Jupiter's moons light up aurora borealis

By Phil Plait | September 17, 2009 7:01 am

One of the most beautiful sights in the sky (at least, so I’ve heard, since I’ve never ^%$#&*# seen one) is an aurora. The Earth has a magnetic field that traps charged particles from the Sun, and due to complicated processes that are still being investigated these particles can slam into our air, causing it to glow (exactly) like a neon sign.

But we’re not the only planet with a magnetic field. And some moons have them, too. Check this image out:

HST ACS image of Jupiter and aurorae

That’s the north pole of Jupiter as seen by Hubble Advanced Camera for Surveys back in 2007 (the vertical black line is part of the detector that blocks the view). It’s an ultraviolet image, since the atmosphere glows brightest in the UV. The big oval is the main aurora, but see the dot with the curved tail to the right? That is due to Jupiter’s mighty magnetic field interacting with the moon Io, a volcanic and violent satellite about the same size as our own Moon. The interaction is truly terrifying: over a million amps of current flows between Jupiter and Io as the magnetic field of the giant planet sweeps past the moon. To give you an idea of how much electricity that is, a typical car battery might crank at about 100-500 amps, so imagine 10,000 cars all lined up, powered only by the current flowing between the planet and its moon…

Io doesn’t have a magnetic field on its own, but the sulfur ions spewed out by Io’s volcanoes get swept up by Jupiter’s magnetic field, linking the two objects. So Io is connected to Jupiter magnetically, and a vast current flows from the moon down to Jupiter’s poles. Where the current hits, the atmosphere glows, and you see that bright dot. As Jupiter spins, that footprint marches across the planet, fading with time, leaving the curved tail of emission.

Ganymede is another moon of Jupiter, and as befits the massive planet, Ganymede is the largest moon in the solar system, bigger than Mercury! It too interacts with Jupiter’s magnetic field, and it too leaves a footprint on the jovian atmosphere. That can be seen on the left of the picture above. Ganymede has a strong enough magnetic field on its own that it has what amounts to a protective bubble around it, much like the Earth’s magnetic field protects it from the Sun’s solar wind. As far as we know, Ganymede is the only moon to have such a bubble.

And now, for the first time, astronomers have been able to measure the size of Ganymede’s auroral footprint and see that it fits the expected size from what’s known of Ganymede’s magnetic field. They’ve also seen that it varies in strength over 3 different time intervals: 100 seconds, 10-40 minutes, and 5 hours. This means there are variables, changes in the magnetic fields of the two bodies, which is modulating the amplitude of the interaction. Most likely the longest is tied to Jupiter’s rotation, which is over 9 hours. The other two may be due to local changes in the plasma environment of Ganymede, and to the way its magnetic field connects with Jupiter’s.

So what does all this mean? Well for one, it means Jupiter’s raw might and power scare the crap out of me. I’m glad it stays 600 million kilometers away! Second, it means that the Jupiter system is incredibly complex and well worth studying, to understand how magnetic fields in space interact and produce the kind of environment we see. A manned mission to Jupiter is unlikely any time in the next century, but the more we understand about such things, the more we can apply them to places we can go, like Earth orbit, and possibly Mars too.

Knowledge gleaned from scientific observations is never abstract; there are always applications for it. That’s because everything really is connected to everything else… whether it’s a physical connection blasting countless electrons into an alien atmosphere, or a connection we ourselves make by applying what we know to what we want to know. That’s science.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (45)

Links to this Post

  1. Go Go Rocket Party » Blog Archive » Sunday Science Sermon | September 20, 2009
  1. You really need to write a book about how dangerous and scary things can be in space.

  2. Once again, my morning dose of BadAstronony does not disappoint. I do have a question, though.

    If Ganymede is so big, and it is bigger than Pluto, then why is it still called a “moon”? Should it not be re-named into something like “co-planet”?

  3. Well, now you know why HAL flaked out.

  4. Brian T.

    FreeSpeaker:

    If I remember right, one of the key definitions of a planet (whatever was decided on when Pluto was re-classified as a dwarf planet) is that it must primarily orbit the Sun. Ganymede is firmly in Jupiter’s orbit, ergo, it is not a planet.

  5. Taf

    Auroras (aurorae?) are just super cool. But using Comic Sans for captions, not so cool ;p

  6. over a million amps of current flows … To give you an idea of how much electricity that is, a typical car battery might crank at about 100-500 amps, so imagine 10,000 cars all lined up, powered only by the current flowing between the planet and its moon…

    Um. There’s a difference between power and current. P = VI as I recall: you need to know the potential difference that the current flows across to know what the power is. What we need is the wattage, not the amperes, to know how many car headlights etc would be kept alight (the usual comparison is with the power requirements of a city you’ve never heard of, of course).

    The only dimly relevant thing I can remember is that this phenomenon was considered as an explanation for Io’s volcanoes and rejected as being about two orders of magnitude too small. FWIW.

    I bite my tongue over the mentions of the Object Formerly Known as a Planet.

  7. Kevin

    I am still reeling at the knowledge that Phil hasn’t seen the aurora in person. I am shocked.

  8. Joe Alvord

    I am truly sorry you have never seen the aurora. Up here in Alaska I see them occasionally and it never disappoints. It’s etherial beauty never grows old. You should really find some excuse to go to Fairbanks some winter. There’s a place near there where you can sit out in hot springs and watch the sky.

    Now, if I just get an extension cord from Io to my Chevy Volt by next year, I’ll be all set.

  9. wildride

    So how to we determine north for non-Earth planets? Direction of rotation? Orientation with respect to its star?

  10. rob

    haven’t seen an aurora? what? that’s not right. i am reeling too.

  11. Sili

    Cue the EU-proponentsists in 10, 9, 8 …

    Presumably North is defined as the side of the plane of Solar System that our Northpole is on. Or does Venus’ Northpole point down?

  12. Looking at arura is simply amazing. One night while flying up north, we also had our NVGs with us. While it made them all green, the image was simply amazing both in extent and of course how alive it all seemed.

  13. John Baxter

    Phil, you MUST arrange to see the aurora. Or better, more than one. Spend a winter well to the North. Take a jacket (or an electric warming tatoo).

    (Southern Michigan worked for me–so did places farther north. Not sure I should have been at the S tip of James Bay in early December, though.)

  14. There must be some way of harvesting the power gradient between our magnetic field and the ionosphere. If it can power the aurora borealis, it can power other stuff, too. Was’t Tesla experimenting with that at Wardenclyffe Tower?

    Update: in researching my own question on wikipedia, it turns out it’s already been done. In 1859 two telegraph operators were able to hold a two-hour-long conversation between Boston and Portland powered entirely by the aurora borealis.

    Wicked awesome.

  15. The Other Ian

    You don’t need to go all the way to Alaska for the aurora. Southern Canada will do.

  16. Ganymede: Aurora Borealis? At this time of year? A this time of day? In this part of the country? Localized entirely within your kitchen?
    Jupiter: Yes.
    Ganymede: Can I see it?
    Jupiter: No.

  17. Ubermoogle

    Wait… you’ve NEVER seen an aurora, Phil?

    Next time you’re up in Canada you’ll have to make a special trip. What’s even better is the twitter account, @Aurora_Alerts, will tell you when the Aurora are active. Hopefully there’ll be some activity while you’re here!

  18. mariana

    You missed the great auroras of, what was it, 1988? 89? Astronomy magazine had pictures of Mexican desert lit up by the northern lights. There were several fantastic displays.

    If you don’t want to do a northern winter trip then right now is a good time to head north. We watched them from central Yukon in early to mid-September in 2005. We were at the main hotel in Beaver Creek and at night we all climbed up onto the theatre roof with the staff and oohed and aaahed over the displays. Our basic digital cameras were able to capture the lights. With the sun being so quiet lately though you might want to wait till it gets feisty again, just to be sure you do see them.

  19. Dirk

    Argh! Comic Sans!

  20. andy

    As a (sometimes) space artist, I wonder what colour the Jovian aurorae would be in visible light.

    As for Ganymede being larger than Mercury, well yes, but Ganymede is mainly made of ice, while Mercury is an iron-rich, rocky planet. Ganymede’s mass is only about 45% of Mercury’s. IIRC this makes it the lowest-mass object in our solar system that is confirmed to have an internal geomagnetic dynamo. (Some of the smaller moons have induced magnetic fields as a result of moving through the magnetospheres of their parent planets, but not intrinsic geomagnetism).

  21. They aren’t easy to see. I’ve been up north a few times and I’ve never been able to catch one. It takes luck.

  22. Torbjörn Larsson, OM

    Electrifying!

    But Phil, come on, you have seen auroras, such as in that picture. Of course, “close up” and real life is the way to go here, trust me.

    a connection we ourselves make by applying what we know to what we want to know.

    Nothing like a bit of Phil-osophy and -oetry to start off a blog!

  23. Torbjörn Larsson, OM

    @ Vagueofgodalming

    I bite my tongue over the mentions of the Object Formerly Known as a Planet.

    You mean like the orbit neighborhood clearing requirement disqualifying moons just as much as the orbit Sun requirement?

  24. IBY

    Wow, how far away is Io and Ganymede from Jupiter? Because if the distance is large, that is some really impressive giant gas magnet there.

  25. Ken Cameron

    The Galilean satellites all orbit in a plane essentially coincident with the equator of Jupiter. These footprints are in very high latitudes. Is the magnetic field between the planet and its moons spiraling upward towards the pole?

  26. David

    Currents flowing in space? I thought that was impossible? ;)

  27. Wayne

    You’ve got to get yourself to somewhere you can see the aurora. I recommend Kiruna, Sweden. As a bonus, there’s also the Swedish Institute for Space Physics (IRF) and Esrange where you can see balloon and sounding rocket launches. Overall, a very spacey place. :-)

    And, no, I’m not from there, but I spent some good times there as a grad student.

  28. Robert L

    Hey Phil,

    Weren’t you in Nth CA in the late 1990s? We saw plenty of aurora (red and green blotch types). Even visible from downtown Santa Rosa!

    I drove out the Petaluma-Pt Reyes highway, and parked to watch an amazing display.

    Of course that was when the Sun was a lot more active. We might have to wait a while to see the like again, what with today’s spotless sun.

  29. EK0r031

    Aurora is beautiful and you should once make a trip to a region where you can see it. In about 2003 or something there was a lot of solar activity and we got to see aurora in the Netherlands. Northern light (noorderlicht). We have a saying ‘met de noorderzon vertrokken’ which means ‘went with the northern sun’, which means ‘gone with unknown destination’. Noorderzon is the name of a ship.

    I never understood this until we had this great view. One night at about 11:30 there was a dawn where it wasn’t supposed to be and it was at the wrong time. There it was: the northern sun. We didn’t see dancing structures since it was quite cloudy and probably too far away, but the view was breathtaking. It was a horizon wide quite bright dawn light with the colors you see when the first sunlight in the sky is visible. It was not like nocturnal clouds that I happen to see for the first time in my life this summer, which is also something you must see at least once.

  30. Plutonium being from Pluto

    @ 13. John Baxter Says:

    Phil, you MUST arrange to see the aurora. Or better, more than one. Spend a winter well to the North. Take a jacket (or an electric warming tatoo). M

    Don’t forget we get them well to the *SOUTH* too. ;-)

    You could also visit Tasmania, Teirra del Fuego, New Zealand / Aoteroa or Antartica.

    Any of those would be quite a trip for you BA with a good chance of seeing an Aurora Australis along the way. Well at least at solar maximum anyhow.

    @ 2. FreeSpeaker Says:
    September 17th, 2009 at 7:40 am
    I do have a question, though. If Ganymede is so big, and it is bigger than Pluto, then why is it still called a “moon”? Should it not be re-named into something like “co-planet”?

    Well Ganymede does go around Jupiter as has been pointed out & is a fraction of Jove’s size.

    However, Pluto does have a lot more claims to planet-hood than Mercury, it boasts three moons (Charon, Hydra & Nix), an atmosphere with active weather (snowfall), probably active geology (cryovulcanism) and even possibly rings.

    So Pluto sure is a lot more “planet” like than Mercury & if the latter is a planet it sure beats me why the former really shouldn’t be!

    Actually, the only reason it isn’t considered a planet is that the IAU made a politically biased, scientifically dubious, highly controversial anti-Pluto decision a few years away where the lead experts like Alan Stern were excluded & only a tiny minority of an already small minority given any real say. It was a disgraceful decison that undermined the IAU’s authority and brought astronomers into disrepute amng the public. I hope sanity soon prevails and that bad mistake gets corrected with Pluto being restored to its proper planetary status.

    The whole “orbital clearing” thing is a nonsense which is ignored for exoplanets which are excluded anyway – it really doesn’t make sense and shpuld be dropped as adefining criterion.

  31. Plutonium being from Pluto

    @6. Vagueofgodalming Says:

    I bite my tongue over the mentions of the Object Formerly Known as a Planet.

    Why? Are you embarrassed by the shame of the IAU’s general crapulence? ;-)

    Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto!
    Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto!
    Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto! Pluto!

    (… & “Vagueofgodalming” & the other IAU henchmen cover their ears & run away screaming!) ;-)

  32. 30. Plutonium being from Pluto Says:
    September 17th, 2009 at 11:18 pm

    The whole “orbital clearing” thing is a nonsense which is ignored for exoplanets which are excluded anyway – it really doesn’t make sense and shpuld be dropped as adefining criterion.
    _____________

    It is a bit strange, now that I think about it. By that definition, the Earth is no longer a planet, since we have objects like 3753 Cruithne crossing our orbit. And Jupiter and Saturn have plenty of Greeks and Trojans at their Lagrangian points. Or do those not count?

    Still, I gotta side with the IAU otherwise – if Pluto’s a planet, so are eight other Kuiper Belt objects. I don’t see why Pluto deserves special status.

  33. pk1154

    Oh my lack of god!!

    I’ve seen auroras and the Bad Astronomer hasn’t?

    WOW. (And I guess I’m not the only one who has had that reaction.)

  34. Helena

    How can you say we probably won’t have a manned mission to Jupiter in this century when we already had a manned mission to Saturn seven years ago?

  35. FC

    I haven’t seen the official definition about “orbital clearing” but I think it’s supposed to be that the object has to be dominant in its orbit concerning mass and gravity. It’s fuzzy but I mean really if just about anything up to a pebble can be a planet (KBOs!!!!) then the word “planet” loses its meaning. I’d rather sacrifice Pluto to KBO hell and have 8 nice major planets than have 15 or 20 “planets” ranging from pebbles to monsters, doesn’t feel right.

  36. wildride

    re: “If Pluto is a planet, so are those KBOs”

    Of course they are. Indeed, that’s why the definition fails. It’s trying to craft a definition to fit a set of data rather than crafting the definition and seeing what data fits it. It’s not a question of Pluto getting special status from other large KBOs it’s why is the number of large KBOs relevent to whether they are planets or not?

  37. Just quickly read this interesting post but noticed slight error in units.

    Power is voltage times current – always. To distinguish both – current is the flow – voltage is the push, or difference. So, in this instance to make it sound funny, we need to know the push to work out the power!

    Claire

  38. Robert

    Whoa!!!

    A car battery can deliver 100-500 amps? Are you sure about that, that seems insanely high to me.

    I suspect that is the current delivered for the spark which ignites the gasoline, in which case the comparison is flawed in my opinion. This current in a car is not running continuously, the current between Jupiter and Io is.

  39. palak

    auroras r really amazzziiin…

  40. Techskeptic

    Yup a car battery can crank out 100-500 amps. But Phil a million amps sounds like a scary number, but it really isn’t. We have I diatribes like electroplating and electrolysis that se something like that amperage in bus bars inside the facility. What makes it even less scary is the cross sectional area of the “conductor” it the size of the sillouette of the moon. I’m sure that makes the current millimps per square inch. Would you even notice anything if you were standing on the moon itself?

  41. Ross Marsden

    Why is Pluto the subject of so many comments? Pluto was not mentioned in the article – Mercury was. ” … Ganymede is the largest moon in the solar system, bigger than Mercury!”

  42. Ross Marsden

    The original article is dated September 17th, 2009 7:01 AM, so is this fork in the comments so important now? Probably not.

  43. Andres Minas

    @ Ross

    BA was meant for continuing education/updates; the reason the comment section was kept open in this article.

    I can say Techsceptic’s current comment is rather enlightening.

  44. Andres Minas

    Oops! Sorry Techskeptic for these lazy fingers.

    Back to Pluto (with leave to Ross), has anyone computed that a couple or a few more billion years from now Pluto would impact one of the eight planets? If so, that will settle the controversy. However, it may turn out that it never will and, on the contrary, Pluto’s orbit may in the future settle to the same orbit as the eight planets.

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »