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There are dozens of speculative proposals to explore Europa, Enceladus, and possibly other Solar System bodies (including Sedna and other Trans-Neptunians!) that may be endowed with liquid water under a crust of ice (due to tidal heating or radioactive decay). One of them (IIRC) was sending crawling robots to exploit (possibly non-existing) cracks in the ice. Another set of rather fantastic-looking proposals hinges on manned exploration and drilling. There are also ice-melting cryobot ideas floating aroung.

I'm interested in cheaper and preferably reliable ways of getting knowledge about the properties of these oceans up to tests for presence of life in situ, without sample return.

  • What is the limit of data obtainable from orbital missions? (please state notional or existing instruments and orbits required, including perhaps multi-satellite formations)

  • How can we get data from inside the oceans? (Can, for instance, penetrators be used - cf. LCROSS?)

References:

Related questions:

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    $\begingroup$ Currently the end of Cold War put a halt on bulk production of nuclear fuel and is a significant roadblock in all space missions. Otherwise, nuclear-powered cryobot would be the simple answer. $\endgroup$ – SF. Jul 26 '13 at 10:45
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    $\begingroup$ @SF. - this is not exactly right, am afraid. Only Plutonium (esp. Pu-238 for RTGs) production has been cut. Uranium is still being produced. $\endgroup$ – Deer Hunter Jul 26 '13 at 10:55
  • $\begingroup$ A proposed method for making a hole in the ice is to first impact something (massive) on the surface to break through it. This might be useful since from what I hear, the water ice of Europa (for example) is so cold that it is as hard as steel. $\endgroup$ – Andrew Thompson Jul 26 '13 at 11:15
  • $\begingroup$ @DeerHunter: NASA uses Pu-238 for its atomic batteries. I haven't heard of Uranium-based atomic batteries and I don't think bringing a sustained-chain-reaction nuclear reactor there would be either wise or reasonably cheap. $\endgroup$ – SF. Jul 26 '13 at 11:37
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    $\begingroup$ @AndrewThompson - considering that both the US and the USSR planned to slam nukes against the Moon but relented once the treaty went into force, we don't have much choice here. I think it's safe to assume that nuking Europa is not on the menu anytime soon. If there is anything biogenic in Europa's ocean, nuking would be an ethically disastrous decision. Mankind shouldn't live with that. $\endgroup$ – Deer Hunter Jul 27 '13 at 4:30
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The best way to explore the subsurface oceans of Europa is to see what comes out from the vents. Basically, along each crack will be a small amount of ocean water. Take a sample, and you could find out a lot about what is below the ocean. One simply has to land on a recent crack, and you should be able to find something out.

Actually getting in to the ice is far more problematic. There are two primary challenges associated with this. The first is simply to get below the ocean layer to begin with. The second is how to communicate with the probe after you have gone deep. Both are problematic.

Let's start with the first one, getting underneath the ocean. The easiest way would be to find a crack forming, and work your way in to it. This is actually quite difficult to do in practice, mostly because you have to know where the crack is going to form before you land, and these cracks change location every 3 days or so. And they are pretty small, for the most part, you might have difficulty fitting a probe in to one. Plus they make the communication more dangerous, as you'll see later on.

The second way is to take a portable heat source and melt the ice away. Basically, this would be a novel use for an RTG of sorts, radioactive heat could melt a hole. This could take a long time, but would work eventually.

Lastly, a hole could be drilled. The most challenging factor with this is the fact that even the best drill bits need to be changed on a regular basis for mining operations. Let's say you could get 300 hours out of a drill bit, at a rate of 3 m/hour, leaves only about a km before the drill bit needs replacing, and that's probably optimistic. Who is there to replace the bit?

Okay, assuming you can get through the surface, what do you do there? Basically, this is in the same realm as submarines, except through ice as well as water. There is a whole Wikipedia article on submarine communications. Method could include buoys to the surface, running a long cable, sound waves, or very low frequency waves. The cable or accustics are the best ideas, ideally there should be a combination of both of them, to give some redundancy in case something fails. Keeping the cable intact, would be difficult due to the moving ice, and acoustics would probably be off a bit as well, but they could be done.

Bottom line is, the only cheap way to study the oceans of Europa is to see what is on the surface, along with technologies like RADAR to see what lies beneath. Truly exploring it would be amazing, but incredible difficult and expensive. Someday we'll get there, however.

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  • $\begingroup$ Do RTG's put off enough heat to even relatively combat Titan's freezing atmosphere? How much heat do those actually put out? $\endgroup$ – Magic Octopus Urn Jun 27 '18 at 20:52
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    $\begingroup$ They actually release a good deal of heat. An RTG would work even better in Titan's atmosphere, as air doesn't conduct as much heat as liquids do. I've heard something in the neighborhood of thousands of watts of heat. $\endgroup$ – PearsonArtPhoto Jun 27 '18 at 21:03
  • $\begingroup$ Wow, that's fascinating, do you have good links to Titan's atmosphere composition, sourced by Huygens? $\endgroup$ – Magic Octopus Urn Jun 27 '18 at 21:04
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    $\begingroup$ I just had a Casinni scientist mention to me that you could use an RTG on Titan to make a hot air balloon. $\endgroup$ – PearsonArtPhoto Jun 27 '18 at 21:43
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I'll have to dig through the decadal survey and LPI proceedings a few times, so I decided I have to make this answer a community wiki anyway. As soon as there is a decent answer by anybody else, I'll accept it.

Three basic mission modes:

  • getting inside the ocean
  • getting onto the surface
  • doing research from low orbit

The main goal: determining habitability of the subsurface ocean (if it exists).

What can be done from low orbit:

  • mapping gravity (yeah, that's what was meant by GRAIL-like twin craft formation-flying missions)
  • doing fine-grained laser altimetry
  • mapping volatiles escaping from the body
  • employing GPR (ground-penetrating radar)
  • characterizing magnetic field
  • studying the plasma around the body (Langmuir probe)
  • undertaking detailed multispectral dynamic stereo imaging to map and monitor the surface (bet you've all seen those dunes move in HiRISE pictures, so you know what I'm talking about)

What can't be done from low orbit:

  • In situ chemistry analysis
  • ...

Getting under the ocean:

  • Melting the ice with heat from a nuclear reactor
  • Drilling the ice (the thick ice idea of drilling 100 km is slightly unappealing)
  • Crashing artificial penetrators into the ice (won't work for thick layers presumably present on Europa)
  • Crashing a captive asteroid into the ice shield (not cheap or quick at all, and may not work for thick ice)

Resources:

Europa lander — preliminary drawing as of mid-2013

(to be continued)

Please feel free to edit the answer if you feel like it.

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  • $\begingroup$ The ice might not be that thick. No citation handy, but I've seen estimates that in places in might be 'only' 10km or less. If it's much less (a few km), we've done that on Earth. // I think it's actually much safer to say that an asteroid would work (if we could actually do that) - there might be regions where the ice is too thick, but there are (lenticular) impact formations that are almost certainly 'punch-throughs'. $\endgroup$ – hunter2 Jul 31 '13 at 10:28

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