http://www.uc.edu/geology/geologylist/cometphotos.htm

DPS Keck Workshop November 10, 2015

Topics

What Do We Want to Learn about Comets with High-Resolution IR Spectroscopy?

The Impact of Keck/ NIRSPEC on the Study of Comets.

Information Obtained from High-Resolution IR

Spectroscopy.

The Chemical Diversity of Comets – Can Comets be Compositionally Classified?

WHAT WE TARGET

Many parent molecules have vibrational transitions between 2–5-µm. (e.g. H2O, CO, CH4, C2H6, C2H2, CH3OH, H2CO, HCN, NH3, NH2, OH).

Emission from gas phase molecules in the coma.

WHAT WE WANT TO KNOW

Relate coma abundances to nucleus ice abundances.

Determine how volatiles are stored in the nucleus and released into the coma.

How chemically diverse are comets? Can they be classified?

What can comet chemistry tell us about the environment of the early solar nebula and how comets evolve?

NIRSPEC Echelle Spectrum of 103P/Hartley 2 (UT 2010 Oct 17)

Keck/NIRSPEC for Comet Studies

Capabilities of Keck/NIRSPEC – Spectral Information

• Sensitivity – Deep chemical searches detect many species.

• Spectral Coverage – Simultaneous sampling of multiple lines of many species.

Allows a robust determination of the chemical inventory in comets.

Capabilities of Keck/NIRSPEC – Spatially Resolved Information

Spatial Information reveals: 1) How volatiles and dust are released from the nucleus. 2) What volatiles may be associated together as ices in the nucleus. 3) The relationship between potential parent and daughter species. 4) Coma excitation conditions.

Classifying Comets by Composition – What is the Prognosis?

Remarkable diversity in only seven measured species.

Fragments of 73P/SW3 are more similar to each other than any other comet is to another.

Some comets show evidence of chemical diversity as a function of Rh, but for most comets there is a lack of Rh coverage.

No distinct taxonomic classes evident.

Trends within the diversity

Interesting trends in comet composition with heliocentric distance.

How are molecular abundances correlated?

The Correlation of Volatile Abundances

C2H6 and HCN abundances positively correlated but CH3OH and H2CO abundances uncorrelated.

Volatile Abundances with Heliocentric Distance

Relative abundances of most species with respect to H2O remain constant with Rh inside where H2O sublimation is fully active (Rh < 2 AU).

There is evidence that the relative abundances of some species increase with respect to H2O at smaller Rh. Could this be evidence for the thermal breakdown of precursor species?

Summary • ~ 30 comets have been observed with high-resolution IR spectroscopy (24

with Keck/NIRSPEC). NIRSPEC has provided almost all IR compositional information on Jupiter-family comets.

• Comets observed at IR wavelengths defy meaningful taxonomic classifications because of the overall diversity in a few measured species. Diversity would surely be more pronounced if more species were measured. But some trends exist ……

• Jupiter-family comets are on average more heavily depleted in the most volatile species than Oort cloud comets. Formation or evolution?

• Abundances of some species in comets are positively correlated, while others are uncorrelated with other species.

• Abundances of some species appear constant with Rh at Rh < 2 AU, while there is evidence that others may increase at small Rh. More data needed to definitively prove this.

• The combination of spatial distributions in the coma and correlations between species and behavior with Rh may provide insights into the composition of the volatile grain components.

END

We target mostly parent volatiles that may be primarily released directly from ices.

Some daughter species that are formed in the coma are also in this wavelength range.

What we can detect and why Sensitivity – Depends on

intrinsic line strength, abundance.

Spectral Confusion – Can you discern emissions among a forest of lines?

Atmospheric Extinction – Can you see emissions through atmospheric windows?

Routinely detected molecules in bold