Europa Orbiter, Speculation, updates and discussion Options

[tedstryk]

Rule 3.5 - un-needed quote removed - Doug

I could have sworn there was an article in a circa 1980 issue of Sky & Telescope
magazine that showed what might have been a plume on Europa. Does anyone
have the details/image?

I remember that. The problem was that it was a single pixel in a lone opnav frame. Therefore, the probability of it being noise, especially given that there were no other frames showing it, is too high to reach any conclusions.

[vexgizmo]

I guess the benefit of Enceladus is that the water's just spewing out into space from open cracks. Europa's got a helluva thick crispy crust protecting its water.

Have a careful read of the Enceladus Science papers (specifically Porco et al vs. Spencer et al.) and you will see that the evidence for water is equivocal, and arguably circular. The prime piece of evidence for liquid water (Porco et al) is the inferred high ice/vapor ratio of the plume (top of p. 1398). This is inferred from scattering models and assumptions of plume particle sizes and argued unlikelihood of particle entrainment in sublimating gas (briefly explained in note 30). Should we hang our conclusions and exploration strategies on that? Instead (Spencer et al), the fractures of Enceladus may simply expose warm (T ~ 180K) ice which sublimates like a comet (p. 1405). Show me the water. (Perhaps discussion appropriate to a different thread.)

[volcanopele]

I guess the benefit of Enceladus is that the water's just spewing out into space from open cracks. Europa's got a helluva thick crispy crust protecting its water.

That does make it helpful in the sense you don't have to drill down as much (and the fact that no one really cares about Europa ). So the material you want is much more accessable. However, if you want to land, there isn't much choice on where you can land as only a very small percentage of the surface has a resonably access water body beneath it (assuming of course that a sub-surface body of liquid water generates the plumes). Europa would presumably have a far greater number of acceptable landing sites.

But that still leaves the problem that no one cares about Europa.

Okay, I'll stop messing with vexgizmo now.

[JRehling]

That does make it helpful in the sense you don't have to drill down as much (and the fact that no one really cares about Europa ). So the material you want is much more accessable. However, if you want to land, there isn't much choice on where you can land as only a very small percentage of the surface has a resonably access water body beneath it (assuming of course that a sub-surface body of liquid water generates the plumes). Europa would presumably have a far greater number of acceptable landing sites.

Is the number of sites a big problem? One is enough, as long as it's not so small as to be hard to target.

I think a bigger problem, even in the best case, the engineering task becomes difficult/uncertain:

Supposing Enceladus does have a pocket of H2O "magma" that spritzes out of a few cracks, the exploration strategy isn't totally clear. There must be some artesian pressure working to force that water out, and whether you want to go in via the existing aperatures or make your own, I don't see how that would work. I see a lander getting fatally blasted by a rocket-fast stream of H2O coming the other way. Maybe we should send two and have one take video of the other one. But that doesn't get us a submarine in that lake.

Or maybe a lander could study from nearby the gush of vapor/ice from a vent, but then what's the point of being on the surface at all?

Even if the optimists are right (and I think they are), I don't see the exploration strategy.

Maybe the way to go is to identify a place where the ice is a desired thickness, like 100 m, and then use a melt-down approach that counts on the tunnel re-freezing behind the probe. Then when it finally taps into the high-pressure soda water, it'll have nowhere to get rocketed back into except the ice at its butt end.

That mission would require a precursor of some sort.

I'd say we might just want to put a lander or even the very same class of lander onto both Enceladus and Europa in the locales of most likely water access and perform a Viking-equivalent mission analyzing the ice for nonice content, and using seismic/sonar means to investigate local ice depth/structure.

It seems like the superset of options would be:

Europa Orbiter
Europa Icepick Sample Return
Europa Surface Lander
Europa Subsurface/Submarine Mission

Enceladus Orbiter
Enceladus Plume FlyThrough Sample Return
Enceladus Surface Lander
Enceladus Submarine Mission

Enceladus science needs to age like a wine before the sense of those missions can be evaluated. Europa Orbiter is clearly a need, even if it's not clear that it's the top need. The two Surface Landers could merit identical design. I can sense Europaists getting itchy at the idea that the two worlds getting anything like a unified approach, but if the two worlds both have the water endgame feasible, then I think the plans ought to converge. On the other hand, we may have an Enceladus without actual liquid... or we might have Europa with a crust so thick as to indefinitely discourage access to the ocean. It would seem that Europa has the advantage of getting the next move, but Enceladus is more likely to get to the endgame this century.

[Guest_BruceMoomaw_*]

I suggest that, since most of the water spewed off Enceladus' plumes immediately falls back onto the moon, the logical course is to touch down a modest distance from the plumes, just collect the falling snow, and then periodically melt, filter and analyze it for organic remains.

After all, even if (as Bob Pappalardo suggests) the plumes may just be due to ice sublimating off the surface of Enceladus, when that surface is heated from underneath by warm liquid water, you're going to get a conveyor-belt recycling over geological time -- the snow will reaccumulate on top of the ice near the plumes, and its weight will cause more of the ice on the bottom layer to sink down to depths where it in turn will be melted before later being refrozen into the bottom of the ice layer. And since Saturn, unlike Jupiter, does not have an intense radiation environment, there's a good chance that any biological or prebiological remains in the warm liquid water will survive for a long time on the surface after having been frozen into the bottom ice, slowly carried upwards by the solid-state convection in the ice, and then expelled onto the surface. (This, after all, is precisely why we're hoping for frozen biological remains in Europa's near-surface ice, once you dig down the relatively short distance below its radiation-damaged upper layer. In the case of Enceladus, the sampling is easier. And if Titan's cryovolcanism is as strong as it currently appears likely to be, a similar search for frozen biological remains in the surface ice from its probable subsurface ocean also makes sense -- and, in fact, the instrumentation for the suggested Titan Organics Explorer WOULD look for chirality in any surface organics that it finds.)

Clearly the first step is to try to (1) get more detailed information on the trace components in Enceladus' plumes; and (2) try to get a measurement of the precise surface temperature at the central source of the plumes. Which, of course, is prcisely what Cassini will be trying to do in its super-low 2008 flyby, which is why its controllers have decided to run that risk. (Notice how Enceladus, so far, has been the ONLY thing found in the Saturn system that has made Cassini's controllers change their original mission plan?) Only then -- and only after Cassini has also told us a lot more about Titan's global surface layout -- will we be able to make any reasonable judgement as to what to do about these three worlds, and the order in which we should do it.

[Bob Shaw]

If Enceladus does indeed have a conveyor-belt snow recycling system, then it might also be that we'd find analogous sites to the Antarctic locations where meteorites fetch up. By sampling along such a 'shoreline' where fresh snow gets trapped and sublimates away, we might have access to the greatest repository of objects from other worlds anywhere we've yet found!

Bob Shaw

[Guest_BruceMoomaw_*]

You misunderstand me. Once the snow lands a certain distance away from the warm regions where the ejection of water vapor occurs (either by direct geysers or by sublimation of ice from the top surface of the warm regions), it stays in place permanently. And the only debris carried up with the water, even if geysering occurs, is possible rocky material from Enceladus' interior -- and then it won't be kicked a long way from the geysers unless it's quite fine material. (If the plumes consist of water vapor just sublimating off a warm ice surface, no other material at all will be spread farther by the plumes, except of course for the trace gases mixed with the water vapor -- but the glacially slow convective "conveyor belt" of warm ice creeping upward in the central warm region, and descending again in the cooler peripheral regions where the snow is accumulating, might carry some rocky debris from the interior up to the surface in the plume regions.)

In any case, those trace gases that get expelled along with the water vapor explain the frozen CO2 and other "light organics" which the VIMS HAS seen spread on the surface for some distance from the vents, along the Tiger Stripes -- and getting a better look at the composition of those other substances with the VIMS must be yet another urgent goal of the coming super-close 2008 Enceladus flyby. (Bob Pappalardo, by the way, obligingly sent me copies of the "Science" articles on the VIMS and mass spectrometer findings, which I'm reviewing right now and will report on later.) If it turns out that the plumes ARE just due to vapor boiling gently off the surface of a quite wide area of relatively warm ice, then it would almost certainly be safe for a lander to touch down right in the middle of the plume region -- and analyze the ice right there to see what else was oozing up from Enceladus' interior along with the warm ice.

[Guest_JamesFox_*]

I just have to ask this question: When it comes to Scientific information, would a mission to Enceladus provide more information than a mission to Europa assuming no life, or complex organics, are found?

I know the liquid water = life argument is trotted out with regularity, but that is still pretty uncertain. At least a Europa Orbiter, or some analagous mission, would provide us with lots of extra information on the Galilean Moons, but this proposed Enceladus mission seems totally focused on life,life,life. If the life does not show up, then it would seem that the mission would be mostly a waste. It seems a bit risky to me.

I would hope that the people at NASA can conceive that -there may be liquid water there-, and -we may get clues to the formation of the solar system- are not the only good reasons to send out space probes. Io, for example, is a fascinating place. There is also a certain satisfaction is getting the first good images of an object that was previously just a dot or a smudge, at least to me. That's pure exploration.

So what I am saying is that I find throwing away a mission that will return valuable, non-search-for-life science , in return for a mission with a remote chance of hitting the jackpot, rather reckless.

[nprev]

I must respectfully disagree with you, James. Although finding life on Enceladus via a lander mission would of course be the scientific coup of Western civilization, in situ observation of the plume processes and associated chemistry would still be valuable in its own right, and certainly enlightening in a wide variety of ways.

Consider: We don't really understand why Enceladus is even active at all at this time. Pure tidal influences seem inadequate; is there something unique about the moon's geochemistry? This is a valid research question with profound implications for the formation of the Saturnian system that alone would justify a lander.

By comparison, Europa does not seem to demonstrate any recent cryovulcanism (although I acknowledge that the Galileo dataset may not have been sufficient to rule it out completely) despite the fact that it's much more massive than Enceladus and also exposed to significantly greater tidal stress.

Therefore, if forced to choose (as we must given the newly austere budget environment), it seems rational to study the more geologically active target in detail first. Finding life would be nice, but it cannot be the entire rationale for such high-risk missions.

[PhilHorzempa]

[size=2]


Back to the subject of the Europa Orbiter, I think that we need to call
Griffin on his remarks last week at the NSS (National Space Symposium).
He suggested that we wait 10 years until the heavy-lift CLV and CaLV
are available. Then, NASA could fly a Jumbo mission to Europa.

This is the same old trick that Goldin, and company, tried in 2000 when
NASA wanted to cancel a mission to Pluto. At that time, it was suggested
that we wait until an advanced nuclear ion propulsion system becomes available.
Then we could launch a Pluto probe on a really fast trip to that planet.
Fortunately, Congress saw through the "smoke-and-mirrors" and added
funds to NASA's budget for the New Horizons probe. As the UMSF community
can see in another post on this site, the advanced propulsion effort, Project
Prometheus, has been cancelled.

I hope that Congress is not tempted to believe that there is merit in Griffin's
suggestions at the NSS. We need to reject Griffin's siren call to wait, and
instead get on with a mission to Europa using technology already in hand.


Another Phil

[Cugel]

As if flying a scientific mission on a man-rated mammoth booster would ever be an option. How many unmanned probes did we launch on the Saturn boosters? Even when they they were ordered and payed for, they were send to a museum rather than to Jupiter or Saturn. Mr. Griffin's suggestions are starting to sound pretty desperate, if you ask me. I still hope ESA can play a leading role in a Europa orbiter, it seems such a logical next step.

[djellison]

Pluto was a special case - the atmosphere is a ticking bomb waiting to go bye bye - we needed to get there by X to have a good chance of investigating it before it froze out.

I agree that Enc and Eur can 'wait' as it were. But only for financial reasons (i.e. we can't afford them at the moment) however - there is no need or requirement for the new heavy LV for those missions. If what you're building busts an Atlas V Heavy, Delta IV Heavy or any other LV mass budget, then you need to be more creative with your mission - slingshots, ion prop - whatever it takes.

Doug

Doug

[ugordan]

I still hope ESA can play a leading role in a Europa orbiter, it seems such a logical next step.

Do you really think ESA can afford funding such a grand mission? From what I gather, Venus Express is their last planetary mission (for the time being) and they're now shifting towards astronomical observatories.
Any involvement in EO would likely be in the form of a joint NASA/ESA mission and I don't know how likely that is given the current state of NASA and its unmanned space exploration. ESA already was once in a position of rescuing a mission from being cancelled (Cassini) and it's questionable whether they are willing to take the risk again and see the U.S. folks bail out on them halfway through the development phase. IMHO, NASA (arguably not due its own fault) just currently can't be taken as a reliable partner.

[ugordan]

How many unmanned probes did we launch on the Saturn boosters?

Saturn V was never a launch vehicle that went into serial production. Contracts were made to build 15 or so of them and then the industrial infrastructure and manpower was laid off. In the end they winded up with a couple of surplus ones and used one to lift Skylab as it was one of the uses a Saturn could have been used as a heavy lift booster. It was much too expensive and useless to continue production because their capability was pretty much overkill once the moon race ended.
Even today, think about its capability - IIRC 30 metric tons to Earth escape velocity. Building a probe that heavy and capable would probably take billions (along with the cost of the vehicle itself) and its obvious these days one can't get even a measly billion for a flagship mission. You'd have an ultra-heavy booster that the market just doesn't need. In that light, it'll be interesting to see where Delta IV Heavy will end up.

[edstrick]

The Skylab booster was the Apollo 20 Saturn 5. The abandoned flight vehicles were the boosters for 18 and 19. However, there are 3 Saturn 5's on display!

All of them incorporate parts from something that LOOKS like a Saturn 5 but was never a flight vehicle: the "Facilities checkout vehicle"... used to test VAB/Crawler/Launch-Umbical Tower/Pad equipment, including fueling exercises. But it didn't have real engines or lots of other stuff.

Things are a bit more complicated than that IRL, but you'd need to read the exhaustively detailed new book on Saturn 5 to find out.