New Hubble Space Telescope images reveal an expanded polar cap on Uranus and another mysterious dark vortex on Neptune.
Once a year, the Hubble Space Telescope checks in on the solar system's ice giants, Uranus and Neptune. These planets haven't seen a spacecraft in their neighborhood since Voyager 2 flew by both planets in the 1980s, but Hubble provides the next best thing with monitoring that captures short-lived storms as well as long-term trends. The seasons in the outer solar system are decades long, so it can take awhile before such trends become apparent.
Hubble's latest image of Uranus shows a vast, bright-white cloud cap over the planet's pole. This wasn't visible when Voyager 2 flew by and has only made itself known in more recent years. Now, Uranus is approaching the middle of its summer season, when the Sun shines almost directly onto the north pole thanks to the planet's extreme tilt. It might be because of seasonal changes in atmospheric flow that this polar cap is as big as it's ever been.
The bright glint at the edge of the polar cap is a compact methane-ice cloud that sometimes becomes bright enough for amateurs to photograph.
In its September 2018 image of Neptune, Hubble discovered another dark vortex, similar to others seen at different latitudes in previous images. The anticyclone is 6,800 miles (11,000 km) across. Voyager 2 also saw dark storms, and this is the fourth Hubble has captured since 1993. Analysis by Andrew Hsu (University of California, Berkeley) and colleagues suggests that the dark storms appear every four to six years, each lasting about two years. They appear dark because the storms are dredging up material from deeper layers within the planet's atmosphere.
White "companion clouds" often appear near the dark storms because, as air flows over and around the dark storms, the methane gas freezes into ice crystals.
The images are part of the Outer Planet Atmospheres Legacy (OPAL) program, led by Amy Simon (NASA Goddard), which calls on Hubble to monitor the outer planets every year when they and Earth are closest in their orbits. You can read more about the images in Hubble's press release.