Scientists have observed a fascinating green glow in the Martian night sky for the first time.
The visible nightglow was detected in the atmosphere of the Red Planet by the European Space Agency's (ESA) ExoMars Trace Gas Orbiter (TGO) mission. The green glow is mainly located at altitudes of between 40 and 60 kilometers (25 to 37 miles) in the planet's polar regions. This is according to a study describing the phenomenon that has been published in the journal Nature Astronomy.
When it occurs, the nightglow should be observable from a Martian orbiter, as well as from the surface of Mars with the naked eye under clear sky conditions. In fact, it could be bright enough for astronauts exploring the Red Planet in future to navigate during dark nights.
This phenomenon is also observed on Earth, although it is quite faint on our planet. Therefore, it is best seen when looking from an "edge on" perspective—as depicted in numerous spectacular images captured by astronauts on the International Space Station.
The nightglow is not to be confused with auroras seen on both Earth and Mars, which can also light up the sky but are a distinct phenomenon.
Auroras occur tend to be highly variable across space and time, whereas the nightglow is more homogenous, spread out evenly over a wide area.
The processes that produce these phenomena are also different. Auroras occur when energetic electrons from the sun hit the upper atmosphere.
On the other hand, the nightglow occurs when two oxygen atoms combine to form an oxygen molecule dozens of miles above the planetary surface.
The origins of the process take place on the dayside of Mars when sunlight splits apart carbon-dioxide molecules, leaving behind oxygen atoms. When the atoms are transported to the nightside by winds and stop being excited by the sun, they recombine into oxygen molecules at lower altitudes and emit light in the visible range.
"This emission is due to the recombination of oxygen atoms created in the summer atmosphere and transported by winds to high winter latitudes, at altitudes of 40 to 60 km [25 to 37 miles] in the Martian atmosphere," Lauriane Soret said in a press release. She is a researcher from the Laboratory of Atmospheric and Planetary Physics at the University of Liège (UL), Belgium, and an author of the study.
"This emission would look similar to an Earth aurora in terms of color and brightness but would also be very different," Soret and her UL co-author Jean-Claude Gérard, told Newsweek. "While Earth aurorae are highly variable because they are directly related to the strength of solar events localized in time, this Martian visible nightglow emission is homogeneous and continuous over time. It thus creates kind of a green dome that is permanently located above the pole located in the current winter hemisphere of the planet."
The green nightglow was expected on Mars, although it was never observed in visible light until the latest research.
Previously, the ESA's Mars Express orbiter had observed the nightglow in infrared wavelengths—which cannot be seen with the human eye—around a decade ago.
Then, the TGO followed up by detecting glowing green oxygen atoms high above the dayside of Mars in 2020. This discovery represented the first time that such a dayglow had been seen around a planet other than Earth.
The latest findings will add to our understanding of the properties of the Martian atmosphere. Understanding these properties will be vital for future missions to the Red Planet's surface. For example, the property of atmospheric density directly affects the drag experienced by orbiting satellites and by the parachutes used to deliver probes to the surface.
"The illumination from the nightglow could be bright enough for future explorers to light the way during the winter polar nights on Mars. Human explorers could likely see the glow as bright as moonlit clouds on Earth," Soret and Gérard said.
Update 11/15/23, 8:46 a.m. ET: This story was updated with additional information from Jean-Claude Gérard.
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