Isotope Ratios in the Martian Upper Atmosphere Measured by MAVEN NGIMS

Isotope ratios manifest escape of the atmosphere to space and thus evolution of the climate through time, because lighter isotopes are removed from the atmosphere at a faster rate than heavier isotopes. Therefore, atmospheres that have experienced extensive loss are enriched in the heavier isotopes of escaping elements. This occurs because the upper atmosphere, the reservoir of gas which escapes to space, is enriched in lighter isotopes due to diffusive separation and lighter isotopes have smaller escape energies. The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) measures the abundances of a number of molecular isotopologues in the upper atmosphere, enabling an investigation of isotope ratios in the upper atmosphere of Mars. MAVEN NGIMS has now collected thousands of vertical profiles of major isotope ratios, such as those of oxygen and carbon in a number of molecules and ions, over a wide range of atmospheric conditions: over the entire Martian day, across most latitudes, and through the seasons of multiple Mars years.

In the upper atmosphere, above the homopause, the ratios of heavier isotopes to lighter isotopes in neutral species decrease with height due to diffusive separation, because lighter species have larger scale heights than heavier species. The expected decrease with height of measured isotope ratios is observed, for example, in the δ¹³C ratio in CO₂ measured by NGIMS using m/z channels 45, corresponding to ¹³C¹⁶O₂, and 44 corresponding to ¹²C¹⁶O₂ (Figure 1). Significant variation of δ¹³C in CO₂ is found with Martian local time (Figure 2). This trend follows the trend in temperature. Where the atmosphere is cooler, δ¹³C is smaller, and where the atmosphere is warmer δ¹³C is larger. The observed trend can be explained by a process which begins with heating on the dayside, causing the upward transport from below of gas relatively enriched in the heavier isotope, followed by subsolar-to-antisolar flow and the downward transport from above of gas relatively enriched in the lighter isotope on the cooler nightside. The isotope ratios in CO₂ are of interest because CO₂ is the main atmospheric species and thus the predominant atmospheric reservoir of C and O atoms which may escape to space. Isotope ratios other than ¹³C/¹²C are also measured, such as ¹⁸O/¹⁶O in CO₂ and O₂⁺.