Saturn's Moon Enceladus Has an Atmosphere

Dimentox writes "The Mercury News reports that the international Cassini spacecraft has discovered that Saturn's moon Enceladus has a significant atmosphere, NASA said Wednesday. The icy moon's atmosphere may be created by volcanism, geysers or gases escaping from the surface or the interior, the space agency's Jet Propulsion Laboratory said. Excluding Saturn's giant moon Titan, which was already known to have an atmosphere, it's the first discovery of an atmosphere on one of the more than 30 moons that orbit the ringed planet."

Comments

Re:Well, for one thing

[jbrader]

Just a guess based on what I know about Titan, but the atmosphere is probably composed of methane and other nasty hydrocarbons.

As to how thick it is, since it wasn't detected until we got "close" to it, it's probably quite a bit thinner than Titan's atmosphere.

Again,even though this is the field I'm in school for right now, I'm pulling all this out of my butt. So usue the usual grain of salt.

significant?

[Anonymous Coward]

*Any* atmosphere on a sub-500km wide moon would be significant. I'm looking forward to a more complete report as more information comes in.

Why am I suddenly hungry for Mexican food?

How unique is this?

[jerkychew]

Please bear with me, as I never paid any attention to astronomy-related stuff. I'm just curious as to how many other planets/ moons/ celestial bodies out there are known to have an atmosphere? Is this a really big deal, or is it a relatively common occurance?

Re:How unique is this?

[jd]

The Gas planets have atmospheres. Well, if they didn't, they'd be a lot smaller.

Europa has H2O in both ice and liquid forms, which is horribly unlikely at near-zero pressure, which means it almost definitely has an atmosphere.

Any comet can be considered as having an atmosphere, whilst it is close to the sun and being frazzled. However, I think there are some who consider that cheating.

Any object close to (or larger than) the size of Mars is going to have an atmosphere, provided at least one of the following conditions is met:

• After the planet formed there was a liquid core capable of generating and sustaining geological activity such as volcanos, geothermal vents, etc.
  
• Gasses or liquids start on the surface where the rate of loss into space is equal to or exceeded by the rate of replenishment. (Replenishment may include geological activity, meteorites containing suitable compounds, etc.)
  
• After the planet formed, it encountered an atmosphere (most likely from a comet that didn't quite hit) that it could then capture and retain.
  
• After the planet formed, it encountered the necessary compounds by actual impacts from cometary fragments, meteorites, etc.

The gas giants can form either from a cloud that coalesces as per a rock planet, but never actually becomes solid, OR when a very large rock planet sweeps enough lighter material to build an atmosphere around it. Jupiter is now thought to be of the first kind, Saturn of the latter.

There may be other ways an atmosphere can form, but these would seem to be a good start on a list.

How can an atmosphere NOT form on a planet?

• It started off with one, but
  • It escaped faster than it could be replenished (planetary spin too high, insufficient gravity and/or too high a temperature)
  • The source material or driving force used to replenish it ran out
  • It froze solid
  • It chemically reacted with something to become part of the surface
• It didn't start with one and
  • There are no processes to generate one internally
  • There is nothing on the surface suitable and at the right temperature
  • It has never encountered suitable frozen material in cometary fragments that could supply one
  • It has never encountered free-floating gas molecules in sufficient quantity AND that is sufficiently massive to be captured and stay on the planet AND isn't moving fast enough relative to the planet to escape.

I don't know what the odds are for any of these, but it would seem reasonable to suppose that 20-30% of all moons will have some sort of atmosphere, and maybe 60-80% of all planets do. We've not found many small extrasolar planets, so we can't tell from that. However, if you go by mechanisms, those percentages feel reasonable enough.

Re:Well, for one thing

[Rei]

The link says that it's made of water vapor, but doesn't give a density. It's interesting, because Europa (a seemingly similar moon - it has the same sort of wrinkled surface) has a *very* tenative (about 1/100,000,000,000th of 1 atmosphere) water vapor and oxygen atmosphere from the sublimation of ice and the breakdown of water from interaction with the solar wind.

I'd imagine that this atmosphere is notably more significant than Europa's, or they wouldn't have described it as they did. And, with less solar energy at these distances, they're speculating that the source is from internal heating causing water geysers. That's really rather fascinating, when you think of it - now we know of another moon with a likely subsurface sea. The moon is a lot smaller than Europa, but it probably has more significant internal heating for its size.

Plus, the saturnian system has a lot of interesting organics - Titan is virtually a drifting mobile organic chemistry lab in its upper atmosphere. Even neglecting Titan, there's the unknown dark organics on Iapetus, Phoebe, and in the rings, among other places.

Re:Nice discovery for the bad news

[Rei]

The liquids on Triton are *nitrogen* (and we have no clue how much there is; it's atmosphere very thin, though). I wouldn't rule out life on Titan, but Triton is even colder, and unlike Titan we have no evidence of any organic chemistry there. Of all of the places in the solar system, why did you pick Triton?

I mean, I can understand people arguing for life on Mars (it had past water, it's huge, lots of solar energy, etc). I can understand people arguing for life on Europa - it has an undersea, tidal heating as an energy source, etc. I can understand people arguing for life on Titan - it has extensive organic chemistry occurring in its upper atmosphere, has a known fluid (even if nonpolar) on its surface, seems to be a geologically active world (and thus has internal heat), etc. I could even understand speculation about life on Io, it being such an energetic world. But Triton? You might as well pick any body in the solar system. Heck, I'd give there better odds of having life in a gas giant than Triton. :P

Re:It would be nice to link to the actual article

[Surazal]

(I RTFA, but couldn't find any info about atmosphere composition... strange, if they detected it, you'd think they'd have a clue what it consisted of too)

The two articles I've read on this subject both indicate the atmosphere is water vapor.

Mercury's atmosphere

[zippthorne]

Is more like a few of those super-ball thingies bouncing around a really roomy tank. The particles don't even interract with each other: the most prevalent collision is by far the particles with the surface. In fact, there's some question about solar pressure 'blowing' away the atmosphere. (though it would be replenished by solar wind particles) On earth would be very difficult to get a vacuum of the quality of mercury's "atmosphere." calling it a trace is extremely generous.

It might.

[jd]

If, under the surface, you have a cavern, or network of caverns, of sufficient size, which are 100% isolated from the primary atmosphere and where the composition of the air is non-trivially different, it would be possible to argue the case for Enceladus having two atmospheres.

Alternatively, since gravity appears to be insufficient to hold the atmosphere in, if the northern hemisphere's atmosphere and southern hemisphere's atmosphere never interact (eg: there's nothing left of either by the time you reach the equator) then you could again argue that they should be considered distinct and not part of a single whole atmosphere.

Of course, these are highly improbable, but this IS Slashdot.

Re:It might.

[ta bu shi da yu]

Can someone mod this up as Interesting?

Re:Nice discovery for the bad news

[Rei]

Heh, it took me a while to understand it too, don't worry :) The basic concept is that yes, the spacecraft's velocity relative to the *planet* will remain the same. However, the spacecraft's velocity relative to other bodies in the solar system (including the sun) will not be.

Picture the following: draw the sun on a piece of paper. Straight up from there, draw a planet moving counterclockwise around the sun. Now, draw a probe heading toward a slingshot to the right of the planet - moving clockwise around the sun and outward, to pass right near the planet and get slung counterclockwise (draw this trajectory).

Now, consider the velocity components of the planet before and after the spacecraft passes. In-plane/out of plane movement will be unchanged, because the spacecraft and planet are both moving in-plane. Toward the sun/away from the sun movement is also unchanged, because the craft is having to approach and then leave.

But what about clockwise/counterclockwise motion? The spacecraft is approaching in a clockwise direction, but leaving in a counterclockwise direction with respect to motion about the sun. This means that the planet has reduced counterclockwise velocity with respect to the sun than before it encountered the spacecraft, and the spacecraft has gained that energy.

In short, the spacecraft leaves the planet with the same velocity that it approaches (with respect to the planet), but with respect to the sun it has gained energy and the planet has lost energy.