New observations from Akatsuki, a robotic spacecraft launched by the Japan Aerospace Exploration Agency (JAXA) in 2010 to orbit Venus, have revealed an equatorial jet in the lower-to-middle cloud layer of the planet’s hostile atmosphere.
An illustration of JAXA’s Akatsuki orbiter successfully tracking lower-altitude clouds during the night with its near-infrared camera IR2. Image credit: PLANET-C Project Team.
Venus is well known for its remarkable, rotating upper atmosphere, which sweeps around the planet once every 4 Earth days.
This is in stark contrast to the rotation of the planet itself — the length of the day – which takes 243 Earth days.
Planetary researchers do not yet fully understand the origin and motor of this phenomenon, known as superrotation, but are working on an answer to the puzzle.
Using observations from JAXA’s Akatsuki spacecraft, Hokkaido University Associate Professor Takeshi Horinouchi and co-authors have now detected fast winds (up to 80 m/s) in the lower-to-middle cloud layer of the planet.
“Akatsuki’s near-infrared camera successfully tracked Venus’ lower-altitude clouds — in particular, thicker clouds between 45 and 60 km in altitude,” the researchers explained.
“This was made possible by observing the silhouettes of clouds that appear when infrared light from thermal radiation originating in the lower atmosphere filter through clouds.”
“Similar observations were previously made by ESA’s Venus Express and NASA’s Galileo spacecraft, but they provided only limited data of the planet’s low-latitude zones. From these observations, scientists speculated that wind speeds at lower-to-middle cloud altitudes are horizontally uniform and have few temporal variations.”
Westward wind speed obtained from Akatsuki/IR2 observations on July 11-12, 2016; longitudinally averaged winds are shown with respect to latitude; the wind speed peaks at low latitude indicating the jet. Image credit: PLANET-C Project Team.
In their study, Professor Horinouchi and colleagues analyzed the data collected by Akatsuki between March and August 2016.
“We employed a cloud-tracking method our team recently developed to deduce horizontal distributions of winds based on data from Akatsuki,” the authors said.
“We discovered an equatorial jet in the wind velocities based on image data from July 2016 and that the jet existed at least two months after that.”
“In March that year, the wind velocities in the same latitude zones were rather slow — thus there was no jet.”
The team’s findings showed for the first time that “wind velocities can be markedly high forming a jet near the equator, which have never been found not only in the scantily observed lower to middle cloud layers but also in the more-extensively studied high layers.”
“Our study uncovered that wind velocities in the lower-to-middle cloud layer have temporal and spatial variabilities much greater than previously thought,” Professor Horinouchi said.
“Although it remains unclear why such an equatorial jet appears, the mechanisms that could cause it are limited and related to various theories about superrotation.”
“So, further study of the Akatsuki data should help glean useful knowledge not only about local jets but also would help address superrotation theories.”
The results are published in the journal Nature Geoscience.
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Takeshi Horinouchi et al. Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki. Nature Geoscience, published online August 28, 2017; doi: 10.1038/ngeo3016
