Some simply amazing images were released this week from the JWST (James Webb Space Telescope) of Jupiter. We’ve had quite a few questions about these images, so here’s a bit of background information.
While many people think of space telescopes like Hubble or the JWST as being deep space instruments, there’s many objects in the Solar System which are also targets for their observations.
Q. Why is Jupiter this colour? Was it taken through some sort of coloured filter?
It was taken through coloured filters, but not what you might be expecting.
If you view Jupiter just with your eyes at night, it will appear a bright, milky white coloured “star”. This is especially true at the moment as Jupiter gets closer to opposition. (More on that later.) This colour is even more pronounced when viewed a telescope, where coloured bands and other features become visible. It’s certainly not the deep shade of brownish purple that we see in these JWST images.
Jupiter imaged with the JWST, Aug 2022/NASA
The reason it appears like this is because the JWST is “seeing” Jupiter in light wavelengths that our eyes can’t see. We can see light in wavelengths from roughly 380 nanometres, the violet end of the rainbow, or spectrum) though to 700 nanometres, or the very deep red end. 1.
(There are one billion nanometres in a metre.)
This Jupiter image was taken using the NIRcam (Near Infrared camera) instrument on JWST. This can detect light from 600 nanometres through to 5,000 nanometres (0.5 microns). Light in these longer wavelengths can’t be seen with our eyes and light from Jupiter in this wavelength can’t penetrate the Earth’s atmosphere. Ground based telescopes even with cameras like NIRcam would not be able to see Jupiter like this.
It’s a combination of three different images taken through different filters that narrows down the wavelength of infrared light. This technique – called narrow band imaging – is something that’s often used by astronomers. We do this because different wavelengths show different features with better clarity.
For these Jupiter images, longtime citizen scientist Judy Schmidt “mapped” or made a judgement on how to translate this light from these infrared images into colours to those we can see. Light wavelengths from one filter which shows the auroras, the lights around the poles, are mapped to reddish colours. Light from another filter is mapped to yellows and greens which is used to show gas swirling around the poles.
The final filtered image sees the swirling clouds around Jupiter’s main belts mapped to shades of blue. All these images are combined to produce the final result we see. If you would like to read more about this process, there’s great NASA blog article here.
The Great Red Spot appears a muddy red to our eyes, but almost white in this image because of the amount of Sunlight it reflects. (A small note – during the exposures, Jupiter would have rotated slightly, and the image was “de-rotated” in software. This is something even amateur astrophotographers carry out using apps like WinJupos.)
Jupiter imaged with the JWST, Aug 2022/NASA
The second JWST image shows a wider field of view of Jupiter showing some of its satellites and faint rings, along with some faint galaxies.
Q. Does this image have lens flares?
One of the first things you might notice is the hazy cross like formations appearing from the southern pole of Jupiter and the spike on the brightest of Jupiter in this image on the left, Amalthea.
These are called “diffraction spikes” and are a result of the JWST hardware. Light can slightly bend around the edge of an object placed in an optic system. The JWST has a centre mirror which holds a secondary mirror to reflect light back into the instruments behind its main mirror. Light bends around the edges of the three struts holding this secondary mirror as well as the edges of the main mirror itself. As this is hexagonal, it produces its own set of six diffraction spikes, but engineers positioned the secondary holder in such a way that they overlap those of the main mirror. The Hubble Space Telescope has a round mirror which doesn’t really contribute much to diffraction spikes. It has a four-arm secondary mirror hold so Hubble images may have four diffraction spikes, as opposed to six in JWST images.
Q. What? Jupiter has rings? I thought only Saturn had rings!
Yes, Jupiter has rings. (The other two gas giant planets Uranus and Neptune also have rings.)
Jupiter’s rings were discovered during a flyby of the Voyager-1 spacecraft in 1979.
They are almost impossible to observe from Earth even with the largest telescopes. This is because they’re thin, very faint and made up of dark material which doesn’t reflect much sunlight. They’re likely made of up dust which was ejected from collisions between some of Jupiter’s inner moons.
The fact that JWST was so easily able to capture Jupiter’s rings with its first serious attempt at imaging the gas giant makes us wonder what future observations will reveal!
Q. When can I see Jupiter?
Jupiter is visible most of the night at the moment (late Aug 2022) and ideally positioned for observation!
Around the evening of September 26th, 2022, Jupiter will be at opposition. This is when the Earth and Jupiter are at their closest point in their orbits during the year. It will be at its brightest and visible all night. While this is a few weeks away, it’s already great viewing.
Please feel free to ask any more questions in the comments below.
1. A very small number of people can see slightly further into the ultraviolet spectrum.