The EPOXI flyby of comet Hartley 2 revealed several new results about the comet's activity, grains, and nucleus. Much of Hartley 2's water comes from icy grains moving tailward, which sublime slowly. The two ends of the comet have very different CO2 abundances, and CO abundance was found to be much lower than expected. The nucleus appears to have ice on its surface and heterogeneous thermal properties. Every cometary encounter continues to surprise scientists with new phenomena and processes that demonstrate comets' diversity.

1. EPOXI @ Hartley 2
Overview
Mike A’Hearn
&
DIXI Science Team
&
Associated Remote Sensing Observers
EPSC-DPS Joint Meeting

2. 2

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4. 4

5. Hartley 2 is Different
• Previously known and published
• Large changes in rotational state
• Activity driven by CO2
• Much water from icy grains
• Very high CO2/CO ~ 100
• Is Hartley 2 the prototype for
hyperactive comets aka comets with
large active fraction?
5

6. New Results - Grains
• Much of the water comes from grains
moving tailward - Knight
• Radiation pressure? Or sunward sublimation?
• Grains sublime slowly - Kelley/Protopapa
• Can now separate icy & refractory grains
- Protopapa
• New measurements of grain trajectories
- Hermalyn
6

7. New Results - Activity
• Two ends have very different CO2 abundance -
Feaga/Besse
• New results on low CO abundance & atomic species -
Weaver/Feldman
• OH spatially separate from other radicals - Knight
• nearly pure icy grains?
• Modeling CO2 Jets - Syal
• Light curves - Bodewits/Jehin/Combi/Meech/
Waniak
• IR spatial distribution - Mumma
7

8. New Results - Nucleus
• Ice on the surface - Sunshine
• Photometric Properties - Li
• Thermal properties - Groussin
• Rotational State & Models - Chesley/
Taylor/Drahus/Mueller/Bowling
(density)
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10. 9

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12. Motion of Chunks
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13. Absolute Abundances
• Data from E-55 h
• FOV large enough to avoid optical depth problems
• Same orientation as encounter (3 cycles earlier)
• CO2 ~20% of H2O at peaks; 10% at minima
• >2x higher than measured with ISO in 1997
• Q(H2O) down 5x from 1997
• Excited rotation illuminates all surface - ends are primordially
different
• CO ~0.2% in Hartley 2 at time of encounter using HST!
• Kawakita et al. with Akari find CO2/H2O 5-30% in many
comets, both Halley-type & Jupiter family, inside 2.5 AU
11

14. Implications
• Waist is probably redeposited
material, including H2O ice
• CO << CO : Not expected in
2
protoplanetary disk
• Tempel 1 CO ~ CO2; Halley CO >> CO2
(extended source? CO ~ CO2 from nucleus?)
• Is the abundance ratio primordial?
• CO2/H2O different in two lobes
• Argues against evolution
12

15. P-P Disk Abundances
• Dodson-Robinson et al. 2008 Icarus 200, 672
(as an example)
• r > 30 AU, disk is isothermal, 20K
• CO and CO2 both mostly ice
• CO/CO2 ~ 104 (in icy mantles)
• CO 40-50K, CO2 ~100K, H2O ~190K
• But see newer work on ice formation (surface
reactions)
• Garrod & Pauly 2011 ApJ 735, 15
13

16. Disk Temperatures
• Disk radial temperature
profiles for first 2 Myrs
• CO2 ice line inside present
Saturn, CO ice line inside
present Uranus
• How does planetary
migration alter this
picture?
• Did SP comets really form
near the giant planets?
Dodson-Robinson et al., 2009
14

17. Global Implications
• Suggest that most comets we see today formed at
10-30 AU
• Both JF and Oort cloud comets, or at least a significant
fraction of them
• Radial migration of giant planets mixed them up
• cf. Walsh et al. 2011 (Nature 475, 206) on migration
mixing up the asteroid belt
• Cometesimals were mixed during aggregation into
comets
• Comets were mixed with scattered disk & classical KB
15

18. Conclusions
• Every visit to a comet has surprised
us
• New phenomena, new physical processes
• Comets are more diverse than we thought
• We are slowly beginning to separate
evolutionary properties from
primordial properties
• Are comets as pristine as we claim?
16

19. Backup

20. P-P Disk Abundances
• Dodson-Robinson et al. 2008 Icarus
200, 672 (as an example)
• r > 30 AU, disk is isothermal, 20K
• CO and CO2 both mostly ice
• CO/CO2 ~ 10 (in icy mantles)
4
• CO 40-50K, CO2 ~100K, H2O ~190K
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21. Hartley 2 vs. Tempel 1
• Nuclear radius ~0.2x Tempel 1
• Activity in OH and CN: 10x Tempel
• Activity in dust: 2x Tempel 1
• Encounter closer to sun by ~1/√2
• All signals from coma stronger, gas 20x, dust 4x
• Activity estimate was pre-encounter, Earth-based
data show secular decrease (as seen in Tempel 1)
• Why is Hartley 2 proportionately so active?
• P ~ 18 h vs. ~40h
rot
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22. Carbon-Chain Depletion
• Still the only correlation
between chemistry and
dynamical history (A’Hearn
et al. 1995)
• Suggests a boundary in the
classical Kuiper belt at
which T passes a threshold
for certain reactions or
condensations
H B W2 T1 H2 CG
C2/CN +0.13 -0.36 -0.21 -0.09 +0.08 -0.31
TJ -0.61 2.56 2.88 2.97 2.64 2.74
T D D T T D
20

23. Carbon-Chain Depletion
• Still the only correlation
between chemistry and
dynamical history (A’Hearn
et al. 1995)
• Suggests a boundary in the
classical Kuiper belt at
which T passes a threshold
for certain reactions or
condensations
H B W2 T1 H2 CG
C2/CN +0.13 -0.36 -0.21 -0.09 +0.08 -0.31
TJ -0.61 2.56 2.88 2.97 2.64 2.74
T D D T T D
20

24. Carbon-Chain Depletion
• Still the only correlation
between chemistry and
dynamical history (A’Hearn
et al. 1995)
• Suggests a boundary in the
classical Kuiper belt at
which T passes a threshold
for certain reactions or
condensations
H B W2 T1 H2 CG
C2/CN +0.13 -0.36 -0.21 -0.09 +0.08 -0.31
TJ -0.61 2.56 2.88 2.97 2.64 2.74
T D D T T D
20

25. DI Flyby Spacecraft
Medium Res camera
(MRI)
•10 µrad/pixel
•8 filters
•High Res Camera (HRIV)
•2 µrad/pixel
•8 filters
•IR Spectrometer (HRII)
•10 µrad/pixel
•Slit 10 µrad x 5 mrad
•1.05 < λ < 4.8 µm
•230 < λ/δλ < 700
21

26. DI Flyby Spacecraft
Medium Res camera
(MRI)
•10 µrad/pixel
•8 filters
•High Res Camera (HRIV)
•2 µrad/pixel
•8 filters
•IR Spectrometer (HRII)
•10 µrad/pixel
•Slit 10 µrad x 5 mrad
•1.05 < λ < 4.8 µm
•230 < λ/δλ < 700
21

27. DI Flyby Spacecraft
Medium Res camera
(MRI)
•10 µrad/pixel
•8 filters
•High Res Camera (HRIV)
•2 µrad/pixel
•8 filters
•IR Spectrometer (HRII)
•10 µrad/pixel
•Slit 10 µrad x 5 mrad
•1.05 < λ < 4.8 µm
•230 < λ/δλ < 700
21

28. DI Flyby Spacecraft
Medium Res camera
(MRI)
•10 µrad/pixel
•8 filters
•High Res Camera (HRIV)
•2 µrad/pixel
•8 filters
•IR Spectrometer (HRII)
•10 µrad/pixel
•Slit 10 µrad x 5 mrad
•1.05 < λ < 4.8 µm
•230 < λ/δλ < 700
21

29. DI Flyby Spacecraft
Medium Res camera
(MRI)
•10 µrad/pixel
•8 filters
•High Res Camera (HRIV)
•2 µrad/pixel
•8 filters
•IR Spectrometer (HRII)
•10 µrad/pixel
•Slit 10 µrad x 5 mrad
•1.05 < λ < 4.8 µm
•230 < λ/δλ < 700
21

30. Deep Impact
• Main Goal: Compare volatiles inside
nucleus with ambient gases released
• Other Goals:Physical properties,
Cometary heterogeneity
• Launch 12 Jan ’05 Impact 4 Jul ’05
22

31. DI Results
• No difference in volatiles down to ~20m
• Surface erodes as fast as thermal wave propagates inward?
• KOSI & theory both say there should be differences
• Dry ice and water ice sublime from different parts of
the nucleus
• Can’t exclude evolutionary process
• Nucleus is very porous (>75%)
• Both locally at impact site & bulk of nucleus
• Layers are ubiquitous
• TALPS model of formation
23

32. CN Anomaly
• An early (Sept)
distraction
• ~800 tons of CN over
2 weeks
• No increase in H2O
• Little increase in
optically important
grains
• Not like most
cometary outbursts
• Instrumental effect?
TCM 19
TCM 20 TCM 21
19 Jul
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33. Movie Flying Under Comet
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34. Movie Flying Under Comet
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