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Active Asteroids: Mystery in the Main Belt

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Active Asteroids: Mystery in the Main Belt ActiveActive Asteroids:Asteroids: MysteryMystery inin thethe MainMain BeltBelt Henry H. Hsieh and David Jewitt Institute for Astronomy – University of Hawaii IAUS 229: Asteroids, Comets, Meteors (Buzios, Brazil) Background:Background: AsteroidsAsteroids && CometsComets n Observationally distinct… - Asteroids: point sources - Comets: fuzzy n Physically distinct… - Asteroids: rocky, inert - Comets: icy, volatile-rich n 1 / 2 n Dynamically distinct … a J é a obj 2 ù - Asteroids: low e, low i, T > 3 T J = + 2 cos( i) ê (1- e )ú J aobj ë a J û - Comets: high e, high i, TJ < 3 n Distinct origins… - Asteroids: formed in main belt - Comets: from Kuiper belt and beyond n Well…not exactly… a=3.16 AU; e=0.17; i=1.39° 133P/(7968)133P/(7968) ElstElst--PizarroPizarro 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)133P/(7968) ElstElst--PizarroPizarro n An active asteroid! 133P/(7968) Elst-Pizarro n Dynamically asteroidal but a = 3.16 AU P = 5.61 yr observationally cometary i = 1.39° e = 0.17 - orbits among Themis family T = 3.16 - dust trail seen in 1996 and 2002 J n Narrow trail, no apparent coma; no gas measurements available - Hsieh et al. 2004, AJ 127, 2997 n 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)133P/(7968) ElstElst--PizarroPizarro Impulsive emission Continuous emission ExplainingExplaining ElstElst--PizarroPizarro n Continuous, recurrent activity à volatile sublimation n Outgassing could be last gasps of comet or recent impact could have exposed near-surface ices n Possibly seasonally modulated n Next emission expected late 2007 ExplainingExplaining ElstElst--PizarroPizarro n EP could be a lost comet, perhaps driven onto its current orbit via non-gravitational outgassing forces n 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 LostLost Comet?...Comet?... n Looks like a comet…could be a comet! n 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 n JFCàMB transition produced in grav. models - D/Pigott à EP-like orbit, though high-i (Fernández et al. 2002) n Transition likely to be rare: EP could be alone ……OrOr IcyIcy Asteroid?Asteroid? n 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 n 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) n Aqueous alteration seen in asteroids - 0.7mm & 3mm features in C asts (review by Rivkin et al. AstIII) - 3.0mm: e.g. Lebofsky 1980, Feierberg et al. 1985, Jones et al. 1990 - 0.7mm: e.g. Vilas & Gaffey 1989, Barucci et al. 1998 ……OrOr IcyIcy Asteroid?Asteroid? n 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) n Ceres may have surface and subsurface ice - Lebofsky et al. 1981: water of hydration (3mm), surface frost (3.1mm) - Fanale & Salvail 1989: moderate-depth subsurface ice stable for Gyrs - A’Hearn & Feldman 1992: OH emission (photodissociated H2O) n Implies other active asteroids should exist! - Where are they? WhyWhy NoNo OtherOther ActiveActive AsteroidsAsteroids Seen?Seen? n Activity is likely weak and transient - Discovery & recovery images typically not deep enough - Few small (km-scale) asteroids studied very intensely n Collisional activation may be necessary - Larger cross-sections preferred à favors larger activated asteroids - Larger asts also better suited for preserving volatiles from Sun n 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 n EP could be bona fide comet FindingFinding MoreMore ActiveActive AsteroidsAsteroids n EP in Themis family: a good place to start! n Family members may share composition - Spectroscopically supported, e.g. Ivezic et al. 2002 - Themis asteroids mostly C-type, e.g. Florczak et al. 1999 n Collision rates elevated in families - Farinella & Davis 1992, Dell’Oro et al. 2001 - suggests Koronis family also a good place to look n 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) FindingFinding MoreMore ActiveActive AsteroidsAsteroids n Hawaii Trails Project - survey of km-scale main belt asts. - from Mauna Kea, CTIO, and Lulin n 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 n No other active asteroids found…yet… FindingFinding MoreMore ActiveActive AsteroidsAsteroids Surveyed so far… FindingFinding MoreMore ActiveActive AsteroidsAsteroids Surveyed so far… ChallengesChallenges n Only one EP known! … Is it typical? n Comet hypothesis difficult to confirm or refute n 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 n Unclear when to look for activity - Need to continue monitoring inactive objects n Lots of possible candidates - Need to refine target selection criteria, if possible - Will be helped by new surveys and other observers stacking LC data ConclusionsConclusions n There’s a comet in the asteroid belt! n EP could be a lost comet à more work on NG evolution needed n EP could be an icy asteroid à more EP-like objects should exist n An open problem…no conclusions! BackBack toto Basics:Basics: AsteroidsAsteroids && CometsComets n Observationally distinct? - Asteroids: usually point sources; EP looks fuzzy though - Comets: can be fuzzy and point source-like at different times n Physically distinct? - Asteroids: direct and indirect evidence of volatile ice content - Comets: extinct comets with exhausted volatile supplies could exist n Dynamically distinct? - Asteroids: asteroids in TJ < 3 orbits probably not all dead comets - Comets: comets seen occupying TJ > 3 orbits n Distinct origins? - Asteroids: cometary interlopers may be possible in main belt - Comets: could come from both outer SS and inner SS SoSo What?What? n Yet another breakdown in traditional comet- asteroid distinctions n Independent demonstration of asteroidal ice n Implications for origin of terrestrial water n Opportunity for studying asteroid interiors - a natural Deep Impact experiment? n Complicates identifying NEO source regions a=3.16 AU; e=0.17; i=1.39°.
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