A team of researchers led by JAXA’s Institute of Space and Astronautical Science has discovered unexpected patterns of slow motion and stationary waves in the nightside upper clouds of Venus. The research is published in the journal Nature Astronomy.
The atmospheric superrotation at the upper clouds of Venus. While the superrotation is present in both day and night sides of Venus, it seems more uniform in the day (AKATSUKI-UVI image, right), while in the night this seems to become more irregular and unpredictable (composite of Venus Express/VIRTIS images, left). Image credit: JAXA / ESA / J. Peralta, JAXA / R. Hueso, UPV/EHU.
Venus is one the most hostile places in the Solar System.
The atmosphere of the planet is composed almost entirely of carbon dioxide (96.5%), with very little water vapor. It has significant amounts of corrosive sulfur-bearing gases and rapidly moving clouds of sulfuric acid droplets.
Despite only 10% of the solar flux reaching the planet’s surface, enough energy is trapped by gases and particles present in the lower atmosphere, to raise the temperature at the surface dramatically. This extreme ‘greenhouse effect’ heats the surface to temperatures as high as 462 degrees Celsius (864 degrees Fahrenheit).
As well as being hot, the Venusian surface environment is dimly lit, due to a thick blanket of cloud which completely envelops the planet.
Walking on the surface would be difficult, like walking under water at 900 m (2,950 feet) depth — the atmospheric pressure is 90 times higher than on Earth at sea level.
Ground-level winds are slow, pushing their way across the planet at painstaking speeds of about 1 m per second (2.24 mph).
However, that is not what we see when we observe Venus from above. Instead, we spy a smooth, bright covering of cloud.
This cloud forms a 20-km (12.4 mile) thick layer that sits between 50 and 70 km (31-43.5 miles) above the surface and is thus far colder than below, with typical temperatures of about minus 70 degrees Celsius (minus 94 degrees Fahrenheit) — similar to temperatures found at the cloud-tops of Earth.
The upper cloud layer hosts more extreme weather, with winds that blow hundreds of times faster than those on the surface.
It rotates about 60 times faster (once every 4 terrestrial days) than the underlying surface (243 terrestrial days) — a phenomenon known as superrotation.
Planetary researchers do not yet fully understand its origin and motor, but are working on an answer to this puzzle.
Examples of new types of cloud morphology discovered on the night side of Venus: stationary waves (Venus Express, upper left), ‘net’ patterns (IRTF, upper right), mysterious filaments (Venus Express, lower left) and dynamical instabilities (Venus Express, lower right). Image credit: JAXA / ESA / J. Peralta, JAXA / R. Hueso, UPV/EHU.
“We were able to relate the stationary gravity waves found at higher altitudes with the surface elevations of Venus,” said team member Dr. Silvia Tellmann, from the Rhenish Institute for Environmental Research at the University of Cologne.
“Hence, the waves can be explained with wind currents caused by topographical obstacles.”
“We assume that these stationary waves are substantial for the continuity of the superrotation in the atmosphere of Venus.”
To study the motions of the night clouds of Venus, Dr. Tellmann and co-authors tracked individual features in thermal emission images obtained between 2006 and 2008 by ESA’s Venus Express orbiter and in 2015 by ground-based measurements with the NASA Infrared Telescope Facility (IRTF).
“We analyzed data generated by Venus Express to investigate components of Venus’ complex atmosphere, including thermal measurements with regard to horizontal and vertical wave patterns,” the researchers said.
“Vertical information in unison with horizontal data help to understand the nature of the observed wave patterns.”
“The vertical information from the VeRa instrument (an atmosphere experiment where radio waves sent by spacecraft Venus Express are being analyzed) could help to identify the observed waves as gravity waves.”
“This, in turn, is crucial for the analysis of atmospheric processes.”
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J. Peralta et al. 2017. Stationary waves and slowly moving features in the night upper clouds of Venus. Nature Astronomy 1, article number: 0187; doi: 10.1038/s41550-017-0187
