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.

[Read about the search for life on Venus and the detction of phosphine in its clouds.]

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.