A Nitrogen-rich Atmosphere On Ancient Mars Consistent With Isotopic Evolution Models

By Keith Cowing
February 11, 2022
Filed under
A Nitrogen-rich Atmosphere On Ancient Mars Consistent With Isotopic Evolution Models
A model for the long-term changes of the free nitrogen reservoir on Mars. The free nitrogen reservoir is comprised of N2 adsorbed in the regolith and N2 in the atmosphere, and it changes over time due to sputtering loss, photochemical loss, ion loss, volcanic outgassing, and nitrate deposition. The regolith and the atmosphere are assumed to exchange isotopes over geologic timescales driven by the temperature variations due to orbital obliquity changes63. We do not include impact additions or removal because the major impacts should have occurred before the modeled period1, 2 (from 3.8 Ga to present). Nor do we include the impact decomposition of near-surface nitrates64 explicitly, as its rate is less than the present-day outgassing and photochemical escape rates by several orders of magnitude (Supplementary Information H).

The ratio of nitrogen isotopes in the Martian atmosphere is a key constraint on the planet’s atmospheric evolution. However, enrichment of the heavy isotope expected due to atmospheric loss from sputtering and photochemical processes is greater than measurements.

A massive, multi-bar early CO2-dominated atmosphere and recent volcanic outgassing have been proposed to explain this discrepancy, and many previous models have assumed atmospheric nitrogen rapidly reached a steady state where loss to space balanced volcanic outgassing. Here we show using time-dependent models that the abundance and isotopic composition of nitrogen in the Martian atmosphere can be explained by a family of evolutionary scenarios in which the initial partial pressure of nitrogen is sufficiently high that a steady state is not reached and nitrogen levels gradually decline to present-day values over 4 billion years.

Our solutions do not require a multi-bar early CO2 atmosphere and are consistent with volcanic outgassing indicated by both geologic mapping and the atmospheric 36Ar/38Ar ratio. Monte Carlo simulations that include these scenarios estimate that the partial pressure of N2 was 60 – 740 mbar (90% confidence, with a median value of 310 mbar) at 3.8 billion years ago when the valley networks formed. We suggest that such a high nitrogen partial pressure could have contributed substantially to warming on early Mars.

Renyu Hu, Trent B. Thomas

Comments: Nature Geoscience, published online on February 10, 2022, this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (; Geophysics (physics.geo-ph)
DOI: 10.1038/s41561-021-00886-y
Cite as: arXiv:2202.04825 [astro-ph.EP] (or arXiv:2202.04825v1 [astro-ph.EP] for this version)
Submission history
From: Renyu Hu
[v1] Thu, 10 Feb 2022 04:07:36 UTC (16,617 KB)

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