Comets in context: comparing comet observations with solar system nebula models

Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system because comets retain ices from the dense regions of the protoplanetary disk (PPD) midplane. We have modeled the chemistry of the early solar nebula and compared the predictions of PPD midplane ice compositions with ensemble compositional properties of 29 comets studied in parent volatiles. Our most important findings are: (1) The range of comet compositions can only be matched if interstellar ices are retained, supporting comet ices preserve heritage from the presolar dense molecular cloud. In contrast simulations where the initial composition of disk material is “reset”, wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. (2) Our fiducial model, in which ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. (3) Using models that combine material processed under different thermal conditions, we find that a combination of warm (CO-poor) and cold (CO-rich) material is required to account for both the average properties of the Jupiter-family and Oort cloud comets, and the individual comets we consider. This could occur by large-scale transport (either radial or vertical) of ice-coated dust grains in the early solar system. (4) Comparison of the models to the average Jupiter-family and Oort cloud comet compositions suggests the two families formed in overlapping regions of the disk, in agreement with the findings of A’Hearn et al. (2012) and with the predictions of the Nice model. Combining such simulations treating the midplane ice inventory with ongoing efforts to disentangle formative from evolutionary signatures in measured abundances of comet volatiles are both essential for understanding comets’ connections to the early solar system.

Reference:

A’Hearn et al. (2012) ApJ, 758, 29

Acknowledgements:

We acknowledge the support of NASA’s Emerging Worlds Program award No. 15-EW15-2-0150 (Willacy, Turner). Integration with comet observations was also supported by NASA EW80NSSC20K0341 (Dello Russo, Vervack), and NSF AST-2009398 (Bonev) and AST-2009910 (Gibb). The project developed out of workshops on “Exploring the Early Solar System by Connecting Comet Composition and Protoplanetary Disk Models,” held the International Space Science Institute, Bern.