Thursday, August 31, 2017

Japan's Akatsuki Spacecraft Detects Savage Winds on Venus

Artist's rendering of the Akatsuki spacecraft orbiting Venus. Credit: JAXA

Japan’s Akatsuki space probe orbiting Venus has spotted extremely strong winds near the planet’s equator, blowing at a speed of over 178 mph. The newly discovered high-velocity winds could provide important hints about the dynamics of Venusian atmosphere.

The winds were imaged by Akatsuki’s IR2 infrared camera in mid-2016. This instrument allowed the group of researchers led by Takeshi Horinouchi of the Hokkaido University in Japan to obtain infrared images of Venus between 1.74 and 2.32 µm through four narrow-band filters. Using a novel automated cloud tracking method, Horinouchi’s team was able to distinguish winds exhibiting a rotational speed maximum near the equator and they refer to this phenomenon as the “equatorial jet”.

“Here we report the detection of winds at low latitude exceeding 80 m/s using IR2 camera images from the Akatsuki orbiter taken during July and August 2016. The angular speed around the planetary rotation axis peaks near the equator, which we suggest is consistent with an equatorial jet, a feature that has not been observed previously in the Venusian atmosphere,” the scientists wrote in a paper published in Nature Geoscience on Aug. 28.

As noted in the paper, the newly detected winds blow at a speed exceeding 80 m/s what corresponds to 178 mph. The jet was spotted at an altitude between 28 and 37 miles (45 and 60 kilometers) above the planet's surface. The team used the infrared camera because these areas are invisible at optical wavelengths due to extremely dense clouds of sulfuric acid.

The equatorial jet revealed by Akatsuki could be crucial for improving our knowledge about the dynamics of Venusian atmosphere, uncovering what is behind the so-called “superrotation” of the planet’s atmosphere.

“Discovery of an equatorial jet is a giant step toward unraveling the mystery of superrotation,” Horinouchi said.

Venus is well known for its superrotating upper atmosphere as it rotates sixty times faster than its surface. The superrotation reaches its maximum near the cloud top, located at around 43 miles (70 kilometers) above the surface, where the rotational periods are three to five days, several tens of times faster than the planetary rotation. Although this phenomenon was the subject of many previous studies, its mechanism still remains a mystery.

“The mechanism producing the jet remains unclear. Our observations reveal variability in the zonal flow in the lower and middle cloud region that may provide clues to the dynamics of Venus’s atmospheric superrotation,” the researchers concluded.

Operated by the Japan Aerospace Exploration Agency (JAXA), Akatsuki is a space probe studying the atmosphere of Venus from the planet’s orbit. The spacecraft was launched into space in May 2010 and failed to enter orbit around Venus during the first attempt in December 2010. Five years later, it was injected into an alternative elliptical Venusian orbit and started regular observations of the planet in March 2016.

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