Ionization processes in the atmosphere of Titan

Abstract

Context. The Cassini-Huygens mission has revealed the importance of particle precipitation in the atmosphere of Titan thanks to in-situ measurements. These ionizing particles (electrons, protons, and cosmic rays) have a strong impact on the chemistry, hence must be modeled. Aims. We revisit our computation of ionization in the atmosphere of Titan by cosmic rays. The high-energy high-mass ions are taken into account to improve the precision of the calculation of the ion production profile. Methods. The Badhwahr and O'Neill model for cosmic ray spectrum was adapted for the Titan model. We used the TransTitan model coupled with the Planetocosmics model to compute the ion production by cosmic rays. We compared the results with the NAIRAS/HZETRN ionization model used for the first time for a body that differs from the Earth. Results. The cosmic ray ionization is computed for five groups of cosmic rays, depending on their charge and mass: protons, alpha, Z = 8 (oxygen), Z = 14 (silicon), and Z = 26 (iron) nucleus. Protons and alpha particles ionize mainly at 65 km altitude, while the higher mass nucleons ionize at higher altitudes. Nevertheless, the ionization at higher altitude is insufficient to obscure the impact of Saturn's magnetosphere protons at a 500 km altitude. The ionization rate at the peak (altitude: 65 km, for all the different conditions) lies between 30 and 40 cm −3 s −1 . Conclusions. These new computations show for the first time the importance of high Z cosmic rays on the ionization of the Titan atmosphere. The updated full ionization profile shape does not differ significantly from that found in our previous calculations (Paper I: Gronoff et al. 2009, 506, 955) but undergoes a strong increase in intensity below an altitude of 400 km, especially between 200 and 400 km altitude where alpha and heavier particles (in the cosmic ray spectrum) are responsible for 40% of the ionization. The comparison of several models of ionization and cosmic ray spectra (in intensity and composition) reassures us about the stability of the altitude of the ionization peak (65 km altitude) with respect to the solar activity.

Planetary space weather: scientific aspects and future perspectives

“…These authors concluded that in order to explain the observed disk emissions, the scattered stellar light from Titan's disk is the most likely scenario. It is worth mentioning, however, that according to Gronoff et al (2011), the ionization by each Z-group of cosmic rays in the atmosphere of Titan results in the formation of an ionization layer peaking at 65 km altitude, independently of the solar activity, in consistency with the Huygens measurements (Hamelin et al 2007;López-Moreno et al 2008). The observed correlation between cosmic ray ionization altitudes and aerosol layers urges for further investigations in this direction in the future.…”

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…However at low altitudes, termolecular chemistry can occur with ions formed by galactic cosmic rays (Gronoff et al 2009a(Gronoff et al , 2009b(Gronoff et al , 2011Molina-Cuberos et al 1999). Anicich & McEwan (Anicich & McEwan 2001;Anicich et al 2000;Milligan et al 2001) studied experimentally termolecular ion-molecule reactions of the main ions of Titan's atmosphere.…”

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets

“…Thus ROCKE-3D cannot directly simulate atmospheric escape processes; this would require coupling to upper atmospheric models specifically intended to simulate ionization and escape processes (e.g. Gronoff et al 2011). Furthermore, since ROCKE-3D (like all GCMs) can only simulate time slices of hundreds to thousands of years, it cannot be used directly to address problems of atmospheric evolution such as water loss in moist greenhouse states near the inner edge of the habitable zone.…”