<figure><img src="image_1.png" /></figure>

HAL
open science

# What goes on inside the Mars north polar vortex?

Kevin Olsen, Bethan Gregory, Franck Montmessin, Lucio Baggio, Franck Lefèvre, Oleg Korablev, Alexander Trokhimovsky, Anna Fedorova, Denis Belyaev, Juan Alday, et al.

► To cite this version:

Kevin Olsen, Bethan Gregory, Franck Montmessin, Lucio Baggio, Franck Lefèvre, et al. What goes on inside the Mars north polar vortex?. EPSC-DPS Joint Meeting 2025, Sep 2025, Helsinki, Finland. 10.5194/epsc-dps2025-1438 . hal-05279611

HAL Id: hal-05279611

https://hal.science/hal-05279611v1

Submitted on 24 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# What goes on inside the Mars north polar vortex?

Kevin Olsen<fnref n="1" />, Bethan Gregory<fnref n="1" />, Franck Montmessin<fnref n="2" />, Lucio Baggio<fnref n="2" />, Franck Lefèvre<fnref n="2" />, Oleg Korablev<fnref n="3" />, Alexander Trokhimovsky<fnref n="3" />, Anna Fedorova<fnref n="3" />, Denis Belyaev<fnref n="3" />, Juan Alday<fnref n="4" />, and Armin Kleinböhl<fnref n="5" />

Mars has an axial tilt of 25.2°, comparable to that on Earth of 23.4°. This gives rise to very similar seasons, and even leads to our definition of Martian time, aligning the solar longitudes ($L_s$) such that $L_s$ 0° and 180° occur at the equinoxes. In the northern hemisphere, between the equinoxes, the north polar region experiences polar days without darkness in spring and summer, and days of total darkness in the fall and winter. The dark polar winters give rise to a polar vortex that encircles the polar region and encircles an atmosphere of very cold and dry air bound within (1-3).

The Atmospheric Chemistry Suite (ACS) mid-infrared channel (MIR) on the ExoMars Trace Gas Orbiter (TGO; 4) operates in solar occultation mode in which the Sun is used as a light source when the atmosphere lies between the Sun and TGO. The tangent point locations of ACS MIR observation necessarily lie on the solar terminator on Mars. At the poles when either polar night or polar day are experienced, there is no terminator, and solar occultations are restricted to outside such a region. The latitudinal distribution of ACS MIR solar occultations during the north polar fall and winter over four Mars years (MYs) is shown in Fig. 1. The furthest northern extent of observations occurs at the equinoxes, and falling northern boundary is seen between, as the north pole points further away from the Sun (similarly in the south, where it is polar day).

While direct observations of the north polar vortex are forbidden with solar occultations, the polar vortex is not perfectly circular (1-3) and occasionally, descends into the illuminated region where we are making observations. The characteristic signs that we are sampling the polar vortex are a sudden drop in temperature below 20 km, the almost complete reduction in water vapour volume mixing ratio (VMR) and an enhancement in ozone VMR, the latter of which is extremely rare (5).

To measure the extent of the polar vortex, we use temperature measurements from the Mars Climate Sounder (MCS; 6, 7) on Mars Reconnaissance Orbiter (MRO). We define the polar vortex as the average temperature over 10-20 km being within a boundary of 170 K (30). We introduce a novel technique to determine this boundary during a 1° $L_s$ period using an alpha hull. We show that we can accurately measure the area of the polar vortex and achieve similar results to (3). The impact of the southern summer and dust activity is clearly visible in the time series of the northern polar vortex extent, leading to maxima occurring at the equinoxes, and shrinking toward perihelion. The impact of global dust storms and the late season dust storms are also pronounced.

We will show the vertical structure of water vapour and ozone VMRs inside and outside the north polar vortex, the results of a search for polar vortex temperatures from the near-infrared channel (NIR) of ACS (along the dark blue dots in Fig. 1), and show whether these results agree with the

<footnote n="1">¹Department of Physics, University of Oxford, Oxford, UK (kevin.olsen@physics.ox.ac.uk)</footnote>

<footnote n="2">²Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS/CNRS), Paris, France</footnote>

<footnote n="3">³Space Research Institute (IKI), Moscow</footnote>

<footnote n="4">⁴School of Physical Sciences, The Open University, Milton Keynes, UK</footnote>

<footnote n="5">⁵Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, CA, USA</footnote>
polar vortex extent measurements using MCS.

<figure><img src="image_4.png" /></figure>

**Figure 1:** The latitudes of ACS MIR solar occultation as a function of time (solar longitude $L_s$) during northern fall ($L_s$ 180-270°) and winter ($L_s$ 270-360°). Data from Mars years (MYs) 34-37 are indicated with colours. The region of interest in searching for polar vortex excursions is highlighted in blue.

**References:**

(1) Streeter, P. M. et al. *J. Geophys. Res.* **126**, e2020JE006774 (2021).

(2) Streeter, P. M., Lewis, S. R., Patel, M. R., Holmes, J. A., & Rajendran, K. *Icarus* **409**, 115864 (2024).

(3) Alsaeed, N.R., Hayne, P. O. & Concepcion, V. *J. Geophys. Res.* **129**, e2024JE008397 (2024).

(4) Korablev, O. et al. *Space Sci. Rev.* **214**, 7 (2018).

(5) Olsen, K. S., et al. *J. Geophys. Res.* **127**, e2022JE007213 (2022).

(6) Kleinböhl, A., et al. *J. Geophys. Res.*, 114, E10006 (2009).

(7) Kleinböhl, A., Friedson, A. J., & Schofield, J. T. *J. Quant. Spectrosc. Radiat. Transfer.* **187**, 511-522 (2017).