Mars' atmosphere pulses in ultraviolet light three times a night

7 August 2020

NASA's MAVEN spacecraft was able to observe, for the first time, large areas of the pulsating ultraviolet night sky of Mars. These data, analysed by researchers from the STAR Institute at ULiège, allow a better understanding of the complex circulation patterns in the Martian atmosphere.

ASA’s MAVEN spacecraft, orbiting Mars since 2015, is the first mission dedicated to understanding the middle Martian atmosphere. The middle atmosphere of Mars, like those of other planets, shows variability in the brightness of the night sky (a glow in the atmosphere that makes the night not totally dark). The IUVS instrument on board MAVEN detected that these pulses occur exactly three times a night from spring to autumn. These new observations also revealed unexpected waves and spirals over the winter poles of Mars, also confirming results from the European Mars Express satellite which indicated that this nightglow was the brightest in the polar winter regions.

“MAVEN’s images offer the first insight into the atmospheric motions in Mars’ middle atmosphere, a critical region where air currents carry gases between the lowest and highest layers," said Nick Schneider, of the University of Colorado at Boulder's Laboratory of Atmospheric and Space Physics and the principal investigator of the Imaging Ultraviolet Spectrograph (IUVS) instrument on board the U.S. probe." This new-generation instrument is capable of repeatedly mapping nightglow and identifying periodic behaviours such as those observed.

This glow occurs where vertical winds carry gases to regions of higher density, accelerating the chemical reactions that create nitric oxide (NO) and produce this ultraviolet glow. "This glow is due to the recombination of oxygen and nitrogen atoms at an altitude of about 70 kilometres. These are produced by the dissociation of carbon dioxide (CO₂) and nitrogen (N₂) molecules on the dayside by sunlight," comments Jean-Claude Gérard, scientific collaborator at LPAP (STAR Institute) of the University of Liège. The emission is concentrated over an area about a thousand kilometres in diameter and is as bright in the ultraviolet as the aurora that can be observed on Earth. "These bright spots intensify each night after sunset and move across the Martian sky at a speed of 300 km per hour.

MAVEN nightglow animation

Animation based on the observations of the MAVEN satellite which shows the night brightness and the pulsations of Mars. The false green to white colours show the brightness intensities of the ultraviolet night glow of Mars measured by MAVEN's Imaging UltraViolet Spectrograph (IUVS) at about 70 km altitude. A simulated view of the Mars globe has been added digitally for context, with the ice caps visible at the poles. Three nightglow brightenings occur over one Mars rotation, the first much brighter than the other two. The three brightenings occur shortly after sunset, appearing on the left of this view from the night side of the planet. The pulsations are caused by downward winds that enhance the chemical reaction that creates the nitric oxide causing the glow. Several months of data have been averaged to identify these patterns, indicating that they repeat every night. Credits: NASA / MAVEN / Goddard Space Flight Center / CU / LASP.

The Martian night glow was first observed by the SPICAM instrument on board the European Space Agency (ESA) Mars Express orbiter, which is still in operation. The team from the Atmospheric and Planetary Physics Laboratory had analysed its vertical distribution and seasonality. "We had shown that studying these bright spots would provide a better understanding of the wind regime and the role of constituent transport in the planet's upper atmosphere," says Jean-Claude Gérard.

This diagram explains the cause of Mars' glowing nightside atmosphere. On the day side of Mars, CO₂ and N₂ molecules are broken up by energetic solar radiation. Global circulation patterns carry the atomic fragments to the night side, where downwards winds increase the reaction rate for the atoms to reform molecules. The downward winds occur near the poles at some seasons and in equatorial regions at others. The new molecules holds additional energy that they emit as ultraviolet light. Credits: NASA / MAVEN / Goddard Space Flight Center / CU / LASP.

Many planets, including Earth, have a nightglow, but MAVEN is the first mission to collect so many images of another planet's nightglow. The team plans to observe the emission "sideways" rather than from above, using data taken by the IUVS instrument just above the limb of the planet. This new perspective will be used to better understand the vertical winds and seasonal changes on Earth's little sister planet.

Scientific reference

Imaging of Martian circulation patterns and atmospheric tides through MAVEN/IUVS nightglow observations, N. M. Schneider, Z. Milby, S. K. Jain, F. González-Galindo, E. Royer, J.-C. Gérard, A. Stiepen, J. Deighan, A. I. F. Stewart, F. Forget, F. Lefèvre, S.W. Bougher, Journal of Geophysical Research, Space Physics, 125 (8), 2020, https://doi.org/10.1029/2019JA027318.

Contact

LPAP - Laboratory of Atmospheric and Planetary Physics | STAR Institute | Faculty of Science

Jean-Claude GERARD

Published on 8/7/20