But while scientists have been talking about developing a Europa mission for some time, so far NASA has not yet sent an orbiter to investigate the moon in detail.

Karl Hibbitts, a research scientist at the John Hopkins University's Applied Physics Laboratory, is working on developing a hyper-velocity impactor that could be carried on a future Europa orbiter.

In this interview with Astrobiology Magazine editor Leslie Mullen, he explains why smashing down into the surface of Europa could provide details about the moon that an orbiter or even a lander could not.

Astrobiology Magazine (AM): You have a proposal for something called the Europa Hyper-Velocity Impactor. Can you give me an overview of how it would work?

Karl Hibbitts (KH): It's a projectile that would go really fast into the surface of Europa, creating a curtain of ejecta. So we could make measurements that reach beneath the solid surface of the moon, measurements that are not possible to make from orbit.

AM: How fast could you get it to fall to the surface of Europa?

KH: 13 kilometers per second. That's how fast Europa is going around Jupiter. You can come in faster than that if you want, but around 10 kilometers per second is reasonable, and that is actually how fast the Deep Impact mission went into comet Tempel 1.

The depth of penetration is largely dependant on the density difference between the impactor and the surface, so you want to make the impactor heavy so it goes down deep. We'd like it to weigh at least 100 kilograms, and much more if we can get it. The ejecta curtain can be a million times more massive than the impactor itself.

The shape of the impactor will also affect the ejecta. You can choose to make the impactor flat, and that will give you an immediate flash from the surface, and then a curtain of deeper material as it burrows down. Or we could make the impactor pointy, which makes it go deeper down but creates a more columnated plume of material.

We want to separate different plume ejecta from each other, so we need to find an equilibrium in how we eject the subsurface material. It's essential for us to do some supporting laboratory work on this to figure out how to provide the best science results. Europa's not a comet, so the results will be different from Deep Impact. We need to simulate its surface composition and properties better.

AM: Some models for Europa say the outer ice shell is between 20 to 100 kilometers thick, with the average thickness of about 40 kilometers. I would not imagine a small projectile could penetrate very far into that.

KH: That's true. It could only go in several meters at the deepest. The goal is to get below the radiation layer.

AM: What is the radiation layer?

KH: It's the layer of the surface that's been irradiated by all the particles and electrons in Jupiter's magnetic field. You can't assume that certain areas are not irradiated, because Europa's ice shell is decoupled from the subsurface. It floats slowly around on timescales of tens to hundreds of millions of years, so at some point in time an area could have been irradiated. So we'd want to target a place that looks like it's been in recent contact with the subsurface.

AM: So underneath that top irradiated layer is a subsurface layer that's not as altered, and maybe more indicative of what the whole moon is like?

KH: Yes. Of course, they are not discrete layers; it's a gradation from heavily irradiated to lesser irradiated. But at some depth, probably more than a meter down, basically it's unirradiated. Any organics would have to be below the radiation layer to survive.

AM: On the surface there're those veins of colored material, and scientists don't know what they are. Is that something an impactor could help figure out?

KH: That is one of the fundamental questions we're trying to answer. Is it a hydrated salt, or is it a hydrated sulfuric acid?

AM: You couldn't figure that out from spectroscopy?

KH: We could if there wasn't so much hydration there. The water ice absorbs the light at some very important wavelengths, so it overwhelms everything. And that gets back to why this technique is so powerful.

Ejecting material from the surface warms it up. Certainly some of it is super-heated, but the rest of it is warmed enough so that the ice would sublimate away and we could look at the non-ice component.

It's very similar to what happened with Deep Impact. Except because Deep Impact hit a comet, the ejecta hung around for hours, while for Europa, the ejecta will only be there for a few minutes. That's because of the difference in gravity -- the gravity of the comet is millimeters per second squared, so things don't move around very fast.

AM: So with the comet, once things sprayed out, there wasn't enough gravity to bring it back home, it was just going to keep spreading out. But with Europa, the ejecta will fall right back down.

KH: Right. And stay closer together, probably. We can study the initial flash, and we can study the material in the plume that shoots out, and those are two different things. The flash will tell us the composition of this non-ice material on the surface. The plume is made from material deeper down, and so the non-ice material there might be different.

AM: One thing we don't understand, given what we know of Europa's gravity and tectonics, is why the surface of Europa is relatively smooth. Why are there no ice mountains on Europa, for instance? Would an impactor tell us something about how the geology works there?

KH: It could potentially, because the way the crater will form in the ice will tell us about the strength of the material which it hits. So if we can image it in high enough resolution in an orbiter's subsequent flybys, then we could learn something about the upper few meters of the surface.

AM: One advantage an impactor seems to have is that it can go to places on the surface that a lander could not. A lander couldn't survive if it fell into a deep crack in the ice, whereas with an impactor you wouldn't have to worry so much about rugged or chaotic terrain.

KH: But even we are susceptible to that problem. So we would not aim for a chaos area; we'd probably want to aim for an area that was as smooth as possible. It would be bad to go into the middle of a crack.

AM: You wouldn't get your explosion?

KH: You would, but it would be very columnated. Or how about if we hit something at a glancing angle, how's that going to affect the partition of the energy into the surface? Our constraints aren't as tight as they are for a lander, but we still don't know exactly what our constraints are. So I have concerns about it that the impact specialists will need to address.

AM: You said that you wanted to go into a recently resurfaced area �- I would think that would be more chaotic than an older one.

KH: It depends on how it's deformed. A young area that's deformed like a mountain-building area on the Earth is going to be chaotic. But how about an area that would have formed very similar to the Columbia flood basalts out West, where you had the ground just open up and the lava flooded everything? That's the kind of area we would like to go into -- young and smooth. Smooth on the scale of tens of meters, so something like pahoehoe flows would be fine for us, but it would really bad for a lander.

AM: What if the ice models are wrong, and the ice isn't thick, and the impactor goes right through the ice into the underlying ocean and disappears with a �bloop!�

KH: That would be brilliant, wouldn't it?

AM: But there'd be no ejecta, right?

KH: No, there'd be huge ejecta. The ocean water's like rock, at those velocities.

AM: So it would be similar to if a meteorite hit the ocean on Earth, creating a huge blast and maybe generating a tsunami.

KH: Those are bigger, but yes, it's the same thing. And, of course, you'd look at an image of the impact crater later and see that we punched through thin ice. So then we would know it would be a good place for a lander to go. That would be a wonderful success, but it's very unlikely.

AM: Are there any contamination issues for the impactor?

KH: No, I already talked to the Planetary Protection Office about that. It self-sterilizes. The impactor would be destroyed because it would disintegrate upon impact. And it would be made of a material like copper, or something that would not be in great abundance on Europa.

AM: Would there be any instruments on impactor itself, or would it just be a big metal ball?

KH: We'd have a camera just like the Deep Impact mission had. A camera would provide some useful information for a lander, because you get surface roughness at a very fine scale at one point. As you approach, your spatial resolution from the camera gets higher and higher.

AM: What are your thoughts about the possibility for a NASA Europa mission, given the current budget issues?

KH: It will happen in the future. Political priorities do waffle, but the science priorities have remained constant for the last 10 years. And Europa has always been a top priority. There are competing destinations, of course �- Titan and now Enceladus, for instance.

But Titan would be a prebiotic Earth �- there's no chance of life there, with the temperature being only 94 Kelvin on the surface. I personally would love to see a mission to Titan for reasons other than astrobiology.

It's easy to get to, you can have a lander, and you can look for some unique things there. But for an astrobiological perspective, Europa is still at the top of the list.

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