Active Asteroids: Mystery in the Main Belt

Active Asteroids:
Mystery in the Main Belt
Henry H. Hsieh and David Jewitt
Institute for Astronomy – University of Hawaii
IAUS 229: Asteroids, Comets, Meteors (Buzios, Brazil)
Background: Asteroids & Comets
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Observationally distinct…
- Asteroids: point sources
- Comets: fuzzy
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Physically distinct…
- Asteroids: rocky, inert
- Comets: icy, volatile-rich
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Dynamically distinct …
- Asteroids: low e, low i, TJ > 3
- Comets: high e, high i, TJ < 3
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Distinct origins…
aJ
 a obj

TJ =
+ 2 cos( i ) 
(1 − e 2 ) 
a obj
 aJ

- Asteroids: formed in main belt
- Comets: from Kuiper belt and beyond
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Well…not exactly…
1/ 2
a=3.16 AU; e=0.17;
e=0.17 i=1.39°
133P/(7968) Elst-Pizarro
UT 2002 August 19, UH 2.2m
UT 2002 September 07, UH 2.2m
UT 2002 November 06, UH 2.2m
UT 2002 December 27, UH 2.2m
133P/(7968) Elst-Pizarro
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An active asteroid!
Dynamically asteroidal but
observationally cometary
- orbits among Themis family
- dust trail seen in 1996 and 2002
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133P/(7968) Elst-Pizarro
a = 3.16 AU P = 5.61 yr
i = 1.39°
e = 0.17
TJ = 3.16
Narrow trail, no apparent coma;
no gas measurements available
- Hsieh et al. 2004, AJ 127, 2997
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Unexplained as impulsive impact
- Recurrent: chances of 2 impacts low
- Persists too long: impulse would fade faster
- Extends too far: impulsive emission could
not reach observed length without detaching from nucleus
133P/(7968) Elst-Pizarro
Impulsive emission
Continuous emission
Explaining Elst-Pizarro
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Continuous, recurrent activity à volatile sublimation
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Outgassing could be last gasps of comet or recent
impact could have exposed near -surface ices
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Possibly seasonally modulated
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Next emission expected late 2007
Explaining Elst-Pizarro
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EP could be a lost comet, perhaps driven
onto its current orbit via non-gravitational
outgassing forces
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EP could be an icy asteroid, or an “activated
asteroid”, a native member of the main belt
with preserved volatiles that have become
recently exposed
Lost Comet?...
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Looks like a comet…could be a comet!
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2P/Encke also has orbit with TJ>3
- thought to be heavily influenced by non-gravitational forces
- Steel & Asher 1996, Fernández et al. 2002, Pittich et al. 2004
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JFCàMB transition produced in grav. models
- D/Pigott à EP-like orbit, though high-i (Ferná ndez et al. 2002)
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Transition likely to be rare: EP could be alone
…Or Icy Asteroid?
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Ice accretion possible in primordial inner SS
- Ice grains from >5AU can drift into 3-5 AU zone and be accreted
- e.g. Cyr et al. 1998, Mousis & Alibert 2005
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Aqueous alteration seen in meteorites
- CI/CM carb. chondrites (Kerridge & Bunch 1979); linked to C-types
- Alteration models (anhydrous parent body + water + 26Al or
electrical induction heating) able to produce observed minerals
(e.g. Grimm & McSween 1989, Rosenberg et al. 2001)
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Aqueous alteration seen in asteroids
- 0.7µm & 3µm features in C asts (review by Rivkin et al. AstIII)
- 3.0µm: e.g. Lebofsky 1980, Feierberg et al. 1985, Jones et al. 1990
- 0.7µm: e.g. Vilas & Gaffey 1989, Barucci et al. 1998
…Or Icy Asteroid?
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Unaltered asteroids/meteorites also seen
- may still have primordial ice, e.g. Jones et al. 1990, Scott & Krot 2005
- thermal models of aqueous alteration suggest persistent ice
(e.g. Grimm & McSween 1989, Cohen & Coker 2000)
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Ceres may have surface and subsurface ice
- Lebofsky et al. 1981: water of hydration (3µm), surface frost (3.1µm)
- Fanale & Salvail 1989: moderate-depth subsurface ice stable for Gyrs
- A’Hearn & Feldman 1992: OH emission (photodissociated H2O)
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Implies other active asteroids should exist!
- Where are they?
Why No Other Active Asteroids Seen?
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Activity is likely weak and transient
- Discovery & recovery images typically not deep enough
- Few small (km-scale) asteroids studied very intensely
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Collisional activation may be necessary
- Larger cross-sections preferred à favors larger activated asteroids
- Larger asts also better suited for preserving volatiles from Sun
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Ejection velocity must exceed escape velocity
- For EP, vesc~vdust~1 m/s à favors smaller active asteroids
- Smaller asts also less affected by 26Al heating; likely less altered
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EP could be bona fide comet
Finding More Active Asteroids
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EP in Themis family: a good place to start!
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Family members may share composition
- Spectroscopically supported, e.g. Ivezic et al. 2002
- Themis asteroids mostly C-type, e.g. Florczak et al. 1999
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Collision rates elevated in families
- Farinella & Davis 1992, Dell’Oro et al. 2001
- suggests Koronis family also a good place to look
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Parent body may have protected interior ices
- Themis family ~2 Gyr old, maybe younger (Marzari et al. 1995)
- suggests younger families also good places to look, e.g. Veritas
(8.3 Myr), Karin (5.8 Myr), Iannini (<5 Myr) (Nesvorny et al. 2003)
Finding More Active Asteroids
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Hawaii Trails Project
- survey of km-scale main belt asts.
- from Mauna Kea, CTIO, and Lulin
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About 200 Themis asteroids surveyed so far
- 17.2 > HV > 13.8 à r ~ 0.5 - 6.0 km à vesc ~ 0.5 - 4.0 m/s
- repeat visits to 19 objects
- objects bracket EP in orbital elements and abs. mag.
- also ~50 Koronis family asteroids
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No other active asteroids found…yet…
Finding More Active Asteroids
Surveyed so far…
Finding More Active Asteroids
Surveyed so far…
Challenges
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Only one EP known! … Is it typical?
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Comet hypothesis difficult to confirm or refute
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Activated asteroids possible but likely rare
- Collisional hypothesis involves size preference conflict
- Not all collisions will necessarily be activations
- Activated sites expected to have finite lifetimes
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Unclear when to look for activity
- Need to continue monitoring inactive objects
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Lots of possible candidates
- Need to refine target selection criteria, if possible
- Will be helped by new surveys and other observers stacking LC data
Conclusions
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There’s a comet in the asteroid belt!
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EP could be a lost comet
à more work on NG evolution needed
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EP could be an icy asteroid
à more EP-like objects should exist
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An open problem…no conclusions!
Back to Basics: Asteroids & Comets
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Observationally distinct?
- Asteroids: usually point sources; EP looks fuzzy though
- Comets: can be fuzzy and point source-like at different times
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Physically distinct?
- Asteroids: direct and indirect evidence of volatile ice content
- Comets: extinct comets with exhausted volatile supplies could exist
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Dynamically distinct?
- Asteroids: asteroids in TJ < 3 orbits probably not all dead comets
- Comets: comets seen occupying TJ > 3 orbits
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Distinct origins?
- Asteroids: cometary interlopers may be possible in main belt
- Comets: could come from both outer SS and inner SS
So What?
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Yet another breakdown in traditional cometasteroid distinctions
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Independent demonstration of asteroidal ice
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Implications for origin of terrestrial water
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Opportunity for studying asteroid interiors
- a natural Deep Impact experiment?
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Complicates identifying NEO source regions
a=3.16 AU; e=0.17;
e=0.17 i=1.39°