What the Heck Is This Giant Bow-Shaped Structure in Venus’s Skies?

It's no secret that Venus, Earth's nearest planetary neighbor, has a penchant for mysterious behavior. But the emergence of a colossal bow-shaped structure in the planet's upper atmosphere, captured by the Japanese Akatsuki orbiter, is pretty weird even for Venus.

Stretching across 10,000 kilometers (6,214 miles), the phenomenon was spotted by the Akatsuki no sooner than it inserted itself into orbit at the planet, on December 7, 2015. The spacecraft monitored the structure until December 12, but then had to shift focus to calibrating its orbit, attitude, and telecommunication equipment. By the time it reopened its eyes to the planet on January 15, 2016, the bow shape was gone. Classic Venus.

So, what was this bizarre archery-themed spectre in Venus's skies? New research published Monday in Nature Geoscience suggests it was likely a stationary gravity wave generated by the flow of air through the planet's mountainous surface topology. Essentially, airflow passing over Venus's mountains was sculpted into waveforms that propagated upward, eventually taking the bow-shape observed by Akatsuki.

Similar mountain gravity waves have been observed on Earth, though they are much smaller than the Venusian version, according to study co-author Makoto Taguchi, an atmospheric physicist based at Rikkyo University in Tokyo.

"For Earth, I know one example of a mountain wave generated in a geographic region over Patagonia, South America, but the length of that wave front and wavelength are 1,000 kilometers and 100 kilometers, respectively," Taguchi told me over email.

In other words, Earth's gravity waves are mere ripples compared to the enormous 10,000-kilometer-scale disturbances Akatsuki saw one planet over. Indeed, Taguchi and his colleagues claim the event was "perhaps the greatest [gravity wave] ever observed in the solar system."

There is an obvious wow factor to beholding such a monster structure, but the wave offers more than just planetary showmanship. It also provides a rare window into Venus's murky atmospheric underworld and surface environment.

Venusian cloud cover is so dense that very little can be inferred about the world below. Only a few robotic probes have braved the journey through its punishing atmosphere, and fewer still survived to transmit information from the ground, so there is hardly any direct evidence of the complex variations in temperatures, pressures, and airstreams from those nether reaches.

Gravity waves, however, can be mined for clues about this hidden world, where they were forged. "Our findings imply that the temperature distributions in the lower atmosphere can be inferred from the bow-shaped pattern seen in the cloud tops," Taguchi told me. "Emergence of the bows reflects the variability of the temperature distributions or climatology of the lower atmosphere."

Still, it will take much more observational data from Akatsuki, and any future Venus probes, to reconstruct the complicated forces at work under this atmospheric veil. "We have found more than 15 bows in the data obtained in one year," Taguchi said. "We are now studying spatial distribution, local time dependence, horizontal scale, and amplitude of the bows. We have to accumulate more data for a statistical study."

Some US-based Venus experts are worried that American missions to this beautiful yet nightmarish world seem to be falling out of fashion, especially in light of NASA's recent decision to fund two Discovery-class asteroid missions instead of two proposed Venus missions.

"Our community is passionate about Venus, but we're getting pretty thin," said geophysicist Robert Grimm, a program director in the space science division of the Southwest Research Institute, in a January 10 Ars Technica article. "We basically have a huge generation gap with Venus, and we really need something to launch in the early-to mid-2020s so we can maintain some kind of continuity."

Akatsuki's extraordinary finds, including its observations of Venus's large-scale gravity waves, show that there really is a fantastic payoff to Venusian missions, if only for the planet's unapologetic WTF-ness.

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