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.



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.

Traces of Mars’ paleomagnetic field?

On Earth’s neighbouring planets, which have no magnetic field that can capture charged solar wind particles, aurorae manifest in a particular way. On Venus, a UV spectrometer on the Pioneer Venus probe, which orbited the planet from July, 1980, to August, 1992, identified a glow from “diffuse” aurorae, i.e. covering large areas. On Mars, “diffuse” aurorae were also observed in the Northern Hemisphere, but the ones in the Southern Hemisphere were “localised” aurorae. So how are these created? L. Soret and J.-C. Gérard investigated these light draperies or arcs, whose morphology is well defined. The phenomenon appears to be related to the solar flares that reach Mars’ atmosphere. “The sky is indeed brighter, but these aurorae are not the result of the same process as on Earth, as there is no global planetary magnetic field,” explains Lauriane Soret.

The study of the SPICAM measurements identified two dozen spots where localised aurorae appear. These are difficult to detect, and they seemingly show up at random, since they only appear temporarily, and on areas of only a few dozen kilometres. The origin of these aurorae is not clear. Professor Gérard explains that they appear in areas where ancient rocks retain traces of a past global magnetic field, trapped for nearly 4 billion years.

The study of Mars’ aurorae is difficult, as they are volatile and do not appear regularly. Many questions remain unanswered. How many aurorae are created? Do they all appear simultaneously? What energy do they contain? In order to better identify the chain of events that results in charged particles, guided by the local magnetic field, colliding with the atmosphere, researchers would need better observations on Mars’ nightside, acquired with a more sensitive instrument. In the meantime, Lauriane Soret has started working on a simulated model in order to better understand the specificity of isolated aurorae. Using this model, she determined that the altitude at which charged particles interact with Mars’ atmosphere and produce aurorae is between 125 and 135 km, with electrons being accelerated down magnetic “funnels”. In comparison, Earth’s aurorae generally manifest at altitudes ranging from 90 to 300 km.

Waiting for NOMAD

The processes resulting in “localised” aurorae appearing in specific areas on the Red Planet therefore remains to be determined. As European probe Mars Express SPICAM instrument is no longer sending data, hopes rest on NOMAD (Nadir and Occultation for MArs Discovery), an instrument carried by ExoMars TGO (Trace Gas Orbiter) satellite, whose 2016 mission is run jointly by ESA and Russia’s space agency Roscosmos. NOMADʼs advantage is that it is a triple instrument, with two IR spectrometers and a sensor for visible and UV light.

During this mission, the LPAP team will take advantage of the opportunity to observe areas of Mars during night time. These observations are not officially in the ESAʼs mission schedule. “The simultaneous detection of atmospheric phenomena in the UV, visible and IR spectra will be a first in Martian exploration,” notes LPAP’s Lauriane Soret, who counts on the ESAʼs time and resources to increase the scale and scope of her observations.

The auroral phenomenon: more common than you might think

How does the discovery of “isolated” aurorae on Venus and Mars advance astronomy or physics? “This is likely a process that is happening in billions of places across the universe, on exoplanets that have lost their global magnetic field,” explains Jean-Claude Gérard. “Every time a planet’s core cools down quickly, especially due to the planet’s small size, its magnetic field disappears. Our discovery on the Red Planet, which no longer has a global magnetic field, provides a model for what might happen on the many exoplanets of low mass and size that have an atmosphere.”

The objective of cataloguing the various types of aurorae will help understanding the interactions between planets, their atmospheres, solar wind, the behaviour of charged particles… these processes cannot quite be simulated in a laboratory. In fact, our Earth is like a gigantic laboratory involving the Sun, its distance from the planet, the planet’s magnetic field, the composition of the atmosphere… Lauriane Soretʼs goal as she studies various aurora phenomena is to gain a general understanding of the processes involved, with universal rules that apply to different conditions. The common theme is the behaviour of the atmosphere in which auroral glows appear.

The study of auroral phenomena will continue to be a part of our exploration of the solar system. In the future more than ever,  LPAP will follow the next scientific missions that will help understand these phenomena, with research on Jupiter’s enormous aurorae being planned using data from NASAʼs and ESAʼs probes. Juno has been en route towards the largest planet in the solar system since August 5, 2011, and it will be inserted into orbit on July 4, 2016. It is equipped with imaging spectrometers UVS and JIRAM (Jovian Infrared Auroral Mapper), which will provide close-up measurements of Jupiter’s aurorae. ESAʼs planned JUICE mission should focus on Jupiter’s aurorae, but also on those of its moons Europa and Ganymede. The latter has its own magnetic field, like Jupiter.

The researchers at LPAP in Liège and BIRA-IASB in Brussels have already taken action to advance the study of light phenomena that occur in planets’ atmospheres, as a part of a collaboration between universities and federal institutes. Belgium’s scientific policy has selected, among the thematic initiatives for interdisciplinary networks, the SCOOP project (towards a SynergistiC study Of the atmOsphere of terrestrial Planets). Starting in March, 2015 and lasting four years, this project will use – among other things – the data obtained from the NOMAD instrument on board ExoMars TGO probe, to be launched in 2016. This means that aurora hunters will have plenty to feast their eyes on for the coming decades.