Transcript Cometary Basics Michael F. A’Hearn Why Study Comets? Medieval copy of Josephus Giotto di Buondoni - Scrovegni Chapel 26/7/07 The Independent - 1997 Mar 22 Bayeux.

Cometary Basics
Michael F. A’Hearn
Why Study Comets?
Medieval copy of Josephus
Giotto di Buondoni - Scrovegni Chapel
26/7/07
The Independent - 1997 Mar 22
Bayeux Tapestry
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More Reasons Why
C/Bennett 1970 II
C/Lee 1999 H1 - L. Sanino
C/Ikeya-Seki
C/Hale-Bopp
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Origin of Comets
• Slow accretion everywhere outside the ice line
• Beyond Neptune - grow to classical KBOs, some KBOs
captured as Plutinos – collisions break off pieces, gravitational scattering hands off to
giant planets successively to make Jupiter-family SP comets
– Others are perturbed to the scattered disk which may feed both
Jupiter-family comets and the Oort cloud (from opposite edges of
the disk) cf. Fernández et al.
• Inside Neptune - some build the giant planets (rapidly),
others ejected to interstellar space (particularly in vicinity of
Jupiter and Saturn), others ejected to Oort cloud, where
subsequent perturbations produce dynamically new comets &
thence LP comets & thence Halley-type SP comets
• All Discussed in Monday’s talks
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Cometary Record of Protoplanetary Disk
• Many (but not all) comets formed small
– No gravitational heating
– Probably no radioactive heating (but some would
argue against this)
• Comets formed far from sun
– No solar heating
• Thus, ices reflect T,P conditions in
protoplanetary disk
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Major Goal of Cometary Studies
• Use observations of comets to constrain
models for formation of the planetary system
• BUT!!!
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Cometary Evolution
• Each perihelion passage heats the outer layer,
modifying ices
– Depth of evolved layer very uncertain
• Each perihelion passage leads to mass loss
– Total 1/2 to 5 m depending on q and r
– unknown fraction of the modified layer - could be
nearly all of it
• Comets in Oort cloud are irradiated by galactic
cosmic rays - break every chemical bond in
outer 10 meters over 4.5x109 yrs
– Leads to very unusual photometric behavior on
inbound portion of first entry to planetary region
– Entire modified layer is lost on first approach
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Nuclear Models
Can we exclude any of these models yet?
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Note the question marks!!!
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Evolutionary Models
Benkhoff-Huebner model has density increasing monotonically from surface to 10s of
meters. Prialnik-Mekler model has a dense layer of water ice at surface with lower
density material below. Ice layer near surface may vary diurnally.
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What We Need to Know
• Nuclear Physical Properties
– Density, strength, porosity
– Heterogeneity
• Evolutionary (onion-skin) layering
• Primordial cometesimal variations
• Nuclear composition
– Chemical composition of ices
• Variation with depth or location?
– Mineralogical and chemical composition of
refractories
– Scale of mixing among ices and refractories
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Context – Comets Unknown
•
Mass – few data
–
Density and Surface Gravity uncertainty still large
–
Measurement from DI (next talk) implies density ~0.4 g/cm3
•
•
•
•
Strength
–
Tensile strength < 103dyn/cm2 at km scale (<102 Pa)
–
Upper limit from DI ==> < 10 kPa, but likely << 10 kPa
Stratification
–
Know only irradiated mantle on new comets
–
Ice to rock ratio unknown
–
Layering clear from DI
Shape
–
•
•
Is it typical? Non-gravitational acceleration models suggests it is typical
4 comets with good images - very different shapes
Photometric Properties
–
Very dark, grey to “pink” in color
–
No mineral absorptions yet detected (except water ice on P/Tempel 1)
Coma dust and rocks very uncertain
–
Very detailed models fit many species
–
Biggest advance is dust particles returned by Stardust from P/Wild 2)
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Differences Among Nuclei
QuickTime™ and a
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Stardust Team
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QuickTime™ and a YUV420 codec decompressor are needed to see this picture.
L. Soderblom
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The Humpty-Dumpty Problem
Species Observed in Comets
Volatile
A toms
Refractory
Diatomic
A toms
Triatomic Polyatomic
Ions
+
(Ar)
Al
C2
(C2H)
C2H2
C
C
Ca
CH
C3
C2H6
CH
+
H
Co
CN
CO 2
CH 3CN
CO
+
Isotopic
Variants
C15N
13
12
C C
13
CN
+
CO 2
+
H2O
DCN
13
H CN
(He)
N
Cr
Cu
CO
CS
H2O
CH 3OCHO
CH 3OH
O
Fe
NH
H2S
CH 4
H3O+
HC N
S
K
NS
HCN
H2CO
HCO +
HDO
+
(N2 )
+
CS2
Mg
OH
(HCO)
H2CS
Mn
S2
HNC
HC 3N
Na
SO
NH 2
HCOOH
Ni
OCS
HNCO
Si
SO2
NH 3
Ti
15
HDO
+
O
+
OH
C++
NH2CO
V
Mg 2SiO4
Must use all possible wavelengths and techniques to sort it out
All measurements are of the coma, not the nucleus
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Coma ≠ Nucleus
• Biver et al.
– Production rates vs.
heliocentric distance
(from mm-wave data)
– Ratios of abundances
vary dramatically
– Thus ratios at any given
time can not be
representative of nuclear
abundances
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Dormant Comets
• Earlier “best” cases (Oljato, Phaethon, etc.) showed
anomalous, possibly cometary properties but were not
good analogs in other ways
• Recent examples very likely to be comets
– Tisserand invariants <3, including retrograde objects
– Low albedos (all <0.1, most <0.06)
– Data from several sources beginning with Harris et al.
1999, Y. Fernández et al. 2003
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Dormant Comets
Convincing evidence for dormant comets!!
(at last)
- cometary nuclei
NEAs & UAs, T<3
NEAs & UAs, T>3
Y. Fernández et al. 2002
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Nuclear Sizes
• Fernández, Tancredi, et al.
– 1999 A&Ap 352, 327; catalog on web
– Wide variety of published data
• Weissman, Lowry, et al.
– 2003 LPSC
– Recent data plus selected older data
• Lamy et al.
– Various papers; chapter in Comets II
– HST data plus selected older data
• A’Hearn - unpublished reanalysis of Lamy et al.
• Recent reanalysis of all data by Tancredi et al. (2006)
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Size Distribution
J. Fernández et al.
First size distribution, data from literature
Jupiter family comets only
Slope +0.54 in magnitude --> slope -0.88 in mass
Albedo assumed to not vary with size
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More Recent Sizes
Cumulative Size Distribution
80
Cumulative Number
40
20
Slope = -1.59 +/- 0.03
(r > 1.4 km)
10
4
54 Jupiter-family comets
2
1
20
10
4
2
1
Radius - km
Weissman & Lowry, 2003 LPSC
Slope -1.59 ± 0.03
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0.4
0.2
Lamy et al., 2003, in Comets II
Slope -1.87 ± 0.02
Detailed studies of individual objects show significant
disagreement with results of Fernández et al., but not
obviously systematic effects
Slopes indicate evolutionary processes other than or in
addition to collisions
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More Size Distributions
Cumulative Size Distribution
2.000
1.500
log(N>R)
Data from Lamy et al. replotted to highlight region
of slope -2.5 for “ecliptic”
comets
Blue = ecliptic comets
Pink = all comets
Chiron & SW1 excluded by
Lamy et al. & here as being
Centaurs
1.000
0.500
0.000
-1.5
-1
-0.5
0
0.5
1
1.5
-0.500
lo g( R )
Results sensitive to assumptions!
- Cutoff for observational selection at small end
- Cutoff for small-number statistics at large end
- Binning vs. counting!
Whether collisions explain the size distribution is still an
open question!
Is depletion at small sizes real? Or is it selection? If real,
is it primordial or evolutionary?
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Slope -2.5
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2
Dormant Comets
Albedo vs. Size - Dead Comets
0.08
Geometric Albedo
0.07
0.06
0.05
0.04
Is the dispersion difference real? I.e., is
there a wider distribution of albedos
among ECs than among NICs?
0.03
0.02
0.01
0
0
2
4
6
8
10
12
Radius [km]
14
16
Are these really dormant comets?
Data from Lamy et al. 2003. In Comets II.
Pink - ecliptic comets (Ecs)
Blue - near-isotropic comets (NICs)
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Dust Trails
• Traditionally discovered in thermal IR
• Dominated by large particles released long prior to
observation
• Spatial resolution limited by observations at thermal
wavelengths
• Recent optical detections provide good spatial resolution
and also provide additional constraints on particle
distribution
– Ishiguro et al., 2003. Ap.J., 589, L101
– Weissman et al., private communication
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Trail from Wild 2
Detailed spatial profiles important for
spacecraft encounters
Estimate 0.5 mg particles
Ishiguro et al., 2003
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Hazards for all close flybys should be
reassessed.
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Cometary Breakup
• Breakup is a long-established process but
usually the largest fragment remains an active
comet at later apparitions
• Comet LINEAR 1999 S4 dissipated rapidly, but
other comets have dissipated on longer time
scales
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Comet LINEAR 1999 S4
Weaver et al., 2001 Science 292, 1329
C/LINEAR - 99S4: breakup for no apparent reason;
Weaver et al. infer most mass in fragments with
500mm < diam < 50m
Previous comets - S-L9, tidal breakup ==> tensile
strength <103 dyn-cm-2 at km scales
Previous comets - few % per passage break up in
some way
Predominant process - one large fragment and a few
smaller fragments (e.g., West, Biela, et al.)
Predominant cause and mechanism - lots of
speculation but really totally unknown
Bottom line - comets are really fragile, certainly at
some scales and probably at all scales
Should landers & other contact missions abandon
trying to work at all strengths and assume low (e.g.
<104 dyn-cm-2 for all strengths and at all scales?
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High-Resolution Spectra
• Optical and IR high-resolution spectra continue to
provide valuable insight and also to provoke new
puzzles
• Most data are for Oort-cloud comets (long-period and
Halley-type; NICs in the terminology of Lamy et al.)
• Missions tend to go to Jupiter-family comets - can we
relate the two? Or is a mission to a NIC essential for
our understanding?
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High-Resolution Spectroscopy
• Two optical examples
– P/de Vico - Cochran & Cochran (not JF)
– C/Hyakutake - A’Hearn et al.
• Near-IR example
– Survey by Mumma et al. (not JF)
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122P/de Vico
Cochran & Cochran 2002
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Allows detailed models of fluorescence which in turn
allows understanding of physical processes - production, motion, etc.
Note huge number of unidentified lines at all wavelengths - new chemistry!!
Similar spectra to shorter wavelengths essential to map C 3 and its effect on lower
resolution spectra
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122P/de Vico
Cochran & Cochran 2002
No features identified in this
segment!!!
We are missing a huge understanding
of the chemical composition!
Beginnings of new identifications in
some segments, e.g., O lines by
Feldman
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C/Hyakutake
Opportunistic observation at small
geocentric distance
Many bands of S2
Highest resolution ever for S2
Trot ~ 70K (collisional control)
Note also prompt emission from hot OH
New formation scenario in inner coma
OCS + S(1D) --> CO + S2
How can missions probe this chemistry?
Is S2 in JF comets? (It is probably in all
non-JF comets; if chemical production is
right, it should be in all comets)
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C/Hyakutake
For the “real” astronomers, note
unidentified feature at 4425-4435Å
DIB at l4430 is wider & diffuse
Are they related - gas phase in
comets and grain surface phase in
ISM?
This is one of many unidentified
features
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Near-IR Spectroscopy
Example of high-resolution, near-IR
spectra obtained by Mumma and
collaborators.
Some species, with telluric counterparts,
can only be measured with large Doppler
offsets.
All species require careful model of
Earth’s atmosphere.
C/Hyakutake - Mumma et al.
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Near-IR Results
Generally similar values except for C/LINEAR (99S4)
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Compare with Older Result
A’Hearn et al. 1995,
Icarus, 118, 223
• Two classes of comets
– Normal abundance ratios (solid)
– Depleted in carbon chain molecules (open) - all are JF
• Reanalysis in progress
• Is Mumma’s depleted comet (C/1999 S4) related to
these depleted comets?
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Near-IR Results
Wide range for CH4 - what does this mean?
CO shows comparable range of abundances, but uncorrelated with CH 4
Other (less volatile) species show much less variation
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Implications
• Most missions will be to Jupiter-family comets
• Can we generalize chemistry from a mission
when our Earth-based ensemble is mostly
Oort-cloud comets?
• Orbiters should map the innermost coma
chemistry to separate native ices from other
sources
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Some Useful Concepts
Optical Depth
• Well known and widely used in astronomy but
often ignored in cometary science
• I/I0 = e-t where t = optical depth
• For simple scattering and absorption
• t=Ns
• where s = extinction cross section
• and N = column density
• A typical photon travels t = 2/3 before being
absorbed or scattered
• We can measure this in the afternoon for DI
ejecta
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Scattering Function
• A single particle (grain of dust or whatever) scatters
light anisotropically
• The phase function describes the distribution of light
with scattering angle, both for microscopic particles and
for large bodies like cometary nuclei and asteroids
• Phase function is often approximated by many different
simple functions of the scattering angle
• For a single particle,
– I = Fsun s p f() where p = geometric albedo and f() is
the scattering function
– Be careful of confusion between geometric albedo and
Bond albedo - geometric is backscattering and Bond is
integrated around the sphere.
– The widely used quantity Afr uses Bond albedo as A,
although this is not well defined for microscopic particles
• We can measure this in the afternoon also
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Typical Cometary Scattering Function
Scattering Functions
Phase Function (normalized)
25
Dust from Ney & Merrill for
comet West (1976 VI)
Nucleus from Lumme &
Bowell model
20
15
Dust
Nucleus
10
5
0
0
50
100
150
200
Phase Angle (180 - scattering)
• See spread sheet for others
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Backup Slides
Deep Impact
Deep Impact -
- An artificial meteorite impact
- 360 kg at 10.2 km/s
- Are there pristine ices at depth?
- What are the surface material properties?
Unlike SL-9 at Jupiter, we will know everything
about the impactor so the only unknowns are in
the target
"It [an asteroid] was racing past them at almost thirty miles a second;
they had only a few frantic minutes in which to observe it closely. The automatic cameras took dozens of photographs,
the navigation radar's returning echoes were carefully recorded for future analysis - and there was just time for a single
impact probe.
The probe carried no instruments; none could survive a collision at such cosmic speeds. It was merely a small slug
of metal, shot out from Discovery on a course which should intersect that of the asteroid.
.....They were aiming at a hundred-foot-diameter target, from a distance of thousands of miles...
Against the darkened portion of the asteroid there was a sudden,
dazzling explosion of light. ...”
____________________
Arthur C. Clarke, 1968. In 2001: A Space Odyssey. Chapter 18
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Scientific Objectives
• Primary Scientific Theme
– Understand the differences between interior and surface
– Determine basic cometary properties
– Search for pristine material below surface
• Secondary Scientific Theme
– Distinguish extinction from dormancy
• Additional Science Addressed
– Address terrestrial hazard from cometary impacts
– Search for heterogeneity at scale of cometesimals
– Calibration of cratering record
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Mission Overview
• 2 spacecraft – Smart Impactor + Flyby
• Fly together until 1 day before impact
– 6-month Earth-to-comet trajectory
• Smart Impactor
– Impactor Targeting Sensor (ITS)
• Scale 10 microrad/pixel
• Used for active navigation to target site
• Images relayed via flyby to Earth for analysis
– Cratering mass (~360 kg at 10.2 km/s)
• Excavates ~100-meter crater in few*100 seconds
– Baseline prediction - other outcomes are possible
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Mission Overview (continued)
• Flyby Spacecraft
– Diverts to miss by 500 km
– Slows down to observe for 800 seconds
– Instruments body-mounted – spacecraft rotates to follow
comet during flyby
• Instruments on Flyby Spacecraft
– High Resolution Imager (HRI)
• CCD imaging at 2 microrad/pixel (0.4 arcsec/pixel)
• 1-5 micron long-slit spectroscopy (R>200, 10 microrad/pix)
– Medium Resolution Imager (MRI)
• CCD imaging at 10 microrad/pixel
• Identical to ITS but with filter wheel added
• Major Earth-based Observing Campaign
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Cometary Dichotomies
•
•
•
•
•
•
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Comets have the most primitive,
accessible material in the SS
Comets must become dormant
There must be many dormant
comets masquerading as NEAs
We know more chemical and
physical details than for other small
bodies in the SS
Abundances in the coma are used to
infer ices in the proto-planetary disk
Comets break apart under small
stresses
•
•
•
•
•
•
We do not know what is hidden
below the evolved surface layers
Is the ice exhausted or sealed in?
We can not recognize dormant
comets among NEAs
We do not know how to use these
details to constrain models of nuclei
Abundances in the coma differ
significantly but in unknown ways
from nuclear abundances
Variation of strength with scale is
totally unknown
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