Saturn weather forecast: rings, with light rain from Enceladus

By Phil Plait | July 26, 2011 1:59 pm

Like any scientist, I love a good mystery. Sometimes it’s fun when they are long, complicated, involve subtle and difficult layers, and require a vast effort to unravel.

And sometimes it’s cool when they are simply stated and simply solved. Like asking "Where does the water in Saturn’s upper atmosphere come from?" and finding out the answer is "It rains down from the moon Enceladus."

cassini_enceladus_nov091

Water has been seen deep in Saturn’s atmosphere before, but a few years back it was detected in the upper atmosphere as well, and that’s a bit weird; there don’t appear to be any ways to get it from deep down in Saturn to the top parts of its clouds. So how did it get there?

Well, the tiny, icy moon Enceladus was discovered to have geysers at its south pole, actively spewing out quite a bit of water into space. Most of it goes into space and is gone forever. Some actually forms a ring around Saturn called the E-ring, and some no doubt hits other moons. Generally, when a moon blasts stuff into space (like Jupiter’s moon Io does with its sulfur volcanoes) the material forms a big donut-shaped region around the planet. It was figured that Enceladus was doing the same thing with water around Saturn, but even the Cassini spacecraft, which is right there, couldn’t detect it. It’s pretty hard to sample.

But astronomers used Herschel, an Earth-orbiting infrared observatory, to observe Saturn. They found a peculiar feature in the infrared spectrum of Saturn, and realized it’s from this Enceladusian water torus. Apparently, about 3-5% of the water from Enceladus’s geysers falls on Saturn, literally raining down in sufficient quantities to explain the presence of the water detected in the ringed planet’s upper atmosphere.

It’s pretty rare to be able to say this confidently in science, but: case closed.

Mind you, I’m not an expert on Saturn, but I read the scientific paper published (PDF) and it looks pretty good. They find that about 2.5 x 1026 molecules of water fall onto Saturn every second, which sounds like a lot until you remember a molecule is pretty small. Reaching back into my dim memories of high school chemistry, I get that this equals about 7.5 kilograms (roughly 17 pounds) of water rain falling onto the planet every second*. That’s not a whole lot! Saturn’s a big planet, and that translates to about 600,000 molecules falling per square centimeter every second.

In a heavy rainstorm on Earth, you get about 1 inch of water an hour falling down. That’s .0007 grams/second falling on a square centimeter of ground, so the rain on Earth is roughly 42 trillion times heavier than on Saturn. I don’t think you’ll need an umbrella if you visit.

Of course, whenever you solve a mystery, another opens up. It’s known that Saturn’s giant moon Titan has water in its upper atmosphere as well. According to the astronomers who made these observations, their models show that the rain from Enceladus onto Titan is not enough to account for what’s seen there; they’d need 5 – 20 times as much water to explain Titan. So clearly there must be some other source of water for that, and whatever it is it doesn’t reach Saturn or else more water would be seen there as well.

What could it be? No one knows.

Did I say, "Case closed"? Yeah, honestly, you can never say that in science. That’s why it’s fun!

Images credit: NASA/JPL/Space Science Institute


* There are 6.02 x 1023 molecules in a mole (from Avagadro’s number), and water has a mass of 18 grams per mole. The rest is just multiplication and division. I’ll note in the original version of this post I used the wrong number for Avagadro’s number, so I simply corrected the numbers above.


Related posts:

- Enceladus sprays anew
- Enceladus on full afterburner
- Enceladus is erupting!
- Season’s E-rings

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (25)

  1. MoleRat

    The footnote at the bottom of the post is incorrect. The number of molecules per mole is 6.022 * 10^23, not 1.02 * 10^23 as reported. Similarly, it should be 7.5 kg/s not 45 kg/s.

  2. Anon

    The water on Titan could be created by microbes digesting the methane seas… Or is that just wishful thinking?

  3. Justin Browne

    Your calculation is off by a bit. There are 6.02*10^23 molecules per mole.
    http://en.wikipedia.org/wiki/Avogadro_constant
    That means about 7.5 kg of water rain down on Saturn per second, if I’m correct.

  4. VinceRN
  5. Sometimes it’s fun when they are long, complicated, involve subtle and difficult layers, and require a vast effort to unravel.

    Unless you watch Fox News… Then it needs to be black and white, and must be explianed in 15 seconds using monosyllabic words…

    On a serious note, I wonder what sort of chemistry would be required to make that Titanian water. Who hoo! A mystery!

  6. zadoc

    Saturn has an amazing system, and I really think that Titan and Enceladus warrant extensive study, with more probes, and perhaps landing devices.

    POLL: What do you think the most interesting place to study is in our solar system?
    http://www.wepolls.com/p/1585898

  7. Nick (Matzke)

    Hi Phil! Great post! Can I ask a dumb question as a non-astronomer? Is there any chance that water from Enceladus has contributed any substantial mass to Saturn’s rings? (other than the E-ring, I mean) Might this explain some of the evidence that they look bright and young?

    Cheers!
    Nick

  8. Jon

    This is nice, since (last I checked) Saturn’s ionosphere “doesn’t work”–electron density isn’t as high as expected–and water quenching was one explanation. (One paper: http://adsabs.harvard.edu/abs/2007GeoRL..3412202M ..Luke Moore has done a lot on this.)

  9. Walter Sobchak

    “Did I say, “Case closed”? Yeah, honestly, you can never say that in science. That’s why it’s fun!”

    That statement is not true either. The Earth’s climate is so well understood and modeled for the next century, that it is case closed. Just ask Ms. LeQuella, she will tell you.

  10. Messier Tidy Upper

    It’s known that Saturn’s giant moon Titan has water in its upper atmosphere as well. According to the astronomers who made these observations, their models show that the rain from Enceladus onto Titan is not enough to account for what’s seen there; they’d need 5 – 20 times as much water to explain Titan. So clearly there must be some other source of water for that, and whatever it is it doesn’t reach Saturn or else more water would be seen there as well. What could it be? No one knows.

    Cyrovulcanism?

    Titan is fairly ice rich isn’t it? If the mantle of Titan has plumes via an active core – like Earth’s but perhaps caused by gravitational tides rather than radioactive element decay then maybe we have Titanian volcanoes erupting water vapour and water “ash” and that might explain that? Or would it?

    Are any Titanian volcanoes known or suspected and have calculations or models been made for this possibility?

    Great write up of fascinating science here too btw. :-)

  11. Messier Tidy Upper

    @2. Anon :

    The water on Titan could be created by microbes digesting the methane seas… Or is that just wishful thinking?

    Well that’s a nice idea but if so how would that water – which you’d expect to freeze instantly into the ice phase rise into Titan’s upper atmosphere? They’d have to be excreting it as steam! ;-)

  12. CR

    @11. Walter Sobchak:
    “Ms. LeQuella” and “she.” Um, you might want to follow Larian’s link a little further…
    …………..
    As for this entry, I am more and more finding the Saturn system to be my favorite within our solar system. Jupiter’s system is also cool, and I’ve always had a spot for Neptune and its big moon Triton, but the more we learn about Saturn & its moons, the more intrigued I get. Man, I wish I could live to be 300 just so that I can know what else future generations shall learn about this (and other) places in our own backyard!

  13. DrFlimmer

    @ #14 CR

    Man, I wish I could live to be 300 just so that I can know what else future generations shall learn about this (and other) places in our own backyard!

    And you would know the dogopus. :D

  14. Case closed? It read in your article I think it’s a case abeirto more than ever, is not it?

  15. Camerasinger

    Ah but water may also be heavier on Saturn. Published values of the force of gravity on Saturn are between 0.98 and 1.1 g.

  16. Stan9fromouterspace

    Huh huh huhhuhhuhhuh… he said “Enceladusian.”

  17. panini

    Love the twist about Titan at the end! IT’S LIFE!!!!!

  18. Peter Davey

    To quote from “A Midsummer’s Night Dream” : “Hot ice and wondrous strange snow.”

  19. JMW

    Hm. Is there a music group of astronomers called the “E-Ring Band”?

  20. Presumably this has been going on for billions if years, yet Enceladus hasn’t disappeared. So how does it replenish its supply of water?

  21. @ ^ Lugosi : It doesn’t. Enceladus just contains alot of it to start with. ;-)

    Would be my guess anyhow.

    BTW. Enceladus clip via AggManUK linked to my name for this comment.

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