EMIRATES MARS ULTRAVIOLET SPECTROMETER’S OBSERVATION
OF ARGON IN THE MARTIAN THERMOSPHERE.
F. Lootah, Mohammed Bin Rashid Space Center (MBRSC), Dubai, UAE, J. Deighan, Laboratory for
Atmospheric and Space Physics (LASP) at University of Colorado, Boulder, CA, USA, M. Fillingim, Space
Sciences Laboratory (SSL), University of California, Berkeley, CA, USA, H. Almatroushi, Mohammed Bin
Rashid Space Center (MBRSC), Dubai, UAE, S. Jain, M. Chaffin, G. Holsclaw, Laboratory for Atmospheric
and Space Physics (LASP) at University of Colorado, Boulder, CO, USA, J. S. Evans, J. Correira,
Computational Physics, Inc (CPI), Springfield, VA, USA, R. Lillis, Space Sciences Laboratory (SSL), University
of California, Berkeley, CA, USA. H. Almazmi, United Arab Emirates Space Agency, Abu Dhabi, UAE. S.
England, Virginia Polytechnic Institute and State University, Aerospace and Ocean Engineering, Blacksburg,
VA, United States.

Introduction:
The Emirates Mars Mission (EMM) has been
launched to Space on July 20, 2020, arriving at Mars
Orbit on February 9, 2021 and commencing its
Science phase on May 23rd, 2021. EMM explores the
dynamics of the Martian atmosphere on a global scale
while sampling contemporaneously both diurnal and
seasonal timescales. EMM has three instruments:
Emirates eXploration Imager (EXI), Emirates Mars
InfraRed Spectrometer (EMIRS), and Emirates Mars
Ultraviolet Spectrometer (EMUS) [1]. The EMUS
instrument is a far ultraviolet imaging spectrometer
that is designed to determine the abundance and
spatial variability of key neutral species in the
thermosphere and the three-dimensional structure and
variability of key species in the exosphere [2].
Argon is a photochemically inert gas with
molecular weight of 40 g/mol - close to CO2's 44
g/mol. Therefore, the Ar/CO2 ratio in the upper
atmosphere can offer a measure of transport from the
lower atmosphere. EMUS provides us with unique
information of Argon globally; observing two Argon
emission lines, Ar I 104.8 nm and Ar I 106.6 nm, as
seen in the spectrum below.

Datasets Specifications:
We will be utilizing data captured through EMUS’
observation strategy 1 (OS1), which performs two
swaths focusing on Thermosphere of Mars [100-200
km]. Until recently, EMUS observations have
occurred during low solar activity. This presentation’s
results utilize the following datasets:
• Dataset 1: Two cases observed on
2021/07/02 at Ls ~66.5, the first taken at
around the subsolar point and the second at
the dusk terminator.
• Dataset 2: One case observed on 2021/12/24
at Ls 146.8 close to subsolar point.
Results:
We have produced disk images of Ar 106.6 nm
from two different viewpoints – the dusk terminator
(a dayside/nightside viewing) and at the subsolar
point for dataset 1 as seen in Figures 2 (a) and (b).

Figures 2 (a) & (b): Disk images taken from dusk terminator and subsolar point.
Figure 1: EMUS spectrum of the Mars disk during transition phase.

This work will showcase how the brightness of the
Argon emission lines vary with different factors, such
as solar longitude (Ls), emission angle (EA), and solar
zenith angle (SZA). In addition, we will be comparing
what we have obtained from EMUS with results from
past missions.

To understand the distribution of brightness
thoroughly we’ve created heatmaps of average
brightness with respect to SZA and EA. The heatmaps
seen in Figures 3 (a) & (b) showcase that the
distribution of Argon across EA seems flat with no
clear
pattern displaying optically thick behavior unlike what has been reported in literature [3]. We
also
notice the dimming of brightness of Ar 106.6
nm at higher SZA, indicating a geometry effect due to
the reduction in solar irradiance. We will also be

exploring whether other factors such as latitude and
local time may contribute to this behavior.

Figures 3 (a) & (b): Distribution of Ar 106.6 nm with respect to SZA and EA in dataset
1 & 2 at the subsolar point.

References:
[1] H.E. S. Amiri, D. Brain, O. Sharaf, P.
Withnell, M. McGrath, M. Alloghani, M. Al Awadhi,
S. Al Dhafri, O. Al Hamadi, H. Al Matroushi, Z. Al
Shamsi, O. Al Shehhi, M. Chaffin, J. Deighan, C.
Edwards, N. Ferrington, B. Harter, G. Holsclaw, M.
Kelly, D. Kubitschek, B. Landin, R. Lillis, M.
Packard, J. Parker, E. Pilinski, B. Pramman, H. Reed,
S. Ryan, C. Sanders, M. Smith, C. Tomso, R. Wrigley,
H. Al Mazmi, N. Al Mheiri, M. Al Shamsi, E. Al
Tunaiji, K. Badri, P. Christensen, S. England, M.
Fillingim, F. Forget, S. Jain, B.M. Jakosky, A. Jones,
F. Lootah, J.G. Luhmann, M. Osterloo, M. Wolff, &
M. Yousuf. “The Emirates Mars Mission.” Space
Science
Reviews
218,
4
(2021)
https://doi.org/10.1007/s11214-021-00868-x

[2] G. M. Holsclaw, J. Deighan, H. Almatroushi,
M. Chaffin, J. Correira, J. S. Evans, M. Fillingim, A.
Hoskins, S. K. Jain, R. Lillis, F. H. Lootah, J. B.
McPhate, O. H. W. Siegmund, R. Soufli, & K. Tyagi,
“The Emirates Mars Spectrometer (EMUS) for the
EMM Mission.” Space Science Reviews 217, 79
(2021). https://doi.org/10.1007/s11214-021-00854-3
Figures 4 (a) & (b): Distribution of Ar 106.6 nm and O 135.6 nm at dusk
terminator with respect to SZA and EA in dataset 1.

Further investigating our hypothesis that Ar 106.6
nm is optically thick, we compare the results with
oxygen 135.6 nm, an optically thin emission,
observed at the same orbit. As seen in the heatmap of
oxygen 135.6 nm Figure 4 (b) there is a clear pattern
where brightness increases with EA, expected from an
optically thin emission. Meanwhile, Ar 106.6 nm in
Figure 4 (a) appears flat, indicating optical thickness.
Comparison with Previous Observations:
We have compared the values obtained from
EMUS with values found from the Hopkins
Ultraviolet Telescope (HUT) and the Far Ultraviolet
Spectroscopic Explorer (FUSE). As seen in Table 1,
values from EMUS are within the value range
detected by HUT, both observed at solar min and.
similar Ls. FUSE values are higher when compared to
EMUS, but that is expected since they were detected
during solar min.

Table 1: Argon values from EMUS compared with HUT and FUSE.

Future Work:
We are working on further understanding the
variation of Argon brightness with latitude, local time
and additional solar longitude. Furthermore, we will
be using data observed through EMUS’ observation
strategy 2 (OS2), which scans both the disk and inner
corona using three swaths. We will be ultimately
comparing the results from both (OS1) and (OS2).

[3] V. A. Krasnopolsky and P. D. Feldman, "Far
Ultraviolet Spectrum of Mars," Icarus, no. 160, pp.
86-94 (2002).
[4] P. D. Feldman, E. B. Burgh, S. T. Durrance, &
A. F. Davidson, A. F. “Far-Ultraviolet spectroscopy
of Venus and Mars at 4 A Resolution with the
Hopkins Ultraviolet Telescope on Astro-2.” The
Astrophysical Journal, no. 538, pp. 395-400 (2000).