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-rba-
09-09-2015, 09:38 AM
Hi all! I 'm probably supposed to post this on the "List your specialist subjects" thread, but for some reason I am unable to load that thread when I'm logged in. I've posted about this in the tech support forum but nothing suggested there has worked. So, mods please forgive me if I'm not posting this in the right place! Anyway, on to the real point of this post:

I'm a planetary scientist! My day job involves zapping rocks on Mars with a laser (plus a lot of programming and email writing). My specialty is Mars geology, but my educational background is in physics and astrophysics. I'm happy to help with questions related to planets, space exploration, physics, astronomy, geology, chemistry, programming, robotics, what it's like to be a scientist, etc. I'm also happy to help with plausibly bending the rules for sci-fi and fantasy writing where you want to be believable but not strictly scientifically accurate. If you think I might be able to help, don't hesitate to ask!

jaus tail
09-09-2015, 10:45 AM
I'm a planetary scientist! My day job involves zapping rocks on Mars with a laser

Do you really shoot laser beams at rocks in Mars(another planet)? Sounds like an adventurous job. Like international cops and robbers.

Maryn
09-09-2015, 07:13 PM
I have always wanted my very own planetary scientist. Sure, I write erotica, mostly, but it's nevertheless a pleasure to have you here. I've set it aside for now, but I have a fantasy with multiple suns I should probably ask you about, specifically whether the resulting seasonal variation they create would be possible.

Maryn, whose kid does planetary-scale work at Lincoln Labs

Dennis E. Taylor
09-09-2015, 07:20 PM
Okay, here's a warmup question: How could you plausibly restart the magnetic field on Mars? I've read commentary to the effect that the Martian core has completely solidified and that it hasn't. I've played with ideas like drilling down a thousand km and loading it with nukes; with setting up a giant magnetic field from orbit and using inductive heating; with using either a giant mirror or a giant microwave transmitter from polar orbit...

GeorgeK
09-09-2015, 07:29 PM
I sent you a PM rba for a non-public question about a plot point

WriterDude
09-09-2015, 07:30 PM
It's about time you turned up.

My story has lots of world's mentioned in passing and I'm trying to have some huge variety while being plausible.

My first world though is an interchange where everyone passed through and switches portals. Like a delta hub, and the human stuff is much like an airport. Outside though, I want the surface of a rogue planet drifting off to the next galaxy. In the nights sky we have the full flat disc of the milky way, but as the world spins, the other side sees nothing.

Questions is:
How do I keep the surface habitable. Cold and harsh but breathable.
How fast can the world spin before atmospheric perturbations become silly.
How bright will the milky way be from a distance great enough to see all of it?
How bright would other galaxies be in the interstellar spaces.
Assuming thus world once had a civilisation before ejected, how long dead would it be?

Tinman
09-10-2015, 12:19 AM
Welcome to AW. Glad to have possible access to your expertise.

-rba-
09-10-2015, 01:55 AM
I do! I'm a member of the ChemCam science team on the Curiosity rover. ChemCam zaps rocks up to ~20 feet away from the rover and we use the light emitted by the (very tiny) spark to figure out what the rocks are made of. Sadly, the laser does not vaporize entire rocks, just tiny parts of them.

It sounds very adventurous and exciting, but in practice my typical day involves a lot of computer programming, a lot of emails, and maybe a teleconference.

-rba-
09-10-2015, 02:08 AM
Hmm, it's awfully hard to do much to the core of a planet. I guess the answer depends on whether you want the rest of the planet to remain intact. You could probably deliver enough energy by smashing a large enough object into the planet (like, another planet sized body), basically vaporizing/liquefying the whole thing. Nukes aren't going to be sufficient, and even if they were, it would be very hard to get them down into the core. Your giant mirror or microwave transmitter could heat up the surface, but that will actually do the opposite of what you want: to get the magnetic field up and running again, you want the core to be liquid and convecting. For convection, you need a temperature gradient from the inside out. By heating the surface, you'd decrease the difference in temperature from the core to the surface, so you might even shut down convection if it was happening. That, plus its slow rotation, is why Venus doesn't have a magnetic field: the surface is too hot!

Inductive heating is the only way I can think of that might be able to heat the inside without destroying the outside, but it's entirely possible you'd still end up depositing most of the energy in the crust or upper mantle. Electricity and magnetism is not my strong suit, so I am not sure how well a magnetic field would penetrate the mantle...

Edit: Oops, this was supposed to be a reply to Angry Guy

Snowstorm
09-10-2015, 02:10 AM
Well, how cool are you and what you do,-rba-! Thank you for offering your expertise. Welcome to AW. See you 'round the boards.

BarII
09-10-2015, 02:47 AM
You're the second NASA guy I met on non-NASA related forums. I used to chat with a guy in a Ross Mathews fan chat room who designed the cockpit for the space shuttle. I'm interested in the limitations of the rovers regarding chemicals they can't detect. I read that "LIBS is relatively insensitive to elements on the other side of the periodic table, including S, Cl, and F...APXS will have significantly better sensitivity for sulfur and halogens." That's not specific (to the layman) about what elements can't be detected or what discoveries we could be missing from that. Could you name an element that can't be detected well by the rovers and that might be important for some exotic form of life?

PastyAlien
09-10-2015, 02:48 AM
I'm a planetary scientist!Damn. That's even better than "I'm Batman."

Thanks for offering your expertise. I'll likely take you up on that at some point.

-rba-
09-10-2015, 03:43 AM
How do I keep the surface habitable. Cold and harsh but breathable.

That's going to be difficult if you're out in intergalactic space away from any star. You could say that when the planet was ejected from its solar system, it was majorly disrupted so it has a lot of heat in its interior. But even then, I'd expect the surface to be freezing cold, with occasional volcanoes. Do you really need a planet surface where people can survive without a space suit?



How fast can the world spin before atmospheric perturbations become silly.

Depends what you mean by "silly"! I'm not an expert in atmospheres, but the wikipedia article on the Coriolis effect is a good place to start. Basically, the faster your planet spins, the more winds on the surface will appear to "turn" as they travel across the planet surface. I wouldn't worry too much about this: any planet without a sun is not going to have an atmosphere: everything will be frozen solid at the surface.



How bright will the milky way be from a distance great enough to see all of it?
It would be clearly visible, though not super-bright. You can go out on a clear night and see our nearest neighboring galaxy, the Andromeda Galaxy with the naked eye as a faint smudge. The milky way as seen from intergalactic space would look like something between what we see the Andromeda Galaxy as, and what we currently see the milky way as. The big difference is that the night sky on your world would have no nearby stars: everything in the sky would be galaxies.



How bright would other galaxies be in the interstellar spaces.

As I said above, we can see galaxies in the night sky right now. They would mostly look about the same. The night sky would be filled with thousands of blurry blobs of light of varying sizes (galaxies) rather than thousands of points of light (stars).



Assuming thus world once had a civilisation before ejected, how long dead would it be?

Depends how fast it was going when it was ejected. There's a good chance that any civilization wouldn't last very long after being ripped away from its star unless they have extremely advanced technology.

-rba-
09-10-2015, 03:47 AM
Multiple suns can make things complicated very quickly, but it's perfectly plausible to have planets orbiting multi-star systems. Generally you have two scenarios: the planet orbits one of the stars in the system and the other stars are quite far away, or the stars are quite close to each other and the planets orbit all of the stars (Tatooine-style).

I know a bit about Lincoln Labs! They would often come to our astronomy and physics clubs in college and try to recruit students. Lots of cool research going on over there!

-rba-
09-10-2015, 03:56 AM
With LIBS, we theoretically can see any element if there is enough of it there. All elements can be ionized and give off light at distinctive wavelengths. The problem is, some elements are much easier to ionize than others, so we are much better at seeing things like alkali earth metals than we are at seeing things like halogens, which tend to hang onto their electrons very tightly. We can still detect sulfur and chlorine, but there has to be a lot of it there. The nice thing about having APXS and LIBS is that they compliment each other. APXS can't measure light elements but ChemCam has no trouble seeing things like Li and H. Meanwhile, LIBS isn't very sensitive to S and Cl but APXS sees those just fine. The SAM instrument is also very good at detecting elements that form gases when samples are heated, so it can easily detect nitrogen, sulfur, chlorine, etc.

So, I don't think we really have a gaping hole in our ability to detect certain elements. The problem is, we don't always get APXS observations because it's a lot more complicated to place the rover's arm on a target than it is to zap it with a laser. And SAM observations are even more rare because it requires drilling into the target. So we have lots of targets where the ChemCam data and the pictures are all we have to work with. We can still do a lot with that information, but we don't always get the whole picture.

GeorgeK
09-10-2015, 08:53 AM
You could probably deliver enough energy by smashing a large enough object into the planet (like, another planet sized body), basically vaporizing/liquefying the whole thing.

Assuming one could move a large planetary body, would it also work, instead of smashing into the larger body, nudging the smaller one into orbit and giving it a moon? Would that churn the insides of the original planet without demolishing the surface? I imagine it would take time, like geologic time? Say one were to put Ceres into orbit around Mars?

BarII
09-10-2015, 06:19 PM
...nudging the smaller one into orbit and giving it a moon? Would that churn the insides of the original planet without demolishing the surface?

Or maybe an impact or explosion large enough to create a ring, which wouldn't be perfectly uniform? An impact may also create a moon.

-rba-
09-10-2015, 07:37 PM
A moon by itself probably wouldn't be enough. It's true that tidal forces can heat the interior of a planet, but if I'm remembering correctly, most of the heat is still deposited in the outer layers (where the deformation is the greatest). And you're right, it would be slow.

You might be thinking of tidal heating because it is so effective on Jupiter's moons Io and Europa. Tidal heating on Jupiter's moons is powerful because Jupiter is so huge, and because the moons are in a resonance, so that they never settle down into perfectly circular orbits. By keeping each other in elliptical orbits, the moons end up with more heating than you would get with a single moon around a small planet.

-rba-
09-10-2015, 07:40 PM
A lopsided ring wouldn't be as effective at heating as a moon: you want as much tidal pull in a single direction as possible to cause maximum deformation of the planet. A ring would cancel out its own gravity (there's a famous problem that physics students are made to do that allows you to show that a uniform ring or sphere of mass has the same (lack of) gravitational force everywhere inside it because things cancel out).

You're right, a giant impact could certainly create a moon. That's the leading theory for how Earth got its unusually large moon.

GeorgeK
09-10-2015, 08:01 PM
Does it also require a metallic core?

BarII
09-10-2015, 08:14 PM
Assuming one could move a large planetary body...

You could also redirect an asteroid to hit the planet, assuming you find one on the right path.

Dennis E. Taylor
09-10-2015, 08:33 PM
A lopsided ring wouldn't be as effective at heating as a moon: you want as much tidal pull in a single direction as possible to cause maximum deformation of the planet. A ring would cancel out its own gravity (there's a famous problem that physics students are made to do that allows you to show that a uniform ring or sphere of mass has the same (lack of) gravitational force everywhere inside it because things cancel out).


Which also works for electrical charges. An electron floating inside a positively charged sphere would feel no particular attraction in any direction. I did that calc in electronics class.

Dennis E. Taylor
09-10-2015, 08:35 PM
The other thing you could do is dig a deep hole, then dump all your nuclear waste in there. Hopefully it would melt its way down to the core and help heat it. Supposedly it's the presence of radioactives in the Earth's core that's helping to keep it hot. 'Course, you'd need a LOT of waste.

-rba-
09-10-2015, 11:12 PM
Aside from the fact that you would need TONS of radioactive material, the trick is getting the radioactive material down to the core. Most radioactive elements prefer to bond with rocks, and so would just incorporate themselves into the crust near where you dumped them, rather than sinking to the core. Also, it's worth mentioning that most of the interior of the earth is solid rock, not liquid, so it's not as if you can just drop your plutonium and it'll sink like a lead weight in a pond.

Dennis E. Taylor
09-13-2015, 08:18 PM
On another subject, rba, how are you with planetary mechanics? Specifically stuff relating to orbital periods and tidal locking?

-rba-
09-14-2015, 04:42 AM
Does it also require a metallic core?

I'm losing track of which question you're referring to here. Tidal heating doesn't require a metallic core: it's just the result of flexing the solid body of the planet (much like how a paper clip heats up if you bend it). A liquid, convecting metallic core is required to sustain a strong planetary magnetic field.

-rba-
09-14-2015, 04:45 AM
On another subject, rba, how are you with planetary mechanics? Specifically stuff relating to orbital periods and tidal locking?

It's not something I do often but I took some classes on it and it's easy enough to look up the equations and remember how they work (at least for simple questions). Orbital periods are (usually) easy, tidal locking is a lot more tricky because it depends on how the planets involved respond to tidal deformation. But since we're talking fiction here, we can make some assumptions and come up with answers that are at least plausible, which in most cases is all you need.

Dennis E. Taylor
09-14-2015, 05:58 AM
Okay, picture a planet that's in a close orbit around a small K star. It's juuuuuuuuust about to go into tidal lock. Each rotation of the planet slows as the heavy end goes around the back side, then speeds up as it swings around towards the star. Just like a lopsided wheel.

Now, finally, the day comes when the last rotation just doesn't quite make it. The rotation stops, then slowly starts to reverse. The planet rotates around 355 degrees or so, and comes to a stop again. Basically, now you have a pendulum. I've found a formula for calculating the period of a pendulum, but I don't think it would be based on the mass of the entire planet. I'm trying to visualize it, but a non-spherical non-homogenous planet would effectively be a bar shape for purposes of calculating periods. The length of the bar could be calculated by determining where the center of mass is compared to the center of geometry, but what would be the effective mass?

Sorry, meandering aside, I'm wondering what kinds of periods you could expect for this kind of cycle? Weeks? Months? Obviously it could be arbitrarily slow, but how fast at the upper end? :chores

-rba-
09-15-2015, 07:20 PM
Okay, picture a planet that's in a close orbit around a small K star. It's juuuuuuuuust about to go into tidal lock. Each rotation of the planet slows as the heavy end goes around the back side, then speeds up as it swings around towards the star. Just like a lopsided wheel.

Now, finally, the day comes when the last rotation just doesn't quite make it. The rotation stops, then slowly starts to reverse. The planet rotates around 355 degrees or so, and comes to a stop again. Basically, now you have a pendulum. I've found a formula for calculating the period of a pendulum, but I don't think it would be based on the mass of the entire planet. I'm trying to visualize it, but a non-spherical non-homogenous planet would effectively be a bar shape for purposes of calculating periods. The length of the bar could be calculated by determining where the center of mass is compared to the center of geometry, but what would be the effective mass?

Sorry, meandering aside, I'm wondering what kinds of periods you could expect for this kind of cycle? Weeks? Months? Obviously it could be arbitrarily slow, but how fast at the upper end? :chores

Oof, you really want me to earn my keep here, don't you? I think you're on the right track, thinking about pendulums. Basically, the first thing to realize for this question is that it becomes much simpler to think about if you use a coordinate frame that is co-rotating with the moon. So basically your planet is stationary and the moon is also stationary but offset from the origin. In this frame of reference, as you say, the moon is essentially a physical pendulum, given the assumption that the force of gravity from the planet is uniform across the moon. We know this is not true (there needs to be a gradient in the force to cause the tides that led to the tidal locking in the first place) but I suspect it doesn't make a huge difference, and we're writing fiction here so it's probably close enough. The other assumption that we are making is that the rotation of the planet does not have any effect, which we also know is not true. The planet's rotation and tidal bulge exert a torque on the moon, which is why it is going into tidal lock in the first place. But if we roll with that assumption too, we can proceed. From here, the trick is: how to you calculate the period of a pendulum that is (a) an extended physical body and not just a mass on a string, and is (b) swinging through a very large angle. Most places you look assume you have a point mass on a string, swinging through a small angle, which simplifies the equation considerably.

First, the problem of the extended mass. Here the concept of "moment of inertia" comes into play. The moment of inertia is like mass, but for rotational motion. If you know calculus, you can calculate it for any arbitrary shape. If you don't, then you can look it up for simple geometric shapes. I think if you want to approximate a moon that is a bit lopsided, you could add together the moment of inertia for a uniform sphere and the moment of inertia for a point mass offset from the origin.

Anyway, once you figure out your moment of inertia, you calculate the equivalent length of your pendulum: L=I/mR

Then you can use that equivalent length in the full equation for the period of a pendulum: https://upload.wikimedia.org/math/6/2/f/62f55d40065dea4bdcaccc5d3bff03d7.png
See the Wikipedia entry on pendulums (https://en.wikipedia.org/wiki/Pendulum), specifically the "period of oscillation" and "compound pendulum" sections.

Bottom line: it's not a trivial thing to calculate. There are probably papers published going into all the gory details, but treating the moon as a physical pendulum will probably get you close.

GeorgeK
09-16-2015, 12:21 AM
I'm losing track of which question you're referring to here. Tidal heating doesn't require a metallic core: it's just the result of flexing the solid body of the planet (much like how a paper clip heats up if you bend it). A liquid, convecting metallic core is required to sustain a strong planetary magnetic field.I was referring to the magnetosphere