Free Astronomy Magazine November-December 2022

45 NOVEMBER-DECEMBER 2022 ASTRO PUBLISHING This planet is characterized as an ice giant due to the chemical make-up of its interior. Compared to the gas giants, Jupiter and Saturn, Neptune is much richer in elements heavier than hydrogen and helium. This is readily apparent in Neptune’s signa- ture blue appearance in Hubble Space Telescope im- ages at visible wavelengths, caused by small amounts of gaseous methane. Webb’s Near-Infrared Cam- era (NIRCam) images ob- jects in the near-infrared range from 0.6 to 5 mi- crons, so Neptune does not appear blue to Webb. In fact, the methane gas so strongly absorbs red and in- frared light that the planet is quite dark at these near- infrared wavelengths, ex- cept where high-altitude clouds are present. Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight be- fore it is absorbed by methane gas. Images from other observatories, including the Hubble Space Tele- scope and the W.M. Keck Observa- tory, have recorded these rapidly evolving cloud fea- tures over the years. More subtly, a thin line of brightness circling the planet’s equator could be a visual signature of global atmospheric circulation that powers Neptune’s winds and storms. The atmosphere descends and warms at the equator, and thus glows at infrared wave- lengths more than the surrounding, cooler gases. Neptune’s 164-year orbit means its northern pole, at the top of this image, is just out of view for astronomers, but the Webb images hint at an intriguing bright- ness in that area. A previously- known vortex at the southern pole is evident in Webb’s view, but for the first time Webb has revealed a continuous band of high-latitude clouds surrounding it. Webb also captured seven of Nep- tune’s 14 known moons. Dominat- ing this Webb portrait of Neptune is a very bright point of light sporting the signature diffraction spikes seen in many of Webb’s images, but this is not a star. Rather, this is Neptune’s large and unusual moon, Triton. Covered in a frozen sheen of con- densed nitrogen, Triton reflects an average of 70 percent of the sun- light that hits it. It far outshines Neptune in this image because the planet’s atmosphere is darkened by methane absorption at these near- infrared wavelengths. Tri- ton orbits Neptune in an unusual backward (retro- grade) orbit, leading as- tronomers to speculate that this moon was originally a Kuiper belt object that was gravitationally captured by Neptune. Additional Webb studies of both Triton and Neptune are planned in the coming year. The James Webb Space Tele- scope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look be- yond to distant worlds around other stars, and probe the mysterious struc- tures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency. I n this version of Webb’s Near-Infrared Camera (NIRCam) image of Neptune, the planet’s visible moons are labeled. Neptune has 14 known satellites, and seven of them are visible in this image. Triton, the bright spot of light in the upper left of this image, far outshines Neptune because the planet’s at- mosphere is darkened by methane absorption at wavelengths captured by Webb. [NASA, ESA, CSA, STScI − Image processing: Joseph DePasquale (STScI)] W ebb’s Near-Infrared Camera (NIRCam) image of Neptune, taken on July 12, 2022, brings the planet’s rings into full focus for the first time in more than three decades. [NASA, ESA, CSA, STScI − Image processing: Joseph DePasquale (STScI)] !