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.
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The source code of the nitrogen evolution model and the associated configuration files used in this study are publicly available at Zenodo (https://doi.org/10.5281/zenodo.5760095).
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We thank Y. L. Yung, B. Ehlmann, B. Jakosky, H. Kurokawa, C. Manning, R. Johnson, M. Grott, F. Gaillard, M. Slipski and C.-Y. Ng for helpful discussions. This work was supported by NASA Habitable Worlds grant NNN13D466T, later changed to 80NM0018F0612. The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
The authors declare no competing interests.
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The free nitrogen reservoir is comprised of N2 adsorbed in the regolith and 2 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).
These scenarios are selected from the evolutionary tracks of CO2 derived in Hu et al. (2015)16 and they are consistent with the present-day pressure and carbon isotopic composition. Scenarios 1 - 3 assume that the photochemical loss rate of carbon depends on the Sun’s Lyman continuum flux to the power of 2. Scenario 1 assumes that carbonate deposition of 40 mbar occurred throughout the Noachian and Hesperian (that is, till 3.0 Ga) in shallow subsurface aquifers. This represents the lower bound of the initial CO2 partial pressure. Scenarios 2 and 3 assume that carbonate deposition of 290 and 600 mbar occurred in the Noachian and early Hesperian (that is, till 3.5 Ga) in open-water systems. Scenario 3 has an initial CO2 partial pressure of 1 bar and is the default scenario adopted in this study. Scenario 4 assumes a power-law index of 3, and that carbonate deposition of 1400 mbar occurred in the Noachian and early Hesperian (that is, till 3.5 Ga) in open-water systems. This represents the upper bound of the initial CO2 partial pressure. Scenario 5 is an endmember scenario where the CO2 atmosphere is assumed to be collapsed at all times and the pressure constant at 7 mbar.
(a) Crustal production rates derived from the photogeologic analysis of volcanic provinces57 from 3.8 Ga. To convert the photogeological analysis (expressed as the total volcanic emplacement in each geologic period from middle Noachian to late Amazonian) to the crustal production rate, we compare the rates derived using the age boundaries from the crater density65 and the chronology model of Ivanov (2001)66 and Hartmann (2005)67, as well as interpolation methods using either step functions or mid-point averages. The labels show the total volcanic activity and the integrated volcanic activity in the last 2 billion years (in parentheses). The midpoint approach would introduce substantially more total volcanism, and so we use the step-function approach. Also in comparison is a volcanic history derived from earlier photogeologic analyses and used in the recent argon isotope study15. The step-function approach on the Hartmann (2015) chronology leads to a baseline model that is very similar to the one used in the argon isotope study15 in terms of the total and the recent volcanic rates. (b) Baseline crustal production rate adopted in this work, based on the global thermal evolution model56 and the step-function interpolation of the photogeologic analysis of volcanic provinces57, whichever is greater. We adopt the model using the Hartmann (2015) chronology as our baseline model, and consider the one using the Ivanov (2001) chronology as the variant. The two models have appreciable difference in the last 2 billion years.
Extended Data Fig. 4 Posterior distributions of parameters from unconstrained MCMC simulations with the parameters and their boundaries listed in Table 1.
The MCMC simulations adopt the five representative CO2 evolution scenarios shown in Extended Data Fig. 2.
Extended Data Fig. 5 Posterior distributions of parameters from constrained MCMC simulations with the parameters and their boundaries listed in Table 1.
The MCMC simulations adopt the five representative CO2 evolution scenarios in Extended Data Fig. 2.
Extended Data Fig. 6 Posterior distributions of parameters from constrained MCMC simulations that fix the sputtering multiplier to be fsp=0.5, 1, and 2.
These simulations adopt the CO2 evolutionary scenario No. 3 with the initial partial pressure of 1.0 bar as shown in Extended Data Fig. 2. The initial partial pressure of N2 is more tightly constrained when the sputtering multiplier is fixed.
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Hu, R., Thomas, T.B. A nitrogen-rich atmosphere on ancient Mars consistent with isotopic evolution models. Nat. Geosci. 15, 106–111 (2022). https://doi.org/10.1038/s41561-021-00886-y
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