To add to the other answers, then it’s actually quite a challenge to land on Europa. I’m not at all sure we should be doing it now even.

First, the surface is unknown at the scale of meters, most of it. One theory for instance is that parts of the surface might be covered in closely spaced vertical “ice blades” or “ice knives” which would make a landing there hard to achieve. On Earth these blades form quickly. On Europa they would take millions of years to form, but it’s the same basic process. As Daniel Hobley said: "Light coming in at a high angle will illuminate the sides of the blades, causing them to retreat away,"

These are called Penitentes. See Penitentes: Peculiar Spikey Snow Formation in the Andes

This video shows how they form on Earth and decline, time lapse:

Here is a photo from the European Southern Observatory site high in the Atacama desert:

Planetary Analogue, see also their Icy Penitents by Moonlight on Chajnantor, and Iconic, Conical Licancabur Watches Over Chajnantor

ICE KNIVES ON EUROPA

On Europa, if they exist, they can potentially be meter scale or higher, and with no atmosphere, the conditions on Europa might well be ideal for their formation. Our missions to Europa so far haven’t taken high enough resolution photos to see them. Ice blades threaten Europa landing - BBC News

They wouldn’t be the result of ice or snow subliming into an atmosphere, obviously. It’s a slightly different process. Instead they’d be the result of the sunlight causing the ice to sublime to water vapour in a vacuum at very low temperatures. Also they would form slowly over much longer timescales, of millions of years.

The surface of Europa is about 50 million years old, so when asking if penitentes can form on Europa, one of the main questions is, how much can the ice there erode under the influence of sunlight in 50 million years? The answer to this question is extremely sensitive to the peak temperatures on Europa, to the extent that twenty degrees can make a difference between formations that are meter scale and ones that are on the scale of millimeters.

In the paper: HOW ROUGH IS THE SURFACE OF EUROPA AT LANDER SCALE? Hobley et al produce this table

So, for a surface temperature of 132 K (about -150 C) it loses about 5.66 meters over the average age of the surface of 50 million years. For a temperature of 128 °K (-154 °C) it loses 1.28 meters in 50 million years, tailing off to 1 cm at 116 °K (-166 °C), and only millimeters at 114 °K

So this is very sensitive to the peak surface temperatures of Europa. Also, the surface is eroded by sputtering from the Jupiter radiation and from bolide (meteorite) impacts. That would counteract the effects of the ice blade formation at temperatures of 126 downwards. They conclude in the paper that the knives could be from one meter to 10 centimeters in height, probably restricted to within 15 or 20 degrees of the equator.

However Europa also has “true polar wander” by which the entire crust moves over the subsurface ocean. This could reduce the size of the blades but also move the ice blades away from the equatorial regions.

UPTURNED ICEBERGS - FOR REGIONS LIKE THERA MACULA - AMONGST THE MOST INTERESTING REGIONS ON EUROPA

Other issues could include a frozen landscape consisting mainly of upturned icebergs. According to some ideas, then hot plumes of melted water rise from the deep subsurface sea and eventually reach the surface and produce these irregular landscapes as icebergs form on the freezing surface and then turn over.

One of the most interesting regions, thought to be most likely to have thin ice over liquid water by the “thin icers” is the Thera Macula

This might be a region of overturned icebergs with, perhaps, liquid water still present only a short distance below the surface. Most of these chaos regions are raised, which suggests the ice below them that lead to their formation has frozen. But Therea Macula is actually a dip in the surface of Europa which may be a sign that it has the denser melted water still beneath it. See Is Europa's ice thin or thick? At chaos terrain, it's both!

POSSIBILITY OF LIQUID WATER CLOSE TO THE SURFACE OR BREAKING THROUGH

So there could also be liquid water close to the surface. Geysers are another possibility. So again there may be a small chance of our lander crashing through thin ice or a soft surface, especially if we land it on the most interesting regions such as Thera Macula. Or it could fall into a crevasse and be unable to communicate.

I know the plan is to orbit Europa for a while before the lander gets there, but what if the orbiter doesn’t find any suitable spot for the design of lander, and decides a different design of lander is needed, or no lander at all? Maybe the lander has to land somewhere uninteresting, or they have to hold back from landing at all for planetary protection reasons?

CAN WE STERILIZE A SPACECRAFT 100%

Then the other problem is that we don’t know how to sterilize a spacecraft 100%. Or more accurately, we can sterilize a spacecraft completely, but the methods that do this, such as prolonged heat, or ionizing radiation, also destroy the electronics so it won’t work any more. That includes of course the ionizing effect of Jupiter’s radiation - although the surface of Europa is riddled with ionizing radiation that would quickly kill any human, any spacecraft there has to survive this, at least up to the landing, which would mean that it is protected sufficiently that microbes could survive also.

If there are some microbes on the lander, and they survive to the landing, then it might impact into liquid, or create a liquid area due to a crash on Europa which might be deep enough to shield microbes so they can reproduce there. Or microbial spores brought to Europa with the lander could eventually in the future over thousands or years find their way into the ocean.

KEEPING SAFE - SAMPLE GEYSERS

I think the best solution here is to focus on sampling any geysers instead. We can already do that with a mission to Enceladus. This is less known but it also may have life.

Geysers on Enceladus (moon of Saturn). A spacecraft could fly through these geysers (Cassini has done so many times now). It could do a detailed analysis and even a life search as according to some theories, the water in these geysers was in Enceladus’ ocean as recently as a few months before they are ejected into space. Europa may have geysers also but with its larger gravity they may not go so high into space, so may be harder to spot.

For these reasons I think with Europa we should focus on an orbiter or flybys first. I think we shouldn’t design a lander at all, but we should put instruments on the orbiter that have the dual capability, as for Cassini, to analyse any plumes found on Europa since that seems a distinct possibility.

FAST FOLLOW UP LANDERS

By the 2030s when the mission gets there, then our technology may be so advanced we can send a follow up orbiter or lander within months or a year or two. In any case I think we simply should not risk a lander at this stage due to planetary protection issues unless we can sterilize it 100%, or somehow can prove that there is no significant possibility of it irreversibly introducing Earth microbes to Europa. Even a 1 in 10,000 chance of contaminating Europa with Earth life, I think would be too high, given what we may be risking there, some unique discoveries that we could never do anywhere else. E.g. it could be some early form of life, not as far evolved as DNA or evolved in a different direction, which might be very vulnerable to DNA based life. And it’s probably impossible to do an accurate assessment of how likely it is that we could irreversibly introduce Earth life to Europa by mistake, we just don’t know enough yet about Europa or about exobiology with no examples yet of any known exobiology to base our decisions on.

Again by the 2030s we may have the technology to sterilize a spacecraft 100% without destroying the electronics. I hope so!

MISSION TO ENCELADUS GEYSERS - AND PERHAPS IDENTICAL MISSION TO EUROPA

Meanwhile one thing we can do right away is to send a mission to Enceladus to analyse its geysers close up, and it would be reasonable I think to send life detection instruments on that mission too. Instruments that would help with analysing whateer is in the particles, able to detect complex organics, and also able to find indications of life too if present.

If funding permitted, perhaps we could also send an identical orbiter geyser fly through mission to Europa “on spec” just in case we find geysers there, to save time. I think that would be less risky than a lander, no danger of crashing, and likely to add to our understanding of Europa even if it has no geysers, by examining the region around Europa just as Cassini did for Rhea etc.

There’s some evidence already of possible water plumes from Europa - though it’s a one off observation by Hubble which hasn’t been repeated. It might have just been a meteorite impact. If it is evidence of geysers, that could be very exciting for search for life on Europa. Water Plumes on Europa: What Lies Beneath?

In any case as I said, I think we should equip any Europa orbiter with similar instruments to Cassini which would help with analysing any dust or ice particles or gas around Europa with the capability of detecting complex organics, which may be in them whether or not Europa has life, and I think we should add chirality detection at a minimum. There’d surely be some dust or gas to analyse even if there are no plumes.

SAMPLE RETURN - WHY QUARANTINE DOESN’T WORK

Later we may do a sample return to Earth. But I think we can do a lot in situ - even just plumes, the composition probably varies depending on where you fly through them, and also on the position in the orbit, may also depend on conditions in the deep ocean, maybe it has the equivalent of algae blooms down there from time to time.And we have many instruments now we can send to do in situ searches, miniaturized “lab on a chip” that just ten years ago would fill an entire laboratory which also have minimal power requirements too.

As for returning it to Earth, if we return a sample likely to contain life or with some chance of life, I think we should return to above GEO, furthest in delta v away from both the Earth or Moon and study it telerobotically from Earth until we are sure what is in it and what precautions are needed if any. I don't think it makes much sense economically, legally, or that we can even do it safely, to try to build a facility to study all conceivable forms of exobiology on the surface of Earth quite yet when we don't yet know of a single example of exobiology outside of Earth.,

Quarantine just doesn't work, see my Case For Moon First - why quarantine doesn’t work - this is for Mars, but the same would apply for life returned from anywhere. There is no way we'd abandon an astronaut permanently in the ISS if they were exposed to Europa samples, I don't think it is ethically or legally possible to do that even if they consented.

A quarantine period only works if you know what you are quarantining against and how long you need to do the quarantine for. And the risks are not just the effects on humans but effects on many other creatures, plants etc as well as long term effects on ecosystems. We can't test that in space just by exposing lifeforms to it. I think there is no alternative to really knowing what is in the sample before you decide whether you can expose humans and other lifeforms to it and what precautions to take. Which means you have to study it somewhere isolated from Earth's lifeforms first and I think you'd need stupendously reliable methods to do that on Earth, far better to do it telerobotically in orbit.

So, I think that’s the way ahead myself, sample the plumes in situ. Return samples eventually but to above GEO. And if we do a lander, it needs to be sterilized sufficiently for biologically reversible exploration. We must not introduce Earth life irreversibly to Europa or Enceladus, in my view. If it’s likely to encounter liquid water, or create a liquid water habitat through impact, hard to see how anything short of 100% sterilization would be sufficient. We need some way to sterilize all the life from a lander while keeping the electronics intact. One promising approach may be to use supercritical CO2 snow in combination with other methods.I think that may be possible in the future but we can’t do it quite yet.

ON HUYGENS AND TITAN

Sorry forgot to cover that - though it’s covered in the other answers. Huygens was an easy experiment if going to Saturn's system anyway. With Titan’s thick atmosphere they could use aerobraking. Also the Titan’s surface is so cold Earth life can’t survive there so there were almost no planetary protection issues. (Titan may have a deep subsurface liquid water ocean but if so, then there doesn’t seem to be much communication with the surface).

And actually there is a remote possibility of life in Titan’s oceans though it would be very exotic for us. First, because of the extreme cold, it would surely rely on chemical reactions that run much faster at those temperatures than the ones in our cells would do - otherwise the life there would be very sluggish. There’d be evolutionary pressure to use faster chemical reactions.

Also it would have the cells kind of “inside out” with non polar molecules facing the methane / ethane oceans, because those are non polar liquids, instead of the usual arrangement. Indeed it might be made of a small polar molecule like acrylonitrile sticking together to form a non polar membrane in the non polar solvent of methane

Normal cell walls are arranged to be polar (having regions of positive and negative charge both inside and outside to attract the water) like this:

The tail repels water (hydrophobic) so naturally meets together in the middle of the cell wall. For details of this idea that life on Titan would need to use a non polar membrane, see Is There a Kraken in Kraken Mare? What Kind of Life Would We Find on Titan? - Universe Today, and Possible oxygen free cell structure made of organic nitrogen compounds that could function at the low temperatures of Titan's ocean.

Glint of sunlight on the lake region around the northern pole of Titan.

Cassini did make some measurements that Chris McKay interpreted as possibly a sign of life processes on Titan, though he listed several other possibilities that may be more likely: Alien Life on Titan? Hang on Just a Minute… - Universe Today

However Hugens was not designed to search for life. Maybe some future spacecraft to Titan’s oceans will take off from where it began?

See also Prebiotic Chemistry on Titan?

SAFE AND EASY LANDINGS FOR EUROPA - “ICE BREAKING” INSTEAD OF “AEROBRAKING”

Huygens was an easy experiment yes, for Titan. We can’t do aerobraking on Europa.

However you could do equally easy experiments for Europa - one idea is a penetrator, using what we could call "ice breaking" to slow it down. I'm not a fan of that myself for planetary protection reasons unless the penetrator can be sterlized 100%.

PLANETARY PROTECTION FRIENDLY VERSION - ARTIFICIAL GEYSER - BERND DACHWALD’S IDEA

However there’s a planetary protection friendly version of it. You could use two spacecraft - a dumb penetrator consisting of just a metal slug, easily sterilized. This sends a plume of ice into space. You could use two “dumb penetrators” with the second one closely following the first for more effect.

In effect, you are creating an artificial geyser here. This would be followed by a low flying orbiter to capture the sample for analysis.

That would have minimal planetary protection issues if the dumb penetrators can be 100% sterile - e.g. just lumps of metal heated beforehand to temperatures where no Earth microbes could survive or otherwise 100% sterilized before impact. This is an idea Bernd Dachwald (head of the German IceMole project) once suggested to me in conversation, which I think is an interesting one.

CHIPSATS FOR EUROPA - COULD THEY BE 100% STERILIZED?

Another interesting idea, here is an old mission idea to send “chipsats” to Europa’s surface, each one rather “dumb” but lots of them, each one consists of just a few sensors on a flat chip. Some would fail but enough would get through, and they would be able to survive impacts that a larger more complex lander couldn’t.

That sounds like a kind of a lander that is so minimal, perhaps it could be 100% sterilized by supercritical CO2 snow or something similar? That’s a technique that can remove all the organics from the surface of an electronics chip without damaging the chip. It’s been shown to work with USB drives. So though it might be tricky to scale up to a complete spacecraft, I wonder if it is good enough to 100% sterilize chipsats? It would have to be 100% reliable.

"ATTEMPT NO LANDING THERE" --New NASA Mission to Europa will Ignore Arthur C Clarke's Warning (2014 Most Popular)

CAN WE ACHIEVE 100% STERILE ELECTRONICS FOR A EUROPA LANDER?

There’s no in principle reason to prevent 100% sterile electronics. You just have to find some process that electronics can withstand and life can’t. If you heat metal to hundreds of degrees C for instance, no life will survive and the result will be 100% sterile. The problem is that this will destroy the spacecraft electronics too. So can we find a way to sterilize it of Earth microbes without destroying the delicate equipment? That’s the big question here.

Also all this might be far easier to do with a chipsat than with a large conventional spacecraft.

First one method being explored by the European Space Agency is Deep cleaning with carbon dioxide. and Science Daily article about it.

• CO2 a liquid at 100 atmospheres and 50 C.
• And then on release of pressure turns to snow and takes the dirt, organics, everything away leaving the surface dry.
• Mixed with Hydrogen peroxide and other chemical to increase effectiveness.
• Can be used even with sensitive electronics. Was used to clean usb drives in testing and they functioned afterwards.
• Surface is left with no trace of organics, not just with dead micro-organisms. Major plus!

Could you remove all traces of organics from the exterior in this way? And - can you also keep exterior and interior separate so there is no chance of leaking contamination from inside the mole?

HIGH TEMPERATURE STERILIZATION

Then also, if you can make the whole thing able to withstand high temperatures, you can just heat it up to a high enough temperature to sterilize all life.

The main issue with sterilizing modern spacecraft is that many instruments are quite delicate, also they can go out of alignment,so even the sterilization temperatures used for Viking of 111 °C for 40 hours is too much for them.

But there are electronic circuits now designed to operate at up to 200°C . High-Temperature Electronics

And there are other developments that should permit temperatures of 200°C upwards :).High-Temperature Electronics Operate at 300 degrees C | EE Times and Designing for extreme temperatures

There’s an economic incentive for developing these electronics, as they are useful in oil wells and motor cars.

I’ve never seen this suggested for a way to keep Europa landers sterile, but it sounds as if it should work!

Back to the drawing board probably for a lot of the designs to make the whole thing uses chips and solders etc that work up to high enough temperatures for 100% sterilization. But it seems like it may be possible! Thanks to Adeel Khan for the suggestion

Is this right? Is it possible to achieve 100% sterilization by heating electronics that’s capable of resisting temperatures of up to 300 C. I wonder if anyone working in the field of spacecraft sterilization has investigated this, either experimentally or in theory. Or is there some other way to achieve 100% sterile electronics such as the CO2 snow approach?

I think we need to look into that myself before we consider sending any probes to habitats that may include liquid water habitable to Earth life. Except of course for the plume flybys. They are safe so long as the ice particles they collect can’t dislodge microbe spores and return them to the liquid water in the subsurface oceans. That sounds likely to be for all practical purposes, zero risk though you’d need to examine it carefully of course.

MULTIPLE METHODS AT ONCE

Perhaps for the best results both can be used one after the other. High temperature to make sure there is nothing viable. Then CO2 snow to remove the organics as far as possible. Heat it up again before it is released from the orbiter for a final precaution to make sure.

Especially for electronics in an impactor / penetrator as that would have to withstand high g force and perhaps high temperatures too, so it would need to use specially hardened electronics. And it needs to be hardened for the ionizing radiation for Europa as well so you are hardly talking about “off the shelf” electronics here.

A RATHER MORE FAR OUT IDEA - 3D PRINTER ON EUROPA PLUS RAW MATERIALS FOR SOME OF THE COMPONENTS

Another idea, just for fun for now - but: land a sterile 3D printer + some raw material feedstock for it, also sterile. The surface would be high vacuum, ideal for electronics. First thing it does is to 3D print a shelter for itself or dig below the surface for protection from the cosmic radiation. Then it sets about printing out whatever you need, including a Europa submarine from the sterile components you supplied it with. If it is a nanoscale printer it can do circuit boards as well. So all you need to do is to send it some sterile chips to attach to those circuit boards, and other hard to print out components pre-sterilized. Most of the rest it does itself.

This is a bit far future perhaps.But perhaps some element of 3D printing could help for an idea of partial in situ construction of devices for helping to study Europa in a sterile way? Especially small chipsat type devices. Sterile electronics plus 3D printing of some extra components to help with mobility or sampling or some such.

IF WE CAN’T ACHIEVE 100% STERILE LANDERS FOR EUROPA

If we can’t do it, I think we simply should not send a lander or submarine to Europa until we can, and should not risk introducing Earth microbes to a habitable environment on Europa.

It is just risking too much to do that. Not just for us, not just for the mission that goes to Europa right now, but for our descendants and indeed all future civilizations on Earth also. It would be just tragic to find some interesting form of exobiology on Europa only to know that we have seeded Europa with microbes that will eventully make it extinct.

It could be very vulnerable to Earth life. The example I like best there is the idea of some primitive early life, for instance RNA based, or even an RNA ocean or autopoetic cells. If Europa was like that, then introduced Earth microbes in a globally connected ocean through exponential growth would surely do short work of converting it all to DNA based life.

WHY NOT JUST SEND EARTH LIFE THERE (MORE DETAIL)?

Because then we won't be able to find out about the life that is already there, if there is any - or pre-biotic or non biotic chemistry - or whatever there is there right now. Especially since our life could make it extinct. About half of Earth's biological history in terms of gene complexity is unknown to us. We just have no idea how the early organic chemicals developed into lifeforms as complex as the simplest microbes. Lot's of sketched out suggestions but no answers and it is way beyond any attempt to simulate in a laboratory.

Well one likely thing to find in the Europa ocean, if life is common, is some early form of life. Maybe RNA based life. Maybe just an RNA ocean. Or maybe autopoetic cells. Or some primitive lifeform that reproduces, sort of, but not nearly as accurately as DNA life does. Or perhaps it's RNA based using ribozymes in the place of rhibozomes, everything done in RNA. And that's just a few examples based on what might have happened in our own planet's past. Europa life may well not be related to Earth life at all. In the entire history of the solar system, at most a handful of rocks may have made it from Earth to Europa. So it could be something else as well.

As those examples show, it could be very vulnerable. An RNA ocean say, or RNA only lifeform could perhaps become extinct after just a few years of exponential growth after the first contamination by Earth life throughout the entire ocean, especially if it is all connected and its ocean has food sources for the life to use. And however quickly or slowly it happens, there is no way we could reverse something like that once it got started. It would be the worst possible anticlimax to all our searches for life in our solar system, to know that Europa was such a biologically fascinating place, until the first probes from Earth landed there, and is no longer like that.

Until we know what's there, I think we have to treat every potentially habitable planet or moon or other habitat in our solar system as if it was the only one of its type in the solar system. Because a lifeform that evolves in Europa's ocean may well not evolve in Enceladus, or Ceres or on Mars or whatever place you study next. It could be our only opportunity for light years in every direction, to study such a lifeform.

• Perhaps they all have different unique lifeforms or types of pre-biotic life.
• Perhaps they all have almost identical independently evolved life (very surprising I think).
• Perhaps life from a previous star seeded them all or most of them.
• Perhaps only one of them has life, or none of them do.
• They are sure to have complex chemistry and we can learn from that also, maybe learn that life evolves only with great difficulty and find out what happens when it doesn't evolve too. We won't know until we find out.

As for experiments in Earth based life in space - we can do closed system habitats to try that out anywhere. For instance the Moon may have vast caves kilometers in diameter, so maybe we do it there. Or in free flying space habitats. There's enough material in the asteroid belt alone to create habitats with a total land area a thousand times that of Earth. There may be many opportunities to do that. We don't need to have as our first priority to turn everything into the closest possible approximation to Earth we can imagine, especially a very poor imitation of it, an ocean covered in kilometers of ice with the harsh environment of Jupiter's radiation on the surface, and too far from the sun for most photosynthetic life to be practical and not at all in its oceans (except for life that uses the heat radiation from hydrothermal vents for photosynthesis).

And meanwhile constructed habitats from asteroid materials can be designed with whatever environment you like, tropical gardens if you like, depending how much sunlight you reflect into it using space mirrors or solar collectors, or simulate conditions on Europa or Mars or other places in our solar system if that's your aim. Or you could simulate some the conditions on an interesting exoplanet. You can use spinning habitats with artificial gravity for whatever level of gravity you want, too.

That's looking forward a bit there - but only decades, centuries at most. You could build a Stanford Torus habitat within a decade or two with the funding and political will to do so even with present day technology. If we want to explore setting up habitats with Earth life in it outside of Earth, I think things like that would be the way to go - starting on a much smaller scale first probably. You could start with small exovivaria in LEO or on the Moon, and experiments with closed system recycling.

While there’s no way we can duplicate the billions of years of Europa’s history and the vast oceans larger than Earth’s oceans. If we mess it up, then the nearest “Europa” analogue may be light years away. And even then, chances are that if Europa and some Europa analogue both have life, even then most likely it has its own unique lifeforms, probably not even the same informational polymer in the place of whatever Europa has - not at all likely that it has the same lifeforms or proto life that evolved on Europa.

SO WHY DID THEY SEND A LANDER TO TITAN INSTEAD OF EUROPA- WRAP UP?

I think it might be partly that they were sending a spacecraft to the Saturn system anyway. In the case of the Jupiter system, then it’s much harder to visit Europa for more than a short time because of the ionizing radiation. Still you could do a penetrator with a fast flyby and that would work much like Huygens. It could communicate back to Earth during the flight to Europa and if it survived the landing, do some experiments and report back during its design life whatever it is.

But it would have many more planetary protection issues to work through than a Titan mission. I think myself best to wait for the orbiter mission first before we decide what to do next. We might well find plumes as for Europa and that would make it really easy to sample it’s ocean with a low flyby or orbiter and then we might not need a lander at all for the first missions there.

SEE ALSO

See also my "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus

You might be interested in the facebook group: Search for ExtraTerrestrial Life - Europa & Enceladus + Subsurface Oceans

See also my As Philae Awakes - Where Might Life Hide In Our Solar System?

EXPANDED VERSION OF THIS ANSWER

I’ve done a longer version of this answer which also looks at the Hubble plume observations and the Europa lander / flyby mission ideas here:

Hubble Spots Europa Geysers Again - How They Did It - And What Next? Flyby? Lander?