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Spacecraft spots giant, 10,000 km long wave on Venus
Tuesday, January 17, 2017, 6:38 PM - We see gravity waves here on Earth all the time, but nothing like what Japan's Akatsuki spacecraft spied on Venus a little over a year ago. It's What's Up In Space!
Venus is not known as a planet of subtleties. Temperatures there soar to over 460oC, the atmospheric pressure on the surface is comparable to the crushing depths of Earth's oceans, it has clouds of sulphuric acid and heavy metal snow may even fall on the planet's mountain peaks.
Now, based on images sent back by Japan's Akatsuki Venus Climate Orbiter in late 2015, our sister planet has racked up another new extreme. Over several days, an immense stationary wave set up in Venus' atmosphere, spanning nearly the entire disk of the planet. This is quite possibly the largest atmospheric gravity wave ever seen in our solar system.
Gravity waves happen all the time here on Earth. We see them spread outwards after disturbing the surface of a pond, and in the bands of cloud that set up downwind of mountain peaks or even large thunderstorm clouds. They're caused by oscillations in the fluid - water or air - as gravity and buoyancy try to restore balance after after the fluid is disturbed. For example, if stable air is forced to rise to flow over a mountain peak, it will attempt to return to its stable flow by sinking in the lee of the mountain. Due to its momentum, though, it will overshoot that stable height, by roughly the same amount it was displaced by the mountain peak in the first place, and then "bounce" back, rising and then falling again, each time a bit less as the waves dampen out. This sets up a standing wave in the air flow beyond the mountain peak, where the bands of rising air produce clouds and the bands of sinking air result in clear areas.
While all of that is going on, however, the energy from these oscillations in the air also cause the waves to propagate upward, from the troposphere, through the stratosphere, and beyond.
Now we've seen this same process, writ large, in the atmosphere of Venus.
On December 7, 2015, the day that Akatsuki finally achieved Venus orbit, after going through a five-year delay in its mission due to a glitch back in December 2010, this is what the spacecraft saw with its infrared and ultraviolet instruments:
Akatsuki's view of Venus, December 7-11, 2015. Credit: JAXA/PLANET-C
Quite the reward for such a long wait for the mission team. This 10,000 kilometre-long wave sat stationary in the Venus' atmosphere for roughly five days, at an altitude of around 65 kilometres - the boundary between the troposphere and mesosphere layers of Venus' atmosphere (Venus doesn't have a stratosphere, like Earth does). Akatsuki lost sight of the wave when it adjusted its orbit around the planet, and by the time it came around for another look, a little over a month later, the wave was gone.
Still, it had collected enough information for researchers to study the phenomenon.
The animation shows this gravity wave as it remains stable in the atmosphere, even while 360 km/h winds - significantly stronger than those of a Category 5 hurricane - howl all around it. The moving lines represent the boundaries of the day and night side of the planet. Credit: JAXA/PLANET-C
The map the gravity wave is projected on is a Venus terrain map. The map below, an altered version of NASA's Venus topography map from the Pioneer Venus Orbiter in 1978, showing the same terrain in colour. The giant gravity wave was set up over the western end of the Aphrodite Terra highlands.
Venus topography. Credit: NASA/S. Sutherland
Speaking to the Daily Mail, the lead researcher, Makoto Taguchi of Rikkyo University said: "We have found more than 15 bows so far, each above the highlands at their centres." These bows can be seen in the large, December 7 images, above.
"Though the mechanism is far different from that of the bows in the Venus atmosphere," Taguchi said, "the stationary thermal structure reminds me of stationary waves on a surface of a shallow river below which an invisible big stone on the bottom prevents smooth flow."
Note that these gravity waves should not be confused with gravitational waves, such as those that were first detected by LIGO in September 2015. Gravity waves are simply a fluid returning to a stable state via gravity and buoyancy. Gravitational waves are the minute squeezing and stretching of spacetime, the fundamental "fabric" of the universe, caused by the cataclysmic merger of two black holes or neutron stars.