A deep dive into the Neptune system with JWST

A major feature that can be seen in all filters is the South Polar Feature (SPF), which is not actually located at Neptune’s south pole, but instead circles it at a latitude of about -70°. It was first reported in 1989 when Voyager 2 did its flyby, and has been a persistent feature at Neptune ever since. The SPF is a clump of clouds so condensed that it usually appears as one feature in most images. In one of the JWST filters (F300M at 3 microns), the SPF is so bright that it caused the only instance of saturation in the Neptune campaign. This saturation can be seen as a black dot in the center of the feature. With the sensitivity of JWST, we can resolve the separate clouds that make up this feature.

JWST detected other structures at the south pole as well. At 2.1 microns (F210M) JWST saw a bright polar vortex that had been observed in the past, as well as a bright continuous band surrounding the pole that had not been detected before — not because we didn’t have the sensitivity, but because it had not been there in the past. The band is around the same brightness as the polar vortex, so if it had existed when the polar vortex was observed it would have been spotted as well. Neptune specialists are excited to continue observations of this new feature and the others accessible by JWST.

Moving from the poles to the equator, JWST’s observing powers continue to yield new insights. The brightness of the planetary disk changes depending on latitude, and such differences are of great interest to atmospheric scientists. The Neptune images from JWST reveal a bright equatorial band at 2.1 and 3 microns, thought to be created by global circulation patterns: the atmosphere sinks at the equator, and as it sinks it compresses and warms up, causing this bright warm band around the planet . At 1.4 microns we see something different and intriguing: a clear discontinuity right at the equator between the bright southern hemisphere and the darker northern hemisphere. These surprising observations are already prompting active discussion among the community of scientists studying the atmospheres of ice giants.

Neptune’s many moons

In JWST’s wide-field image of Neptune you can see its largest moon, Triton. Triton’s odd retrograde (backwards) and tilted orbit leads planetary scientists to speculate that this object is not a native moon of Neptune. Rather, Triton is likely a Kuiper Belt Object like Pluto that strayed just a little too close to Neptune sometime in the distant past and was captured by the ice giant’s strong gravitational field. Future JWST observations will focus on a detailed examination of the infrared colors of Triton. We hope to learn more about its surface composition, its thin atmosphere, and how it compares to other large Kuiper Belt Objects such as Pluto, Eris, Makemake, Haumea, and others that JWST will observe.

Triton aside, at least six other known moons of Neptune are definitively detected in the JWST images. Despina and Galatea are embedded within the rings and are thought to be responsible for helping to craft the detailed ring structure. Naiad and Thalassa are found within the inner, dustier parts of the rings system. External to the rings, the moons Proteus and Larissa can be easily detected. Hippocamp, a tiny moon detected in 2013 is tentatively visible; moon specialists are working on enhancing the images to confirm this detection. The other known moons orbit so far from Neptune that they do not appear even in the wide field images, like Neptune’s moon Nereid, which orbits at an average distance of more than 5 million kilometers (more than 3 million miles). Future JWST images may observe some of these more distant moons.

More JWST Neptune data still to come!

All of the JWST data shown above come from less than two hours of observations, taken to demonstrate the imaging capability of JWST. The best is yet to come. There is an entire JWST science program devoted to the Neptune system, and another planned for Uranus. These two programs also hope to use the Hubble Space Telescope alongside JWST to do near-simultaneous observations in the visible and infrared spectra, expanding the scope of ice giant studies.