Enceladus-3 (March 12, 2008) Options

[Guest_BruceMoomaw_*]

I found Jason's blog entry from almost a year ago ( http://volcanopele.blogspot.com/2005/03/en...resolution.html ) quoting Zibi Turtle as saying that "we will probably not get better images than 25 m/pixel, despite coming within 100 km of Enceladus in March 2008 (which would yield sub-meter resolution images if ISS took images at C/A)." A pity, but we can't have everything.

[Guest_RGClark_*]

...
Composition of reservoir: INMS observations of the plume suggest that it isn't pure H2O. Their composition measurements suggest 91% H2O, 3% CO2, 3.3% N2 (though CO is possible), and 1.6% CH4.

Jason, does Cassini have the capability to do an isotopic measurement on that methane?


- Bob Clark

[EccentricAnomaly]

I found Jason's blog entry from almost a year ago ( http://volcanopele.blogspot.com/2005/03/en...resolution.html ) quoting Zibi Turtle as saying that "we will probably not get better images than 25 m/pixel, despite coming within 100 km of Enceladus in March 2008 (which would yield sub-meter resolution images if ISS took images at C/A)." A pity, but we can't have everything.

why is that a pity? There are other instruments you know. And it seems like plume science is better done with in situ measurements than remote sensing. Remote sensing would be better for looking at the vents more closely... but E3's geometry doesn't allow that, so imaging on E3 would be a big waste INMNSHO.

[JRehling]

One Enceladus mission I could see flying would be a sample return that flies through the plume (assuming it is "on" all the time -- what a horror if it weren't!) and brings the stuff back to Earth on a free return trajectory, or cruising there and back with NEP. Moreover, the same mission could grab some upper atmosphere stuff from Titan and/or the rings and/or Saturn (though surely not all four). Several add-ons could be imagined: an imaging impactor, etc.

[Guest_BruceMoomaw_*]

The NEP idea -- which would orbit Saturn and fly through the Enceladan plume several times to collect a sample for return -- is what Oberg was talking about in his article. But it would obviously be expensive as hell.

The vastly cheaper idea of flying by Enceladus once and then returning to Earth is a longer range version of Chris McKay's "Europa Ice Clipper", and it might very well be worth doing even for one sample. It wouldn't even need to include the complex requirement, for Europa, of dropping an impactor on Europa and then making sure it flew through the impact-debris plume -- and it wouldn't necessarily need nuclear power; there's already been a proposal for a solar-powered Saturn entry probe flyby mission based on the "INSIDE Jupiter" design, but with 4 solar panels instead of two (and without the big JOI propulsion system). Basically, we would just be flying a duplicate of Stardust -- but with much bigger solar panels -- through the Saturn system and then back to Earth. We're definitely talking about a New Frontiers-class mission, and I have no doubt that people are already thinking about proposing it for the next New Frontiers AO. The catch might be whether Saturn's gravity would be strong enough to allow a hairpin-turn direct return to Earth -- but it might well be workable, instead, to arrange a gravity-assist flyby of Jupiter on the way back INTO the Solar System to finish bending the returning craft's orbit all the way back into the inner Solar System. (The craft would initially have been launched directly from Earth to Saturn, which our stable of ELVs could easily do.)

Parenthetically, there was still some interest at the Europa meeting in dropping an impactor onto Europa and either analyzing or actually collecting the resultant debris plume. C.A. Hibbitts gave a talk on a concept whereby Europa Orbiter would drop a large impactor onto Europa (maybe during one of its multiple orbit-trimming flybys of Europa in Jupiter orbit, before it finally brakes into orbit around Europa itself), take spectra of the resultant flash, and later photograph the fresh crater from Europa orbit -- that is, an Europan version of Deep Impact.

Afterwards I had a long and interesting talk with him about variations on this idea. His idea is to try to analyze material from below the radiation-modified layer -- but he agreed that it's open to question whether long-range IR spectra could properly analyze any organics dug up by the impact. It might be better to have the craft actually fly through the debris plume and analyze it directly with a dust-impact mass spectrometer. (In any case, his impactor would be a substitute for a small piggyback lander on Europa Orbiter -- but see more on that below.)

And it might be better still to have a separate mission that would fly through the Jupiter system without stopping, drop such a really big impactor onto Europa, fly through and collect some of the impact plume debris, and then double back to Earth with it -- that is, Europa Ice Clipper, but with a much bigger (and actively guidable) Deep Impact-type impactor. I've been thinking about this idea for some time, and it turned out that he has too. Again, this is a New Frontiers-class mission -- and if Europa Orbiter really does get hung up fiscally, it might be advisable to fly it first.

One thing we finally did agree on, though, is that one of the optional goals we'd both been thinking about for such an impactor -- using a descent camera on it to get those super-closeup views of Europan surface terrain which may be important to properly design the landing system for a big Europa Astrobiology Lander -- probably wouldn't be practical. In order to have time to return the last few really close descent images just before the craft hits the surface, you need a lander that can survive landing and play back the images after landing -- a crash-lander probably wouldn't be able to return any images of Europa's surface better than the final ones from the Ranger missions, whose resolution wasn't any better than the images we can get from orbit anyway using a HiRISE-type camera of the type that's already very likely to be carried on Europa Orbiter.

[Guest_BruceMoomaw_*]

why is that a pity? There are other instruments you know. And it seems like plume science is better done with in situ measurements than remote sensing. Remote sensing would be better for looking at the vents more closely... but E3's geometry doesn't allow that, so imaging on E3 would be a big waste INMNSHO.

Oh, I'm most emphatically not saying that such very low-altitude flybys aren't very much worth doing for Cassini -- probably several times. Besides mass spectrometric analysis of the denser plume cloud and better gravity data, the CIRS could probably nail down with great accuracy both how wide the tiger-stripe vents really are and how warm the fluid coming out of them really is -- and, if it was warm enough, the NIMS could also provide more data on the composition of the erupting fluid when it first emerges. It's just a pity that we couldn't also get really high-res photos of the vent sites as well -- especially since, according to Scalbers in the "Stardust Mission to Saturn" thread below, the region will soon be in seasonal shadow until 2030.

In that connection, I was puzzled by the failure of Porco's "Science" article to mention at all one thing that positively jumps out at the viewer even in Cassini's first Enceladus close-ups: the moon's "freckles". Quoting Helfenstein ( http://www.aas.org/publications/baas/v37n3/dps2005/450.htm ): "Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits." But there's not a peep about these in Porco's otherwise very detailed "Science" article. The fact that they're dark makes it likely that we're looking at the dark products of methane and the other organics expelled from the vents and then radiation-processed by solar UV and Saturn's own radiation belts -- although it's possible that they might also contain some bits of rocky debris from Enceladus' interior, or salts dissolved from that rock into the water and then redrying on the surface after expulsion. Maybe they're mentioned in some of the other "Science" articles, since I only have Porco's and Spencer's?

[volcanopele]

In that connection, I was puzzled by the failure of Porco's "Science" article to mention at all one thing that positively jumps out at the viewer even in Cassini's first Enceladus close-ups: the moon's "freckles". Quoting Helfenstein ( http://www.aas.org/publications/baas/v37n3/dps2005/450.htm ): "Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits." But there's not a peep about these in Porco's otherwise very detailed "Science" article. The fact that they're dark makes it likely that we're looking at the dark products of methane and the other organics expelled from the vents and then radiation-processed by solar UV and Saturn's own radiation belts -- although it's possible that they might also contain some bits of rocky debris from Enceladus' interior, or salts dissolved from that rock into the water and then redrying on the surface after expulsion. Maybe they're mentioned in some of the other "Science" articles, since I only have Porco's and Spencer's?

No, they are not mentioned. The dark leopard spots are found in Samarkand Sulci in the equatorial sub-Saturnian hemisphere. to keep the Science article manageable, it was limited to South Polar geology and plume analysis.

[ugordan]

I was puzzled by the failure of Porco's "Science" article to mention at all one thing that positively jumps out at the viewer even in Cassini's first Enceladus close-ups: the moon's "freckles".

Are you referring to features seen in this and also this raw image?
If so, the statement that the features seen are indeed dark isn't backed up by strong evidence, IMHO. They really look like topographic shading effects on small dome-like or spike-like features in the grooves. Several of the larger ones, specifically in the second image I linked, at the top center clearly give the impression that we're looking at their shadowed regions. They otherwise don't look any darker than the surrounding ice to me.

[Guest_AlexBlackwell_*]

"Officially," the new reference trajectory is still in review at the Project/Program level; a decision should be forthcoming very shortly. I won't speak for Jason, but unofficially, the 25 km figure is the new Enceladus-3 flyby C/A altitude under all options that were being considered.

Well, the Enceladus-3 flyby C/A distance is official in one sense: Wikipedia is now reporting it

[volcanopele]

The planetary society Cassini tour page now has the new value. Emily says she got he new trajectory from Dave Seal on the navigation team, so this is about as official as it gets.

[Guest_AlexBlackwell_*]

The planetary society Cassini tour page now has the new value. Emily says she got he new trajectory from Dave Seal on the navigation team, so this is about as official as it gets.

Yeah, it's been "official" for several weeks now; I got it from Seal's MP presentations, I believe, in late December 2005. In fact, the 25 km figure is popping up everywhere except on the Cassini public website

[Guest_AlexBlackwell_*]

I just read the materials in the 060323 Reference Trajectory Update package that Cassini Mission Planning (MP) released today and the 25 km C/A altitude for the 61En targeted flyby does not appear to be set in stone (viz., "not finalized yet" according to MP).

Having said that, the new tour tables do show 25-31 km C/A figures but it looks as if MP may revisit this issue before 2008.

[Guest_BruceMoomaw_*]

Can you tell us anything more about just what they plan to accomplish with such a low-altitude flyby?

[Guest_AlexBlackwell_*]

Can you tell us anything more about just what they plan to accomplish with such a low-altitude flyby?

The precise science observing schedule is still TBD, which is not surprising due to fact that 61En has brand new flyby geometry. As EccentricAnomaly pointed out earlier in this thread, one of the working assumptions is that INMS will be pointed to RAM near or during C/A, so I presume INMS plume data is going to be a high-level goal, assuming, of course, the 25 km figure holds up.

I'll only note in passing that the Rev-28 Enceladus RSS plume occultation is shaping up to be a very important observation.