Science & Nature

Akatsuki Observes Nightside Equatorial Circulation in Atmosphere of Venus

Akatsuki Observes Nightside Equatorial Circulation in Atmosphere of Venus thumbnail

Venus is well known for its 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. According to new research, the nightside cloud-top circulation discovered by JAXA’s Akatsuki orbiter may fuel the Venusian superrotation.

The three main weather patterns on Venus. Image credit: Imamura et al. / JAXA.

The three main weather patterns on Venus. Image credit: Imamura et al. / JAXA.

“Venus — a terrestrial planet with a size similar to Earth — is covered by a thick atmosphere mainly composed of carbon dioxide,” said University of Tokyo’s Professor Takeshi Imamura and colleagues.

“Sulfuric acid clouds, which are thought to be produced from sulfur dioxide abundant in the atmosphere, cover the entire surface at altitudes of about 50-70 km (31-43.5 miles).”

“A global westward wind called the superrotation prevails with a maximum speed of about 100 m/s around the cloud-top level of 65-70 km (40.4-43.5 miles), corresponding to a rotation period of about 4 days, in contrast to the planetary rotation period of about 243 days.”

“Superrotation also exists in the atmosphere of Saturn’s moon Titan and is thought to occur on tidally locked exoplanets.”

Professor Imamura and co-authors used thermal infrared images taken by Akatsuki, which is sensitive to an altitude of about 65 km (40.4 miles), to measure winds in the nightside Venusian atmosphere.

“Small-scale cloud patterns in the direct images are faint and frequently indistinguishable from background noise,” Professor Imamura said.

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“To see details, we needed to suppress the noise. In astronomy and planetary science, it is common to combine images to do this, as real features within a stack of similar images quickly hide the noise.”

“However, Venus is a special case as the entire weather system rotates very quickly, so we had to compensate for this movement, known as superrotation, in order to highlight interesting formations for study.”

“We were finally able to observe the north-south winds, known as meridional circulation, at night,” he said.

“What’s surprising is these run in the opposite direction to their daytime counterparts.”

“Such a dramatic change cannot occur without significant consequences. This observation could help us build more accurate models of the Venusian weather system which will hopefully resolve some long-standing, unanswered questions about Venusian weather and probably Earth weather too.”

A paper describing this research will be published in the July 22, 2021 issue of the journal Nature.


Okay. Fukuya et al. 2021. The nightside cloud-top circulation of the atmosphere of Venus. Nature 595, 511-515; doi: 10.1038/s41586-021-03636-7

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