Isotopes of nitrogen on Mars: Atmospheric measurements by Curiosity's mass spectrometer

[1] The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) measured a Mars atmospheric¹⁴N/¹⁵N ratio of 173 ± 11 on sol 341 of the mission, agreeing with Viking's measurement of 168 ± 17. The MSL/SAM value was based on Quadrupole Mass Spectrometer measurements of an enriched atmospheric sample, with CO₂ and H₂O removed. Doubly ionized nitrogen data at m/z 14 and 14.5 had the highest signal/background ratio, with results confirmed by m/z 28 and 29 data. Gases in SNC meteorite glasses have been interpreted as mixtures containing a Martian atmospheric component, based partly on distinctive¹⁴N/¹⁵N and⁴⁰Ar/¹⁴N ratios. Recent MSL/SAM measurements of the⁴⁰Ar/¹⁴N ratio (0.51 ± 0.01) are incompatible with the Viking ratio (0.35 ± 0.08). The meteorite mixing line is more consistent with the atmospheric composition measured by Viking than by MSL.

The vibrational dependence of dissociative recombination: cross sections for N2+

Theoretical ab initio calculations are reported of the cross sections for dissociative recombination of the lowest four excited vibrational levels of N2(+) at electron energies from 0.001 to 1.0 eV. Rydberg vibrational levels contributing to the cross section structures are identified as are dissociative channels contributing more than 10(-16) cm(2) to the total cross sections. In contrast to the prior study of v = 0 (S. L. Guberman, J. Chem. Phys. 137, 074309 (2012)), which showed 2(3)Πu to be the dominant dissociative channel, 4(3)Πu is dominant for v = 1. Both 2 and 4(3)Πu are major routes for dissociative recombination from v = 2-4. Other routes including 2(3)Σu(+), 3(3)Πu, 2(1)Πu, 2(3)Πg, 2(1)Σg(+), 1(1)Δg, and b('1)Σu(+) are significant in narrow energy ranges. The results show that minor dissociative routes, included here for N2(+), must be included in theoretical studies of other molecular ions (including the simplest ions H2(+) and H3(+)) if cross section agreement is to be found with future high resolution dissociative recombination experiments. The calculated predissociation lifetimes of the Rydberg resonances are used in a detailed comparison to two prior storage ring experiments in order to determine if the prior assumption of isotropic atomic angular distributions at "zero" electron energy is justified. The prior experimental assumption of comparable cross sections for v = 0-3 is shown to be the case at "zero" but not at nonzero electron energies. Circumstances are identified in which indirect recombination may be visualized as a firefly effect.

On the strong and selective isotope effect in the UV excitation of N2 with implications toward the nebula and Martian atmosphere

Isotopic effects associated with molecular absorption are discussed with reference to natural phenomena including early solar system processes, Titan and terrestrial atmospheric chemistry, and Martian atmospheric evolution. Quantification of the physicochemical aspects of the excitation and dissociation processes may lead to enhanced understanding of these environments. Here we examine a physical basis for an additional isotope effect during photolysis of molecular nitrogen due to the coupling of valence and Rydberg excited states. The origin of this isotope effect is shown to be the coupling of diabatic electronic states of different bonding nature that occurs after the excitation of these states. This coupling is characteristic of energy regimes where two or more excited states are nearly crossing or osculating. A signature of the resultant isotope effect is a window of rapid variation in the otherwise smooth distribution of oscillator strengths vs. frequency. The reference for the discussion is the numerical solution of the time dependent Schrödinger equation for both the electronic and nuclear modes with the light field included as part of the Hamiltonian. Pumping is to all extreme UV dipole-allowed, valence and Rydberg, excited states of N(2). The computed absorption spectra are convoluted with the solar spectrum to demonstrate the importance of including this isotope effect in planetary, interstellar molecular cloud, and nebular photochemical models. It is suggested that accidental resonance with strong discrete lines in the solar spectrum such as the CIII line at 97.703 nm can also have a marked effect.

On the origin of Titan's atmosphere

The present atmosphere of Titan exhibits evidence of extensive evolution, in the form of rapid photochemical destruction of methane and a large fractionation of the nitrogen and oxygen isotopes. Attempts to recover the initial inventory of volatiles lead toward a model in which nitrogen was originally supplied as NH3, essentially unmodified from its relative abundance in the outer solar nebula. Titan's atmospheric methane, in contrast, appears to have been formed from carbon and other carbon compounds, either by gas phase reactions in the subnebula or by accretional heating during the formation of Titan. These conclusions can be tested by further studies of abundances and isotope ratios in Titan's atmosphere, augmented by studies of comets. The possible similarity of carbon and nitrogen inventories on Titan to those on the inner planets makes this investigation particularly intriguing.

On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

We analyze recently published nitrogen and hydrogen isotopic data to constrain the initial volatile abundances on Saturn's giant moon Titan. The nitrogen data are interpreted in terms of a model of non-thermal escape processes that lead to enhancement in the heavier isotope. We show that these data do not, in fact, strongly constrain the abundance of nitrogen present in Titan's early atmosphere, and that a wide range of initial atmospheric masses (all larger than the present value) can yield the measured enhancement. The enrichment in deuterated methane is now much better determined than it was when Pinto et al. (1986. Nature 319, 388-390) first proposed a photochemical mechanism to preferentially retain the deuterium. We develop a simple linear theory to provide a more reliable estimate of the relative dissociation rates of normal and deuterated methane. We utilize the improved data and models to compute initial methane reservoirs consistent with the observed enhancement. The result of this analysis agrees with an independent estimate for the initial methane abundance based solely on the present-day rate of photolysis and an assumption of steady state. This consistency in reservoir size is necessary but not sufficient to infer that methane photolysis has proceeded steadily over the age of the solar system to produce large quantities of less volatile organics. Our analysis indicates an epoch of early atmospheric escape of nitrogen, followed by a later addition of methane by outgassing from the interior. The results also suggest that Titan's volatile inventory came in part or largely from a circum-Saturnian disk of material more reducing than the surrounding solar nebula. Many of the ambiguities inherent in the present analysis can be resolved through Cassini-Huygens data and a program of laboratory studies on isotopic and molecular exchange processes. The value of, and interest in, the Cassini-Huygens data can be greatly enhanced if such a program were undertaken prior to the prime phase of the mission.