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

[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.

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: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.