Mars Unveils Its Nighttime Secret: Mesmerizing Green Glow Discovered

by Hiroshi Tanaka
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Martian Nightglow

Mars Reveals Its Nocturnal Enigma: Fascinating Green Luminescence Unearthed

This depiction portrays what the phenomenon of nightglow might appear as to an astronaut amidst the polar winter realms of Mars during the nighttime hours. The emerald radiance emerges when high-altitude oxygen atoms combine to form oxygen molecules.
This simulated visual representation was crafted using an authentic, albeit darkened, image of the Martian surface captured by NASA’s Opportunity rover’s panoramic camera, alongside a synthetic nightglow illustration mirroring the actual color of the oxygen emission. Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.– E. W. Knutsen

ESA’s ExoMars Trace Gas Orbiter has stumbled upon a verdant nightglow phenomenon within the Martian atmosphere, furnishing invaluable insights into atmospheric processes and potential illumination for forthcoming Martian expeditions. This unique occurrence, distinct from auroras, signifies a substantial leap forward in comprehending Mars.

When future astronauts venture into Mars’s polar territories, they will be greeted by an emerald luminescence gracing the nocturnal heavens. For the very first time, ESA’s ExoMars Trace Gas Orbiter (TGO) mission has detected a visible nightglow within the Martian atmosphere.

Under clear skies, this luminescence could be sufficiently bright for human observation and for guiding rovers during the dark Martian nights. Nightglow is a phenomenon also observable on Earth, but on Mars, it was an anticipated yet unobserved spectacle until now.

The Martian Nightglow

The atmospheric nightglow materializes when two oxygen atoms unite, forming an oxygen molecule, at an altitude of approximately 50 km (around 30 miles) above the planet’s surface.

These oxygen atoms embark on a journey, originating on Mars’s sunlit side where solar radiation energizes carbon dioxide molecules, causing them to disintegrate. When these oxygen atoms migrate to the night side of Mars, where they are no longer excited by the Sun, they reassemble and emit light at lower altitudes.

“This emission arises from the recombination of oxygen atoms generated in the summer atmosphere and transported by winds to high winter latitudes, at altitudes ranging from 40 to 60 km in the Martian atmosphere,” elucidates Lauriane Soret, a researcher from the Laboratory of Atmospheric and Planetary Physics at the University of Liège in Belgium, and a member of the team responsible for publishing this discovery in Nature Astronomy.

The luminosity emanating from this nightglow could be intense enough to illuminate the path for future Martian explorers, rendering the night sky as radiant as moonlit clouds on Earth.

“These findings are unexpected and hold great significance for forthcoming missions to the Red Planet,” remarks Jean-Claude Gérard, the lead author of the recent study and a planetary scientist affiliated with the University of Liège.

Airglow, a similar phenomenon, is observable within Earth’s atmospheres when sunlight interacts with the atoms and molecules present. In an image captured by astronauts aboard the International Space Station (ISS) in 2011, a green band of oxygen glow can be discerned over Earth’s curvature, while portions of northern Africa are visible on the surface, featuring evening lights along the Nile River and its delta. Credit: NASA

Tracing the Emerald Glow

The international scientific team’s curiosity was piqued by a prior discovery made using Mars Express, which had observed nightglow in infrared wavelengths a decade ago. The Trace Gas Orbiter followed up by detecting glowing green oxygen atoms high above Mars’s sunlit side in 2020 – marking the first instance of this dayglow emission being witnessed around a celestial body other than Earth.

These oxygen atoms also migrate to the night side of Mars and then recombine at lower altitudes, resulting in the visible nightglow unveiled in the recent research publication.

Artist’s impression of ESA’s ExoMars Trace Gas Orbiter detecting the green glow of oxygen within the Martian atmosphere. This emission, identified on Mars’s sunlit side, resembles the night glow observed in Earth’s atmosphere from space. Credit: ESA

Orbiting the Red Planet at an elevation of 400 km, TGO managed to monitor Mars’s nocturnal hemisphere utilizing the ultraviolet-visible channel of its NOMAD instrument. This instrument encompasses a spectral range spanning from near-ultraviolet to red light and was positioned towards Mars’s edge to provide a better view of the upper atmosphere.

The NOMAD experiment is under the leadership of the Royal Belgian Institute for Space Aeronomy, collaborating with teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University), among others.

Scientific Significance

The nightglow functions as a tracer of atmospheric phenomena, yielding a plethora of data regarding the composition and dynamics of a region of the atmosphere that is challenging to measure, as well as the density of oxygen. It also offers insights into how energy is deposited by both the Sun’s illumination and the solar wind, the stream of charged particles emanating from our star.

A snapshot captured from space showcasing the Milky Way and Earth in conjunction beyond the International Space Station. The Milky Way stretches beneath Earth’s curvature, accompanied by a faint green airglow. The galaxy’s central bulge appears with starfields punctuated by dark rifts of interstellar dust. This image was taken by NASA astronaut Scott Kelly in 2015 during his one-year mission in space. Credit: NASA/Scott Kelly

Understanding the characteristics of Mars’ atmosphere is not only a matter of scientific intrigue but also a pivotal consideration for missions destined for the Red Planet’s surface. Atmospheric density, for instance, directly influences the resistance experienced by orbiting satellites and the parachutes employed for delivering probes to Mars’s surface.

Nightglow vs. Aurora

While nightglow is also observed on Earth, it should not be conflated with auroras. Auroras represent just one way in which planetary atmospheres become illuminated.

Auroras, both on Mars and Earth, manifest when energetic electrons from the Sun collide with the upper atmosphere. They exhibit variations across space and time, whereas nightglow offers a more uniform display. Both nightglow and auroras can display a wide spectrum of colors, contingent upon the prevailing atmospheric gases at various altitudes.

The green nightglow observed on our planet is relatively faint and is best observed when viewed from an ‘edge-on’ perspective, as frequently depicted in stunning images captured by astronauts aboard the International Space Station.

Reference: “Observation of the Mars O2 visible nightglow by the NOMAD spectrometer onboard the Trace Gas Orbiter” by J.-C. Gérard, L. Soret, I. R. Thomas, B. Ristic, Y. Willame, C. Depiesse, A. C. Vandaele, F. Daerden, B. Hubert, J. P. Mason, M. R. Patel, and M. A. López-Valverde, published on November 9, 2023, in Nature Astronomy.
DOI: 10.1038/s41550-023-02104-8

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