Japanese Spacecraft Spots Massive Gravity Wave In Venus' Atmosphere (theverge.com)

An anonymous reader quotes a report from The Verge: The Japanese probe Akatsuki has observed a massive gravity wave in the atmosphere of Venus. This is not the first time such a wave was observed on the Solar System's second planet, but it is the largest ever recorded, stretching just over 6,000 miles from end to end. Its features also suggest that the dynamics of Venus' atmosphere are more complex than previously thought. An atmospheric gravity wave is a ripple in the density of a planet's atmosphere, according to the European Space Agency. Akatsuki spotted this particular gravity wave, described in a paper published today in Nature Geoscience, when the probe arrived at the planet on December 7th, 2015. The spacecraft then lost sight of it on December 12th, 2015, because of a change in Akatsuki's orbit. When the probe returned to a position to observe the bow-shaped structure on January 15th, 2016, the bright wave had vanished. What sets the huge December wave apart from previously discovered ones is that it appeared to be stationary above a mountainous region on the planet's surface, despite the background atmospheric winds. The study's authors believe that the bright structure is the result of a gravity wave that was formed in the lower atmosphere as it flowed over the planet's mountainous terrain. It's not clear how the wave exactly propagates to the planet's upper atmosphere, where clouds rotate faster than the planets itself -- four days instead of the 243 days it takes Venus to rotate once.

Comments

Note: Gravity wave != Gravitational wave

[xyrw]

It's worth pointing out that the article talks about a gravity wave, which is a material wave that arises out of a disturbance due to gravity. This should not be confused with gravitational waves, which are ripples in spacetime due to the movement of masses. (The article and summary aren't wrong, but the terminology itself is confusing.)

The effect observed on Venus is in fact quite massive, while gravitational waves are tiny and difficult to observe.

Re:Note: Gravity wave != Gravitational wave

[ASDFnz]

It is a bit of a picky difference since a gravity wave can be caused by wind.

It is essentially the same thing as a wave on the sea which is also caused by wind (primarily) but can be caused by other things, for example, an explosion could cause waves in the sea and it could also cause a wave like this in an atmosphere.

It is the difference between cause and effect.

The odd thing about these waves (they have been seen before) no wind process (or any other process) that we are familiar with can cause a wave of this magnitude yet here they are.

Re:Note: Gravity wave != Gravitational wave

[silentcoder]

Your conspiracy theory is flawed on a number of points.
- The terms come from different sciences. Gravitational waves is from relativistic physics (and were first predicted by Einstein over a century ago). Gravity waves come from fluid dynamics and has an entirely different history.
- Their sources differ. Gravity waves were observed, then named in the theories that explained them. Gravitational waves were predicted by a theory but not observed for another 110 years despite constant attempts along the way.

Re:Note: Gravity wave != Gravitational wave

[nadaou]

The odd thing about these waves (they have been seen before) no wind process (or any other process) that we are familiar with can cause a wave of this magnitude yet here they are.

The basic phenomena is equivalent to a hydraulic in a river downstream of a rock. Kayakers have lots of fun playing in them. If the vertical density gradient in the atmosphere is small enough the waves can grow quite large without too much trouble. With a softly defined upper Venetian atmosphere I'd view it more as an internal wave than a surface wave, then it isn't too surprising.

I am surprised at the apparent lack of Coriolis and no sign of something similar to Hadley cells at latitudes less than 60 degrees, but then again IANAPAP.

Re: Note: Gravity wave != Gravitational wave

[Anonymous Coward]

I'm the same AC who tried to explain the concept of gravity waves. Parcel theory is a common assumption in meteorology (see http://www.srh.noaa.gov/jetstream/upperair/parcels.html as one of a huge number of examples), even though its conditions are usually violated to some degree in the real atmosphere. Sure, parcel theory isn't perfect, but it does a pretty good job of explaining a lot of things in the atmosphere. It's also extremely common to use it in meteorology to determine things like the amount of instability in the atmosphere or to estimate vertical motions resulting from isentropic upglide and downglide. Anyone who has ever looked at a skew-t diagram from computer model forecast (extremely common in thunderstorm and severe weather forecasting) is applying parcel theory. There area a lot of things we do in meteorology that simplify how the atmosphere works to make forecasting easier. Parcel theory is one of those things, and yet it does a pretty damn good job when we apply it to the real atmosphere.

Yes, the conditions of parcel theory are violated in the real atmosphere. Yes, mixing is one of the ways parcel theory is violated. However, it still works quite well and is extremely common in forecasting. So, although you're technically correct that parcel theory is violated in the real atmosphere, it's usually a close approximation. Entrainment usually isn't significant enough to make a big difference most of the time. That's because usually large volumes of air are displaced, and entrainment generally only occurs around the edges. For example, in a growing cloud, there's entrainment of dry air at the edges, but the interior of the cloud is unlikely to experience much impact from entrainment. The cloud is large enough that the entering is mostly insulated from the effects of entrainment.

As for your cows comment, fuck you.

Re: Note: Gravity wave != Gravitational wave

[Anonymous Coward]

A gravity wave is not a compression wave. It is either called a gravity wave or, occasionally, a buoyancy wave. Such waves arise from the vertical displacement of a fluid in stable conditions. Gravity is the restoring force, causing the displaced fluid to return to and overshoot its original location, causing the wave. That's why it's called a gravity wave, a term that is extremely common in meteorology. Your post is factually incorrect.

Note that what's large...

[Rei]

.... is the size, not the intensity. The air moves only slightly faster or slower than the surrounding atmosphere as one passes through the wave.

They weren't expected on Venus, though. Venus's surface is dozens of kilometers down, thick and "soupy" there, transitioning to thinner layers above. It was surprising to see that surface features that far away, in a fluid that can compress, would still make clear phenomena like gravity waves in the high atmosphere.

Re: Note that what's large...

[Rei]

Venus has multiple "tropopauses" and "stratospheres", depending on how you define them. The atmosphere is like a layer cake with multiple convection zones (like Earth's troposphere) separated by areas of dynamic stability (like Earth's stratosphere). And again, ~50-70km is an awfullly long way from the surface, and surface winds are weak. But, there's a lot about Venus that we don't understand.