Energy transfer in O collisions with He isotopes and Helium escape from Mars

Abstract

[1] Accurate data on energy-transfer collisions between hot oxygen atoms and the atmospheric helium gas on Mars, are reported. Anisotropic cross sections for elastic collisions of O( 3 P) and O( 1 D) atoms with helium gas have been calculated quantum mechanically and found to be surprisingly similar. Cross sections, computed for collisions with both helium isotopes, 3 He and 4 He, have been used to construct the kernel of the Boltzmann equation describing the energy relaxation of hot oxygen atoms. Computed rates of energy transfer in O+He collisions have been used to evaluate the flux of He atoms escaping from the Mars atmosphere. Atmospheric layers mostly responsible for production of the He escape flux are identified. Our results demonstrate that strong angular anisotropy of scattering cross sections increases the collisional ejection of light atoms and is critical in the evaluation of He escape from Mars, Venus and Earth. Citation: Bovino, S.,

Energy Relaxation of Helium Atoms in Astrophysical Gases

“…Figure 6 shows both the ab initio data as well as the fits. Total cross sections computed by Bovino et al (2011) using the same interaction potential as the current work, but without the modified core, are also shown in Figure 6 between 1 and 5 eV with good agreement to both ab initio and fitted data. To obtain good fits to the data, the large interval was split up into several smaller intervals, each with its own fitting parameters.…”

“…The interaction potential of Bich et al (2007) was used for a description of helium collisions and its isotopes 4 He+ 4 He, 3 He+ 3 He, and 4 He+ 3 He. The potential used by Bovino et al (2011) was employed in our calculations for He+O while the ground-state interaction potential for He+H was constructed using multi-reference configuration interaction (Werner & Knowles 1988) and coupled cluster (Knowles et al 1993) methods, with the final potential being further extrapolated to the complete basis set limit (Varandas 2008). The aug-cc-pv5z basis set was used in the calculation of the He+H ground-state potential.…”

“…(A color version of this figure is available in the online journal.) low probable, large-angle collisions: for example, determination of the back scattering intensity (albedo) of helium ENA fluxes from planetary atmospheres (Futaana et al 2006) or investigations of high momentum-transfer collisions in the escape process (Bovino et al 2011). Large-angle (small impact parameter) scattering of atomic particles at high collision energies requires consideration of inelastic scattering channels even for cases when the total elastic cross section is significantly larger than inelastic ones, which is out of the scope of the present investigation and plays a minor role in the energy relaxation of fast helium atoms.…”

“…Escape of energetic oxygen and helium atoms, and their isotopes, is important for investigations of the evolution of planetary atmospheres (Bovino et al 2011;Zhang et al 1993;Hunten 1993). Helium gas is a major constituent of the upper atmospheres of the terrestrial and gas giant planets (Fox & Hać 2009;Krasnopolsky & Gladstone 2005).…”

He2+ transport in the Martian upper atmosphere with an induced magnetic field

“…To approach these cross-sections, we used the same analytic approximations of s el as for H + + O 2 , N 2 , and O taken from the model of the proton aurora at Earth [Gérard et al, 2000] in the energy range [100 eV to 10 keV]. The only calculation we found in the literature was cross-sections for elastic He + O collisions up to 5 eV [Bovino et al, 2011]. Therefore, we normalized the analytic approximation for the elastic collisions He 2+ + CO 2 to be equal to the calculated one at 1 eV, i.e., by the factor s el (He + O; E = 1 eV)/s el (H + +O 2 ; E = 1 eV) = 3.8 Â 10 -15 /3.02 Â 10 -14 = 0.119.…”