The US and European space agencies have drawn up plans for a major space mission to the Jupiter system, to launch in 2020, a talking point at last week's Lunar and Planetary Science Conference in Texas.
The Europa-Jupiter System Mission will focus on Jupiter's icy satellites Europa and Ganymede, investigating their chemistry and geology.
Dr Robert Pappalardo from Nasa's Jet Propulsion Laboratory has led a study to scope out the venture. He told BBC science reporter Paul Rincon why this mission could yield "spectacular results".
Europa may harbour an ocean beneath its thick crust of ice
PR: Why is Europa such an attractive target for planetary scientists?
RP: Europa rises to the top of places that we want to explore in the Solar System, because we're trying to understand whether it's an environment where life could possibly exist. Europa probably has a subsurface ocean of liquid water, where that ocean is in direct contact with rock below it and that ocean is below an ice shell that is relatively thin.
So Europa may have the ingredients for life. It almost certainly has liquid water and probably has the molecules from which life can be built. And a big question is whether it has the chemical energy that can allow for life in that ocean below the surface.
EUROPA JUPITER SYSTEM MISSION
Nasa: Jupiter Europa Orbiter could launch on Atlas rocket in 2020
Esa: Jupiter Ganymede Orbiter (above) lofted by an Ariane
Probes use Venus gravity assist to arrive six years later
Orbiters conduct joint observations at other Jupiter moons
Would finally settle into orbits around dedicated targets
Studies will focus on Europa's and Ganymede's interiors
End destructions will allow unique measurement opportunities
PR: What kind of life might survive on Europa? Are there any analogues on Earth that could give us clues?
RP: If there's life at Europa, we're not expecting it would be big fish or whales or anything, we're expecting it would be microbial life - single-celled organisms. That's the picture of what life could be like there. I don't want to give the wrong impression.
This mission isn't to find life, but to understand whether Europa has the environmental conditions that might allow for life. Then we could follow up in the future with missions that could actually search for whether there is evidence of life.
PR: What shape will this mission take?
RP: The mission as a whole is bigger than just Europa. It is exploration of the whole Jupiter system with two spacecraft. One will end up going into orbit around Europa to study it in detail and the other will go into orbit around Europa's neighbour Ganymede to study it in detail.
By comparing these two icy moons, both of which probably have oceans below their surfaces, we'll learn much more than the sum of the parts. Ganymede's ocean is probably deeper beneath the surface, not in direct contact with the rocky mantle.
It might not be as hospitable to life but that's the kind of thing we need to understand better. We have an idea of how these moons work, but we don't understand in detail how they work. With two spacecraft in orbit around each moon, we can learn a lot about the interior structure of the satellite.
Scientists would like to get down on to the surface of Europa
We can learn about the depth to the ocean and the thickness of those oceans and we can map the surfaces and the geological processes that have affected these satellites in detail. We'll also understand a lot more about their chemistry.
Then we'll understand Jupiter system as a whole by understanding the interactions of these moons with the magnetosphere of Jupiter, we'll know more about the chemistry of Jupiter itself.
Even the rings of Jupiter and the small satellites and what do they tell us about the history of the system as a whole. Both spacecraft will spend about two-and-a-half years in orbit around Jupiter, making close flybys of the Galilean satellites before entering orbit around their respective target moons.
PR: In terms of sending a probe down to the ice on Europa - or Ganymede - will you be able to do that on this mission?
RP: There is some consideration as to whether it will be possible to send a penetrator - which the UK is looking into - to the surfaces of Europa or Ganymede, or maybe both. That would try to understand the ice properties and get in more detail at the chemistry.
Using seismology it would get at the exact thickness of the icy shells. It's uncertain whether that will come to fruition, but these spacecraft will set up the ability to send landers to these moons in the future.
So, to follow up on Europa and whether there really could be life there, one would want to send a lander to the surface of Europa that could dig down beneath the top layer of ice which is very processed by the radiation environment at Europa and get below that radiation-processed layer and understand what's there. Are there organic materials in that ice?
PR: You've mentioned the radiation environment, how difficult will it be to operate a spacecraft at Europa? It seems like one of the most hostile places to go in the Solar System.
RP: The radiation is the principal challenge to the Europa spacecraft. We have to design a spacecraft that can operate in this very severe radiation environment.
The kinds of radiation-hardened parts for such a mission do exist. The real challenge is to educate the community who will be proposing to build instruments for this mission on how to construct instruments which will be able to operate in this radiation environment.
We've done it before - the Galileo spacecraft made many flybys of Europa and even of Io, which is even deeper in the radiation belts of Jupiter. But never before has a spacecraft spent so much time - as this spacecraft will - in the vicinity of Europa; something like a year we want the spacecraft to be able to function.
So, it's a challenge, but it can be overcome and it's going to return spectacular results, really opening our eyes to how this icy satellite works and, for that matter the mission as a whole, as to how the Jovian system works.
Scientists want to understand whether Europa's ice shell is thin (l) or thick (r)
PR: How will the mission assess Europa's habitability? What are some of the ways in which tidal heating affects the moon, for example?
RP: This idea of how material moves from the bottom to the top (of the ocean) is pretty important. That we could try to get at with radar sounding - (to determine) what's going on inside the ice shell.
With regards to the issue of tidal heating, and how it's distributed, if we can understand the 3D nature of the icy shell, we'll understand better what these "chaos" regions are, for example. One idea is that these chaotic regions are places where the ice has completely melted through.
Another idea is that they're areas where tidal heating has been concentrated in pockets of warm ice that get tidally heated to the point where it runs away and partially melts. By using gravity, we might be able to get at the topography of that seafloor and the local thickness of the ice shell and how it varies across the satellite.
Those get at issues of tidal heating. If it's extreme tidal heating like we have at Enceladus in the Saturn system, we might be able to measure the heat coming off the satellite itself. In specific regions you might be able to find hotspots. Again, exchange processes are very important: how do you get stuff from the oceans to the surface? Is it by melting? Is it by convection - where warm "blobs" of ice rise from the bottom to the top?
If convection is going on, the ice shell must be about 20km thick. We want to understand if that means that material from the ocean can still get up to the surface. Convection is a way that can happen - to dredge material up from the ocean to the surface.
If the ice shell's thin and it's melted in spots, it's a direct transport. But if it's thick, we want to understand if convection is getting us material from the ocean to the surface. And for that matter whether it can transport surface material rich in oxidants - which could be a fuel for any life that's there - down to the ocean.
We want to try to understand the chemistry of Europa's ocean by both infrared techniques and by trying to understand the composition of particles that make it from the surface up to the spacecraft. Charged particles hit Europa's surface and knock particles off to make a very thin atmosphere. We can measure the composition of that atmosphere directly from the orbital spacecraft around Europa.
The ice shell of Europa has been cracked and shifted about
PR: What do you think of the prospects for finding sub-surface oceans on the moons Ganymede and Callisto?
RP: The evidence from the Galileo spacecraft is that those satellites do also have subsurface oceans. But they haven't been well characterised. It's the same as Europa: we really need to confirm it and characterise those oceans. The best evidence for the oceans comes from the Galileo magnetometer experiment that says the subsurfaces of these moons are behaving like conductors, so there must be a salty water layer down there.
But Ganymede and Callisto are different beasts because they have thicker H20 layers in total (ice plus water). They are so thick that beneath the ocean layer there is more ice, but a higher density ice - a different form of ice that sinks relative to water.
So it's an ocean sandwich, with the ocean at about 150km depth, and then regular ice above and higher density forms of ice below. Those oceans aren't expected to be in direct contact with rock below, which makes Europa's ocean a more attractive place to think of in terms of habitability.
[Ganymede is] the only moon with its own magnetic field. That implies a hot, convecting metal core down within Ganymede today
Also, with Ganymede and Callisto, the oceans are much deeper below the surface. Europa's is thought to be maybe 20km below the surface, wheras Ganymede's and Callisto's should be something like 150km. It's more difficult to characterise those oceans; it's much less likely - probably impossible - that radar would penetrate all the way to the oceans.
But we'll be able to get at the shallow subsurface structure. With magnetometry, we could better characterise the depth and thickness of those oceans. We'll be able to get some hints at composition, but it's not yet clear if those oceans have erupted on to the surface somehow.
It's much less likely that they'll have melted all the way down, but maybe convection can dredge up material. So those oceans are a little more mysterious, a little harder to get at. But it makes an important comparison to Europa. The real value will be to understand the context: how all these worlds relate, how they're similar, how they're different.
With Ganymede, a really important objective is to understand its magnetic field. It's the only moon with its own magnetic field. That implies a hot, convecting metal core down within Ganymede today.
So understanding that magnetic field is going to tell us a lot about why is Ganymede hot and how does that relate to past activity of all the satellites with the goal of bringing together the big picture of the Jupiter system and how these moons relate to one other.
Io is the most volcanic place in the Solar System
PR: The mission will also investigate Jupiter's moon Io. What do we want to know about Io and the volcanic activity there?
RP: Io is the most volcanically active body in the Solar System. The plan is for the Nasa spacecraft to make three or four encounters with Io. Getting at the composition is key. From a fast flyby we should be able to directly get at the composition of the atmosphere. We might even be able to fly through a plume and understand directly what is that stuff coming off Io.
Then we can understand whether that is the stuff polluting Europa and understand the transport processes from Io to Europa. Understanding the interior of Io is key. We are going to have to see what is realistic from the close flybys, but there are ideas that might be able to test whether the interior of Io is really molten - as one model suggests. How melted, how hot is that interior?
Then, by understanding the distribution of volcanic centres, of the heat coming off Io and where the volcanoes are, combining with past data from Galileo, we can get a better picture of where the tidal heating is distributed within Io and a better idea of how the tidal heating process works.
That doesn't need to be done in close flybys - that comes from monitoring Io over time. We have two-and-a-half years in the Jupiter system before going into orbit around the satellites. One can even get a handle on the composition from distant observations, in the ultraviolet and the infrared, by watching Io erupt.
PR: The community of scientists who study icy satellites has been wishing for a Europa mission for decades. What does this mean to them?
RP: It's been a long, arduous effort to get to this point - to make the scientific case for the importance of Jupiter system exploration - and Europa and Ganymede in particular. The scientific community is just thrilled to be able to move forward in exploration of the outer Solar System.
We see this as the next in a series of flagship missions to explore the outer Solar System. In following up on Cassini's discoveries at Saturn - especially the geysers of Enceladus and the methane-ethane lakes of Titan - we want to send a mission to explore Titan in detail and orbit that moon in future.
But it takes a long time and lots of money to get to the outer Solar System, so it will be little while before we're ready to get back to Saturn and Titan, but it will happen.