I am a planetary scientist and computational physicist specializing in giant planet atmospheres. I currently teach undergraduate physics. Ask me anything!

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We still don't know a whole lot about what the interiors are like, but with more missions like Cassini and Juno, we'll be getting a much better picture.

Let's travel from the outside inward. The planets have the same layers as on Earth, though their natures vary.

The outermost layer of the atmosphere is the thermosphere (well, technically the exosphere, but I consider that to be effectively space). It's called the **thermo**sphere because temperature increases a lot in this region. Most people would call this a total vacuum, but as scientists, we know that it's close to a total vacuum, but there's still plenty going on. We have very little idea what the winds are like here. This is where we see aurora. There are significantly less types of chemicals to keep track of, which is nice for us. ;-)

Near the bottom of the thermosphere is the ionosphere, which is where a lot of interesting chemistry happens. There, radiation from the Sun breaks apart molecules and frees electrons from atoms — a process known as "photoionization". The result is a partial plasma layer. On Earth, this is partially responsible for reflecting certain radio communications back down towards Earth (more info). This is an important layer because it helps mediate the atmosphere below and the magnetosphere above. Powerful electrical currents come back and forth from the magnetosphere, and they interact with the atmosphere here.

The mesosphere and stratospheres are the next two layers. (Though, evidently Jupiter lacks a mesosphere!) There, temperatures typically decrease with height. Things called gravity waves and planetary waves often play a significant role.

The troposphere is where the cloud deck is. Lots of stuff happening there! Finally, the atmosphere is thick enough that we can it! We can infer wind speeds by tracking clouds!

Now we're getting past the traditional atmospheric layers. The pressure is getting too much for us. Lots of clouds everywhere. I wonder what it would look like! The Galileo atmospheric probe only got to 132 km below the 1 bar level, so we've never *directly* probed below this. What we know is based on indirect measurements.

We expect that dynamics will play a big role. Other measurements suggest that the wind systems (jet streams) we see from the clouds extend much deeper into the atmosphere — likely thousands of kilometers (source)! There's also a lot of different chemical species and tons of different kinds of chemical reactions going on here. It's a really complex place.

You've probably heard of exotic things down below, like weird chemicals "raining" down. Recent evidence suggests helium rain might fall in Jupiter (more info). The pressure here is mind-boggling: many millions of times higher than Earth's surface pressure.

**Is there a" surface"?** It's a complicated question.

At deeper levels and higher pressures, the pressure pushes gas molecules so close together that it becomes a "supercritical fluid", with no distinction between the liquid and gaseous phases. Different chemicals experience these sorts of phase changes at different levels, so it'd be a really unusual mix of some things being solid, some things being liquid, and some things being gaseous — possibly like the worst smog you can imagine in the middle of a blizzard (where the solid stuff is the snowflakes; except a really hot blizzard, and not snow).

The "mantles" are deeper than we know, but our models can help give some good guesses, based on other indirect observations we've made. The exact composition depends on the planet. Jupiter and Saturn have large layers of metallic hydrogen surrounded by a thinner liquid hydrogen layer. We believe Uranus and Neptune have more water, methane and ammonia.

We do expect that the cores contain some mixture of "rock" and metal. This makes sense: rock and metal are pretty dense, and would sink to the bottom. At this point, perhaps there'd be a solid core? AFAIK, this is still poorly understood.

More great info is available here: https://lasp.colorado.edu/outerplanets/giantplanets_interiors.php.

Did you go to bed right after starting this?

so much respect for that

Thanks! I'm trying to get better at that.

I did! I expected maybe a dozen or so comments, but not 136!

EDIT: over 200....

What are some things in the universe that exist, yet, based on our understanding of physics, should not?

Saturn's magnetic field is — as far as we have been able to see — *perfectly* aligned with its rotation axis. All of the other planets have their magnetic field at an angle (or even offset from the center!). Uranus and Neptune are especially wonky (image). Dynamo theory — which represents our understanding of how rotating planets generate planetary magnetic fields through the rotation and convection of their fluid interiors — says that this shouldn't happen for Saturn. Why it is so closely aligned is a big mystery!

Another thing! We see a lot of matter in galaxies, but the galaxies behave as if they have a lot more material than we see. This is borne out from several different types of experiments. We infer that there must be a lot more material there than we can see, but since we can't see it, we call it "dark matter". We still have no idea what it is!

Oh goody!

I've always wondered, since most stars are binary, it's jupiter our sun's failed binary star? How much more matter would jupiter need to become the smallest type of star?

That's a neat question! I don't know if I'd call it a failed binary star...it's a bit too small for that. But that's an insightful way of putting it!

In planetary science, we actually use Jupiter as a standard of mass. The mass of Jupiter is...**drumroll**..."*one Jupiter mass*" 😂. The minimum mass an object needs to be to begin fusing hydrogen in its core — and therefore be considered a star — is about 80–90 Jupiter masses.

However, something with 13 Jupiter masses can be a brown dwarf!

More info here.

Thank you so much for the reply!

I had no idea jupiter is used as a standard of measurement, that makes it a lot easier for a layperson to imagine how just many more jupiters you'd need to become a star lol

Not just a layperson — we use those units for the same reason! :-)

And if we shot nuclear waste into jupiter, would it possibly cause it to go fusion?

I can answer this one. no.

what causes the fusion is mass, creating pressure which creates the fusion.

you might not be aware but that little spot on Jupiter's surface is around 3x the size of earth. so if you threw our entire planet into Jupiter it wouldn't really notice it.

things in space at very very large, and we are very very small on those scales.

If we shot OP's Mom into Jupiter, would it cause it to go fusion?

Haha, no. But I'm pretty sure I'd go nuclear.... ;-)

Correct!

Although, Jupiter would very much notice if Earth cannonballed into its Great Red Spot! It'd affect the dynamics in a powerful way for thousands of years — and there'd probably still be echoes of it for hundreds of millions of years, or longer.

Interesting.

If that is the case is it possible to work backwards from what we see now to figure out if there have ever been any large impacts like that in the past?

Yes and no.

It's hard to imagine what we'd be able to notice. If you accidentally dropped a wedding ring into a mucky pond, you wouldn't know where or how to start looking for it. The same is true for a planet with a pretty opaque atmosphere, especially if it happened many millions of years ago.

We expect, based on our understanding of how solar systems form, that all the planets in the solar system should be orbiting with similar inclinations (how tilted their orbit is) and similar spin axes. This is less true of far-out planets. An impact from another massive planetary body, however, could alter their rotation in much the same way as how a stably-spinning top changes how it spins when you give it a whack. This is one of the leading hypotheses as to why Venus and Uranus rotate backwards compared to their orbit and compared to the other planets. So this is one possibility. (More info here.)

Many suspect that Saturn's rings are (at least in part) the remnants of something that got caught, or perhaps the remnants of its accretion disk as it formed. We don't know yet, though.

It's easier to see evidence for this in the terrestrial planets because we can see the surface and infer their geological history.

Mars has a very striking dichotomy in its crust. The crust in the southern hemisphere of Mars is 1–3 km higher than the crust in the northern hemisphere. The southern crust averages 58 km thick while the northern crust is only 32 km thick. On top of that, the "northern lowlands" are relatively smooth (indicating a younger formation age) while the "southern highlands" are heavily cratered (indicating more exposure to impacts, implying an older age). One of the leading theories for this is that a massive object struck Mars, knocking half of its crust off, or knocking it to the side. (More info on Wikipedia.)

We also believe that our own Moon formed when the young Earth was struck by a Mars-sized planetary body. This is called the Giant-Impact Hypothesis (Wikipedia). There's a neat visualization of this happening here. We got some extremely compelling evidence for this when we analyzed the Moon rocks from the Apollo missions. Scientists were amazed to find that the Moon rocks had almost identical isotopic signatures as rocks here on Earth. In geology, isotopic chemistry helps identify rocks in the same way as fingerprints help identify humans. The fact that the Moon rocks had the same isotopic abundances is the "smoking gun", implying that they have the same origin.

What if we shot a lot of nuclear waste? Like from beyond earth?

again no, but mostly because nuclear waste(I presume you meant fission fuel waste and not irradiated coolant) is very dense already

most solar ignitions (I'd say all but I do t know for sure) are hydrogen based, and hydrogen is the lightest, least dense element there is. typically denser materials are produced late in a stars life and are the sign that the star is nearing its end. you wouldn't be able to create a star with radiation either. it really is all so much simpler than that. it's good old fashioned squashing that does all the work.

There are actually lots of different elements that can fuse in a star! Hydrogen is the first in a series. Helium is the second. For a full list, see https://www.thoughtco.com/stellar-nucleosynthesis-2699311.

Note that all stars burn hydrogen. Not all stars get past that point. The more massive the star is, the more different kinds of elements it will be able to fuse in its lifetime.

For more info, check out Wikipedia's article on Stellar Nucleosynthesis. It's really neat!

You could if you shot enough mass at it! But "enough mass" would be almost 90 times the mass of Jupiter.

How much science fiction do you read? Do you find that having a really good understanding of subject matter can hamper your enjoyment of such? Also, favourite fiction book?

I would LOVE to read more! Unfortunately, I'm a pretty slow reader, so I typically read only one or two books per year. I'm also an avid gamer, so that limits things, too. ;-)

Right now, I'm reading Lord Kalvan of Otherwhen, by H. Beam Piper. It was given to me by a dear friend.

Favorite? Hmm..back when I read a lot more as a teen, I was all over Star Wars books. The Han Solo trilogy was one of my favorites. I'm still bear a grudge against Disney for summarily dismissing the established canon. I think one of my other favorites was The Martian Chronicles, by Ray Bradbury. Again, I wish I had time to read so much more!!

Favorite fiction book overall, however, is definitely The Lord of the Rings. :-)

What do you think is the most interesting celestial body and why?

Also any advice for someone older who wants to become a physicist?

I went to the DPS/EPSC conference for planetary sciences two years ago, and they were passing out name tags that had a space for your name and your favorite planet. I wrote in "the one with all the people" (i.e. Earth), because we live on such a neat place! I love plants, animals, large bodies of water, and so much more that seems to be special to us. I love people most of all, though sometimes they make it hard.

My second favorite is Saturn — the one I studied for my dissertation. There's so much we know and don't know, so it's a fun, satisfying and exciting place to do research!

My third favorite is Titan, because it is so familiar and yet so incredibly alien. It has streams, huge lakes, clouds, storms, dunes, erosion, waves, lightning, and so on — all things that we're used to. But the lakes are liquid ethane or methane, not water. The dunes are probably ice crystals or frozen hydrocarbons, or maybe sand (I'm honestly not sure). The clouds are similarly hydrocarbons, not water-based. The atmosphere is pretty thick, as on Earth, but the temperature is far colder.

Do you think that planet X exists? Thank you in advance :)

There's a building body of evidence that something is out there! One of the planetary scientists studying this is Dr. Renu Malhotra (website). She and others have evidence from orbital resonances that suggest the existence of two large objects the size of planets beyond Pluto. AFAIK, they're still only hypothesized at this point!

Though, there are quite a number of "planet X"s, in some sense. There are several large objects beyond Pluto that are much bigger than Pluto. They were part of the reason we had to come up with a definition of a planet back in 2006 — and why Pluto was ultimately "demoted". ;-)

What does a career on your general field look like? Like what are the general options? Asking as I took the "practical" route of food science and always regret not getting into astrochemistry. So I'm curious if I went back for a PhD in physical chemistry, where would that take me?

Food science is awesome! I love cooking, and as you know better than I, there's a lot of neat stuff that goes into cooking that most people never think of!

You'd apply to several grad schools, and then if they liked you enough and had funds to support you, you'd get accepted to one or more programs.

During your first two years, you'd probably focus 60–80% of your time on classes. I was taking 2-3 classes per semester. Most programs have a "core curriculum" (kind of like in undergrad) that you need to fulfil, but you also need to take electives related to your field. Class sizes are usually much smaller than in undergrad. The rest of your time would probably be spent as a teaching assistant (TA), lab assistant or research assistant (if you showed interest and aptitude for research). Occasionally grad students are even selected to be lecturing TAs or even instructors. If you do research early on, you'll be reading a lot of scientific papers to get your feet wet and begin to understand what kinds of things people have been doing in your field and what sorts of problems are unanswered.

After about the two-year mark, you'd take a written and/or oral comprehensive exam. Some programs require one or the other, while other programs require both. The written exam is usually to demonstrate that you learned enough from your core classes. The oral exam may also do that, but for many programs is also a chance to demonstrate that you're ready for research. I took both kinds of oral exams and two written exams, since I was in both a physics program and then later a planetary science program. The focus of my second oral exam was to present two research projects (a primary and a backup) that I wanted to pursue for my doctoral research.

For the next 1–3 years after those exams, you're expected to focus on research while also finishing up your coursework obligations. Since you've probably completed all the core classes by now, the classes you're taking now tend to be electives related to your research. Rather than packing in the classes as before, you tend to take less (1-2 per semester) for the next two or so years, until you fulfill the class requirements for your degree.

The last 1–3 years are spent doing almost entirely research. For me, that meant extending an existing simulation of Saturn's upper atmosphere, and then analyzing the results. At this point, many people read an average of 1–3 new research papers per week. You start writing papers, too — hopefully as "first author" (the primary author who coordinates the study and does most of the writing). A lot of grads publish 2–5 papers during their PhD work, but a friend of mine published over 20! 😲

By the end of your PhD work, most people have read hundreds of scientific research papers, and they begin writing their dissertation. This can take several months to a year or two. Some programs allow doctoral candidates to "staple three papers together, add an introduction and conclusion" and call it a dissertation. Other programs require the dissertation to be a separate entity of unpublished work.

Side Note: A lot of grad students struggle with mental health in various ways. Most struggle at some point with imposter syndrome: thinking they don't have what it takes to do this, or that everyone else is better than them, or that one day they will be revealed as being totally inadequate. Others struggle with extreme anxiety, often related to the stress they have to deal with. It's a really tough thing for anyone to go through. It's something that needs addressing.

Hello Dr. Jess Vriesema, how large a planet would have to be before the atmosphere pressure is too much for human breathing? Or too hight pressure for life to exist?

Hi! Happy cake day!

Great question!

From the biological side: Humans need about 1 atmosphere of pressure. We can live with a little bit more and a little bit less. (I'm not sure about the exact requirements, but I do know people can, with time, adapt to living in lower pressure or higher pressures.)

Generally speaking, larger planets have more atmosphere (and vice-versa), but there are lots of exceptions. Venus has a huge atmosphere that is more dense than Earth's, and is slightly smaller than Earth. Venus's atmosphere — besides it being highly toxic and extremely hot — is many times the pressure that humans do well in. Mars is a bit smaller, but has significantly less atmosphere than Earth. Titan is one of Saturn's moons, but it has an atmosphere that, at its surface, is 50% higher pressure than Earth's.

So, at least in our solar system, there are too many outliers to really say. It depends on how the planet formed and on its history more than just its size. Outside our solar system, we're finding thousands of exoplanets. However, we don't know those well enough yet to really say that, either.

I fail to imagine how Jupiter works. How does it's surface works? Would we sink on it? Fall through it and get to the other side? And how storms work over there?

Hi!

Jupiter, like the other giant planets in our solar system, doesn't really have a surface. It's basically all atmosphere, almost all the way down. So, if you dropped an iron cannonball into any of them, it would probably keep going down until it got to the core. As it fell through the atmosphere for tens of thousands of kilometers (many times Earth's size!), it would heat up and probably melt (like a meteorite), but let's ignore that for now. As it fell, friction with the atmosphere would slow it down, but it would keep on going down. Going deeper, the atmosphere would get thicker and thicker, increasing the amount of friction on the cannonball in the same way that water is thicker and therefore harder to move through for us than air. Eventually, it would stop when it hit the core. Even if it had no solid core and it did crash through the core, it would go very far on the other side because friction would be so very high. It'd be like "swimming" in a pool of very thick syrup: the cannonball would slow down and get pulled back down to the center of the planet.

1. Do you teach computational physics to undergrads? If yes, do you use lots of examples from your field of research?
2. Programming skills vs mathematical rigor (in numerical analysis). Which one is more important in a computational physics course?

1. I do not yet, but I'd like to someday! I did teach computer science (CS), both to CS majors and to science majors. If/when I do, I would definitely use some examples from my field of research! (It's too cool not to!)
2. It depends what you're going for. In a typical undergrad course, I imagine the programming skills would get you further. However, you would need a certain degree of mathematical ability to solve problems. After that point, it depends what you want to do. My area of research required heavy amounts of both — perhaps more so of both than it did for any of my peers (by choice: this is what I like!). Whether you need more programming skills in the field or more numerical analysis depends on what you plan to do with it — and after classes, that's largely up to you.

I was always engaged in citizen science projects like Zooniverse. I've spent many hours classifying galaxies, moon craters etc. With AI becoming more advanced: Do you think this will eventually render these citicen science projects obsolete? Would AI be capable to process massive survey imagery like the data from WISE and SLOAN? Are there plans to let AI reevaluate older survey data? I find this an extremely interesting topic. Thanks for your AmA!

Neat question! (Also, you're welcome! This is fun!)

I don't think it will render those projects obsolete.

AI classification algorithms require training. Humans have to give it a bunch of initial data and say, "this is a picture of X, and that is a picture of Y." With enough exposure to pictures of X and Y, the system can begin predicting whether a new picture shows X or Y. So, humans will still have to do some initial classifications to train the AI.

Also, once we're done classifying some of the big things, we'll look for other, smaller things to classify. Maybe we'll eventually classify all the moon craters down to a certain size. (Cool!) I have a feeling human curiosity will compel us to classify something else next. :-)

To answer the second part of your question: I expect AI will play a large role in determining what data gets brought to the attention of scientists. There's far too much data to process by hand, so scientists will require a sieve to determine what gets human attention. I imagine that sort of technology will come from machine learning.

I don't know of plans to reevaluate older survey data, but I imagine people have considered it.

What are your beliefs in relation to the simulation theory? Do you hold any religious or spiritual beliefs?

By simulation theory, I take it you mean something like the Matrix: we're all lab rats in somebody's experiment. If so, I don't really like that view so much. It doesn't feel right. At any rate, how would we have any way to know or test it?

I am a Christian. I believe in theistic evolution — the idea that God used natural processes of planet formation billions of years ago and evolution more recently to create the universe we live in. One of my passions is to talk about science and faith. I very strongly believe that science and faith are not mutually exclusive. Scientists use various types of faith to do science all the time, whether they realize it or not. I believe people of faith (e.g. Christians, others) ought to refine their religious beliefs with many of the same tools we use to understand other fields of study: skepticism, inquiry and reason. I think too many religious people rely too heavily on blind faith, to their detriment.

do you believe in astrology?

Definitely not!

I actually teach my students about this in introductory astronomy classes. We do an activity where I give them a bunch of horoscope readings (without the sign) from the previous day or two and ask them to rank each one on how accurate it was for them the past day. They don't know which horoscope reading actually is supposed to correspond to them. I also ask them what their astrological sign is. Then, I figure out which horoscope prediction actually corresponded to each student and analyze the data. If astrology were true, then we would expect that students would, on average, rate the horoscope reading that corresponds to their astrological sign as more accurate than the other signs. The result, however, is that there's no significant difference: students rate all the horoscope predictions equally, whether it's supposed to correspond to them or not. Other people do this activity and arrive at the same conclusion. It's a fun activity!

More than that, there's no scientific reason to expect astrology to be true. Why should planets that are so far away from us be able to alter the destinies of humans? They give us a teeny pinprick of light in the night sky, and they exert an immeasurable amount of gravity. How would these forces possibly be able to influence our love life, our wealth, or our health? There are much more significant forces going on here on Earth that directly impact these things!

It always breaks my heart a little bit when somebody comes in to office hours with "ASTROLOGY" (instead of ASTRONOMY) written on the spine of their binder.

Such a Virgo response

LOL!

I've read science fiction that postulates the ability of humans to inhabit gas giants. This strikes me as not just impossible due to hostile environmental conditions but undesirable due to gravity. What are your thoughts about the idea of settling on a planet with no solid surface?

I totally agree!

One possibility is to have some sort of submarine-like spacecraft made of some miraculously light and inconceivably strong material. That could inhabit the depths like a submarine...or, perhaps more accurately, like a blimp.

A much more likely possibility is to have space stations orbiting the planet. Such stations could exist in a stable orbit if they moved fast enough around the planet — just like space stations here on Earth. They would be in freefall and would experience microgravity. One problem with this is that the Van Allen radiation belts could be problematic for people living in those space stations. Strong electric currents and radiation going from one pole of the planet to the other would cause serious dangers for humans and electronics.

Agreed. Nothing about living on or even near Jupiter is in any way good for humans in terms of long term health or survivability. We have robots for a reason.

So where should humans settle? Mars kinda sucks and Venus is far worse.

Yeah...we don't have any great options that way.

Moon: orbits near Earth, so it's "more convenient" to get to — at least compared to other planets! It's also in the habitable zone, but due to its slow rotation rate (once per orbit, since it's "tidally locked" with Earth), its temperature extremes are worse than Mars's. Not having an atmosphere makes things dangerous, and also makes cooling difficult, since radiators can't exchange heat with the air like in your home's AC system. One neat possibility is the underground lava tubes on the Moon — they would provide some degree of shelter from the crazy temperature extremes.

Mars: More temperate than the Moon, and it has an atmosphere. But, it has dust storms that can mess with things, and the atmosphere isn't particularly great for humans. Also, the lack of a magnetic field seems dangerous (I'd have to investigate this more before I speak authoritatively). I would want to colonize Mars first for this reason, and most of the scientific community seems to agree.

Venus: ["next!"]

Mercury: We might be able to hang out in one of the polar craters that still contains ice, since those craters are always in the shadow. Otherwise, it's far too hot for us.

Asteroid Belt: Maybe? It's not nearly as dense as what it often is portrayed in movies. It's getting cold out here, too.

Jupiter and beyond: It's cold, but nuclear fuel cells could help power heating systems.

Heya Jess!! Thanks for doing an AMA! With the James Webb, will we be able to gather information on exoplanets via spectrometry or other methods? If yes, what kind of kind of data can we gather, how can we use it, and how does it compare to what we have today?

Also, what other advantages will the James Webb bring us?

You're welcome! Sorry it took so long to respond.

The JWST has an instrument called NIRSPEC, its Near InfraRed SPECtrograph, so it will be able to gather lots of info via spectrometry! That will help it study lots of tiny, distant galaxies. It's actually designed to study 100 such objects at once, if they're all in the field of view!

It has a couple other instruments that will help. Its Near-InfraRed Camera (NIRCam) will help it take pictures of faint, distant objects around bright, distant objects. This camera, developed by the University of Arizona (my alma mater!), is perfect for observing distant planets!

I'm not that familiar with all the details. TBH, I've got my browser looking at https://www.nasa.gov/mission_pages/webb/about/index.html and https://jwst.nasa.gov/index.html to look up info.

The takeaways are that it should help us observe a ton more exoplanets, kind of like the Kepler mission — but more so. Also, it has other features that let it observe things that are longer-wavelength. Longer wavelength observations are important because longer wavelengths are able to get through clouds of dust and gas more easily than shorter wavelengths. Although there's more at play in the following example, that's part of why the Sun appears red at sunset: shorter wavelength light (e.g. blue, green, purple) gets scattered more easily than red light. Because of this, we can use long wavelengths to peer into stellar nurseries and into the ancient formation of the universe.

It should be really neat!

What are your thoughts on Pluto? My way of remembering the order and name of the planets (My Very Easy Method Just Speeds Up Naming Planets) doesn't have the same ring to it without Pluto's P

I learned it from this: "My Very Educated Mother Just Sold Us Nine Pizzas". Since Pluto was demoted, I've heard "My Very Educated Mother Just Sold Us Nothing". I agree. It lacks the ring. (Any suggestions, Reddit?)

In 2006, the IAU basically said that a prospective planet has to meet the following 3 criteria to be considered a planet:

1. It must be big enough to be round. (This knocks out most of the asteroids!)
2. It must be the biggest thing in its neighborhood. (Sorry, Moon and asteroids!)
3. It has to orbit its host star — for our solar system, the Sun.

For #1: The bigger an object is, the more round it gets. That's because the bigger something is, the more of an effect gravity has in shaping it, and gravity wants to bring everything towards the center.

For #2: The prospective candidate must have cleared out other objects in its orbit.

I agree with the IAU's decision to classify Pluto as a dwarf planet. When it was first discovered, we thought it was MUCH bigger than it actually was. In the decades after we discovered it, our estimates of its mass got smaller, and smaller, and still smaller. It's still big enough to satisfy #1, and it certainly satisfies #3, but the problem is with #2. It's more accurate to say that Pluto and Charon orbit each other: the center of mass for them is actually outside Pluto's surface. (See here for a nice description of what this means.)

Also, there are things that are much bigger than Pluto that are further out. If Pluto were to be considered a planet, those other things would also have to — and they might be more deserving (in some sense) than Pluto.

EDIT: There's a nice article here that explains this: https://www.loc.gov/everyday-mysteries/item/why-is-pluto-no-longer-a-planet/.

What are our best ideas for how to penetrate through more of the atmosphere to learn about what's underneath? Sending probes down into it? How would they transmit data through all of that?

Yup! Probes are the way to do that. The problem with them is that they're one-time-use, like a needle used to take your blood sample. We get a reading, but don't get the probe back, and space probes tend to be kinda pricey....

They need to point their antenna towards Earth, or towards a relay satellite (e.g. an orbiter spacecraft) as they go down. This is difficult for a probe to do, just like it's difficult to point a camera straight up when you're skydiving. The signal needs to be somewhat stable (though some "cleaning" can be done on our end, after the fact), it needs to be strong enough (bigger radio dish --> more efficient transmitting, I think!), the spacecraft needs to last long enough before it breaks apart, and the signal needs to be able to get through the upper layer of the atmosphere.

For that last issue (signal getting through the atmosphere): that's not a problem high in the atmosphere, because there's not much between it and Earth. It's more of a problem below the ionosphere, because then plasma from the ionosphere can scramble, reflect and hide the spacecraft's signal. This can be avoided somewhat by broadcasting at frequencies the ionosphere doesn't impact as much, but I am less familiar with the details of this.

The Cassini spacecraft, during its final plunge, probably didn't make through the ionosphere before we lost contact with it. (Actually, the exact location of the ionosphere is a little unclear, but it probably touched the very top or perhaps made it to somewhere near the middle of the ionosphere.)

For more info on the Cassini Grand Finale — and especially it's Final Plunge — there's an award-winning video here: https://www.jpl.nasa.gov/videos/cassinis-grand-finale. Since Cassini burned up as it was sampling the thermosphere — the region of space I study — I often show this video when giving public talks. That video makes me tear up in front of my audience every single time I show it. But it's too good not to show.

Have you seen the expanse? What are your thoughts on the show?

I AM LOVING IT! I actually just got done watching an episode before posting this last night. My wife and I are in Season 3. :-)

I really, really appreciate the science bits I see:

• Ships turning backwards and thrusting as they are approaching their destination
• Convoluted orbital paths to make use of gravity assists
• Complications arising from high-G maneuvers. If stuff isn't tethered down, things can fly around due to their inertia. Humans can only take so much acceleration before it hurts us. (There's an awesome example of one of these things in Season 3, Episode 2. Spoiler: One of the main characters does not tether down some supplies, and when the spaceship goes into combat with extreme twists, turns and sharp accelerations, the tools fly around the ship, nearly killing its passengers who are strapped in.)

The imagination is awesome! It's just really well done science fiction.

Why is there such a big push to leave Earth?(ie. Bezos,Musk,Branson) is it just rich guys doing rich guy stuff?

We all kind of expect that humanity will outgrow Earth. What drives us away? Perhaps it's the search for other materials. Perhaps it's war or famine. Perhaps its a solar event or collisional event that we need to escape from. Or, perhaps it's just our innate drive to explore. Whatever it is, we should be ready to colonize other worlds.

I don't think it's so much that we're wanting to leave Earth behind, as much as we're wanting to expand and also be elsewhere.

I think a lot of it is rich guys doing rich guy things. But from the science side of things, it's an investment in humanity. We can learn a lot from being on other planets, just like how the utility person can learn more about a power outage by visiting your house rather than just sitting in their office. We also develop a lot of really useful technology along the way that benefits humanity in ways we didn't expect or imagine. It's also something that helps unite people across the world: we get this sense of working together that helps bring perspective and — hopefully — peace.

A nun in Zambia once wrote to a NASA administrator, asking him why the US government spent billions of dollars on space exploration rather than on humanitarian aid on Earth. The administrator wrote a letter in response, and it's one of the most powerful letters I've ever read. You can read it here: https://lettersofnote.com/2012/08/06/why-explore-space/.

Are floating cities possible?

https://tvtropes.org/pmwiki/pmwiki.php/Main/WorldInTheSky

Real Life section

It's totally conceivable that we could have space stations that orbit a planet and seem to float. People in those space stations would be in a continual state of freefall and would be experiencing microgravity. I suppose with enough thrust, the space stations could be made to stay "upright" — though that would break the freefall and it would probably feel somewhat disorienting.

The floating city trope seems to rely on some kind of antigravity technology. We don't know of any way to counteract gravity yet, and all indications point to it being impossible. Who knows?

Another possibility is a "lighter than air" city — basically a city that is a giant submarine or is suspended by colossal balloons. This is more possible, but it would require an atmosphere that is incredibly dense. That, in turn, would require incredibly lightweight and strong materials. It's beyond our tech, but not inconceivable.

I recently read about microbes found deep under the surface of the ocean bed. stuff that has scientists redefining what life is. what are the chances of life... any life living on the surface of a gas giant?

Hi!

Scientists are discovering extremophiles — microorganisms that thrive in extreme conditions — in all kinds of places. Tardigrades, for example, can withstand extreme temperatures, extreme pressures (high and low), radiation, dehydration, and starvation (Source: Wikipedia).

Astrobiologists study these because 1) they're freakin' awesome, and 2) we think that our best chance for discovering life beyond Earth might be some kind of extremophile, since there are so many hazardous places outside of Earth.

There's not really a "surface" to the gas giant planets — they're basically planets that are gigantic atmospheres. It's hard to imagine them living in the upper atmosphere, and the lower atmosphere is pretty deep and fairly hot. According to this study, some extremophiles on the bottom of the ocean thrive in pressures of 110 MPa — almost 1100 times Earth's pressure at the surface. That's still pretty high up in Jupiter's atmosphere, where the density is still a bit less than water. I suppose something could live there, floating around, but it would presumably have to be able to reproduce, eat, and do all the things it would need to do.

Bottom Line: It seems unlikely, but what do I know? ;-)

Hello Dr. Vriesema! Thank you for doing this AMA. I’m curious about how you incorporate computation into your study of giant planetary science. For example, do you make simulations of weather patterns on Jupiter?

Hi! You're welcome. Thanks for your patience!

I have helped develop a simulation of Saturn's upper atmosphere. The simulation, called the Saturn Thermosphere-Ionosphere Model (STIM for short) is written in Fortran. It's a suite of different kinds of models for different aspects of this part of the atmosphere. It's a 3D, global model. Here, 3D means that it has a grid that includes all three dimensions. Global means that the grid covers the whole planet — as opposed to local simulations that are often done for weather-prediction here on Earth and only model hundreds or thousands of kilometers.

Some of the things it models:

• Fluid dynamics — the coupled, non-linear, 3D Navier-Stokes equations of momentum, energy and continuity for a number of neutral and positively-charged chemical species
• Magnetohydrodynamics — electrical currents and magnetic fields interact together in fluids (this was my big contribution!)
• "Magnetobraking" in the auroral regions — the magnetosphere leaves an imprint on the upper atmosphere near the poles, causing winds to move slower than the planetary rotation rate. This simulates one-way forcing from the magnetosphere.
• Chemistry — for the top 5-7 chemicals only
• Photochemistry — how radiation from the Sun affects chemistry
• Chemical transport — winds blow chemicals around, just like winds on Earth blow smoke across continents
• Plasma diffusion and transport — how plasma moves around in the domain
• Ring shadowing — (usually turned off, but the rings can block light from the Sun that drives photochemistry)

Currently, this model is serial, meaning that it only runs on one core of a computer at a time. Typical test simulations at low resolution take 2–6 days of computer time. If given time and funding, I'd love to make it run in parallel, so it would run faster. There are higher priorities, however. I love my teaching, but it doesn't leave me as much time to work on this simulation as I'd like!

How did you get a teaching job? Plenty of phd are jobless

My wife and I lived apart for two years after she graduated with a PhD in educational psychology. She did a postdoc position in Santa Barbara, CA while I finished up in Tucson, AZ. (I took the train to visit her as often as I could!)

After that, she got a job at our current institution. I wasn't finished with my PhD, so I wasn't ready to try for a spousal hire. She did mention me, however, and the math department offered me a one semester teaching appointment to teach statistics. After that, they had a dire need of computer science teachers, so I taught two CS courses. Then, I accepted a 1 year position in the Physics+Astronomy department, which was renewed for the current year.

So I don't have a permanent position, either. I'm still working on it! :-) I love what I do and where I am, so I'm happy to keep this up as long as they'll have me.

Who lives in a pineapple under the sea?

SPONGE BOB SQUARE PANTS!

Can I hire you for my Stellaris playthrough?

LOL Sounds fun! Alas, I have too much to do already! I've also never played it yet....

Do you ever get flat earthers/ climate change deniers in your classes? How do you handle that?

I never have yet! I'd love to chat with them in office hours about it, though! I love being able to gently build bridges.

I had a student in a class I was TAing for who was a Young-Earth Creationist and didn't think that Earth was billions of years old. As a Christian and a scientist myself, I wanted to talk to this student about so many things! One of my passions is discussing issues of science and faith. I don't see them as incompatible.

How many traits do you share with Sheldon Cooper?

I like gaming. I don't pick up on sarcasm well. I think I'm pretty smart, but I'm not egotistical like he is. Some might say I'm oblivious to a few things in social situations, but nowhere near what he is. I think we all share a lot of traits with Sheldon in lesser ways: we all do weird things sometimes, we all are stubborn sometimes, etc. He's just all those things and more — for dramatic effect.

I honestly don't watch that show much. I found a lot of its stereotypes to be kind of offensive. Sure, there are a lot of funny parts, but it felt like almost every line was supposed to be a wisecrack — as evidenced by the canned laugh track playing after almost every line.

What is the actual likelihood that we’re going to get hit by an asteroid that ends life on earth?

I don't know! It depends on what time. What's the probability that it happens in the next decade? Extremely small!

What about in the next million years? Pretty small!

What about in the next billion years? Possible. 👀

What are your thoughts on Pop science?

It's fun! Sometimes I get my news from pop science, especially for other fields of study. It's also a really great way to get the public interested in our work! We're funded by taxpayers, so we really want to let people know what we find. Why? 1) We think it's cool and think other people might agree, 2) if taxpayers fund us, then we have an obligation to report back to them, and 3) without taxpayer support, we don't get paid to do what we love. (I think most of us would put those things in that order, but realistically, it depends on the person and on the day! 😂)

Sometimes it's annoying, though, because the authors focus on one thing and ignore the important stuff. Or worse, they misunderstand something and get things wrong (caricature of this from XKCD). The "Science News Cycle" comic by Jorge Cham really hits home, too: http://phdcomics.com/comics/archive.php?buffer_share=381dc&comicid=1174.

what do you think of the books of "Dune" by Frank Herbert?

I very vaguely remember reading a little bit of Dune. I think I was borrowing it from a friend and had to give it back in middle school. Alas, I have not read all of it!

I hear it's good, though.

Where is Planet 9?

RemindMe! 10 years

We don't know yet, but somewhere beyond Pluto!

Where are you published? I don’t think AMAs count for tenure.

I don't think they do, either.

Here's my Google Scholar profile: https://scholar.google.com/citations?hl=en&user=lceYsQEAAAAJ

You can also see my CV on my website: https://www.lpl.arizona.edu/~vriesema/curricula_vitarum/current_cv.pdf.

I only have one first-author paper (published by Icarus), which is pretty low for somebody in my position. I'm far from a "rockstar" planetary scientist. I've got another article I'm working on....