring water &
Organics to Earth
The Oort Cloud

Oort, J. (1950) B.A.N. 408, 91-110.
Oort J. H. & M. Schmidt (1951) B.A.N. 419,
259-270
17th century physics:
Brahe, Kepler & Newton


Eorbit = -m/2a
Distribution of 1/aoriginal


22 long-period comets
Strongly peaked 


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Source 50,000-150,000 AU
Contains 1011 comets
Width very narrow

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Fading Problem
“Volatile Frosting”
Different chemistry
The Modern Oort Cloud

Outer Oort Cloud 15,000-105 AU


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Stellar perturbations > 104 AU
Inner Oort Cloud 2000-15,000


Galactic Tides


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Dynamically inert 50-2000 AU
Kuiper Belt
35-50 AU


Stable, dynamically active
Classical, 3:2, scattered
Dynamically new
Long Period P > 200 yr
Short Period P < 200 yr



1/aorig < 100x10-6 AU-1
Halley family – Oort cloud origin
Jupiter family – KBO origin
Centaurs transition objects
The Evidence for Fading

Different types of evidence



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Really bright comets are all long-period
Distant comets  narrow tails (large dust)  volatile gases
New comets tend to split more frequently (more volatiles)
Non-gravitational motion (jets)
Problems


Non uniform data sets
Non-linear detectors
Great Comet 1577
Morehouse 1908 III
Halley 1910
Delavan 1914
SP Comets 3.4-14.5 AU
Comet Activity Levels  Trends
Evidence for Differences

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Dots = All SP obs
Squares = Halley
Triangles = DN comet
Sublimation of Volatiles?

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Delsemme’s original work: albedo too high
Water-activity out beyond Jupiter
Water Ice Physics

Phase I: P < 2700 atm

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High P forms: II to XIV
Amorphous Tcond< 100K

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Ih – hexagonal
Ic – cubic (low T, low P
phase)
Traps gases
Clathrates

Mechanical trapping in
cages
Comet Formation
100K
64K
31K
0
10
100 AU
Low Temperature Condensation

Ices in comets
condensed T< 100K

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Release of gases

CH4
N2
Ar
CO

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Amorphous form
Trapped other gases
Amounts depend on r
137K amorphous 
crystalline phase change
Annealing (30-35K)
Sublimation 160-180K
Gas release at large distances:
controlled by Water
Heat Transfer
in Comets

Conduction low

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Depends on porosity
(unknown)
Radiation
Gas phase conduction
(recondensation)
Sintering

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Changes the
conductivity
Volatile re-distribution
Insulating layers
The Halley Outburst

Gas Laden amorphous ice model

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Heat from perihelion penetrates to ice layer
Exothermic transformation (137K)
Released gases build up pressure  outburst
Chiron’s Behavior

Amorphous ice model

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60% dust
40% amorphous ice
0.1% trapped CO
Matches observations

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Density < 0.4 g/cm3
Mass loss rates & dust
CO fluxes match obs
Tsurface matches obs
Activity sporadic  not
refreshing surface
Hale Bopp

Active at large r

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Discovered 7.2 AU
(1995)
Pre-discovery image
13.0 AU (1993)
Dynamically young
Large CO fluxes seen
Molecules of different
volatilities appear at
similar times
Thermal models: Comet Hale Bopp


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Amorphous ice crystallization model
Porosity 0.65
4% by mass trapped CO
Activity at Larger r?
C/2003 A2 Gleason
q = 11.43 AU
1/a = 42 x 10-6 AU-1

Distance for T ~ 137K

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Beginning near 10 AU
Mechanisms at r > 10 AU

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Solid volatiles (e.g. CO, CO2)
sublimation
Annealing
KBO1996 TO66 – Activity?
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Orbit

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Lightcurve period
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Q = 48.6, q = 38.5
q: 5/3/1910 Q: 2/1/2054
1997: 2 peak 6.25 +/- 0.03
hr, Dm = 0.12 mag
1998: single peak, Dm = 0.33
Consistent with activity
Blue colors

Vary with rotation in 1999
Observations

Subaru 8m + Suprime Cam

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8x12 K CCD mosaic
0.2”/pixel, 0.25o FOV
Target Selection

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15 blue-neutral objects
Select smallest r = 33.8 AU
1997QJ4: V-R = 0.296 (Plutino)
r = 33.8 AU, Hv=7.5 (rad = 80 km)

October 3, 4 2002 UT

Nt 1 phot, Nt 2 clouds
Sensitivity

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S/N = 3, V=28 12000s
Composite
Image

Single exp,
400 sec
Composite
Image

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Single exp,
400 sec
12000s sum
Composite
Image

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Single exp,
400 sec
12000s sum
(zoomed 80”)
Composite
Image

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Single exp,
400 sec
12000s sum
(zoomed 80”)
Median
combined
Composite
Image

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Single exp,
400 sec
12000s sum
(zoomed 80”)
Median
combined
Shift & sum
for KBO rate
Composite
Image

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Single exp,
400 sec
12000s sum
(zoomed 80”)
Median
combined
Shift & sum
for KBO rate
Median star
subtracted
Surface Brightness
Result: Q < 0.01 kg/s
F = Sopagr2pvQf / 2r2D2vgr
Constants: So , p , r , D, f
Assume:
agr = 0.1 mm (max lifted off)
pv = 0.04, vgr = 0.1 km/s (CO)
Comet Paradigms

“Comets are the most
pristine things in the
Solar System”

“Comets tell us about
the formation of the
Solar System