For a few days, Venus smiled — sideways.
When Japan’s Akatsuki spacecraft pulled into orbit around Venus in December 2015 and turned on its instruments, it almost immediately discovered a bow-shape feature in the atmosphere stretching 6,000 miles, almost pole to pole — a sideways smile.
More remarkably, while Venus’s winds blow at speeds up to 250 miles per hour and clouds whip around the planet every four days, this gargantuan sideways smile did not move, but remained fixed above the ground for four days. Because of Akatsuki’s large looping orbit, the spacecraft could not make more observations for a month.
When the spacecraft looked at the same region again, the smile had disappeared. Except for a few brief glimmers in April and May last year, the smile has not returned.
In a paper published Monday by the journal Nature Geoscience, scientists working on the mission describe their observations in detail and suggest it was a “gravity wave” — a disturbance in the winds caused by the underlying topography that propagated upward.
The bow-shape arc appeared above Aphrodite Terra, a highland region about the size of Africa that rises up to three miles from the surface. Scientists working on data from the European Space Agency’s Venus Express reported finding a similar disturbance in the atmosphere.
The authors of the new paper said that numerical simulations provided preliminary support for the idea, but that they still could not explain how the gravity wave forms and propagates in the lower atmosphere. Or why the prominent smile was seen in December 2015 and not since.
Scientists also cannot yet answer the big question Akatsuki was sent to investigate: Why do the winds blow so fast on Venus to begin with?