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Credit: NASA/JAXA

There is a darkness coming over Venus, but what exactly is it?

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Sep 1, 2021, 3:59 PM EDT

Venus has a dark secret, and no, it has nothing to do with the mythical goddess and her many, many lovers.

This is much weirder than any deity love triangle. Venus (the planet) has always been overshadowed by what came to be called the Giant Dark Cloud, suspected in observations from Earth and first identified by JAXA’s Akatsuki orbiter. It would appear and vanish without an explanation. Unfortunately, Akatsuki wasn’t equipped with the right instruments to investigate further. What it needed was a spectrometer to really shed light on this darkness.

When ESA’s Venus Express mission flew over with its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), it could see in infrared like Akatsuki, but it was also able to figure out some of the chemical reasons behind the Venusian specter. Now planetary atmospheric scientist Kevin McGouldrick of University of Colorado Boulder is looking deeper into that cloud than ever.

“The advantage of using a spectrometer is that you get the whole spectrum, and you can use radiative transfer models to identify what combinations of abundances of the atmospheric constituents are most likely to be responsible for the variations in brightness seen as a function of the wavelength,” he told SYFY WIRE.

McGouldrick led a study, recently published in The Planetary Science Journal, that analyzed VIRTIS data to explain what Akatsuki saw. Akatsuki was designed to observe hourly changes in the atmosphere with multiple cameras, but its filters did not cover the entire spectrum of light. The VIRTIS instrument on Venus Express had multi-wavelength vision that could see parts of the spectrum Akatsuki could not. It was able to see Venus in many more wavelengths and then image it by scanning the spectrometer’s field of view over as much of the planet as it could.

An observation (left) and simulation (right) of the clouds of Venus. Credit: JAXA

Because VIRTIS could see across the spectrum, it could make sense of the radiation it observed and how it was related to the atmospheric chemical composition of Venus, including what might be causing such drastic variations in brightness. The only issue was that it could see no more than 10% of the full disc of Venus at its furthest distance from the planet. That is where Akatsuki had an advantage—it had a spectacular view of the planet. Virtis needed to take multiple shots to get that view, but combining data from both spacecraft was most critical.

“What the Venus Express/VIRTIS data showed was that the environment in the vicinity of the giant dark cloud was one that appeared to have a lower altitude of the cloud base, and initially somewhat smaller cloud droplets,” McGouldrick said. “The acidity of the droplets also increased, while water vapor in the air decreased.”

Venus is swirling with toxic clouds of sulfuric acid, VIRTIS showed the amount of mass from sulfuric acid was increasing as water vapor mass decreased. Put that together with how fast the menacing cloud overshadowed Venus and then receded, and it started to suggest to the researchers that this cloud could be driven by a Kelvin wave. This is a phenomenon that also happens on Earth. Kelvin waves are huge gravitational waves that move one way towards a boundary such as the equator. After its amplitude peaks there, it dies down as it moves away.

Kelvin Waves are affected the rotation of a planet and, at least on our planet, often end up trapped in coastlines or mountain ranges. It turned out to be an observational advantage that there are no such geological features on Venus. Equatorial Kelvin waves are trapped by the rotation of the Coriolis effect on Earth, which deflects rotating objects not attached to the surface.  Because the rotation of Venus is much slower than Earth's, there is hardly any Coriolis effect, but the planet's winds are enough to trap Kelvin waves, which extend further up and down the planets and are larger overall. Whether the cloud is actually being caused by a Kelvin wave has not been proven yet, but finding out more about what is in the atmosphere of Venus could demystify it. Analyzing the VIRTIS data again and going through data from spectrometers observing Venus from Earth could help, but what we really need is another spacecraft.

"Studing Kelvin waves on Earth may give us insight into what the role of Kelvin waves on Venus is on certain planetary-scale oscillations critical to understanding the climate of Venus," coauthor and JAXA Akatsuki team member Javier Peralta also told SYFY WIRE. "Venus can also help us to better understand Kelvin waves with larger spatial scales."

On Earth, Kelvin Waves influence the adjustment of the tropical atmosphere to convective latent heat, meaning heat energy that is released or absorbed when there is a phase transition (like liquid to gas) in water and other substances, such as when water condenses. They also impact oscillations, or fluctuations between extremes, in the atmosphere. Latent heat on Venus is released when vaporized sulfuric acid and water vapor condense. Its effects are also not nearly as strong as those of water condensation, but there are several atmospheric ocillations on Venus suspected to be caused by Kelvin waves. Becuase we have so many limitations when it comes to getting up close to Venus, it has been difficult to study its atmospheric phenomena, which is why more research is needed in the future.

“It is much more difficult to observe Venus from Earth than it is from a spacecraft—and more difficult than many other planets,” said McGouldrick. “Many of the constituents of the Venus atmosphere also exist in Earth's atmosphere, making analysis of the spectra difficult. We are now attempting to simulate the clouds of Venus with a computer model of cloud formation.”

It might sound far off, but computer simulations can actually bring the effects of minor atmospheric changes to light and rule out some potential causes of the Giant Dark Cloud. Something like that may finally bring it out of the dark.