Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011apj...741l..10i&link_type=abstract
The Astrophysical Journal Letters, Volume 741, Issue 1, article id. L10 (2011).
Astronomy and Astrophysics
Astronomy
Planets And Satellites: Atmospheres, Planets And Satellites: Formation, Planets And Satellites: Individual: Titan, Shock Waves
Scientific paper
Titan is the only satellite that possesses a thick atmosphere, composed mainly of N2 and CH4. However, its origin and evolution remain largely unknown. Knowledge of the acquirement of a N2 atmosphere on Titan would provide insights into nitrogen evolution in planetary atmospheres as well as the formation of satellite systems around gas giants. Previous studies have proposed that the atmospheric N2 would have been converted from NH3 via shock heating by accreting satellitesimals in the highly reducing proto-atmosphere composed of NH3 and CH4. Nevertheless, the validity of this mechanism strongly depends on both the composition of the proto-atmosphere and kinetics of shock chemistry. Here, we show that a CO2-rich oxidizing proto-atmosphere is necessary to form N2 from NH3 efficiently by atmospheric shock heating. Efficient shock production of N2 is inhibited in a reducing proto-atmosphere composed of NH3 and CH4, because CH4 plays as the coolant gas owing to its large heat capacity. Our calculations show that the amount of N2 produced in a CO2-rich proto-atmosphere could have reached ~20 times that on the present Titan. Although further quantitative analysis are required (especially, the occurrence of catalytic reactions), our results imply that the chemical composition of satellitesimals that formed the Saturnian system is required to be oxidizing if the current atmospheric N2 is derived from the shock heating in the proto-atmosphere during accretion. This supports the formation of regular satellites in an actively supplied circumplanetary disk using CO2-rich materials originated from the solar nebula at the final stage of gas giant formation.
Ishimaru Ryo
Matsui Takafumi
Mousis Olivier
Sekine Yasuhito
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