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This is an old revision of this page, as edited by Volcanopele (talk | contribs) at 23:23, 2 December 2005 (→Major changes coming). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

English pronunciation .

Atmosphere

An atmosphere exists around every larger body of the solar system. The substantial information which is missing here is the pressure / density of the atmosphere. 193.171.121.30 18:52, 18 Mar 2005 (UTC)

The press release announcing the discovery gave no information on the atmospheric properties, only that it is "substantial", whatever that means. --Jyril 20:13, Mar 18, 2005 (UTC)

Although the planetary society article says "a million million times less" which means a factor of 10, it's actually a factor of 10 if the number of 200 to 300 million atoms/molecules per cm stated in the same article is correct. I think one order of magnitude does matter. 194.166.218.217 01:55, 23 September 2005 (UTC)

False color image

I definitely prefer the old picture over the new false color image. I will revert, if nobody objects! Awolf002 13:15, 27 Apr 2005 (UTC)

Done. Awolf002 13:55, 28 Apr 2005 (UTC)

Yes, on Titan (moon) the concensus was that the main image of a body should look as realistic as possible. Specifically, false-colour images were considered inferior. In fact, apparently you were the one who reverted that one.  :-) --P3d0 17:52, Apr 28, 2005 (UTC)

I'm not at all sure that the current image has true colors whatever that means. Enceladus, with its extremely high albedo, is actually bright white or gray, not brown like in this image which looks like a colorized one. But I've to admit that the current image looks much better.--Jyril 19:45, Apr 28, 2005 (UTC)

Yes, there should be a better picture, since this is an old Voyager image, I believe. I will take a look at the JPL/Cassini image archive. Awolf002 19:58, 28 Apr 2005 (UTC)

I can only find raw images, close-up images of the surface, or false color images that are better than the Voyager image. All other visible light images from the whole moon have less resolution. However, those pictures show a white surface! Should we replace? Awolf002 20:14, 28 Apr 2005 (UTC)

Personally I would include both on the page. Most of the images I have added following my revisions earlier this month are high resolution images that don't show the whole disk and perhaps one of the full disk images from Cassini could be added to highlight the blue cliffs of Enceladus (to go along with the high resolution image I have for that purpose. Volcanopele 17:37, Jun 20, 2005 (UTC)

Mass and Density Figures

I have updated the mass and density numbers for Enceladus based on a presentation given by Zibi Turtle last month (not my only source of that information, but it is the only public mention of the change that I am aware of). The density figure is now 1.6 g/cm, compared to the previous value of 1.3. This update is based on the Cassini NAV team's assessment of the reconstructed trajectory of Cassini following two recent flybys. This makes Enceladus denser than the other Saturnian mid-sized icy satellites, suggesting that it is composed of more iron and silicates that those other moons. I'm planning on making a mention of this in the article itself. Volcanopele 22:06, 01 Jun 2005 (UTC)

There have been a few updates to the mass numbers lately by other members, mostly because I forgot to change the source on some more recent figures. I have once again put in the more recent numbers (1.08x10^20 kg; 1.61 g/cm^3) based on the GM numbers in the Cassini pck SPICE kernels. I have now linked to the source of those numbers, but the calculations are my own. If others want to check my work, go ahead, just make sure to read the documentation in the source document on the units used for GM (gravitational constant times the mass), so you do the right conversions. I also used a different radius for density, escape velocity, and surface gravitational acceleration from the March 9 flyby. Volcanopele July 6, 2005 19:54 (UTC)

Major changes coming

I am working on some major edits to the Enceladus article after having spending the last week working on Enceladus images. The edits will be confined to the "Physical Characteristics" section, plus a new section on the history of Enceladus exploration. The Physical Characteristics section will have the following outline:

• Interior
• Surface
  • General Geology
    • Voyager 2 results
    • New Insights from Cassini
  • Impact Craters
  • Tectonics
  • "Planitia"
  • Cryovolanism?
• Atmosphere

This would generally follow an inside out approach to discussing this satellite. The atmosphere section will largely remain intact as it is now, though I may de-emphasize the "current events" nature of the section.

Just wanted to give a heads up to the changes I am working on.

- Volcanopele 19:29, Jun 3, 2005 (UTC)

I have updated this article again, to attempt to fold in some of the results from the July encounter and from the plume images from November. As such, I have removed the atmosphere section. It is now becoming increasingly clear that the Magnetometer, UVIS, and INMS instruments didn't see an atmosphere, per se, but a plume emanating from the south pole, made of water vapor and dust. So, I have folded the information in the atmosphere section into the cryovolcanism section, where I feel it belongs.

Temperature

What is Enceladus' surface temperature? Googling seems to mostly repeat the tidbit mentioned here, that is to say 70 K. But that's way too hot for the albedo of 0.99! 70 K implies an albedo of 0.67-0.68; an albedo of 0.99 implies 29 K (but that value is very sensitive to the actual albedo at that point). Anyone have an actual measurement they can quote?

Urhixidur 2005 July 5 03:43 (UTC)

This says it was measured in February. What was the result?

Urhixidur 2005 July 5 03:45 (UTC)

I recently read a source with temperatures derived from Voyager IRIS data. I'll need to find it again and post the information here. In terms of Cassini results, I don't think the Cassini CIRS results from the flyby have been posted, and certainly not temperature data, AFAIK.

Also, don't forget that other factors contribute to surface temperature, including surface roughness, as well as albedo.

Volcanopele July 6, 2005 19:44 (UTC)

Okay, found it, from Cruikshank et al. (2005):

"The High albedo of Enceladus results in a colder surface than most of the other satellites, with a calculated subsolar temperature of 75 +/- 3 K, and an average temperature of ~51 K. Volcanopele July 6, 2005 23:07 (UTC)

I have found that the original source of the 75 +/- 3 K value comes from a paper by R. Hanel et al. (1982)., using infrared data from the IRIS instrument. This temperature results in a bolometric bond albedo of 0.89±0.02 and a phase integral of 0.89±0.09 (as opposed to the geometric albedo of 0.99). Volcanopele July 7, 2005 20:53 (UTC)

I've updated the page with the temperatures from the refererence above. I'll look into the albedo issue some more, particularly how that temperature was estimated, and see if I can find a better estimate, but that is the only mention of surface temperature I have found thus far in the literature. Volcanopele July 7, 2005 20:35 (UTC)

Using an emissivity of 0.9, a geometric albedo of 0.91 gives a mean T of 51 K; if we stick to a geometric albedo of 0.99, then the emissivity has to go down to about 0.1 to reconcile the temperature. Not altogether impossible, just unlikely (the calculation is, understandably, very sensitive to the geometric albedo value as it nears 1).

Urhixidur 2005 July 8 00:24 (UTC)

That calculation isn't supported by the geometric albedo observed by both ground-based and spacecraft observations which consistently show an albedo of 0.99±0.01 (Franz and Millis 1975, Cruikshank 1979, Smith et al. 1982, Buratti and Veverka 1984, Buratti 1988). For temperature measurements, it is the bond albedo that is important here, not the geometric albedo. Bond albedo is also known as the spherical albedo, and is the fraction of the total incident solar radiation—the radiation at all wavelengths—that is reflected or scattered by an object in all directions. As I said above, this value is 0.89±0.02, which is consistent with the value you came up with. Volcanopele July 8, 2005 19:01 (UTC)

Found an even better reference, Verbiscer and Veverka 1994, which reported that the single-scattering albedo (another way of saying geometric albedo), is 0.997±0.001 for disk-resolved images (value an average of all terrains). Volcanopele July 8, 2005 19:37 (UTC)

Another problem has occured to me regarding the temperatures reported here. What exactly constitutes a max, mean or min temperature. Is the max the average sub-solar temperature, the min the average midnight temperature, and the mean the average between the two? Is the max the maximum surface temperature (as a result of differences in thermal inertia/volcanism), the mean the average sub-solar temperature, the min the average nighttime temperature? What is the convention at wikipedia, if there is any? Volcanopele 17:30, July 20, 2005 (UTC)