Proton auroras observed for the first time on Mars!
Artist's impression of Mars Express. The background is based on an actual image of Mars taken by the spacecraft's high resolution stereo camera. credit: ESA/ATG medialab.
A research programme conducted at the University of Liège in collaboration with the Royal Observatory of Belgium and the University of Versailles (1) has just observed for the first time proton auroras on Mars, although it has no global planetary magnetic field. The researchers analysed the data collected between 2004 and 2011 with the SPICAM ultraviolet spectrometer on board of the Mars Express probe (ESA).This is the first time that researchers have observed the existence of this type of aurora elsewhere than on Earth.
ur Sun releases a continuous flow of electrically charged particles into space (electrons, protons, ions), the so-called the solar wind. These particles reach the planets of the solar system and interact with their global magnetic field (if present) and their atmospheres. On Earth, this interaction leads to the acceleration of particles trapped in the geomagnetic field towards the upper atmosphere, which results in light emissions, better known as polar auroras. These extremely colourful and quite spectacular curtains of light occur around the magnetic poles of our planet and sometimes even at lower latitudes during periods of increased solar activity.
On Earth, there are three types of auroras: discrete and diffuse auroras - both triggered by electrons - and proton auroras - equally diffuse and which, as their name suggests, are triggered by protons - each with a different shape and light translation. These three types of auroras have long been observed on our planet, while on Mars the proton aurora has just been observed by two research teams, including one led by Jean-Claude Gérard of the University of Liège. « Until now, proton auroras had only been observed on our planet, explains the researcher from the Laboratory of Atmospheric and Planetary Physics (LPAP) - STAR Research Institute. « Mars does not have a global magnetic field like the Earth, which strongly limits the appearance of auroras. But the planet does have regions on its southern hemisphere that have retained the imprint of the magnetic field that reigned in past geological periods and therefore allows the appearance of this type of physical phenomenon. »
The discrete aurora, triggered by electrons, had been mapped on Mars in 2013 by the same ULiège team using SPICAM - an instrument of the European Space Agency (ESA) designed to study the characteristics of the atmosphere of Mars. The second type of aurora, the diffuse aurora, was discovered in 2015 by MAVEN, a NASA Mars exploration space probe. These auroras are much weaker on Mars than on Earth and are generally observed on the night side of the planet during periods of enhanced solar activity. The recently detected proton aurora is visible only on the day side of the planet.
Proton auroras on Mars
"The Martian proton aurora was theoretically predicted as early as 2001, but it is only now that we have found observational evidence," says Birgit Ritter, first author of the article published in Geophysical Research Letters (1) and researcher at LPAP. Examination of seven years of data (2004 to 2011) collected from Mars Express revealed that when a higher than usual proton flux from the Sun reaches Mars, the proton aurora is detected as an increase in the brightness of the atmosphere between 120 and 150 km altitude. This phenomenon can occur anywhere on the sunlit side of the planet and is not related to any magnetic field structure. A higher solar proton flux can for example occur in a coronal mass ejection (CME) during periods of increased solar activity, during which a huge amount of solar plasma is ejected into interplanetary space at once. These CMEs can carry dense clouds of solar wind protons far into the solar system."
LEFT: Auroral observations superimposed on a Martian magnetic field intensity map. The white stars indicate the newly observed proton aurora observations from Mars Express, while the black stars show the locations of the discrete aurora previously detected from Mars Express. RIGHT: Emission altitude profile of a proton aurora observed with SPICAM. The black curve represents a Lyman-alpha emission profile in the Martian atmosphere without a proton aurora and the blue curve shows the observation with the auroral signature between 120 and 150 km. The magenta curve shows the difference.
These observations give us a new insight into how the solar wind interacts directly with Mars and other planetary objects, surrounded by a neutral corona. The Trace Gas Orbiter (TGO) satellite - that is carrying a combination of spectrometers capable of detecting the visible emission of the proton aurora - has just begun its observation of the atmosphere of Mars.
The observation of the proton aurora on Mars provides us, as it is often the case in space research, with information on the general nature of physical processes. It also indicates that the proton aurora could be very common in the universe and that, unlike traditional vision, the presence of a planetary magnetic field is not required to produce the auroral phenomenon. Any planet in orbit around an active star is exposed to its stellar wind and thus subjected to a similar bombardment of protons.
To better understand the polar auroras
A polar aurora is called « borealis » in the northern hemisphere and « australis » in the southern hemisphere. Polar auroras occur in areas near the magnetic poles and result in the appearance of coloured veils in the night sky. Polar auroras are caused by the interaction between charged particles emanating from the sun (solar wind) and the Earth's space environment. This includes charged particles from the solar wind itself and from the ionosphere, the Earth's upper atmosphere. The solar "wind" is obviously not a draught air as we usually understand it, but it is made up of streams of electrically charged particles from the Sun that constantly scan the interplanetary environment. This wind is very light (5 to 10 particles per cubic centimetre), fluctuating and very fast (400 to 800 km/s). It is composed mainly of electrons, protons and helium nuclei and, curiously, it carries a part of the original solar magnetic field as frozen within : the interplanetary magnetic field that crucially conditions the interaction between the solar wind and the space environment of planets with an intense magnetic field.
The colour and shape of the polar aurora depend on the particles involved (electrons or protons as projectiles, atmospheric constituents as targets). The most commonly observed emission is the green line stimulated by energetic electrons that excite oxygen atoms in the upper atmosphere, but the characteristic emissions range from the X-rays to the infrared.
When protons interact with atmospheric constituents on Earth (mainly nitrogen and oxygen), they often capture an electron from these neutral components and transform into neutral hydrogen atoms. A fraction of these atoms are in an excited state and produce an ultraviolet emission (Lyman-alpha) when these atoms relax by emitting light.
Scientific reference :
(1) B. Ritter, J.-C. Gérard, B. Hubert, L. Rodriguez, and F. Montmessin (2018). Observations of the Proton Aurora on Mars with SPICAM on board Mars Express, Geophysical Research Letters, 45. https://doi.org/10.1002/2017GL076235.
LPAP - STAR Research Institute
Dr. Birgit Ritter - firstname.lastname@example.org - Tel : +32 (0)4 366 97 82
Pr. Jean-Claude Gérard - JC.Gerard@uliege.be - Tel : +32 (0)4 366 97 75