The First Europa Lander, What can be done first, cheapest & best? Options

[dvandorn]

My thoughts exactly -- it's going to be a very energy-intensive process. Just bringing on a few electric heaters isn't going to get it done. We're talking about *extremely* cold ice here, for the most part. Ice, like any other material, needs to be raised all the way to its melting point before it will melt. You can't just ignore the hundred-plus degrees of heat that you have to apply just to get the ice up to its liquifaction point.

This is going to be harder and more energy-intensive than, say, melting ice cubes that are already within 10 degrees of their melting point.

-the other Doug

[hendric]

Well, RTGs actually put out quite a bit of heat, and it wouldn't be unreasonable to send one capable of 2-5kW of heat (100-200We of electricity) as part of the descending module. Should be plenty to get down as deep as you'd like. Plus, you don't need to melt the ice, per se, sublimating it away is just as good. Also, your heat source never goes away, it's working all day, every day.

Besides, look at what simple dirt/dust does with comets and Iapetus. I think a 5kW RTG would be more than able to get a reasonable descent speed.

[nprev]

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.

I wouldn't be too quick to discount life even if the water chemistry is more hostile than that of a flooded open-pit mine...the durability & adaptibility of life on Earth surprises us rather frequently...

Like this idea a LOT, though. Heck, don't forget a microscope; could be some tiny critters like plankton frozen in there as well! In fact, if we found any degree of biodiversity in frozen microorganisms, that would be a very strong indicator that macrofauna (macro-somethings, anyhow) were quite possible...and watch that funding flow!!!

[DDAVIS]

Wow, this discussion is great!
I am imagining an RTG mounted vertically at the bottom with the radiating fins shaped so as to taper like a blunt harpoon pointed down. The narrower parts of the radiators may be hotter than the wider upper parts which should, along with the weight of the RTG itself, send the probe tunneling more or less straight down. The probe would be a modest cylinder mounted atop the RTG and slightly narrower, with various 'windows' for admitting water samples, and various sensors. Two aspects which intrigue me are what direction to mount the camera or to have multiple cameras. The other is the data return question. I wonder about having the probe unreel a long durable cable as it drops. What is the smallest size a cable spool a few km long can be made? How thin is the thinnest ice on Europa? Can the figures for the thinnest ice and the longest cable overlap?

Don

[tty]

Let's do some order-of-magnitude calculations here. Let's say that a sinking probe needs to melt a 0.5 square meter hole in ice at an average temperature of 173 K.

The density of ice at 173 K is 925 kg/m3. The specific heat of ice goes from about 2.0 kJ/kgK at 273 K to 1.4 kJ/kgK at 173 K. I'll assume 1.7 as an average. To melt 1 kg of ice at 273 K requires 333 kJ. Melting your way one meter into Europan ice would thus require 463 (333 + 100x1,7) = 233 000 kJ = 233 000 kWs. That means that an 5 kWT RTG would need about 13 hours to melt its way through one meter of ice at 100% efficiency.

However efficiency will be very far from 100%. While the specific heat of ice goes down with temperature the thermal conductivity goes up, from 2.2 W/mK av 273 K to 3.5 W/mK at 173 K, so a lot of heat will be wasted in heating ice well away from the probe. Also the RTG will need to keep the entire outer surface of the probe at >273 K, otherwise it will almost immediately get stuck (at 173 K that water will re-freeze fast).

If we rather optimistically assume 50% total efficiency then a 5 kWT RTG will be able to melt its way about 3 feet per day. Melting your way through, say, 50 km of ice is clearly not on. Even a couple of kilometers would take several years.

Another big problem is communications. Even if the hole does not re-freeze behind the probe, the pressure will almost certainly close it, unless it is lined in some way, long before the probe is through it. Radio communications are probably out, particularly as there is likely to be some electrically conductive materials mixed up with the ice. However sound conducts pretty well through ice, so some kind of sonar link would probably be possible. It would require leaving a relay station on the surface though.

[mchan]

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."

That and the time it takes for an RTG (even a 5kW one) to melt its way thru 10's of Km of ice brings back to mind an old UMSF thread:

Nuking Europa

Back closer to reality:

JPL Cryobot

Before sending these fancy gizmos to Europa, can we even get a submarine robot thru 3 Km of ice here on Earth:

Lake Vostok

[nprev]

Gotta say that I'm pretty sold on the pristine ice-sampling concept; that could conceivably tell us most of what we want to know, plus it's a million times more technically feasible.

TTY's calcs convince me that unless we find some really thin ice, a Europan sub mission is a hundred years off or more, awaiting highly advanced hypothetical technologies like high-efficiency directed energy or fusion generators.

Hmm...slightly OT here, but if there are weak spots at all on the surface, they must pop open from time to time. A future orbiter should look REALLY hard for very transient geysers. (I suspect that, if they exist, they don't last very long at all due to freezing/self-sealing).

[edstrick]

Both from a science and an engineering perspective.. ultra-high priority from a Europa orbiter or multi-flyby mission <much less good at this> is to map the global ice thickness, and search for thin spots.

[centsworth_II]

At the least, a first Europa mission should map the global ice thickness,
at the most, it would include a lander that could drill a meter into the surface.
Discussion of getting a probe into the ocean is way off topic for a first mission thread.

[hendric]

Let's do some order-of-magnitude calculations here. Let's say that a sinking probe needs to melt a 0.5 square meter hole in ice at an average temperature of 173 K.

That's an 80cm diameter hole. I'm thinking a 20cm probe, with a hole maybe ~25cm diameter. More like .05 square meters.

So to purely melt the ice would now require only ~23,300 kWs, about 1/10th the previous number.

But, some of the ice will directly sublimate before reaching 273K, reducing the heat needed, and also any contaminants will depress the melting point as well.

Assuming 50% efficiency, that gives us ~10 meters a day. So a 90 day mission would get you almost a kilometer down. Plus I bet the ice gets warmer as it gets deeper, accelerating the rate of descent.

I was using the 5kW RTG number for a suspended probe from a wire. For a dumb-hot-rock, I think you could get 50kWt or more and only slightly increase the volume of ice you need to melt.

For a real submarine probe, I actually agree with you, it's probably not going to happen because of the time to get to the ocean would fry any relay left on the surface. Let's say you got a spiffy cool melting drill that went twice as fast, 1 km in 45 days would still take over a year to get to 10km. Your surface relay will likely be toast well before that.


Let's try working it backwards. Say you want 30 days in the ocean, and 60 days descent through 10km of ice. that's 1 km every 6 days, or almost 7m/h of ice. Given a .5 m^2 borehole (reasonable for a sub), your numbers give 1.6 MWt/h. You'll need to bring along a small nuclear reactor, apparently, to make it through the ice!

[tty]

Your point on impurities lowering the melting point is well taken. However I don't think sublimation will have any effect on the energy required. The melting will happen in a thin water film around the probe and I can't see how sublimation could work in this environment. There may be some sublimation behind the probe if the hole stays open, but that does not do you much good. Also the ice is actually going to be a lot colder than 173 K at least to start with. Another complication is that several of the (at least) 12 phases of ice may be present at depth on Europa, and I'm very uncertain how this would affect the energy requirements and how they would react to heating.

As for the surface relay being fried by radiation, how about this: the complete probe lands on Europa and deploys a dumb rad-hardened antenna that anchors itself to the surface (easy, you turn on a little heater in each leg for a while, and then turn it off again). Then the complete probe starts melting itself into the ground unwinding the antenna cable. When it has gone several meters into the ice the probe separates into two parts. The relay section (which has a small (or well-insulated) RTG) freezes into place while the deep-sinker section with a big, hot RTG keeps on going down and unwinding either a thin metal wire or a an optical filament to keep in contact with the relay. Note that the bobbin must be on the sinker since the filament will inevitable get frozen in.

Using a long thin wire for communication may sound outlandish, but both missiles and torpedoes use this kind of wire for guidance over very considerable distances (tens of kilometers) and at very high speeds. The Swedish Navy even has a variety of the TP61 torpedo that can be used for reconnaissance/intelligence gathering. A submarine launches it and then lies doggo on the bottom while the torpedo runs in slowly and quietly towards e. g. a harbor. Then the torpedo also puts itself on the bottom and starts listening and sends what it hears back through the wire quite indetectably.

The main problem with using wire communication is that if there is much internal movement in the ice it will probably break the wire. However the amount of movement in ice can be determined seismically, so in addition to measuring the thickness of the ice and the composition, we will need to land at least one seismometer on Europa.

[JRehling]

Assuming 50% efficiency, that gives us ~10 meters a day. So a 90 day mission would get you almost a kilometer down. Plus I bet the ice gets warmer as it gets deeper, accelerating the rate of descent.

For a real submarine probe, I actually agree with you, it's probably not going to happen because of the time to get to the ocean would fry any relay left on the surface. Let's say you got a spiffy cool melting drill that went twice as fast, 1 km in 45 days would still take over a year to get to 10km. Your surface relay will likely be toast well before that.

The home run we've got to try for is to find a place where the crustal integrity is considerably less than average and where we can get a much greater descent for the same effort. There is a lot of evidence in the geomorphology that some places on Europa's crust get more interaction with the subsurface than others, on time scales that are sub-geological. The hope is that we could get our probe into the right place at the right time -- which may range in difficulty from pretty easy (if there's a warm slushy spot overlying some permanent, active volcano) to absolutely impossible (if, say, such eruptions usually do not take place, and the mean time between them is measured in centuries or worse). And this is exactly why we have to commit to serious reconnaissance of Europa sooner rather than later. If we get the negative result, then the rest of the jovian system "wins" and can enjoy receiving the next /n/ missions. If we get the positive result, then Europa deserves the preferential (Marslike) treatment.

On the surface-relay issue, I think as long as we're burrowing down, a worthwhile goal would be to at least "countersink" the relay so that it faces open sky, but is surrounded by europan ice on five sides. That would a priori seem to cut the radiation load by 75-80% of what would be experienced by a Europa orbiter, and perhaps more if the landing site is not on the trailing side (which gets the most radiation pummeling). Imagine a relay sunk a few meters in with a skyward hole that sweeps past the Earth's location in the sky once each orbit. If a good landing site were near the center of the leading face, the radiation exposure might be cut an order of magnitude or more thanks to the shielding provided by Europa itself.

[brellis]

ENDURANCE probe is to be tested in the next few weeks. An untethered robotic submarine. It's a followup to Depthx.

[Vultur]

This might well be too hard to pull off, but what about this:

The lander falls toward Europa, firing retrorockets at low impulse. At some point close enough to the ice surface where the trajectory is very predictable (less than a meter or so of error, hopefully), the lander fires an explosive-tipped impactor. The explosive goes off on contact with the ice, making a crater down into the levels of unirradiated ice. The lander lands in the crater; if you're lucky, there'll be pulverized bits of unirradiated ice to put in your analyzers even before you rasp it off the crater wall!

[djellison]

Sound more dangerous, unpredictable and dynamic than it needs to be. A sub-surface drill is a far better idea.