Artist impression of the geysers and subsurface ocean, credit NASA / JPL / CalTech

So I suggest just as a concept for discussion, that we could have a mission end sterilization cobalt60 ionizing radiation source or similar. Shielded from the mole - and the orbiter of course. But the shielding need not be that heavy because it just needs to prevent radiation in the direction of the mole itself, not in all directions. Just a plate in between of sufficient thickness, maybe on a boom if necessary to reduce the cross section and give some extra attenuation.

Then at end of mission, to remove the shielding, and the mole then remains there, but irradiated by several thousand Grays a week or whatever is feasible.

The most radioresistant life can withstand around 50,000 Grays. So to be safe, ensure that that amount of radiation is received in a short enough time so that there is no risk that it lands in the ocean with viable life. But it might need to be more than that if you also have to make sure that it doesn't include even dead fragments of life. Eventually ionizing radiation would break most of the bonds of any complex molecule until all you have left are gases. So just leave it to sterilize itself for all future time.

Sample return with sterilized sample

Then that's the same, after you receive the sample, use an ionizing radiation source, to sterilize it. So we will get material back that is split up into many pieces by the radiation. But still, we can detect biosignatures even after a lot of radiation damage. And as well, we have in situ measurement to inform our analysis.

Potentially, if totally sure it is adequately sterilized, can return to the Earth.

Sample return with unsterilized sample

You'd think this was easy, but it turns out to be surprisingly complex and difficult. I'm going to suggest it is best to find a way to just bypass all these issues. But first let's see what the issues are.

Back in the 1990s then the general idea was that we can just return samples to a glove box facility in a biohazard 4 laboratory.

But as studies were undertaken, the precautions needed got more and more complex. Ending with ideas that need hundreds of millions of dollars facilities with capabilities never tested before.

The problem is that it is easy to contain a known pathogen, say smallpox, or anthrax, or the Ebola virus etc. And that's the general situation for a biohazard laboratory.

But what can you do when you don't know what is in the sample, what its capabilities are, what size it is, or even what biochemistry it has? Quite possibly doesn't use DNA?

XNA based life in the sample return may be sub optical resolution as well.

Smallest size for cells not containing all the machinery of modern DNA generally estimated as about 40 nm. (But depends on advances of science, like all of this, and see the 3 nm theoretical example above).

So successive studies by the National Research Council (NRC), USA (two studies) then the European Science Foundation (ESF) (one study) gradually lead to more and more stringent requirements.

First, reduction to 200 nm by the NRC after discovery of the ultramicrobacteria

Then reduced to 10 nm by the ESF. as a result of discovery of the profligate way that archaea can share their DNA through the tiny Gene Transfer Agents (GTAs)

Credit: Zina Deretsky, National Science Foundation

The red colouration of this pea aphid comes from a unique ability to generate carotenoids itself. It got this ability through horizontal gene transfer from a fungi.

Archaea can also transfer genes between phyla that are as different from each other as fungi are different from aphids. It is an ancient mechanism and so may also be able to transfer genes from life that had last common ancestor with us in the early solar system.

In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day

So if the life is at all related to Earth life, you have the possibility of this exchange of DNA bringing new capabilities to Earth microbes from space. Even if the microbes themselves don't survive.

Recommendations of the ESF:

"Unsterilised particles smaller than 0.01 µm would be unlikely to contain any organisms, whether free-living self-replicating (the smallest free-living self-replicating microorganisms observed are in the range of 0.12–0,2 µm, i.e. more than one order of magnitude larger), GTA-type (the smallest GTA observed is 0,03 µm, i.e. three times larger) or virus-type (the smallest GTA observed is 0,017 µm, i.e. almost twice as large). This level should be considered as the bottom line basic requirement when designing the mission systems and operation.

...
 

"Any release of a single unsterilised particle larger than 0.05 µm is not acceptable. The ESF-ESSC Study group considers that a particle smaller than 0.05 µm would be unlikely to contain a free-living microorganism, but that larger particles may bear such an organism. As self-replicating free-living organisms are likely to be the main concern following a release event, the study group considers that the release of a particle larger than 0.05 µm is not acceptable under any circumstance.

This is one of the designs they came up with in 2008, the one with the telerobotics.

The LAS sample receiving facility uses a fully robotic workforce, including robotic arms that manipulate samples within interconnected biosafety cabinets. Carrier robots would transport the samples around the facility. Credit: NASA/LAS

This is for just a less than 1 kg of samples returned most likely, yet you have to build something like this,

Also their conclusions have to be reviewed continually, based on new research, as they conclude at the end.  For instance there is much active research at present into into a semi synthetic minimal living cell or an artificial minimal cell. Does the 40 nm size limit still apply based on the recent research?

And it doesn't address at all issues of human error, accidents, terrorism, a plane crashing into the facility etc (as they also say in the study). So that needs to be considered as well.

Meteorite extinctions

You might think that we don't need to worry, because so much material arrives on Earth from outer space through meteorites.

However

• Material even from Mars, the closest, has spent hundreds of thousands of years in transit (most recent impact on Mars able to send material to Earth) radiated by cosmic radiation
  
  Zunil crater on Mars, diameter 10 km, a young candidate crater source for Mars meteorites on Earth. A crater needs to be around this size or larger to send material to Earth. Most of the material arrives at Earth between 16,000 and 20 million years after impact, and most of it is thoroughly sterilized by cosmic radiation and solar flares in transit.
• Capsules can return microbes that could never get here on a meteorite
• No known Earth meteorites from the Jupiter or Saturn system,, and chance of a sample returning that way from Europa or Enceladus is tiny.

And how do we know that meteorites don't cause extinctions over timescale of many millions, even hundreds of millions of years?

The NRC looked into it in assessment of a Mars sample return, and they concluded:

Certainly in the modern era, there is no evidence for large-scale or other negative effects that are attributable to the frequent deliveries to Earth of essentially unaltered martian rocks. However, the possibility that such effects occurred in the distant past cannot be discounted.

What do we need to concern ourselves with?

Most science fiction scenarios are implausible and wouldn't work. E.g. a virus able to infect humans.

But there are things we should be concerned about

• XNA based life that Earth life can't recognize as a hazard, able to take up residence in our ecosystems, our animals, soil, the sea, or even ourselves (like legionnaires disease in our lungs, uses same mechanism it uses to infect amoeba to infect human lungs).
• XNA life could create molecules that closely resemble life molecules but are not identical and taken by mistake. Like BMAA.
   L-serine, resembles  BMAA which is created by green algae.
  It's been suggested that BMAA can be misincorporated to cause  tangle diseases like Alzheimers.Perhaps in a similar way (my own suggestion here) an XNA based lifeform could generate organic molecules that resemble amino acids used by Earth life and be misincorporated to cause protein misfolding and tangle diseases. It doesn't need to be adapted to Earth life to do this.
• XNA life that is  better than Earth life, more efficient metabolism say. Even better in all respects than DNA, smaller cell, more efficient systems of replication and metabolism. It could take over from other micro-organisms in Earth ecosystems. And may function differently from them.
• GTAs that tranfer new capabilities to Earth life.

On the first possibility, Joshua Lederberg who first characterized the Archaea said:

"On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse"

Not saying at all that it is going to happen, or is inevitable. As Carl Sagan said once for Mars,it could be that you can ingest kilograms of Enceladus life without any ill effects.

But if you don't know what is in it, you have to be prepared for anything

Legal complexities

Margaret Race covered these in an excellent paper, there's far more to it than you'd think. Back in days of Apollo, then the quarantine rules for the Apollo 11 return were only published on the day that they launched to the Moon. That would simply not be permitted today. And the Apollo regulations have lapsed BTW.

And there are many domestic and international regulations to be negotiated and new laws to be passed. She considered the whole process likely to take ten years or more, and it can also potentially involve the domestic laws of nations that are not receiving the sample, because of the potential effect of the worst case scenario could impact on all nations.

First idea to bypass all this - characterize before return

So this whole baroque complexity can be bypassed completely if we simply don't return it to Earth quite yet.

So the simplest solution is to characterize the sample before you return it. If we start with in situ missions first, then by the time we do a sample return, we may know what is in it so well that we don't need to do anything much. Either we know it is harmless, or we know it is hazardous but know what needs to be done to prevent harm.

Second idea, to return to HEO

If we do return and are unsure, then I think safest return is to High Earth Orbit. Above geostationary, or geostationary.

With L1 or L2 it could potentially spiral out and then after flybys of Earth and Moon leave the Earth and enter independent orbit around the sun and eventually return and impact Earth.

In HEO then it's orbit is stable for the indefinite future. So we have plenty of time to decide what to do.

If so that could be a low cost sample return from Enceladus.

We don't need to build a receiving facility in orbit. The capsule needs to be returned with trajectory biasing, obviously not aerobraking in LEO, and need a way for it to get into HEO without close approaches to the Earth that endanger collision with the Earth.

Once there in a stable orbit, we may enclose it in a larger "holding" spacecraft. Then anyone  can send their own robotic missions up there to study it.

This is for the 2030s. So by then we can send up equipment far more capable than our current Mars rovers to study it.

Also with the sample so close to Earth we have near to real time telepresence.. By then probably already be using telerobotics routinely for repairing satellites I think. Dextre is gradually step by step working towards satellite repair capabilities, as is Darpa, involved in an initiative for repair of civilian spacecraft in GEO

And by then with heavy lift we'll be able to send hundreds of tons into HEO.

And telerobotics surely much advanced. We already have telerobotic surgery as routine.

FIGURE 1: The Da Vinci Telerobotic Surgical System permits the surgeon to perform an operation on a patient from a remote site. Currently, the FDA requires the surgeon to sit physically in the same room as the patient on whom he is operating.

The first ever trans-continental telerobotic surgery was done in 2001, when a patient in France was operated on by a surgeon in the US in order to test the feasibility of intercontinental operation - a successful operation to remove a gall bladder. (Surgeons were at hand in France to step in and halt proceedings if needed, but they were not required)

So that should completely safe for planetary protection so long as material is only transferred one way. And no chance of impact with Earth.

So that then would simplify everything. No need to convince general public and ourselves that we have everything covered and that nothing can possibly escape containment. Because it is in a stable orbit at GEO, or just above, obviously can't impact on the Earth.