A view of nightglow from Venus Express. The top shows the oxygen nightglow at about 60 miles over the surface of Venus, while the bottom shows the weaker glow of nitric oxide 68 miles up.
Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA
An eerie nightglow on the planet Venus has appeared at least twice to a European spacecraft, though a human would need infrared eyes to spot it.
Previously-reported glows have revealed the chemical breakdown of Venus' atmosphere. But this latest sighting represents a newly discovered nightglow caused by nitric oxide in the planet's atmosphere, and has proven useful for observing Venus' atmosphere.
"The nightglow can give us a lot of information,? said Antonio Garcia Munoz, a former researcher at Australian National University who has since relocated to the Instituto de Astrof?sica de Canarias in Spain. "It can provide details about the temperature, wind direction, composition and chemistry of an atmosphere."
Europe's Venus Express spotted the nightglow with its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument to record the nitric oxide levels on the planet below. On Earth, the molecule can be found as a pollutant from cigarette smoke, car emissions and power plants
The phenomenon speaks to the ongoing violence of the sun's ultraviolet light pummeling the planetary atmosphere, which breaks up molecules into simpler molecules and atoms. Chemical fragments may recombine into molecules such as nitric oxide, and give off energy in the form of light during the recombination phase.
Sunlight normally outshines any faint glow on the day side of Venus. But the dark side provides ample opportunity for Venus Express to catch an infrared show. Nitric oxide has also made a showing on Earth and Mars in the shorter ultraviolet range, as opposed to infrared.
Previous Venus nightglows from oxygen and hydroxyl molecules have been detected from 56 to 62 miles (90-100 km) up in the planet's atmosphere. Nitric oxide light emissions have appeared slightly higher at the 68 to 75 mile (110-120 km) range.
"Luckily for us, Venus has a temperamental atmosphere," Munoz said. "Packets of oxygen and nitrogen atoms are blown around."
He explained that the whirling collection of atoms sometimes became dense enough to boost the brightness of nightglow and make it visible to VIRTIS. Venus Express can even pick up on all three types of nightglows simultaneously.
However, when and why each of the three nightglows appears remains a mystery for now. Full results on this research appear in the January issue of the journal Proceedings of the National Academy of Science.
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