Friday, July 29, 2005

UVIS Directly Measures Enceladus' atmosphere


After not sensing the atmosphere during an occultation in February, UVIS finally observed the atmosphere first observed by the magnetometer instrument. The UVIS detection came during another stellar occultation, this time of gamma Orionis, or Bellatrix. This star turned out to be a much better one to use than the star used in February, Lambda Sco. During the occultation, UVIS measured the intensity of the star as it passed behind Enceladus and its atmosphere, then reappeared following the occultation. Measurements during ingress (or approaching occultation) indicated a UV absorber between Bellatrix and Cassini over Enceladus south polar region, indicating an atmosphere composed of water vapor. Measurements during egress (or following the occultation) over nightside equatorial regions of Enceladus did not show nearly as strong of a UV absorber. This measurements fits well with the MAG models indicating that the atmosphere is not uniform over the surface of Enceladus and is likely concentrated over the south pole. The lack of a detection during egress could also result from the atmosphere "freezing out" at night.

The presence of water vapor in Enceladus' atmosphere and the lack of atomic oxygen indicates that the atmosphere is produced from the venting or evaporation of water ice from the surface rather than from magnetospheric sputtering, as is the case for the rarefied atmospheres of Europa and Ganymede. Instead the atmosphere has more in common with Io.

9 Comments:

Blogger Mossbury said...

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7/29/2005 02:27:00 PM  
Blogger Mossbury said...

Jason, I am perplexed by the Cassini web page text re the temparature map. The last para, says that the e-ring and dust around Enceladus is due to micro-meteor impacts. How can they be so sure here. Surely the temporary atmosphere around the south pole would escape to form the e-ring.Am I missing something?

7/29/2005 02:32:00 PM  
Blogger Jason said...

I think we maybe looking at both. E-ring particles appear to be produced by meteorite impacts. The atmosphere is produced through outgassing, not sputtering or impacts (since atomic oxygen was not found). More on this in a bit.

7/29/2005 02:54:00 PM  
Blogger Mossbury said...

Is there any vibe why there is no ring associated with Mimas if Enceladus has one due to sputtering?

7/29/2005 03:18:00 PM  
Blogger Bruce Moomaw said...

Mossbury has asked the Million Dollar Question. I see no way that the E Ring could be due to particles blasted off Enceladus' surface by micrometeoroids without the other moons expelling E Ring material themselves -- it has simply got to be material expelled from the geysers (or vents, or whatever you choose to call them). I suspect it's initially expelled as water vapor, which then refreezes into microscopic ice particles after escaping from the moon.

7/30/2005 10:22:00 PM  
Blogger Mossbury said...

I seem to recollect reading that the e-ring consists mainly of micron size particles that were all the same size and spherical, as if condensed from a cooling gas. Perhaps we are talking about 2 different things here. The spherical dust cloud around Enceladus is due to sputtering but the e-ring is mostly the `vented` atmosphere escaping. Steve Taylor London

7/31/2005 03:47:00 AM  
Blogger Jason said...

CDA measured 2 populations of particles, E-ring ring sized particles and impact-generated dust right around Enceladus. They find that neither population has a localized source, like the atmosphere. the source is uniform across the surface. Thus the second population is interpreted as being due to micrometeorite impacts. Given the similarity in the two particle populations, except size, their interpretation of their data is that the E-ring is produced by the impact-generated dust, not volcanic activity.

Personally, I don't really care anymore. Enceladus scientists focused on the E-ring as indirect evidence for acitivity before this flyby. Now we have evidence from multiple instruments demonstrating that it is active, so we don't need the E-ring anymore. Plus, it does help us get around how Tethys can be a secondary source. It can just be impact-generated dust and not activity on Tethys.

Though there is that pesky issue of Nitrogen...

7/31/2005 11:51:00 AM  
Blogger Bruce Moomaw said...

The central problem is this: why aren't the other moons also central sources of the E Ring if it's made of impact-detached particles? (And why didn't Galileo see the same thing around Jupiter's moons -- where it detected modest numbers of impact-detached particles around all the moons, but no more than that?)

As for the apparently uniformity with which the E Ring particles emitted from Enceladus are spread around its vicinity: could we be looking at liquid water/ammonia mixture that vaporizes in the vacuum of space and quicly spread evenly around Enceladus, with the water vapor then refreezing separately as tiny ice particles? (This would also greatly accelerate the rate at which the now-separate ammonia -- still vaporized -- was broken down into N2 and H2 by solar UV, explaining Cassini's failure to detect NH3 vapor near Enceladus. And it would nicely explain why ALL of Enceladus' surface, including its inactive portions, are covered with that very high-albedo water snow.)

7/31/2005 04:52:00 PM  
Blogger Bruce Moomaw said...

Cryptic comment from that new Planetary Society article you mention above:

"A different in-situ instrument, the Cosmic Dust Analyzer (CDA), had measured the composition of Saturn's E Ring particles in previous trips through the ring plane. The water vapor in the atmosphere detected by UVIS and INMS does not match this composition, Spilker said. "The water vapor is very different from the E ring particles themselves. So we have this sort of cloud, patchy atmosphere over the south pole, and then the E ring particles seem to be coming uniformly off of Enceladus, probably through micrometeorite impact kicking up particles. So the vents are not the source of the E ring."

No decription yet, though, of HOW th composition of the E Ring particles as measured by the CDA differs from the water vapor cloud around Enceladus.

Meanwhile, the only data we get on this from the abstract for the upcoming DPS paper on the CDA analyses of the E Ring ( http://www.aas.org/publications/baas/v37n3/dps2005/576.htm ) is as follows:

"The Cosmic Dust Analyzer (CDA) onboard the Cassini mission measures the properties of micron sized dust particles in the environment of Saturn. Since its arrival at Saturn in July 2004, the CDA detector recorded many thousand dust impacts within the E ring of Saturn. Data analysis revealed enhanced dust densities until a radial distance of at least 16 Saturn radii. The dust densities and the mass distribution of the ring particles is investigated. Furthermore, compositional measurements indicate a clear dominance of water ice particles in the size range between 0.5 and 2 micro cmeter. The discovery of the extended E ring changes the former understanding and modeling based on remote sensing observations."

7/31/2005 05:04:00 PM  

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