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

[nprev]

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.

[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.

[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!