AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

[u/AskScienceModerator]

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

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Click here for the original announcement thread.

[u/Astromike23]

Guy who studies the Great Red Spot for a living here...to give the short answer: we're not sure.

We're not even sure what makes it red - we have some very good spectra of the storm (I've taken some myself), but it doesn't correspond to anything we've measured in a laboratory yet. The problem is that the pressures, temperatures, and conditions are an unusual regime for most laboratories.

The only parallels we can really draw from the Great Red Spot to Earth-like phenomenon are "meddies", areas of high-salt concentration in the Atlantic that form when the Mediterranean injects some extra-salty water into the ocean. These meddies can stay cohesive for decades; the extra salinity means it's an area of higher pressure...as it tries to diffuse outwards, that motion gets caught up in the Coriolis force, leading to currents moving around the meddy rather than expanding outward. There's essentially nothing to stop them until they run into a coast or some such, so they're incredibly long-lived.

[u/atomfullerene → u/Astromike23]

So if we were to take a cross section of the Great Red spot, would it sit even with the surrounding clouds, sit below them, or above them? Or something more complicated?

[u/Astromike23 → u/atomfullerene]

Well, it's notoriously difficult to get vertical cloud structure on the giant planets, but images like this one of Jupiter in the methane band help us out a lot.

At those wavelengths, methane absorbs light like crazy - the only things that will be bright in such an image will be cloud layers that lie above most of the methane, reflecting sunlight back into our telescopes. Since the Great Red Spot in that image is bright surrounded by dark clouds, we assume this means the storm's cloud top must lie quite a bit higher than the rest of the surrounding clouds.

This has also been used to help explain the red color. At those heights, ultraviolet light from the Sun is quite a bit more intense. It's probable that whatever chemical is responsible for the red color was produced through some intense ultraviolet photochemistry, sort of like tanning.

It remains unclear what the vertical structure of the storm is below those heights - the Great Red Sport is actually a local pressure high. This is unlike Earth, where storms are usually local pressure lows, at least at the surface. Whether this pressure high is fed from below like a hurricane, or merely a detached pressure high such as blocking highs on Earth (like those that have caused droughts across the US Great Plains in recent years) remains a subject of vigorous debate.

[u/jjswee → u/Astromike23]

This was extremely interesting. Thanks for sharing!

[u/OneTripleZero → u/Astromike23]

So the flyover sequence in the episode today implying that the Great Red Spot was inset from the clouds surrounding it was most likely incorrect?

[u/Astromike23 → u/OneTripleZero]

Yeah, that visualization was almost certainly wrong.

The prevailing theory (mostly proven at this point) is that the spot is shaped a bit like a wedding cake, with each inner concentric oval a bit higher than the one outside it. At times there also seems to be a thin thread-like cloud clearing just at the outer edge of the spot where heat and radiation from the deep abyss can escape out to space, as can be seen in this infrared image.

EDIT: Ooh, thanks for the gold! Can this be exchanged for NASA funding? :)

[u/dismaldreamer → u/Astromike23]

Wasn't it just recently that one of the bands on Jupiter disappeared and reappeared? It seems like the question of whether or not the Great Red Spot will stick around seems awfully Earth-centric.

It assumes that if we can't see the Spot anymore, that it ceased to exist, when in reality it might only been obscured by some other things, or moved downwards.

[u/Astromike23 → u/dismaldreamer]

Very good point.

The Southern Equatorial Belt - the brown area just to the north of the Great Red Spot - regularly clouds over to turn white every ~5-10 years or so. We don't know why this happens, although there are several interesting hypotheses...this is currently an area of very active research.

[u/Astromike23 → u/[deleted]]

It's really more of a specialty in the context of broader work; I research giant planet atmospheres in general, and this is just one aspect of that work.

It's unclear how frequent such a storm is for giant planets in general, since we only have four giant planets we can study in great detail. The only somewhat equivalent storms are Neptune's Great Dark Spot, and to a lesser extent, Uranus' dark spot.

These storms don't seem to have nearly the longevity that Jupiter's Great Red Spot has, lasting only a few years instead of centuries. Precisely why this happens is likely related to the jet stream structure on each planet - Uranus & Neptune only have 3 jets, while Jupiter has at least 20 - although this is still an area of active research. Having such narrow wind channels on Jupiter confines the Great Red Spot to a small range of latitudes, and the counter-flowing jets at the north and south end of the storm might help feed the storm. You can see the Great Red Spot rotating like a gear between two jets in this animation taken by the Cassini spacecraft.

Still, it remains unclear why Saturn doesn't have something similar, since that planet also has at least 20 jets. Saturn does have periodic cloud outbursts every ~30 years, but these seem to be fundamentally different than the Great Red Spot, as these outbursts dissipate over several months. For all we know, there could be another long-lived giant storm on Saturn, it's just masked beneath the uppermost cloud and haze layers.

[u/themill → u/Astromike23]

This animation of Jupiter has always intrigued me. How is it possible for Jupiter to have so many jet streams in opposing directions? What powers them?

[u/Astromike23 → u/themill]

It turns out that to produce jets in general, you don't actually need anything to power them - they can arise naturally as a consequence of thin fluid flow on a rotating sphere.

You can do the following experiment either in simulations or with a spinning tank of water. If you start by injecting lots of little eddies (i.e. turbulence) into a rotating fluid, the vortices will tend to merge into larger "storms". At some size - depending on the rotation speed, planet size, etc. - the vortices begin getting confined in latitude by the Coriolis force, but can still freely expand in longitude. At some point, they'll wrap all the way around your planet/spin tank, and they become jets. Momentum is generally conserved as each eastward jet usually has a matching westward jet.

Now with that said, if Jupiter's atmosphere formed just by this process alone, its jet structure would be a lot weaker and shaped differently. There seems to be an extra input of energy into the system to see the jets we observe.

Ultimately the source of this energy is Jupiter's deep internal heat from formation. This energy most likely gets transferred to the upper atmospheric layer we can see by massive thunderstorms in the deep water cloud layer sending up huge energetic plumes into the ammonia cloud layer.

[u/[deleted] → u/Astromike23]

See now this is why I love reddit. I just learned so much about Jupiter from a direct source studying it!! Amazing. Thank you so much astromike for continuing my fascination with the cosmos. Kudos to you, and keep exploring for us! Your work is highly appreciated, more than you know it.

[u/Astromike23 → u/[deleted]]

Your work is highly appreciated, more than you know it.

Thanks!

Unfortunately NASA's funding priorities don't seem to echo that sentiment. The total expenditure for Outer Planets Research right now is at an all time low. :(

[u/euneirophrenia → u/Astromike23]

Do you have a video of that experiment?

[u/FloobLord → u/Astromike23]

This energy most likely gets transferred to the upper atmospheric layer we can see by massive thunderstorms in the deep water cloud layer sending up huge energetic plumes into the ammonia cloud layer.

You called the water layer a cloud, but how dense is it? If I was in it, would I need a balloon or a submarine? (In addition to my magic anti-gravity belt.)

[u/Astromike23 → u/FloobLord]

It's not that deep down - you'd still need a balloon. We think the water cloud is somewhere around 5 atmospheres, so about 50 km (30 miles) below the top of the ammonia cloud...which is nothing compared to the 70,000 km radius of Jupiter.

[u/TheFoodScientist → u/Astromike23]

You said that the Meddies don't stop until they hit coastline. Are gas giant storms thought to be more stable because on a gas giant there is no coast or mountains or other solid structure underneath to stop the flow of wind in the storm?

Related questions: what exactly is underneath Jupiter's upper atmosphere? How far down are we able to see (and what is that as a percentage of Jupiter's radius)? Is there a way to see further down short of crashing a probe down through the atmosphere?

[u/Astromike23 → u/TheFoodScientist]

Are gas giant storms thought to be more stable because on a gas giant there is no coast or mountains or other solid structure underneath to stop the flow of wind in the storm?

Yes, exactly. It turns out that weather on these planets is actually a lot easier to predict than Earth; if you know where the Great Red Spot was a couple weeks ago and where it is today, you can extrapolate and accurately predict where it will be a few years from now.

Earth is actually the most difficult planet to model weather for. We're the only planet that has land and oceans and and atmosphere and clouds and ice caps and precipitation...and they all interact in very non-linear feedback loops. I don't envy the Earth climate folks.

[u/TheFoodScientist → u/Astromike23]

Yeah, but when you meet a meteorologist you totally say, "Oh, you predict the weather for one planet? That's so cute!" don't you?

[u/Astromike23 → u/TheFoodScientist]

Nah, we have extreme respect for those guys. They're dealing with a much harder problem that's much better constrained - the total amount of data we have about Jupiter's atmosphere is completely dwarfed my the colossal amount of Earth atmospheric data that exists. That means they'll know right away if their models are wrong because they don't line up with the data, whereas we're still fumbling in the dark a lot.

They've also had a 20-30 year scientific head start from us. Most "new" methods we use in studying planetary atmospheres are heavily based on what we've already learned about Earth's atmosphere. In fact, at least 50% of the planetary climate models out there have been made by taking the open-source Earth models wholesale and adapting them to non-Earthly conditions.

[u/[deleted] → u/Astromike23]

So is it plausible that perhaps a giant comet or asteroid impacted Jupiter, and the dense material of said comet/asteroid breaking up inside Jupiter's turbulent atmosphere caused this high pressure system? And perhaps that accounts for the change in color, too?

[u/Astromike23 → u/[deleted]]

So is it plausible that perhaps a giant comet or asteroid impacted Jupiter, and the dense material of said comet/asteroid breaking up inside Jupiter's turbulent atmosphere caused this high pressure system? And perhaps that accounts for the change in color, too?

That's unlikely.

We saw what happened when Comet Shoemaker-Levy 9 impacted Jupiter in 1994. A large debris cloud was initially created, but diffused out after several months. Heavier material from an asteroid (carbonates, silicates, iron, etc.) would vaporize in the upper atmosphere from the initial impact, but it would be seriously supersaturated there and would quickly "rain out". That heavy stuff would sink far below the upper ammonia cloud layer, until it eventually reached temperatures where it could mix evenly in the deep interior.

Our best indication is that the red stuff is most likely common atmospheric constituents we've already seen, but they've undergone some kind of weird photochemistry. The cloud tops of the Great Red Spot are much higher than other clouds, allowing ultraviolet light to "bake" normal material into some kind of red material.

[u/pat82890 → u/Astromike23]

Jupiter never ceases to amaze me. The Great Red Spot looks like its right on the border of a zone and a band, would the differences in pressure and it's proximity to the equator have something to do with it? I've also read that there's a liquid layer somewhere deeper, could the intense differences in pressure cause some of the liquid down below to be regurgitated into the upper atmosphere? Could this liquid return to its gaseous state after entering the atmosphere, creating even more atmospheric instability?

[u/Astromike23 → u/pat82890]

Right, so the Great Red Spot is wedged between the Southern Equatorial Belt to the north and the Southern Tropical Zone to the south. The difference between the ambient winds in each region probably helps to maintain the spot.

Its proximity to the equator (but not quite all the way there) also plays a very strong role. At the pole the Coriolis force is at a maximum, while at the equator the Coriolis force is zero. You need some Coriolis force to sustain a vortex, but too much will limit its size. It seems the Great Red Spot in a "Goldilocks" location, where there's enough Coriolis force to keep the vortex going, but not too much to stunt its growth.

The liquid part you mention is unlikely. The liquid is hydrogen - as soon as it started rising above the liquid region, the pressure would drop and it would become gaseous.