MONITORING OZONE AND AEROSOL IN THE MARTIAN MESOSPHERE
FROM MAVEN/IUVS STELLAR OCCULTATION OBSERVATIONS BETWEEN
MY 32 AND 36.
A. S. Braude, F. Montmessin, L. Verdier, Z. Flimon, F. Lefevre, LATMOS/CNRS/UVSQ/Sorbonne/Paris-Saclay,
Guyancourt, France (ashwin.braude@latmos.ipsl.fr), S. Gupta, S. K. Jain, N. M. Schneider, J. Deighan, LASP, U.
Colorado, Boulder, USA, F. Y. Jiang, R. Yelle, LPL, U. Arizona, Tucson, USA.

Introduction
In this talk, we will present vertically-resolved profiles
of ozone and aerosol in the mesosphere, as retrieved
from bimonthly stellar occultation campaigns carried
out between MY 32 and 36 by the Imaging UltraViolet Spectrograph (IUVS) instrument on board the Mars
Atmosphere and Volatile Evolution Mission (MAVEN)
[1, 2]. Ozone is heavily correlated with other products
of the photolysis of water vapour in the Martian atmosphere (notably HO x ) that are more difficult to sense
remotely (e.g. [3, 4]) but which play a crucial role in
the stability of Mars’ CO2 atmosphere. Since preliminary data from MY 32 and 33 was presented in [5] we
have refined the process of filtering out false detections
of ozone in the data, which has allowed us to provide
more reliable vertical profiles not only of ozone number
density, but also of aerosol optical depth in the middle
atmosphere together with an associated Ångström coefficient that provides an approximate measurement of the
size distribution of sub-micron particles (e.g. [6, 7]). In
particular we wish to focus on regional variations in the
data that can be decoupled from general differences in
latitude, local time and season.

Data and Methods
The MAVEN probe has been in orbit around Mars since
2014. Since then, IUVS stellar occultation observation
campaigns have taken place approximately once every
two months, with each campaign typically consisting of
roughly 50-100 individual stellar occultations optimised
to provide substantial longitudinal coverage for a given
latitude and solar longitude. IUVS makes use of a beam
splitter to direct incoming radiation towards two separate detectors: one sensitive to the FUV (110 - 190 nm)
and one to the MUV (180 - 300 nm), with a spectral
sampling of 0.3 nm and 0.7 nm respectively following
spectral binning. The FUV component is mostly sensitive to CO2 and O2 absorption above around 80 km,
while the MUV component contains an ozone absorption feature at 250 nm (the ‘Hartley Band’) that provides
information on ozone number density between approximately 20 and 60 km altitude. In addition, systematic
absorption in the MUV can be used to retrieve aerosol

optical depth τ(λ=250nm) while the spectral slope α can be
used to determine the approximate size distribution of
small (<0.5µm) particles.

Figure 1: Example retrieval of aerosol and molecular column
densities from an IUVS occultation obtained near the Equator at aphelion. Left: retrieved aerosol parameters τ(λ=250nm)
(aerosol optical depth, black) and α (Ångström coefficient,
magenta). The orange cross-hatched region marks lower altitudes at which the spectra are saturated due to the presence
of dust. The increase in α with altitude indicates the presence
of a haze or ice cloud layer at high altitude. Right: retrieval
of CO2 , O2 and O3 column densities. Black dots indicate individual retrieved ozone column densities. The light blue line
indicates significant detections of ozone, while ozone column
densities that are below the 2 sigma detection limit are discarded from the vertical inversion.

We selected 510 individual occultations in our analysis out of the 40 observation campaigns that have been
processed as of writing, according to the availability of
MUV data and the quality of calibration and straylight
correction. In practice this also limits us to using nightside occultations where the effect of straylight on the

Figure 2: Comparison of vertical profiles of ozone local density at aphelion, between measurements from IUVS (blue) and modelled
values from the LMD MGCM (red).

MUV due to scattered sunlight is minimised. We first
perform a spectral inversion procedure in order to simultaneously retrieve vertical profiles of CO2 , O2 and
O3 column density, together with τ(λ=250nm) and α (Fig. 1).
Variations in CO2 and O2 observed by IUVS are mostly
only relevant to the upper atmosphere well above the altitudes at which IUVS is sensitive to ozone and aerosol,
and so will be mostly beyond the scope of this talk.
Following spectral inversion, ozone upper limit values are estimated at each altitude by progressively injecting ozone column density values into the forward model
until a given reduced χ2 threshold is reached [8, 9], so
that ozone detections of below 2σ significance can be rejected to minimise the number of false detections due to
fitting noise or artefacts in the data. A vertical inversion
procedure is then performed on the resulting column
density profiles to retrieve profiles of local density of
O3 .
For more details on the instrument specifications as
well as the calibration, spectral and vertical inversion
procedures, refer to [10, 1, 5].

Ozone - Preliminary results and perspective
We identify positive ozone detections in approximately
38% of IUVS stellar occultations, mostly concentrated
around aphelion. In Fig. 2 we display some preliminary
retrievals of ozone around aphelion in equatorial and
mid-latitudinal regions, and compare them with corresponding profiles generated from the LMD Mars General Circulation Model (MGCM) [11]. We generally

find good correspondence between the modelled and
observed night-time ozone layer at around 40 km. We
also retrieve a near-surface layer of ozone below 20 km
altitude in the mid-latitudes where observations near the
surface are less limited by the presence of aerosol. These
ozone layers are generally well-attested in other measurements of ozone made both by the SPICAM and NOMAD instruments in the UV (e.g. [12, 13]), and with the
ACS instrument in the mid-infrared [14]. However, we
also observe more variability in altitude than is reflected
in modelled ozone densities. In this talk we will further discuss differences between observed and modelled
ozone profiles more broadly across the latitude and temporal range covered in this analysis, and identify their
possible origins.

Aerosols - Preliminary results and perspective
The exact composition of aerosol layers cannot be directly determined from observations in the ultraviolet,
but measuring vertical correlations in Ångström coefficient and temperature together with some knowledge
of microphysics can provide some constraints on their
morphology to first order. As was found in data from
SPICAM-UV ([7, 15], we also identify a substantial
number of observations featuring aerosol present in two
or more discrete vertically-resolved layers (‘detached
layers’), as well as some observations with mesospheric
cloud layers present at very high altitude [16, 17]. We
find that, like in the work of [15], the predicted decrease
of Ångström coefficient with height (indicating smaller

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particle radii) is common, but not universal. In this
talk we will seek to characterise the general variation
of aerosol with latitude and season in more detail, and
map the distribution of detached aerosol layers with an
additional focus on identifying regional and longitudinal
variations not linked to differences in latitude, season or
local time. We will also examine their impact on longitudinal variations in observations of ozone on Mars.
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