ABSTRACT 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

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. Access through your institution Buy or subscribe This is a preview of

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TECTONICS Article 26 October 2023 STABLE CARBON ISOTOPE EVOLUTION OF FORMALDEHYDE ON EARLY MARS Article Open access 17 September 2024 PHOTOCHEMICAL DEPLETION OF HEAVY CO ISOTOPES IN THE

MARTIAN ATMOSPHERE Article 11 May 2023 DATA AVAILABILITY The data needed to generate all figures in the main text (Figs. 1–3) and Extended Data Figs. 2 and 3 are publicly available at Zenodo

(https://doi.org/10.5281/zenodo.5760095). CODE AVAILABILITY The source code of the nitrogen evolution model and the associated configuration files used in this study are publicly available

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ACKNOWLEDGEMENTS 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. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA,

USA Renyu Hu & Trent B. Thomas * Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Renyu Hu * Department of Earth and Space

Sciences/Astrobiology Program, University of Washington, Seattle, WA, USA Trent B. Thomas Authors * Renyu Hu View author publications You can also search for this author inPubMed Google

Scholar * Trent B. Thomas View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS R.H. designed the study and the evolution model, interpreted the

results and wrote the manuscript. T.B.T. implemented the evolution model, carried out the simulations and interpreted the results. CORRESPONDING AUTHOR Correspondence to Renyu Hu. ETHICS

DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. PEER REVIEW PEER REVIEW INFORMATION _Nature Geoscience_ thanks Ramses Ramirez and the other, anonymous,

reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin; Xujia Jiang; Stefan Lachowycz. ADDITIONAL INFORMATION PUBLISHER’S NOTE Springer

Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. EXTENDED DATA EXTENDED DATA FIG. 1 A MODEL FOR THE LONG-TERM EVOLUTION OF THE

FREE NITROGEN RESERVOIR ON MARS. 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). EXTENDED DATA FIG. 2 REPRESENTATIVE BACKGROUND CO2 EVOLUTIONARY SCENARIOS ADOPTED IN THIS STUDY. 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. EXTENDED DATA FIG. 3 BASELINE CRUSTAL PRODUCTION RATE ADOPTED IN THIS STUDY. (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. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION Supplementary Information A–H, figures and

tables. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE 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 Download citation * Received: 27 May 2020 * Accepted: 16 December 2021 * Published: 10

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