Since its launch on December 25, 2021 and subsequent commissioning, the James Webb (JWST) has become the largest space telescope in history. Although its main objectives are the study of the early universe, galaxies and star formation processes, bodies in our Solar System have also been observed. The images obtained are impressive. Maybe some expected a higher resolution, but that’s what the laws of optics say: with a telescope 6.5 meters in diameter, it’s very difficult to exceed the resolution obtained by a space probe, especially when operating in the infrared. Nor is it easy to outperform observations from larger ground-based telescopes such as the Keck telescopes (with a main mirror larger than 10 meters). The last object observed by JWST is Uranus, which looks like this in this spectacular NIRCam instrument image taken on February 23 of this year:
The image is simply beautiful and it’s the first time we’ve seen the complete eleven rings directly from Earth’s orbit. But, as we said, it cannot be compared to the detail that a space probe can achieve. Just look at the following image taken by Voyager 2 —the only probe that passed by Uranus— in 1986:
Despite the lower resolution, the first thing that strikes you in the JWST image is Uranus’s bright rings. While in visible light they are thin and dim, JWST can see them clearly, giving the planet a very curious “target” appearance. The reason is, once again, that the JWST observes in the infrared, a region of the spectrum where relatively cold objects stand out, such as the rings of Uranus, formed by rocks, water ice and dust. The next thing that stands out is that in the Voyager 2 image, Uranus appears to us as a featureless blue ball, whereas in the JWST image we see clouds and other structures. This has nothing to do with JWST, but when the space probe flew by the ice giant, the planet was remarkably calm.
Prominent in the JWST image is Uranus’s so-called “polar cap”, a lighter-colored region around the subsolar point in the planet’s northern hemisphere. Remember that Uranus rotates with its axis “lying down”, so in the image we are seeing the northern hemisphere of the planet (at the moment the axis of rotation does not point directly towards the Sun). This polar cap forms in the Uranian summer and disappears in the fall (the year on Uranus lasts 84 Earth years). It was first discovered in 2014 thanks to the Keck Observatory in Hawaii and has a counterpart in the southern polar cap, which disappeared before the Uranian equinox in 2007. It is believed that this cap is not formed by clouds, but by its hue. lighter is due to the lower proportion of tropospheric methane (the bluish color of Uranus is due to the presence of methane in its atmosphere). Also visible in the JWST image is a bright cloud at the edge of the cap – likely made of methane ice crystals – and other clouds scattered across the planet. When Voyager 2 passed Uranus, the northern hemisphere was in shadow, so the spacecraft saw only the southern hemisphere. Interestingly, with nearly half a Uranian year having passed since then, the perspective of the JWST image is very similar to that seen by Voyager 2 as it approached Uranus. In addition to Uranus itself, JWST captured six of its moons, which appear relatively bright due to the fact that they are covered in water ice.
Previously, JWST has already surprised us with an image of Neptune in which it is best to appreciate that both ice giants appear relatively dark in infrared images. And it is that, as we said, Uranus and Neptune are blue due to the presence of methane, which absorbs red and infrared light. As in the case of Uranus, there are bright methane clouds that reflect sunlight, although their number is greater than that of the neighboring planet (Neptune, despite being farther from the Sun, has greater atmospheric activity because it emits more internal energy ).
We can see a bright line right on Neptune’s equator, probably formed by the descent and heating of atmospheric gases (mainly hydrogen and helium). Neptune’s north pole also appears extraordinarily bright, despite the fact that it is winter in that area for unknown reasons. As in the case of Uranus, the JWST image of Neptune is the best we can see this planet’s rings since Voyager 2 passed by this world in 1989 (Hubble and Keck also saw the rings). rings of Uranus and Neptune, although with less detail). Triton, Neptune’s largest moon, is also visible. Triton is covered in nitrogen ice so that it reflects 70% of incident light, making it brighter than Neptune in the infrared.
We’re still waiting for the JWST images of Saturn to be released, but we’ve already managed to see its largest moon, Titan, with this telescope. Thanks to its infrared vision, on November 4, 2022, the JWST was able to break through the opaque outer layer of organic haze that envelops this fascinating world with lakes of methane. The space telescope has detected two large methane clouds in Titan’s arctic zone, right where the northern hemisphere’s seas and lakes meet. The detection of these clouds confirms the atmospheric models that predict greater activity at the end of the local summer. Near-infrared images taken two days later by Keck Observatory revealed that the clouds had not moved from their position, although they could have been different clouds, as cloud structures on Titan do not last long.
Due to its large apparent and actual size, JWST images of Jupiter are of high resolution, although they may not be as surprising to the general public because the gas giant has been observed before with various instruments in the infrared. Most impressive about the JWST observations is the brightness of the Jovian auroras, as well as the fact that we can clearly see the planet’s thin rings. Storms, including the Great Red Spot, appear bright because they are overhead and reflect a significant amount of sunlight. Dark zones are regions without cloud cover that allow us to see deep into the atmosphere. JWST observations of Jupiter are complicated, paradoxically, by their large apparent size, as the planet’s rapid rotation – about ten hours – makes it difficult to stack the different images taken in the different filters.
JWST cannot see Mercury and Venus because they are too close to the Sun in the sky, but it is able to point to Mars. If we are used to seeing images of Jupiter, we are more in the case of the red planet, so the JWST finds it difficult to surprise us in this regard. As with Jupiter, studying Mars with the JWST is not easy due to its high brightness, which requires imaging with very short exposures and using specific data analysis techniques. The NIRCam camera and NIRSpec spectrometer observed Mars on September 5, 2022. Details of the Martian surface can be seen in images taken with the NIRCam’s 2.1 micron filter, but in the 4.3 micron filter instead of at the surface we only see the heat emitted by it, modulated by the Martian atmosphere (this is why the Hellas basin, at lower altitude, appears darker).
While there are instruments in space and on the ground capable of looking in the near infrared, most are not able to see the longer wavelengths that JWST can study, or at least not with its resolution. This means that JWST’s study of Solar System objects will continue to yield surprises… and generate spectacular images in the process.