[X] My Assistant

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I think that many people in this forum would agree that somebody's going to have to land on Europa someday before the rather elaborate schemes to penetrate the outer ice layer will ever fly, if for no other reason than to get some relevant ground truth before committing to such an elaborate, expensive, and risky mission.

EO seems to have ruled out any surface science package for that mission (though it would be nice to change their minds! ), but I think that there is a valid requirement at some point to directly assess the surface properties of Europa in an inexpensive yet creative way. Some candidate instrument payloads might be:

1. A sonar transducer/receiver set embedded within a penetrometer to determine crust density and examine the uniformity of the ice layer within the operational radius of the instrument (looking for cracks and holes, in other words).

2. A conductivity sensor again embedded inside a penetrometer to measure the native salinity of the surrounding material and possibly derive some constraints on the composition of metallic salts in the European crust (saltiness has a major effect on ice properties, in addition to the obvious need to derive the salt content of any underlying ocean).

3. A seismometer for all sorts of reasons.


How does this sound? Any critiques, additions, or subtractions? I omitted a surface imager not only because of bandwidth/extra complexity considerations but also because it seems desirable to penetrate the crust in order to minimize as much as possible reading any contaminants from Io during surface measurements. The orbiter data could be used to sense and subtract this from the penetrometer readings.

[dvandorn]

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