UNDERSTANDING
THE
EVOLUTION
OF
ATMOSPHERE THROUGH NITROGEN ISOTOPES

THE

MARTIAN

H. Pieris, B. M. Jakosky, LASP, University of Colorado, Boulder, CO, USA (heshani.pieris@colorado.edu)

Introduction:
The 14N/15N ratio of the Mars atmosphere was
first measured by Viking and then later by the Mars
Science Laboratory (MSL). MSL measured a 14N/15N
ratio of 173±11 which corresponds to an enrichment
of 1.57 times the terrestrial value of the 15N isotope.
This type of enrichment can only be explained by a
significant loss of N to space, with the lighter isotope
escaping more efficiently. A quantitative understanding requires analyzing all of the sources, sinks and
exchanges of N2. We employ the processes of sputtering, photochemical escape by dissociative recombination and photodissociation, and volcanic outgassing to understand the evolution of N2 through time.
Sputtering and photochemical escape are isotopically
selective escape processes that preferentially remove
the lighter isotope creating an enrichment of 15N/14N.
We use a forward time-marching model starting at an
initial abundance at 4.4 Gya in time steps of 1 Myr to
calculate the N2 abundance and 15N/14N enrichment
through time. This first set of plots allow us to quantify the amount of N2 lost under varying CO2 histories and understand the evolution of N2 in the atmosphere through time. We also vary the initial N2 abundance, initial CO2 pressure and rate of loss or input
of each process (by a constant multiplicative factor;
i.e., rate factor) and explore which combinations of
these parameters allow solutions that satisfy today’s
nitrogen composition and isotopic ratio of the Martian atmosphere.
A similar study of the Martian nitrogen abundance and isotopic evolution was recently published
by Hu and Thomas (Nature, 2022). While this work
does a thorough analysis of the nitrogen evolution
models, our work expands on this by extending the
evolutionary timeline from 3.8 Gya back to 4.4 Gya.
We believe that observing the evolutionary models throughout this most stable period of the Martian
atmosphere is key to obtaining a more complete picture of the early atmospheric environment of Mars.
Our work, when completed, will also entail examining the effects of impact supply and loss processes
(discussed further in the following section) which
were not included in the model of Hu and Thomas,
2022.

Box Model:
To investigate the evolution of the Martian atmosphere through nitrogen isotopes, we constructed
a box model allowing simultaneous inputs and outputs of nitrogen gas into the atmosphere. As dis-

played in Figure 1, the processes that remove nitrogen from the atmosphere are sputtering, photochemical escape (dissociative recombination and
photodissociation), impact erosion and formation of
nitrates. Further, nitrogen can also be replenished in
the atmosphere by volcanic outgassing, impact erosion of nitrates, and supply from incoming interplanetary dust particles (IDPs) and meteorites. This current model will only consider the processes of sputtering, photochemical escape and volcanic outgassing. The rest of the processes will be investigated in
future continuation of this work.

Figure 1: A schematic of the atmospheric reservoir
and its exchanges with the surface and space. The green
arrows portray the loss processes and the orange arrows
represent the methods of replenishment.

CO2 History:
Escape rates of sputtering, dissociative recombination and photodissociation depend on the mixing
ratio of N2/CO2 in the bulk atmosphere. To account
for the time varying abundance of CO2, we employ
two scenarios where the amount of CO2 in the atmosphere decays with a linear or with a power-law
function. These two functions are set to begin at 3.8
Gya and decay to the present day CO2 partial pressure of 6 mbar. The power-law decay is of the form y
= k.xa, where y is the CO2 abundance at each time, x
is the time value and k and a are determined by the
initial and final CO2 pressures.
In both of these evolution scenarios, we utilize a
constant CO2 pressure for the time period of 4.4-3.8
Gya, as shown by Figure 2, in order to account for a
stable atmosphere during the Noachian period. The
initial CO2 pressures we employ for this study range
as 6 mbar (assuming CO2 pressure has remained con-

stant over time), 10-100 mbar in increments of 10
mbar and 100-1000 mbar in increments of 100 mbar.
We use this simplified version of CO2 decay in
the atmosphere as an alternative to the more realistic
approach of CO2 loss which would entail studying
the processes of escape to space by impacts, sputtering and photochemical loss, carbonate formation, and
polar cap deposition alongside the loss processes of
N2, which is beyond the scope of this project.

Figure 2: Linear and power-law decay of CO2 with
constant pressure through the Noachian epoch. The initial
pressures vary from 6 mbar to 1000 mbar with all decaying to the current atmospheric pressure of 6 mbar.

Input Parameters:
This box model takes inputs of initial N2 abundance and initial CO2 pressure. The initial N2 abundance of the Martian atmosphere is not a value that is
known since it is not plausible to measure this value
in the current depleted atmosphere of Mars. For our
analysis of the processes at some starting abundance,
we use an initial value of ~9x1042 molecules (108
mbar) which is the net abundance of the current N2
budget of Earth's atmosphere scaled to the mass of
Mars. Here, initial CO2 pressure is set to vary from 6
mbar to 1 bar for both evolution histories of linear
and power-law decay.
For the second set of plots, which explore the solutions that are able to reach today’s nitrogen abundance and 15N/14N ratio, the initial N2 abundance is
input as a free parameter. For this step of the model,
we also introduce a rate factor, which is a multiplicative factor of the rate of each of the processes that
varies over a set of values given as 0.25, 0.5, 1.0,
2.0, 5.0 and 10.0.
After the initial N2 abundance is input into the
model, an initial 15N/14N ratio is used to obtain the

starting abundances of 28N2 and 29N2. For this, we use
the terrestrial 15N/14N ratio of 0.00367 with the assumption that Earth has not undergone significant
fractionation of nitrogen that would have resulted in
the current ratio being an enrichment of its starting
ratio

Results:
We considered each process separately at first, in
order to understand each one’s behavior. After gaining this intuitive understanding, we included multiple
processes simultaneously. Figure 3 portrays the simultaneous operation of all four processes of sputtering, dissociative recombination, photodissociation
and volcanic outgassing starting at an initial N2
abundance of 108 mbar at 4.4 Gya. Here, the CO2
evolution follows a constant then power-law decay as
shown by the bottom panel of Figure 2. It is clear
that the highest N2 loss occurs for lower initial CO2
pressures and for higher initial pressures, there is a
much less significant amount of N2 lost through time.
This occurs as a result of the escape rates being dependent on the N2 mixing ratio which means that
higher concentrations of CO2 decrease N2 escape.
The sudden increase in N2 abundance at ~ 1Gya seen
for the lower pressure curves of 6 mbar and 10 mbar
is caused by a sharp increase in volcanic outgassing
at the early-Hesperian epoch. For lower initial CO2
pressures, we see a steep initial decrease in the escape rates allowing volcanic outgassing to take precedence.
The bottom panel of Figure 3 shows that the lowest initial CO2 pressure curves (6-20 mbar) have a
rapid increase in enrichment within the first billion
year, corresponding to the large decrease in N2 abundances seen in the top panel. However, these peaks
are short-lived and quickly reduce to arrive at an
enrichment that falls below today’s atmospheric
measurements. Here the orange dashed lines represent the 15N/14N measurement range within error bars
made by Viking. We consider the value measured by
Viking to allow room for slightly more uncertainty
(Viking’s 8.8% compared to Curiosity’s 6.4% error).
Under the specific assumptions of a CO2 evolution of
power-law decay and a starting N2 abundance of 108
mbar, the N2 evolution curves that are able to reach
today’s enrichment are those of 600 mbar - 1bar initial CO2 pressure.
Figure 4 depicts the solutions that satisfy today’s
N2 abundance and 15N/14N enrichment ratio under
varying initial N2 values and rate factors of each process varying as 0.25, 0.5, 1.0, 2.0, 5.0 and 10.0.
Here, the initial CO2 pressure is set at 1 bar and decays under the power-law evolution scenario described by the bottom panel of Figure 2. The presentday range of N2 pressure values are obtained by calculating a 30% seasonal variation of today’s abun-

dance of ~0.125 mbar, which provides a range from
0.0875 – 0.1625 mbar. This variation is an estimated
consequence of seasonal CO2 condensation at the
polar caps. According to this figure, the initial N2
pressures that are able to reach today’s atmospheric
conditions with an initial CO2 pressure of 1 bar, vary
from 0.0005 mbar to 3 mbar. With volcanic outgassing being the sole process of N2 input assumed so
far, solutions with lower initial N2 quickly rise to ~1
mbar at which point they converge with the rest of
the solutions to evolve to the present-day abundance.
It should also be noted that this model was run for
initial CO2 pressures of 6, 10 and 100 mbar as well
(not shown here), and while the 100 mbar case had a
few solutions that were able to satisfy today’s composition there were none for 10 and 6 mbar.

Figure 3: N2 abundance (top) and 15N/14N enrichment
(bottom) as a result of combining sputtering, photochemical escape and volcanic outgassing as CO2 decays in a
power-law function after 3.8 Gya. The dashed orange
lines describe the present day measured enrichment within
error bars.

Conclusions:
This current model employing the escape processes of sputtering, dissociative recombination and
photodissociation and the input process of volcanic
outgassing is able to produce solutions that satisfy
the measured abundance and enrichment of N2 in
today’s Martian atmosphere. Under the assumption
that only these four processes were in play in the
evolution of nitrogen through time, the initial N2
pressure would have been in the range of 0.0005
mbar to 3 mbar. The initial converging of the solutions show that the scenarios that are able to reach

today’s abundance are somewhat independent of the
initial N2 abundance. A similar case in the enrichment plot shows that the different solutions that satisfy today’s enrichment are independent of the 15N/14N
ratio up to 1 billion year after formation. As noted in
the previous section, since the number of viable solutions decreased with decrease in initial CO2 pressures, it could be concluded that larger initial CO2
pressures (~1 bar or more) are a more likely depiction of the early atmospheric conditions of Mars.
Completion of this project will entail studying the
evolution of N2 through time under all input and output processes described in Figure 1. This would allow us to obtain a more complete picture of the atmospheric evolution of Mars as well as a more accurate understanding of the initial atmospheric conditions at 4.4 Gya.

Figure 4: N2 abundance (top) and 15N/14N enrichment
(bottom) solutions that satisfy today’s abundance and
enrichment at an initial CO2 pressure of 1 bar. The dashed
red lines depict the present day measured abundance and
enrichment within error bars.