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The First Europa Lander, What can be done first, cheapest & best?
nprev
post Dec 3 2007, 12:46 PM
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Wow. smile.gif

Don, only thing I can think of is that the penetrator/melter should be fluted in some way to allow water vapor to escape to space around it instead of re-freezing in the tunnel. (Exactly how long would it take that thing to melt its way through several kilometers of ice, anyhow?)


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JRehling
post Dec 3 2007, 07:05 PM
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QUOTE (DDAVIS @ Dec 3 2007, 03:17 AM) *
This discussion is of particular interest to me as I am doing an animation sequence soon of a Europa lander. I have visualized such a thing in the past for the PBS show 'Life Beyond Earth' as a surveyor type lander carrying a probe underneath which would be weighed at the bottom with a plutonium 'lens', with the instruments above this. This probe resembes a blunt version of the Galileo probe entry shell. When released the heat from the lower probe lining melts a hole below it and the probe merrily tunnels its way to the liquid far below.
Any thoughts about this general means to get to the deep ocean with a probe as opposed to other spacecraft schemes being contemplated?

Don


One of the worries is that impurities in the ice would accumulate at the bottom of the hole, eventually creating an obstruction. Even if the fraction of impurity were very low, accumulating one meter of it after ten thousand meters of ice could be a show stopper.

I'm curious about what would happen when you actually hit the water. Would it gush out and rocket your sub into a geyser far above the surface -- which, obviously, would disable it as well as prevent access to the ocean. One of the problems I see with Enceladus is that if what we're seeing are geysers, then it may be pretty well impossible to push into the ocean, since it's trying to get out with considerable force. Which makes sense -- there must be nonzero pressure in water under a shell of ice, whereas the pressure above is the vacuum of space.

Maybe the saving grace would be if the hole re-freezes above the craft, and then when it reached the ocean, there'd be nowhere for it to go up. Somehow, it would have to handle that pressure transition, though, from surface to ocean.

Another issue is how to maintain radio contact. Leaving a chain of repeaters has been mentioned, but that sounds like a lot of mass, not to mention one heck of a lot of failure points.

And once the craft got to the ocean, there'd be a lot more to accomplish. A vertical descent to the rocky mantle would be fascinating, but might entail rather tremendous pressures.

I think realizing this vision is, however, one of the most exciting goals in solar system exploration, and finding the best location (if there is a best location) to make this descent is the real imperative of the next Europa exploration from orbit. Alternately, if a lander were only to sit on/near the surface and analyze the crust, it's still important to pick a good spot.

Good spots Galileo looked at include these:

http://photojournal.jpl.nasa.gov/catalog/PIA01405

http://photojournal.jpl.nasa.gov/catalog/PIA01640

http://photojournal.jpl.nasa.gov/catalog/PIA01177

But given how little of the surface Galileo imaged very well, it's imperative to do a more comprehensive survey.
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dvandorn
post Dec 4 2007, 07:54 AM
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I would say that not only is it imperative that a melt-hole reseal itself, it's really unavoidable. The extreme cold of the ice surrounding the melt-hole will refreeze any liquid water very quickly, for the first 80% of the hole at least, so if you don't keep re-heating the walls of the hole, it'll freeze solid again within hours of the passage of the ocean probe.

I'm tempted to think that the first 20 or 30 meters of the hole might have to be drilled "dry", the probe dropped in, and the whole thing filled in with the excavated shavings. You then very *slowly* melt out enough liquid water around the probe to *seep* up through the shavings, consolidating them and establishing a pressure-tight seal.

You think that's going to be an engineering feat? That's not even the biggest challenge -- the biggest problem is how to maintain communications through the ice crust between the probe as it descends (and of course after it reaches the ocean) and the lander on the surface, which is of course the comm link between the probe and the outside Universe.

You can't just lower the thing on a cable -- the cable would probably have to be a few km long at the shortest, which would be pretty massive on a spacecraft which will likely have an extremely tight mass budget. Not to mention avoiding snag and jam issues on whatever payout device you design, and the fact that 99% of your cable would be frozen into the resealed hole for most of the descent. You'd have to keep the cable heated for its entire length for it to move through the ice as it pays out, and that wouldn't let you truly seal the melt-hole and avoid that nasty geyser that will otherwise spray your entire mission into a Europan sub-orbital trajectory.

The best design I've seen (and it's likely been discussed here) was one in which the descending probe would leave relays every few tens of meters, each relay capable of talking with the two above it and the two below it. (It's easier to transmit across 30 meters of ice than it is to transmit across a few km... and you want to be able to lose one or two and, as long as they're not next to one another, you still maintain your overall link to the surface.)

The bigger issue, of course, is that any such ocean probe is going to have to literally sink the entire way through the ice crust. That means that the probe is going to have to create a bubble of superheated (for its environment) liquid water that will unfailingly *sink* through the entire crust. Given the likelihood (almost certainty) that the ice is likely not homogenous but will have impurities (such as, oh, I dunno, maybe house-sized rocky boulders), this is going to be very, very difficult to pull off. The whole descent process is going to be extremely energy-intensive, and the probe will have to take that energy down with it -- no cables, remember? And each comm relay is going to have hefty power requirements, too. (And, of course, in such an eternally dark ocean, we'll need to bring some awfully bright lights all the way down into it, just to see what's there...)

The odds are that this isn't going to work the first time we try it. It may not work the first several times we try it. It may be almost impossibly difficult to do.

Whatever the odds, though -- we simply must try.

-the other Doug


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nprev
post Dec 4 2007, 12:19 PM
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Nihilistic, but IMHO painstakingly plausible, oDoug... blink.gif

Convinced me utterly that we gotta do a VERY thorough search for cracks, weak spots, whatever sort of (relatively) easy access points there may be...


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Cugel
post Dec 4 2007, 01:06 PM
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Why would you go all the way down to the liquid water (if any is present at all)?
All that ice has to come from somewhere and it didn't come falling out of the blue sky.
It's the same ocean that everybody is looking for, just in a frozen state!
Now, the top 1 meter or so might be seriously altered by Jupiter's radiation,
but just 2 or 3 meters down and you're in the ocean.
Melt it, pump it up to the lander and analyse it for organics and other ingredients.
It will probably cost you a billion or 3 but at least it is technically doable.
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JRehling
post Dec 4 2007, 03:02 PM
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QUOTE (dvandorn @ Dec 3 2007, 11:54 PM) *
The bigger issue, of course, is that any such ocean probe is going to have to literally sink the entire way through the ice crust.


I hadn't thought about this angle before, but of course if it's much denser than water, it's going to descend very rapidly (and without much controlled mobility) once it hits water. If it's not, then then it's not going to be a lot of force driving the descent through the ice. Unless the heated front end is jettisoned (or, I should say, flotsomed) at that point. Another failure point.
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JRehling
post Dec 4 2007, 03:06 PM
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QUOTE (Cugel @ Dec 4 2007, 05:06 AM) *
Why would you go all the way down to the liquid water (if any is present at all)?
All that ice has to come from somewhere and it didn't come falling out of the blue sky.
It's the same ocean that everybody is looking for, just in a frozen state!
Now, the top 1 meter or so might be seriously altered by Jupiter's radiation,
but just 2 or 3 meters down and you're in the ocean.
Melt it, pump it up to the lander and analyse it for organics and other ingredients.
It will probably cost you a billion or 3 but at least it is technically doable.


Icebergs on Earth also come from the ocean, but I bet they don't contain many squid, clams, or blue whales.

The "3 meter down" lander may have to be the next step, but it will leave a lot to be desired.
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centsworth_II
post Dec 4 2007, 03:55 PM
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QUOTE (JRehling @ Dec 4 2007, 10:06 AM) *
Icebergs on Earth also come from the ocean, but I bet they don't contain many squid, clams, or blue whales.

But do icebergs contain detectable evidence of the existence of squid, etc. ?
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marsbug
post Dec 4 2007, 04:35 PM
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I dont know about squid but there are many studies ( these two are a random selection) showing that microorganisms can survive entombed in ice, which would at least tell us there was something down there. Europas ice sheet might prove to be an interesting habitat in its own right, and much more accesible than the ocean itself. smile.gif Investigating the ice at increasing depths first would give us a chance to test out various approaches, giving us a better chance when we feel ready to, er, take the plunge into the ocean itself. Although if we could come up with an approach that would let us do it tomorrow I'd say go for it, even if it were a shot in the dark!
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Attached File  lifereallyinice.pdf ( 324.6K ) Number of downloads: 2494
 


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hendric
post Dec 4 2007, 04:55 PM
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QUOTE (JRehling @ Dec 4 2007, 09:02 AM) *
Unless the heated front end is jettisoned (or, I should say, flotsomed) at that point. Another failure point.


I always thought the "heated front end" would be the RTG you take down with you to get power. After all, the only source for energy would be the RTG, and the waste heat has to go somewhere! Unless you plan to throw away your RTG and run on batteries the rest of the mission? Be a real shame to limit yourself to a few days in the ocean. Besides, as the probe gets closer to the bottom I think it will run into water bubbles and caves within the ice. Dumping your drill the instant you hit free water might not get you to the open ocean. You'll want active bouyancy control once in the ocean anyways, so a slightly larger ballast tank should take care of any "falling like a stone" concerns.

Would the water refreeze before it sublimates? Speaking of which, how will the motherprobe handle being on top of a geyser? Could it get encrusted in ice? Maybe the motherprobe needs to have wheels, or a crane, so that it doesn't need to be directly over the probe hole. Just a couple of sensor ports so it can sample the outflow.

A precursor mission could be a connected hot drill, just a 20cm 100W RHU that gets lowered on a wire. Being on a wire, you can control your descent rate so that the hole stays as clear as possible, without water refreezing above you. There would still be problems with dirt/rocks/pebbles, but you could probably go for dozens of meters minimum. Just put a KM on the spool in case of an extended mission. smile.gif If you put it on an arm (I know, KISS), or rover, you could probe in multiple locations. Of course, money and UMSF being what it is, I doubt even a simple lander that just drops an RHU on the ground could be made for a New Frontiers budget.

Here's a thought: What if the lander WAS an RHU? Basically, a hard lander that had a very large RHU with a minimal a la Deep Impact camera. Make your own instant geyser for a dedicated orbiter to sample over the following months. A 250-500kg hard lander with mostly plutonium could make one hell of a hole over time. Could it reach the ocean itself turning into a mini cryo-volcano? The mind boggles!

Anyways, enough of the random ramblings. Back to work!


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centsworth_II
post Dec 4 2007, 05:58 PM
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I think it's safe to say that the first Europan lander will not attempt to reach
the ocean, but will hopefully attempt to reach un-radiated ice.
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dvandorn
post Dec 4 2007, 07:12 PM
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I don't think I'd be good with just opening up a hole and creating a geyser to study. There is a whole wide range of assumptions we can make about what's in that ocean -- but if there are living organisms, I think it's incumbent on us to explore without endangering them.

After all, we don't want UMSF to get the bad reputation that the military had at one point -- you know, the reputation reflected in the old slogan, "Join the Army, go to strange new places, meet strange and interesting people -- and kill them."

rolleyes.gif

-the other Doug


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Cugel
post Dec 4 2007, 07:30 PM
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QUOTE (JRehling @ Dec 4 2007, 04:06 PM) *
Icebergs on Earth also come from the ocean, but I bet they don't contain many squid, clams, or blue whales.

The "3 meter down" lander may have to be the next step, but it will leave a lot to be desired.


At least, a water sample from 3 meters down would tell you the saltness and acidity of the ocean, which could be bad enough to make any form of biology impossible. And if we're lucky and some organics are found in the water it would be a lot easier to get the funding for the real thing. Here's a bit from Bruce about this topic: http://www.spacedaily.com/news/life-00p5.html
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tty
post Dec 4 2007, 07:42 PM
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QUOTE (JRehling @ Dec 4 2007, 04:06 PM) *
Icebergs on Earth also come from the ocean, but I bet they don't contain many squid, clams, or blue whales.


Couldn't that possibly be related to the fact that the ocean water has been evaporated (=distilled), then fallen as snow, lain around at subzero temperature in an icecap for thousands of years before finally being calved back into the Ocean?

If you analyzed sea-ice that has actually frozen in/on the ocean you wouldn't have any problems finding organic material and even an occasional small squid frozen into it. Clams are bottom animals and whales sink after death so I wouldn't expect finding them.
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charborob
post Dec 4 2007, 08:03 PM
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A probe melting its way all through Europa's ice crust? That would demand an enormous amount of heat. Much more than a small RTG can supply, I'm sure. Unless you land on thin (a few meters at most?) ice.
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