Jupiter is a very photogenic planet.
That's a big reason scientists installed the JunoCam instrument on Juno, a mission orbiting the ginormous planet. Juno's mission is to study the atmosphere and interior of Jupiter, and has a lot of sophisticated instruments on board to do that.
But mission-critical hardware didn't really include a visible-light camera. That would be unfortunate, so they built JunoCam specifically to take snapshots of the planet for the public. The highly elongated orbit of Juno takes it out as far as 8 million kilometers from Jupiter down to a hair-raising 4,200 kilometers above the cloud tops, at which point the mighty gravity of the immense planet whips Juno past it at 60 kilometers per second.
It's on a polar orbit, so it drops down from space over Jupiter's north pole heading south, and as it does the JunoCam snaps image after image of the clouds below.
On 11 September, 2019, it got shots of an astonishing sight: a total solar eclipse! But it wasn't anything like an Earthy one. Instead of the Earth's Moon blocking the Sun, it was Jupiter's moon Io, and the shadow it cast on the cloud tops … well, see for yourself.
I know. Wow.
This was taken on Perijove 22 — the 22nd time it passed close to Jupiter after Juno first entered orbit on 6 July 2016 — and the image was processed by software engineer and data visualization artist Kevin Gill.
What you're seeing is literally the shadow of Io cast on the clouds of Jupiter. The shadow is about 3,600 km across, roughly the same size as the moon itself. That's actually pretty tiny compared to Jupiter, which is over 140,000 km wide at its equator. In this image, though, the shadow looks big because JunoCam uses a fisheye wide-angle lens, which distorts more distant objects near the edge of the field of view. If you had a regular lens the edge of Jupiter would look much straighter than it does here, but in this case it's warped by the camera. You're only seeing a very small section of the planet here.
Juno was 7862 km above Jupiter's clouds when it took this shot, at a latitude of 21°. Io orbits over Jupiter's equator, but the tilts of the planet and Io's orbit put the shadow at a latitude of roughly 10° north. Using the law of cosines (I love math like this!) I get that the shadow was about 16,400 kilometers away from the spacecraft (assuming it was due north of the shadow) when this shot was taken. More or less. That's comfortably larger than Earth's width!
On Jupiter, you can be a whole planet away from something and still call it “close.”
Thinking about this image, I couldn't help it: I wondered what it would look like if you were in a balloon, floating above Jupiter's clouds, and that shadow happened to pass over you. What a sight! It would be something like a solar eclipse here on Earth … but with some major differences.
By coincidence, from Jupiter's cloud tops Io is about the same apparent size as our Moon is in our sky: 0.6° (compared to 0.5° for the Moon). From Earth, the Moon and Sun are about the same apparent size, so in a solar eclipse the Moon almost perfectly covers the Sun and we can see the solar corona, its magnificent atmosphere, nearly all the way down to the Sun's surface.
But Jupiter is five times farther from the Sun than the Earth is! So the Sun is 1/5th the size (0.1°), making Io far larger than the Sun in Jupiter's sky. It would easily block the entire Sun, plus quite a bit of the corona. So that's one difference.
Another is how long the eclipse can last. The best you can do standing on Earth's surface is about 7.5 minutes — it depends on how fast the Moon orbits the earth, how fast the Earth rotates, and some other factors.
But doing the same math for Io, I calculate that a solar eclipse can last almost 13 minutes! This was a rough calculation, based on Jupiter's larger size and faster rotation, and Io's faster orbital speed than our Moon. That would be amazing.
I think, though, the biggest difference would be due to Io itself. It's the most volcanically active object in the solar system, with constant eruptions occurring all over its surface. With Io blocking the Sun, it would appear as a dark disk in the sky, but covered with bright red evilly glowing pinpricks — each an active volcano spewing sulfur into the sky! And surrounding that would be the Sun's corona, wisps and feathers reaching out across the sky.
Yegads. To see such a wondrous thing … I hope that our descendants decades hence get to view this sight. I can only imagine how life-altering it would be.
And while I can daydream about seeing such things for myself, thanks to Juno and JunoCam — and folks like Gill — we can still get a pretty good glimpse of what it would be like.