THE ATMOSPHERIC EVOLUTION OF MARS INDICATED BY C, N, AR,
AND H ISOTOPES
R. Hu, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
(renyu.hu@jpl.nasa.gov), T. B. Thomas, Department of Earth and Space Sciences, University of Washington,
Seattle, WA, USA, E. L. Scheller, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA, D. J. Adams, Division of Geological and Planetary Sciences, California Institute
of Technology, Pasadena, CA, USA, B. L. Ehlmann, Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA, USA, and Y. L. Yung, Division of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA, USA.

Introduction:
The initial thickness, evolution, and loss of the
Martian atmosphere have been the subject of much
debate, as these have major implications for the
evolving climate and habitability potential of the red
planet. Recently, new clues to this problem have
been found by integrating atmospheric and geological processes into the models of the isotope budget
of the planet. The isotopic composition of carbon,
nitrogen, argon, and hydrogen in Mars’s atmosphere,
together with the extent of carbonates, nitrates, and
hydrated minerals found on the planet, provides information about its atmospheric evolution. We have
developed a comprehensive model to trace the evolution of the Martian atmosphere, including the impacts of outgassing, escape to space, and sequestration into the crust (Figure 1). As new features, the
model integrates the atmospheric and geological
sources and sinks and enables the joint analysis of
the C, N, Ar, and H isotopes. Recognizing that evolution involves a trajectory in a multi-dimensional
phase space of many variables, we have used a million-model approach to rigorously perform quantitative tests against the isotopic composition of the atmosphere and the size of crustal volatile reservoirs.
Results:
The current atmospheric 13C/12C ratio points to
an evolutionary history of CO2 that started from a
moderately dense atmosphere (i.e., <~1 bar) at approximately 3.8 billion years before present (i.e.,
when most valley networks are thought to have been
formed). Only scenarios with large amounts of carbonate formation in open lakes permit higher pressures up to 1.8 bar. We also show that a previously
unknown family of evolutionary solutions can explain the isotopic composition of nitrogen, without
invoking a multi-bar early CO2 atmosphere or excessive outgassing in the Amazonian. This new family
of solutions suggest that Mars had an N2-rich early
atmosphere, with a partial pressure of ~0.3 bar. The
outgassing models are consistent with the constraints
from Ar’s isotopic composition and suggest mantle
sources with the oxygen fugacity more reduced than
IW. Furthermore, we show that ocean-scale volumes
of water on ancient Mars are simultaneously compat-

ible with the evidence of widespread hydrated minerals, H escape rate estimates, and D/H measurements.
Implications:
Collectively, these empirical constraints indicate
a multi-component atmosphere in the late Noachian,
with the CO2 partial pressure not much larger than
about 1 bar and N2 as an important component of the
atmosphere. Additionally, the crustal sequestration
of water and the outgassing from reduced mantle
sources may give rise to a substantial component of
H2 in the atmosphere. This new picture of the Martian paleoatmosphere provides a new path forward to
explain the geologic records that suggest intermittent
conditions for liquid water on ancient Mars’s surface
and to understand the redox conditions of these past
surface environments.

Figure 1: A comprehensive model for the evolution
of the Martian atmosphere, including the impacts of
outgassing, escape to space, and sequestration into
the crust. The model traces the mass and isotopic
composition of the volatile reservoirs to the present
day, and the results are compared with the isotopic
composition of the current atmosphere. The model
also takes into account the information about the
content of carbonates, nitrates, and hydrated minerals in rock and soil from in situ and remoting sensing
measurements, and MAVEN’s characterization of
the upper atmosphere processes.