Brightness temperature and UV brightness of the Venus disc. © Planet-C

When the Japanese space probe Akatsuki slotted into Venus orbit in December 2015, it was greeted with a smile – of sorts.

A giant wave emerged from infrared images that peered through the hot, turbulent, thick atmosphere of our nearest neighbour. And what’s behind it remains a mystery.

Tetsuya Fukuhara from Rikkyo University in Tokyo and colleagues in Japan think it might be a gravity wave generated between two layers of different density, perhaps thanks to mountains on the Venusian surface. But if that is the case, they posit in Nature Geoscience, the winds in the deep atmosphere must shift differently than planetary scientists first thought.

When the planets align, Venus is only 38 million kilometres from Earth. Yet it’s far less explored than Mars, in part because it’s more like hell than the sultry goddess after whom it’s named.

A dense carbon dioxide atmosphere topped with sulfuric acid clouds swaddle a rocky, hot planet. The entire upper atmosphere rotates faster than the ground below and at the cloud tops, winds whip along at around 100 metres per second.

The Japanese Space Agency’s Akatsuki probe was launched on 20 May 2010 to explore Venus’ climate. It reached its destination less than seven months later but technical troubles meant it failed to pop into orbit.

Only five years later was it manoeuvred into an elliptical orbit – and then its Longwave Infrared Camera saw the wave.

Sequential images of cloud-top temperatures. You can see the wave remains stationary. © Planet-C

It extends from the north pole, across the equator, and down to the south pole. It’s more than 10,000 kilometres end to end and, at around 500 ºC, is hotter than its surrounds.

So what’s going on? The team suggests it’s “induced by an atmospheric gravity wave” where mountains below interrupt the rushing atmosphere. (Not to be confused with gravitational waves, ripples in space-time produced by a cataclysmic event such as colliding black holes.)

Such phenomena are seen on Earth, albeit at much smaller scales. And “from conventional knowledge” of Venus’ atmosphere and their own numerical models, the researchers write, formation of mountain waves “appear difficult”.

It remains a mystery. And with NASA earlier this month funding asteroid missions but nothing for Venus, we might have a wait ahead.