The arrival of spring and fall on Earth is marked by milder temperatures and the respective sprouting and shedding of leaves. Spring and fall on Mars, in contrast, is signaled by the Martian atmosphere weirdly emitting exactly three pulses of ultraviolet light every night, according to a new study.
These clockwork “nightglow” pulses on Mars during equinoctial seasons reveal hidden circulation patterns in the Martian atmosphere, which is about 100 times thinner than Earth’s skies. If humans could see in ultraviolet light, these nocturnal shows would be as bright as the Northern Lights.
In addition to measuring the pulses, a team of scientists reports the discovery of a polar sky spiral and “a remarkably bright spot” at the equator, according to a study published on Wednesday in the Journal of Geophysical Research: Space Physics.
Led by Nick Schneider, a planetary scientist at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP), the researchers observed Mars with NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), an orbiter that arrived at the planet in 2014. MAVEN’s Imaging Ultraviolet Spectrograph (IUVS) instrument is able to capture fine details about the middle of the Martian atmosphere, which is about 70 kilometers (approximately 40 miles) above the surface.
The data revealed that the ultraviolet flashes are generated by an eastbound atmospheric wave that intensifies into three peaks over the course of one Martian rotation (called a sol). The third peak is just a repeat of the first one that gets carried back into MAVEN’s view before one sol is completed.
“We say we see three peaks by looking at the same local time, say midnight, and counting how many peaks come by over the course of one planet rotation,” explained study co-author Zac Milby, a professional research assistant at LASP, in an email. “They are not at all random: we see consistent behavior when they appear in local time and where they appear in longitude.”
The glow itself is caused by chemical reactions driven by the shift from day to night. Carbon dioxide and nitrogen molecules in the atmosphere get split up into constituent elements by sunlight during the day. When night falls, nitrogen and oxygen molecules descend into the middle atmosphere, where they hook up to create nitric oxide—a union that releases ultraviolet light. The pulses occur shortly after sunset, according to NASA.
Mars is enveloped by lots of different types of atmospheric waves, but the one that produces the pulses in spring and fall is particularly influenced by seasonal processes.
“The reason these waves exist only during equinox seasons (and near the equator) is due to solar heating which is modulated by surface topography, surface thermal inertia, albedo, and dust loading,” said Milby. “So, really it’s a variety of factors that come together during equinox to provide the right atmospheric conditions for this kind of wave.”
Milby and his colleagues plan to continue probing the mysterious dynamics of the middle atmosphere, including its ultraviolet nightglow, to refine models of the Martian skies.
For instance, MAVEN has captured a huge number of “limb scans,” which are observations the orbiter has collected from about 150 kilometers of altitude—the closest point in its orbit around Mars.
Those extreme close-ups can provide scientists with a far more intricate view of Martian atmospheric processes.
“We have thousands of orbits’ worth of these limb scans covering a huge range of latitudes, longitudes, and local times,” said Milby. “[W]e’d like to use these data to characterize how the layer changes on daily and yearly cycles and expand on a previous study of a limited set of these types of observations.”
Update: This article has been updated with comments from study co-author Zac Milby.