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Main-Belt Comets as Tracers of Ice in the Inner Solar System. Henry H. Hsieh University of Hawaii Hubble Fellows Symposium Space Telescope Science Institute Baltimore, Maryland 2013 March 4. Asteroids vs. Comets. Observationally different. Physically different. Dynamically different.

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main belt comets as tracers of ice in the inner solar system
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
asteroids vs comets
Asteroids vs. Comets

Observationally different

Physicallydifferent

Dynamicallydifferent

Different source regions

main belt comets
Main-Belt Comets

Observationally and (probably) physically cometary

Dynamically indistinguishable from main-belt asteroids

main belt comets1
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
main belt comets2
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?

slide6

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)

slide7

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)

slide8

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)

slide9

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)

slide10

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)

slide11

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
slide12

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)

slide13

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)

slide14

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)

slide15

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?
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