Neptune’s shrinking vortex

Neptune, the eighth and farthest planet from the Sun, was visited for the first and last time by NASA’s Voyager 2 mission in 1989. Since then, the NASA/ESA Hubble Space Telescope has been attempting to unearth the myriad mysteries surrounding this cool, majestic behemoth — including deciphering why it has the fastest wind speeds of any planet in the Solar System, and what lies at its centre.

These new Hubble images reveal one of the standout features of Neptune’s strange atmosphere: a rare dark spot, or dark vortex — a whirling high-pressure atmospheric system usually accompanied by bright “companion clouds”. This particular dark spot is named SDS-2015 (Southern Dark Spot discovered in 2015), and is only the fifth observed so far on Neptune. Although it appears to be slightly smaller than previous dark spots, observations of SDS-2015 from 2015 to 2017 revealed that the spot was once big enough to almost swallow China before rapidly diminishing in size.

Each of the five dark spots found on Neptune have been curiously diverse, but all have appeared and disappeared within just a few years — as opposed to similar vortices on Jupiter which evolve over decades. Bright clouds form alongside dark spots when the flow of ambient air is disturbed and diverted upwards over the spot, likely causing gases to freeze into methane ice crystals.

Only Hubble is currently powerful enough to image Neptune’s dark spots, and produce striking images such as these; these views were taken over the course of two years using Hubble’s Wide Field Camera 3 (WFC3).