Main-Belt Comets as Tracers of Ice in the Inner Solar System

1. Main-Belt Comets as Tracers of Ice in the Inner Solar System • Henry H. Hsieh • University of Hawaii • Hubble Fellows SymposiumSpace Telescope Science Institute • Baltimore, Maryland • 2013 March 4

2. Asteroids vs. Comets Observationally different Physicallydifferent Dynamicallydifferent Different source regions

3. Main-Belt Comets Observationally and (probably) physically cometary Dynamically indistinguishable from main-belt asteroids

4. Main-Belt Comets • Activity likely driven by sublimation • Sublimation implies ice is stillpresent today in main belt • MBCs likely native to the main belt- All are decoupled from Jupiter so unlikely to be captured from outer solar system

5. Main-Belt Comets • Can use MBCs as tracers of ice in inner solar system- Discovered from limited & biased data so many more likely exist- Size & distribution of dormant (icy but inert) population unknown- Could constrain protosolar disk models and give insights into primordial terrestrial water delivery • To do so, need to answer a few key questions! Are they really icy? Are they really from the inner solar system? If yes to both, how do we move forward?

6. Are MBCs really icy? • Long duration inconsistent with impact generation- Emission behavior can be determined using Finson-Probstein dust modeling Hsieh et al. (2012b) • Repeated MBC activityon successive orbits- Multiple impacts on single objects unlikely to be so frequent / reliable • Evidence is indirect and notuniform for all MBCs, however Hsieh et al. (2010, 2011b)

7. Are MBCs really icy? • Other evidence of water (at least in past) on asteroids- Meteorites and primitive asteroids known to have hydrated minerals; thermal models predict some ice should survive- Ice detected (maybe) on (24) Themis- Unambiguous present-day water ice detection in main belt still elusive • Thermal models indicatethat water ice could actually be long-lived- Much of main belt lies beyond “buried snow-line” and could have subsurface ice that has survived for Gyrs- MBCs associated with asteroid families (at least 4) could have surfaces that are even younger- Near surface ice on extremely young asteroids could be abundant and easily accessible to impacts Rivkin & Emery (2010) Beck et al. (2011) Schorghofer (2008)

8. Are MBCs really icy? • Likely asteroid impacts have been observed- P/2010 A2 (LINEAR): detached tail, unusual dust tail structures, located in inner part of asteroid belt- (596) Scheila: well-known asteroid, large nucleus (d=113km), unusual morphology- P/2012 F5 (Gibbs): ~10 arcmin tail; active near aphelion • Determined to be impact events- P/2010 A2: Long-lived dust tail but highly unusual morphology suggests impact- Scheila: Dust cloud consistent with hollow impact ejecta cone; confirmed by dust modeling- P/Gibbs: Dust modeling indicates dust ejected as a single impulse, not as a prolonged event; consistent with impact, and not sustained sublimation Jewitt et al. (2010) Ishiguro et al. (2011) Hsieh et al. (2013, in prep)

9. Are MBCs from the inner SS? • Dynamical simulations indicatethat most MBCs are stable for>100 Myr (where most cometshave lifetimes of ~104-105 yrs) • Some (e.g. 238P/Read, 259P/Garradd) not completely stable though- Interlopers from outer solar system?- Or from elsewhere in the main asteroid belt?- Critical to know if MBCs are to be used as compositional probes Hsieh et al. (2012c) Haghighipour (2009); Jewitt et al. (2009)

10. Are MBCs from the inner SS? • Nice model suggests main beltmay have substantial TNOcontamination, however • Meanwhile, Grand Tack modelsuggests that the entire outerbelt could consist of outersolar system material Levison et al. (2009) • Better understanding of origin of main belt imperative for correct interpretation of MBCs! Walsh et al. (2011)

11. How do we move forward? • Need to discover more MBCs • Need to confirm presence of ice or gas in a main-belt asteroid • Need to physically characterize known and newly-discovered MBCs • Need robust theoretical dynamical and thermal models for interpreting findings

12. Discovering more... • Crucial for understanding true extent and distribution • All-sky surveys (e.g. Pan-STARRS, LSST) provide deep all-sky sampling, but reliable comet detection is challenging • Two MBCs associated with very young asteroid families; could be a promising method of candidate identification Hsieh et al. (2012b)

13. Confirming gas/ice... • Attempts to detect gas in MBCs unsuccessful so far- Searches for CN (3880 A) in P/Garradd, P/La Sagra, P/2006 VW139, and P/2012 T1 conducted with Keck & Gemini upon discovery; estimated limits of QH2O ~ 1026 mol s-1 • Attempts also made to search for H2O (557 GHz) in 176P and P/2012 T1 with Herschel/HIFI; QH2Olimits similar to previous work • Detailed study may require spacecraft visit Jewitt et al. (2009); Hsieh et al. (2012b,c); Hsieh (2013, in prep)

14. Physical characterization... • Need to confirm sublimation as likely cause of activity • Want to know characteristics of MBC population (e.g., activity levels, orbital dependence, etc.), and how MBC nuclei compare to background main-belt population • Can we detect [subsurface] ice in absence of activity? What is distribution of ice (not just MBCs)? • What physical conditions facilitate ice preservation, and how is activity triggered and modulated? Hsieh et al. (2012b) Hsieh et al. (2012c)

15. Theoretical modeling... • Want to better understand thermal and collisional evolution of MBCs and effects on ice preservation... How much ice do they contain now? How much ice did they contain originally? • What part(s) of the solar system do MBCs really sample? Can we develop observational tests to answer this? • How large a role did MBCs actually play in the primordial delivery of terrestrial water? Can we develop observational or experimental tests to answer this?

16. Thank you!