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Tuesday, 6 July 2021

Space probe observes incredible nocturnal aurora on Mars

The United Arab Emirates' Mars Hope probe, orbiting the Red Planet since February, has captured images of a very rare and spectacular event: a nocturnal aurora. The probe is equipped with an ultraviolet spectrometer, designed to study the process of escape into space of oxygen and hydrogen from the atmosphere of Mars; the sensitivity of the instrument made it possible to capture by chance a magnificent discreet aurora in the Martian night.

The nocturnal auroras of Mars are particularly difficult to detect and scientists have been trying to study the phenomenon in detail for several decades. Just five months after arriving, Mars Hope is already making a major discovery, which was not even part of the mission's original science goals: Images released last week show large, luminous structures in the planet's dark skies.

“[These auroras] aren't easy to grasp, and that's why seeing them right away with Emirates Mars Mission was quite exciting and unexpected,” said Justin Deighan, planetologist at the University's Atmospheric and Space Physics Laboratory. of Colorado and scientific leader of the Hope mission. This discovery could help better understand the Martian atmosphere and how it interacts with solar activity.

"Mini magnets" spread all over the planet
On Earth, the polar auroras are linked to the magnetic field of our planet; they occur when charged particles from the solar wind collide with the magnetosphere. These high-energy particles are captured and channeled by the lines of the earth's magnetic field to the poles; there, they excite and ionize the atoms of the upper atmosphere which, returning to a more stable state, emit a photon. A “curtain” of various colors then appears in the sky, due to the various gases present in our atmosphere.

Images taken by Hope’s onboard spectrometer (left-hand panel) and an artist’s impression (right) show discrete auroras on Mars’s night side.Credit: Emirates Mars Mission

On Mars, the process is very different: Auroras are not limited to the poles, but can appear all over the planet. Unlike Earth, Mars does not have (or rather, no longer has) a global magnetic field, comparable to that of a giant right magnet. On the surface of the red planet, however, scientists have detected plates of rock containing the signature of a magnetic field that has now disappeared. “It's like you've taken a bag of magnets and thrown them into the crust of the planet. They are all pointed in different ways and they have different strengths,” says Deighan .

Finally, these disjointed magnetic fields act a bit like lenses guiding the particles of the solar wind towards different parts of the Martian atmosphere; these particles then collide with atoms and molecules in the upper atmosphere, generating the glow of the aurora. Scientists have observed three types of aurora on Mars. One of them occurs exclusively on the side lit by the Sun; the other two occur at night. Among these nocturnal phenomena, one only occurs during extremely powerful solar storms. Discrete auroras, such as the one captured by Hope, are not limited to intense solar activity and only occur in certain areas of the night side of Mars, where traces of ancient magnetism are particularly dense.

Elucidate the disappearance of the atmosphere of Mars

Although it was not designed specifically for this use, the EMUS (Emirates Mars Ultraviolet Spectrometer) on board the Hope probe had every chance of capturing the phenomenon. "The aurorae are weak and all visible light sensitive instruments on Mars are designed to take pictures in daylight," explains Justin Deighan. But EMUS can make observations in the far ultraviolet and its position offers a global view of the planet from several angles.

The study and comparison of the terrestrial and Martian auroras is of particular interest to scientists, since the two planets have very different magnetic properties. On Mars, the residual magnetism of the planetary crust is up to 30 times that of the earth's crust, especially in parts of the southern hemisphere. These new images provided by Hope can notably help to understand how Mars lost its atmosphere, which was much thicker billions of years ago. “The question is how does the energy get into the system and move away from the upper atmosphere, and the auroras are right at the heart of that,” says Deighan.

The team still doesn't know precisely which charged particles create these auroras. Deighan believes that the electrons are responsible, although these can come either from the Sun or from Mars itself; it is in all cases certain that their energy is limited.

NASA's MAVEN probe can also take pictures of Martian aurorae when its elliptical orbit takes it far enough away from the planet; it can also directly measure and identify the solar particles causing the spectacle when it is nearby. But it does not have the capacity to perform both measurements simultaneously. So by coordinating the Aurora photographs from the Hope spacecraft with the particle measurements from MAVEN, planetologists might be able to better understand the nocturnal lights of Mars and determine where the charged particles are coming from. phenomenon.

The Hope mission is scheduled to last one Martian year (or 687 Earth days), which will allow the team to collect much more information about these particular auroras. “We are able to capture these kinds of images on a very regular basis, so we would expect more frequent images of the aurora each time they occur, and this will allow us to further study the discrete auroras and why they occur. produce”, explains Hessa Al Matroushi, responsible for the mission.

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