Bad astronomy | Hubble spies on Jupiter, Saturn, Uranus and Neptune


Each year, the Hubble Space Telescope is pointed at each of the four giant outer planets in our solar system – Jupiter, Saturn, Uranus, and Neptune – as part of a program to monitor their atmosphere.

All four turn quickly, in well under an earth day*, the images are therefore taken over several hours to be able to create global maps of the planets, allowing planetologists to examine as much as possible the atmospheric systems of the planets. They are also done the same way every year to provide a consistent database over time.

It’s called the Outer Planet Atmospheres Legacy program (or OPAL), and each year gives us very beautiful images of these huge planets. 2021 is no exception.

First, of course, is Jupiter, the largest planet around:

This image was taken on September 4, 2021, when Jupiter was approximately 605.5 million kilometers from Earth, shortly after it found itself in opposition, the position in its orbit when directly facing the Sun in our sky and closest to us.

The Great Red Spot is its most obvious feature, an ancient storm (at least a hundred years old) that has been the subject of intense study in recent years. The Juno probe, currently in orbit around Jupiter, uses infrared imagery to scan deep into the atmosphere to understand what is happening below the surface (the Spot passes at least 500 km below cloud tops!), And Hubble’s observations like this show that the winds in the Spot have increased even as the Spot itself has decreased in size. These annual OPAL images will help scientists understand why all of these things are happening.

Dozens of other smaller storms are visible across the face of the gas giant, including a barge, the long brownish cloud meandering halfway to the pole of the equator. It is a low pressure cyclone with warmer air inside; barges are usually not that large and can appear in both hemispheres. All of these features interact with the atmosphere around them, changing size, shape, composition, etc. It is only through repeated regular observations that we hope to understand what is out there.

Oh my God, Saturn.

The planet with the rings is obviously aptly nicknamed here, with the large A and B rings visible and the large Cassini division between them. There is a fairly thick layer of haze in Saturn’s upper atmosphere that typically dulls the bands (which are so clearly visible on hazy-free Jupiter), but the filters used in this image see the light passing more easily through the haze. , allowing the stripes to shine.

The huge and bizarre (but understood well enough) north polar hexagon can be seen in brownish red, a huge wind pattern 25,000 km in diameter (almost wide enough to hold two Earths side by side). Geometric patterns are common in polar vortices, a result of how the flowing air behaves in a fast rotating system.

Four moons are visible if you look closely (check image caption). The moons revolve around Saturn so quickly that even in a few hours they can travel quite a long distance; I struggled to identify which moon was which, but luckily the NASA Hubble site has an annotated version of the completed image.??

Uranus is a strange planet. Most planets orbit the Sun with their axes of rotation close to the right of their orbits. Earth is a bit of an exception with a tilt of 23 ° (Mars also with roughly the same tilt), but Uranus is tilted to 98 °. No one knows why, even though one idea is that a massively huge grazing impact has knocked it over.

In this image, Uranus’ north pole is quite white – this is called its polar hood – possibly due to chemical changes in the atmosphere at the start of the North Polar Spring, and the Sun’s ultraviolet light has a stronger effect. The fact that it is white means that it reflects light at all wavelengths, so this may be due to methane clouds; that is, methane condensing in the atmosphere. Uranus is usually a light greenish blue in color due to methane, which absorbs red light and reflects other colors back to us. But if a lot of clouds form, that could change.

By the way, Uranus can smell rotten eggs. Do whatever you want with it.

Speaking of methane, Neptune is the furthest major planet from the Sun – 4.3 billion km from us when this image was taken on September 7, 2021 – and its methane atmosphere makes it a striking deep blue. We learned a lot about this during the passage of Voyager 2 in 1989, including that the atmosphere can have huge dark storms. One Voyager seen was completely gone by the time Hubble launched and took a look, but over the years more have been seen; you can spy on one at the top left of this photo.

These dark spots form in both hemispheres and tend to migrate through latitudes to the equator where they disappear. We recently saw one of them turn around and start heading a way from the equator. Their behavior is strange and poorly understood.

But that’s the point of these observations in the first place, isn’t it? These huge worlds, eclipsing our own, are difficult to understand. They don’t have surfaces like the Earth, they have atmospheres hundreds of miles deep that merge into a liquid ocean and behave in ways that are very difficult to reproduce in the laboratory. So we study their winds, their air, their cloud tops, their rings, their moons and everything we can see to better understand them. And, in this way, to better understand the Earth too.

One last remark: If you go out in the next few weeks and look south, Jupiter is the brightest object in this part of the sky (currently Venus dominates in the southwest). Saturn is weaker but not too far west. Uranus and Neptune are also standing, although you will need binoculars for the first and a good scope for the second.

It’s been ages since I looked at them for myself through my “scope”. It is by no means Hubble, but knowing that these photons traveled from the Sun to these planets, reflected off them, then traveled hundreds of millions or billions of miles back to go straight into my eye?

Yes. It’s pretty cool too. It’s time to get out.

*As a longtime sci-fi fan, being able to write “Earth Day” and having it necessary and precise is always a little exciting.

??My thanks to the astronomer and planetologist Amy simon (Principal Investigator for OPAL), who pointed out the annotated image to me.


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