Marsʼ aurorae revealed

Following an in-depth analysis over the course of ten years – from 2004 to 2014 – of data from the SPICAM instrument placed on board European orbiter Mars Express, Lauriane Soret, researcher at the University of Liège’s laboratory of atmospheric and planetary physics, and her colleagues have identified aurorae in Mars’ atmosphere. This rare phenomenon remains largely unexplained, and has sparked considerable interest in the scientific community. Similar phenomena are likely common throughout the universe, on exoplanets that have lost their magnetic field.

As early as the 1960s, the University of Liège’s institute of astrophysics, headed by professor André Monfils, was pioneering European space exploration, using sounding rockets to carry out experiments studying in situ the aurora borealis. His expertise in UV (ultraviolet) spectroscopy gave way to an entire school of researchers.

Aurore boreale

Following in the footsteps of Jean-Claude Gérard, whose PhD thesis was devoted to the auroral process, LPAP (Laboratory of Planetary and Atmospheric Physics), which is part of the ULg’s AGO (Astrophysics, Geophysics and Oceanography) department, gained international fame for its insight into these colourful celestial draperies (green, pink, red and violet): they are produced by excited molecules resulting from the interaction between atmospheric particles and charged solar wind particles that are caught and accelerated in the Earth’s magnetic field. The auroral glow is produced when these particles collide with atmospheric gases, which means a planet without an atmosphere cannot produce aurorae.

Later, LPAP studied the very bright aurorae appearing on giant planets Jupiter and Saturn. One of the laboratory’s researchers, Arnaud Stiepen, studied Mars’ atmosphere using data from the US probe MAVEN (Mars Atmosphere & Volatile EvolutioN), which has been orbiting the Red Planet since September 21, 2014. His analysis of NASA data led to the discovery of “diffuse” aurorae in Mars’ Northern Hemisphere. Aurorae were also identified on Uranus, appearing as spots. “The auroral phenomenon reveals the existence of a magnetic field around a planet,” explains J.-C. Gérard. “This is the general rule, but we have just identified some exceptions, namely the Earth’s neighbours Venus and Mars.”

The Red Planet does have aurorae, described as “localised” or “isolated” as they only cover very limited areas of the planet, unlike “diffuse” aurorae which can cover a large area. They are restricted to the regions in the Southern Hemisphere where Mars orbiters have detected residual magnetism. Lauriane Soret, researcher at LPAP, has been hunting them down. This French engineer in physics identified them by analysing ten years of data – 2004 to 2014 – from the SPICAM instrument (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) placed on board the ESAʼs Mars Express probe, which has been orbiting Mars since December, 2003.

SPICAM is a spectrometer that was partially designed and built at BIRA-IASB (Royal Belgian Institute for Space Aeronomy) and whose function is to take atmospheric measurements in the ultraviolet (0.118 to 0.320 µm) and infrared (1 to 1.7 µm) spectral ranges. Its main objective was to establish a 3D map, taking altitude into account, of the composition and temperature of Mars’ atmosphere. By reviewing the data collected during Mars flybys, Lauriane Soret was able to identify some twenty auroral events in the planet’s Southern Hemisphere. These are indeed the result of atmospheric collisions that produce light.

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