Jupiter from Hubble: Enormous, magnificent, and… fading at the edge?

Jupiter is a big fella.

The largest of our solar system's planets is so big that you could toss all the planets inside it and still have room leftover for all the moons and asteroids, too. Even though it's five times farther from the Sun than we are, and therefore only gets 4% of the light we do here on Earth, it's so flippin' big that it reflects a lot of that sunlight back toward us, making it one of the brightest objects in the night sky. If you go outside after sunset tonight and look south you'll get an eyeful of that fact; it's looming over the southern horizon for northern hemisphere observers, bright and obvious (and if you do go out to look for it, the waxing gibbous Moon and bright red star Antares are nearby, too).

On June 27, 2019, Jupiter was just under 650 million kilometers from Earth. That's a long way on a human scale, but for the Hubble Space Telescope that's practically breathing down our necks. So when the space observatory was aimed at the big planet that day, this is what it saw:

Jupiter on June 27, 2019 as seen by Hubble Space Telescope. Credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley)

Oh yeah.

The image was taken as part of an ongoing program called OPAL, for Outer Planet Atmospheres Legacy^*, which monitors changes in the cloud tops of the four giant planets (Jupiter, Saturn, Uranus, and Neptune) over time.

The Great Red Spot, the semi-permanent anticyclonic high-pressure system in Jupe's southern hemisphere is prominent (though less so every year, it seems), as are the overall bands stretching all the way around the planet. Smaller — if larger than our entire planet Earth can be described that way — storms dot Jupiter's cloud tops, too.

But one thing jumped out at me that I want to take a moment to point out. While the center of the disk is bright and lovely, when you look toward the edge you may notice features washing out and the overall brightness dims. That's not an illusion or some weird observation effect. It's real, and it's called limb darkening.

We're not seeing a solid surface of Jupiter here — it doesn't have one. We're seeing the tops of the clouds, which run hundreds of kilometers of kilometers deep into the planet. The atmosphere doesn't just stop at some height; it thins out gradually. Light from the Sun passes through the thinnest stuff and hits the clouds, where some of it is absorbed and some of it reflected back to us.

As we look out into the solar system at Jupiter, it's far enough away that to it, the Earth and Sun are close together in the sky, so it's like we're seeing Jupiter with the Sun behind us. This affects the geometry of how light behaves.

Sunlight moving straight down through Jupiter’s atmosphere (red line, left) passes through less air than it does near the edge of Jupiter’s face (red line, right). That means more light is absorbed by the air darkening the edge as seen from Earth. Credit: Phil Plait

When we look straight down into Jupiter's face, the pathway through the upper atmosphere is shortest. But when we look toward the edge that pathway gets much longer (another way to think of it: from those spots on Jupiter the Sun is low to the horizon, so the light has to pass through a lot more air).

That means that near the edge of Jupiter there's more air to absorb sunlight, so there's less to reflect back. Also, haze (small particles) in the upper atmosphere of Jupiter scatter sunlight; when light hits them they send it off in random directions. That also means there's less light to reflect back to us. So the limb (the part near the edge of the disk) look dimmer. Limb darkening!

Jupiter's atmosphere has a lot of methane, which is good at absorbing red light. Other complicated effects also rob more of the red light than blue when seen from Earth, giving the edge of the planet's face a slightly blue tinge.

Through a small telescope Jupiter is so bright that the limb darkening can be hard to see until you use higher magnification. I seem to recall noticing it through the eyepiece on occasion, but I don't think I've ever noticed the bluer color. Even here with Hubble it's a subtle effect.

But, with Jupiter still high in the sky for me after sunset, maybe I'll take a look and see if I can spot it. Even if I can't, that's OK. Jupiter is still quite something to just sit and stare at for pretty much any reason.

^*I thought for a moment it was actually for Outer Planets Alliance Legacy. Sorry, beltalowda.