Eighteen months on, the Juno mission to probe the otherwise little-known poles of Jupiter is producing incredible dividends. Check this out.
This is just so cool on so many levels. One of the things that NASA and JPL have done is to publish the raw Juno-Cam images sent back by the probe, for everyday people like you and me to process. This one – taken between 9.54 and 10.11 pm PDT on 15 July, was colour-enhanced by Gerald Eichstädt and Seán Doran, and – well, wow!
To me it looks like Vincent van Gogh on steroids. But it isn’t. It’s weather – on Jupiter. Woah. Let me explain how the colour gets there. The photo isn’t exactly what the naked eye would see – but it begs the question. How come Jupiter has a kind of bluish, bisque-brown, green palette, when Saturn is more sienna-tan, Uranus ice-green and Neptune a pale blue.
The answer’s mostly in the chemistry of their atmospheres – and the temperatures. Jupiter’s atmosphere is made of about 86 percent hydrogen, 13.6 percent helium, and 0.4 percent other stuff. The other stuff includes a smidge of water (less than 0.04 percent), ammonia (0.06 percent), methane (0.2 percent) and other gases including hydrogen sulphide. Yup – Jupiter, the largest planet in the solar system by far, with the thickest atmosphere… smells of really bad fart.
The clouds we see, near the upper part of the atmosphere, are ammonia ice crystals, with smidges of water around (that’s really further down though). At less than about 150 degrees Kelvin (-190 fahrenheit, -120 celsius), the result is swirling clouds of red, brown, orange, white and – as we see in these pictures – blue.
Just to put that in perspective, if Jupiter was orbiting the Sun at Earth’s distance, the temperatures in its atmosphere would be high enough to form water clouds instead. And Jupiter would look pretty much the same as Earth does. Only bigger. A number of the exoplanets found so far fall into this class, including possibly Gliese 876 b and c.
Saturn has much the same atmosphere as Jupiter, but it’s subtly different (including phosphene) and a good deal colder – meaning the colours are a lot more muted.
Uranus, by contrast, has a good deal of methane in its atmosphere, which – at the even lower temperatures out there – absorbs a lot of red. Consequently it shows up green. And Neptune, colder yet again, has less methane; so it turns up blue.
You’ll notice I used the word ‘mostly’. There is one other factor colouring these planets: Rayleigh scattering, which is a process by which certain wavelengths of light are scattered by an atmosphere. In most circumstance this renders their atmospheres visually blue – and this is also why the sky looks blue to us in daylight. I say ‘most circumstance’ because the exact same phenomenon also makes Mars’ sky red, and our own skies red at sunset. And yet, you can also see blue if you look at the right angle – as in this Hubble photo, where the edges of the planet have a distinct blue tint.
It’s all a function of physics. And that brings me back to Jupiter and the Juno probe, which is still orbiting Jupiter and returning way more than just pictures; there’s also some fantastic science, including measurements of Jupiter’s magnetic field – and insights into the planet’s interior structure. It turns out that this is very different from what was previously imagined.
See what I mean when I say all this is so cool?
Copyright © Matthew Wright 2018