My Ice Fracture Explorer concept for getting a probe down into Europa’s subsurface ocean – one of the likely places in our Solar System to find extraterrestrial life – was just one way to dig beneath the ice crust. Other concepts often involve melting through the ice crust. However, I thought, what if we can take advantage of the places where Europa’s geologic dynamics allow access to the ocean without tunneling through the ice?
I can think of two surface features on Europa that mark potential exposure of the ocean to space. One is the “chaos,” which may be formed when ocean-floor volcanoes or rising blobs of warm water melt through the ice crust all the way to space. However, we don’t yet have a good way to predict when chaos features would form – unless the impact theory of chaos formation, my personal favorite, is correct, and we can track a large meteoroid on its way to hit Europa. The second, the double-ridge features marking cracks in the ice crust, are potentially more predictable so it makes better sense to plan a mission around penetrating the crust through these fractures.
My IFE concept involved a disposable probe landing on a double-ridge, rolling to the center, and hanging over the crack as it opens up under Jupiter’s tides. The hanging probe could then drop a penetrator into the fracture, to punch through the thin layer of ice below and dive into the ocean water.
A number of readers left me comments here and on io9 pointing out various challenges with this design. Getting the lander to hang, suspended, in the middle of the crack might stretch our space-tether technologies a little too far. Timing transmissions to an orbiter before the closing crack crushes the lander is a problem. Communications from the penetrator are also an issue: since those have to cross a water/space boundary, I wanted to just reel out a long data line from the hanging lander to the penetrator – but the length of this cable could be an issue if the ice crust ends up being 100 km thick. And, since the probe would probably have to be powered by an RTG, when the fracture closes and squishes the probe, we’d be dropping radioactive gunk on the Europan natives. While I don’t think any of the stages of the IFE concept stretch our technologies much more than, say, the Mars Science Lab’s Sky Crane, it certainly wouldn’t hurt to make things easier on ourselves!
One such idea might be to drop the miniature penetrators directly from orbit. There wouldn’t be any suspended platforms, data cables, or rolling around on an unpredictable surface. There also wouldn’t be as much of a challenge in lining up the orbiter for receiving data, since the penetrator came from the orbiter in the first place. However, determining which ice fractures are opening and closing, and timing the drop from orbit to coincide with those events, might be tricky. Landing on the surface first adds an additional safe-hold point to the mission: controllers can wait to establish good telemetry from the lander after it’s on the surface and before ordering it to commence penetrator deployments.
Another suggestion might be to keep the lander on one side of the double-ridge interior. It could shoot a cable across to the other side, and reel the penetrators out to the middle before dropping them:
This concept buys the IFE a number of things: first, it can drop more than one penetrator. My original concept called for several IFE’s to be dropped in tandem to several ice fractures to increase the chance of success. However, if each IFE can deploy more than one probe into the ocean, then the mission managers can get several chances to successfully drop the mini-probes as the crack opens and closes and opens again. Second, the lander won’t be crushed, meaning that we won’t have to worry about radioactive contamination of the ocean (as long as the penetrators run on batteries) and we’ll get the chance to have the landers keep performing science operations after all the penetrators are expended. Third, the lander can buffer data from the penetrators and uplink the information to an orbiter at leisure – no rushing to time the drop for an orbiter pass!
One thing scientists don’t really know yet about Europa is how wide these cracks open up. The tether-based ideas I’ve outlined work as long as the crack is big enough to admit the penetrators – but they have the advantage of working if the fractures end up being many meters wide. However, that might not be an advantage the spacecraft needs if the cracks are very narrow. In that case, why not just have the lander come down with footpads on either side of the fracture?
As the crack opens and closes, damped mechanical joints in the legs could take up the motion and keep the lander centered over the crack. This lander would also be able to buffer data, survive for many tidal cycles, and be able to drop as many penetrators as it has packed into its body.
I think the biggest issue with my designs is that data line: the images and biochemical experiment results from the penetrator have to get transmitted to the lander somehow. (From there, they can get to orbit and then to Earth.) Direct transmission via radio or optical signal could be very difficult from beneath the alien waves, and speed is a factor, so I opted for a hardline. But how long does the cable need to be? At least tens of meters. Probably around ten of kilometers. But maybe as long as 100 kilometers, which could be prohibitively long! One possible solution might be to drop a two-segment penetrator into the crack: the upper segment would have floats – probably some sort of inflated bags – and a radio transmitter. The lower segment would contain the ice-shattering hard shell and all the science instruments. The two halves of each probe would be connected by an unreeling data cable. So, the probe would drop from the lander, smash through the ice, and then split into halves – with one half floating on (and them freezing on to) the ocean surface while the other half continues its plunge into the depths. The probe would collect its data, then zap that data up the cable to the surface unit. From there, the data would travel via radio to the lander, which would relay it to the orbiter and then the Deep Space Network.
Certainly, any concept for a Europa mission strains our ingenuity. But that is one reason why it’s so fun!