The ancient oceans of Jupiter's Europa and Saturn's tiny Enceladus are hidden beneath an ice sheet kilometers thick. They may have ET microbes, even multicellular swimming creatures around hydrothermal vents. Or they could have imperfectly reproducing "protocells"; a window into the first stages of evolution.
These conditions, which make them so habitable, and interesting for astrobiology, may also make them especially vulnerable to invasive species. Cassini orbiter found geysers at the south pole of Enceladus, continually venting sea water from its ocean into space, as ice particles. This may give us a wonderful opportunity to look at ET life in our solar system without interference from Earth life.
We can also send ice moles and mini submarines to explore these oceans. This may be more of a challenge, for planetary protection. Can we find a way to explore these oceans without any risk of introducing Earth life to them?
I'll focus on Enceladus because we know so much more about its geysers, after many Cassini flybys and close observations. We don't have anything like Cassini in the Jupiter system and probably won't until 2030 when the ESA orbiter JUICE will do a couple of extremely close flybys of Europa before settllng down into an orbit around Ganymede.
WHY ENCELADUS IS SO INTERESTING
Here is a photograph (not an artist's impression) of two of its geysers.
As you can see, Cassini has good cameras. It discovered the geysers in 2005, and since then has flown through them several times, from 2008 onwards. It has one more fly through scheduled for October 28 2015, within 30 kilometers of the surface. This one is also timed to pass through when the plumes are at maximum output (a first for the mission). With its suite of instruments including imaging spectrometers it can do spectrographic observations from a distance and can also capture and examine the material in the plumes close up.
As a result of its observations, we now know that the plumes erupt continuously, and are easily accessible from flybys.
This is what we know about the Enceladus geysers.
3D model of the geyser basin of Enceladus showing the location and tilt of 98 of the 101 geysers identified in 2014. Five of these jets have images taken too close together to determine the tilt precisely - these are shown with dotted lines.
- 101 geysers counted in 2014
- Typical vent is a hot spot of about 10 square meters.
- Geysers occur along fracture lines
- They contain ice crystals, methane, ammonia, and silica
- The silica tells us an astonishing amount about the conditions there
The geysers come from an under surface ocean which has been detected indirectly by gravity anomalies.
Water is denser than ice and the anomalies are explainable by a liquid ocean below the surface of the south pole at a depth of about 30 to 40 km and with a thickness of 8 to 10 kms. Extends up to a lattitude of 50 degrees south, though it could also be global. It also has an internal rocky core beneath this made up of silicates, density about 2.4 grams per cubic cm, so there's the potential for water / rock interactions there. Sapienza Università scientist Luciano Less explains in this video.
From the silica we can deduce (Gabriel Tobie, March 2015 Nature):
- Enceladus subsurface sea has at most 4% salt - so not too salty for life
- It's pH is between 8.5 and 10 so moderately alkaline.
- Originates at an interface between rock and water at temperatures of 90 C and pH greater than 8.5
- The silica particles probably get to the geysers within a few months of formation at the vents, or at most, within years.
These conditions are almost identical to those in the "Lost City" hydrothermal vent in the mid Atlantic..
These habitats occur when mantle rock (the layer below the Earth's crust) is exposed to the surface. The chemistry supporting this life is serpentization: olivine rock plus water and CO2 reacts to create serpentine + magnetite + brucite + ammonia gas + hydrogen gas + hydrocarbons. Then the sulfate in seawater is reduced by hydrogen to produce hydrogen sulfide
Microbes may be using anaerobic methane oxidation as methane consumers (methanotropes) There may also be methane producers (methanogens)
Temperatures of the inside of these pillars: 20-90°C and pH 9-11 (water emitted at 300°C but rapidly cools down). The inside of the pillar is dominated by biofilms of - Lost City Methanosarcinales (LCMS) Microbial community to a large extent independent of surface conditions
These hydrothermal vents show how you can have an ecosystem not dependent on sunlight in any way. This community could probably survive on Enceladus. Also this is one of the habitats suggested for the evolution of life in early Earth, so it seems promising as a place to find life in the Enceladus oceans.
Another article however questions whether silica particles would form in this way in the Enceladus ocean. They reason by analogy with carbonaceous chrondites, which when mixed with water form solutions that are undersaturated in silica.
Another line of research, combining geophysical modeling, and geochemical analysis of the plumes, suggests a different picture of the Enceladus oceans, that it may be very alkaline, about pH 11, with sodium carbonate ("washing soda") present in addition to the sodium chloride.
Interior of Saturn's Moon Enceladus. New research suggests pH of 11 so it may potentially resemble a soda lake.
This would make it similar in pH to a soda lake.
These are also habitable on the Earth, so it could remain habitable to many species.
Artemia Salina, a species of brine shrimp - popularly known as "Sea Monkeys" can survive in soda lakes such as Mono Lake (pH 10) (Wikipedia article on Mono Lake).
Kenya's alkaline lake Magadi also has a fish, the Lake Magadi Tilapia, Akolapia Grahami, adapted to live in water at pH 10, and up to pH 11. It does this mainly by excreting all its nitrogenous waste as urea instead of ammonia.
Lake Natron in Tanganyika. It's pH can sometimes reach as high as 12. The rocks that weather to make it salty have carbonates, but little by way of magnesium and calcium. Because the water is so caustic, it's an ideal breeding site for flamingos that nest there, protected from predators by the alkaline waters. (More about the geology and chemistry of Lake Natron)
This paper has an interesting section at the end about the possibility of life in the Enceladus oceans. Some of their conclusions or suggestions include:
- It's possible that Enceladus never developed a differentiated core (if it did, basalt / water interactions would make the oceans less alkaline)
- In these conditions, all the salt from the undifferentiated interior of Enceladus probably leached into the ocean
- Without the driving forces of sunlight and continental drift, the ocean could come into a near equilibrium with the core.
- Hydrothermal vents are attractive for origins of life because of the wide variety of physical and chemical gradients
- Hydrothermal vents don't by themselves guarantee that life evolves, because carbonaceous chrondites show evidence of hydrothermal vents in the parent bodies - but no evidence of life. Perhaps these chondrites were on the way to life but hadn't yet evolved it, because they didn't have liquid water for long enough.
A study in 2013 showed that, though there is a chance that life on Enceladus or Europa could be related to Earth or Mars life, opportunities for transfer would be rare. In their simulation, between one and ten meteorites get from Earth to Enceladus over the entire history of the solar system. The figures are similar for Europa, and similar figures also apply for transfer from Mars to Enceladus or Europa.
WHAT ABOUT EUROPA?
Europa has water plumes too, but we don't know much about them as they have only been spotted from Earth by Hubble.
Water vapour plumes over Europa, spotted by Hubble in 2014. They haven't been spotted since then so it may be a temporary event.
We don't have any way to go close to Europa and look at them right now, though we will be able to soon, with the ESA's Juice at Europa, and NASA's Europa Multiple Flyby mission proposal. The ESA mission will only make two flybys of Europa, in 2030, because the budget didn't stretch to the levels of radiation hardening needed for more flybys of Europa - but will do this with much more capable equipment than Galileo had.
The Europa plumes could be as interesting as the Enceladus plumes or more so, but we don't know yet. We can only observe the very largest events from Earth.
Also - Europa has much more gravity than the tiny moon Enceladus, so any water geysers travel briefly into space but fall back and never reach escape velocity. (Europa has twelve times as much gravity as Enceladus, and the plumes reach a maximum of 125 miles before they fall back to the surface).
The plume seen by Hubble could be a temporary event (e.g. result of meteorite impact opening a fissure, or a short lived plume), and if so, an orbiter may have to do many flybys before it is lucky enough to catch a plume in action. A re-analysis of Cassini's observations of Europa during its distant fly past of the Jupiter system in 2001 found that Europa doesn't send much material into space, with Io likely to be the main contributor to the ions at the distance of Europa's orbit, and Europa contributing around 25%.
They found no evidence of water close to Europa, though they could find it easily with similar observations of Enceladus. Co author of the study Amanda Hendrix says: "It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus".
Of course, this is the result of limited close up observation of Europa. It may all change with the ESA observations in 2030, and also with the results of the NASA mission if it is approved - with new close up images of Europa and better understanding of its water plumes and terrain. But for now Enceladus seems the surer bet, at least for a flyby mission to study the material in the geysers.
MYSTERY OF HEAT SOURCES OF ENCELADUS
Originally there was a mystery about how Enceladus got enough heat to keep its ocean liquid. Back in 2012, it was still quite a puzzle to get the heat budget to add up.
First, Enceladus is tidally locked. But it's also kept in an eccentric orbit, and as a result has eccentricity tides.
How the eccentricity tides work: because its orbit isn't perfectly circular, it orbits Saturn more quickly when it is closest. But it has to spin at a constant rate. As seen from Saturn, it will wobble back and forth slightly. This creates subtle tides, the eccentricity tides.
The hot spots are very hot compared to the rest of its surface, but also very concentrated along the narrow "tiger stripes" fractures.
This is a heat map taken by Cassini in 2008 and later in 2009.
Zooming in on Heat at Baghdad Sulcus Temperatures measured at up to 180 Kelvin (-93 C) at the hottest parts, confined to an area of a few square meters. The rest of its surface averages -201° C. So the vents are over 100° C warmer than the rest of its surface and the interior of the vents may be even hotter, perhaps hot enough for liquid water.
There is no internal heat in violet coloured areas. Most of the heat from the warm flanks of the fractures, and the interior of the fractures may be warmer than that, possibly warm enough for liquid water just below the surface.
Plumes wax and wane with the tides. But with a 5.7 hour delay which is a challenge for the models. It may be due to a lag in response of the ice to the tidal effects amongst other suggestions.
This is the latest model of the surface processes. Vapour and salty liquid droplets rise and condense near the surface. Latent heat of condensation helps heat up the geysers. In this model, there may be liquid water near the surface, kept liquid by the latent heat of condensation of the vapour from below.
There are two main solutions suggested.
One way is through the lag in the eccentricity tides.
Another way, favoured by the 2014 paper, is due to longtitudinal libration of the crust over a global ocean.
Shows libration of our Moon, both latitudinally and longtitudinally. Small longtitudinal librations of 0.8 degrees (beyond the resolution of studies so far) of its ice crust over a subsurface global ocean may be a source of heat for Enceladus, but this is a prediction of a theoretical model not yet confirmed.
IS ENCELADUS'S OCEAN TEMPORARY OR LONG TERM?
Originally it was thought to be a temporary ocean, because the heat budget didn't add up to keep it liquid. But these newer models favour the idea of a long term ocean.
If Enceladus was completely frozen through, there wouldn't be anything like enough heat from the tides to create an ocean. But once an ocean forms, by whatever method (e.g. impact) then the tides have much more heating effect and the oceans stay liquid. So it's thought that Enceladus can be in either of two long term states - either completely frozen, or with a liquid ocean. And it so happens to be in the state with a liquid ocean.
With these new models it has probably had a liquid ocean for billions of years.
Other recent research looking at ridge patterns in the surface in equatorial regions suggests that the equatorial regions may have had thin layers of ice in the past, now frozen to greater depths - if so - the ocean may be gradually freezing. This doesn't mean that the plumes will stop next year or next century, but over a few more hundred million years it may freeze solid. If so then it may be a temporary ocean, perhaps an event that happens from time to time in its geological history - created as a result of changes in eccentricity of the Enceladus orbit from time to time, age of order hundreds of millions of years, maybe a billion years.
WHAT MIGHT WE FIND IN THE EUROPA AND ENCELADUS OCEANS?
This next graph may not seem that interesting, just some dots on a straight line, but it is potentially quite huge in its implications. This is from a paper which charted the growth in complexity in DNA.
The authors' insight was to measure only the "non redundant" DNA. So they left out junk DNA and also DNA duplicates such as redundant chromosones. They got a nice straight line and tracing back to find out when the life first started to evolve, you'd expect it to hit the origin at 4.5 billion years in the past. But instead, this is what they found:
So, there are two ways to take this. One is to use the now respectable theory of Panspermia. Perhaps life on Earth originated around another star.
If it did start around another star, perhaps it was an orange dwarf. Why start on a planet orbiting such a rare and short lived star as a yellow dwarf like our sun? When it could start on a planet around the much more numerous, longer lived, more stable, and equally habitable orange dwarf stars?
Anyway, wherever it started, the idea is that this precursor star passed through the collapsing gas cloud when our solar system was forming.
Our sun and planets formed in a "stellar nursery" like this one, the Eagle Nebula's "Pillars of Creation". where gas clouds are currently collapsing to create star systems. Perhaps another earlier star passed through the collapsing nebula and brought life to it. Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)/J. Hester, P. Scowen (Arizona State U.)
Or maybe it passed through at a later stage, when the solar system was partly formed, and there was still a lot of material to bombard its planets and transfer material to the young Earth?
TW Hydra disk - a protoplanetary nebula imaged with evidence of a gap where a giant planet may be forming. This is what our solar system probably looked like soon after it was "born". Perhaps life could be transferred to it by another star system passing close to one of these protoplanetary nebulae in a stellar nursery.
One way or another, it may be possible that ancient life in our solar system has a common origin with Earth life from a previous star.
The other way to take it though, is that perhaps evolution proceeds far more slowly once DNA is involved with all the complexity and error correction machinery of the modern cell. Perhaps all that evolution can happen in just a billion years or so, amongst the simpler precursors to modern life. Or perhaps it evolved more quickly due to special conditions.
Either way, the diagram helps us to see the huge gap in our understanding of biology. Presumably the missing first half of this diagram has as many stages as the second half.
We are probably missing events as momentous as:
- development of cells with a nucleus (Eukaryotes),
- Creatures with a backbone
But we have no idea what these missing steps are. Our experiments in laboratories only just edge in a bit from the left of this diagram.
In between you have the vast area of "Don't know". Many theories, no data.
Another thing that suggests the same conclusion is the immense complexity of the modern cell, with its million chemicals all coming together in a complex dance. There is no way that could just form from chemicals without many intermediate steps.
- RNA polymerase used to decode DNA to mRNA, present in all living cells.
Every cell of our body, and every cell of every living creature is using exactly the same process, and same DNA, same bases, same amino acids. It's all rather "Heath Robinson" or "Rube Goldberg".
It works, and presumably is a good solution to the problems, but it is surprisingly complex, with details piled on details, parts of the "machinery" error correcting other parts, until you get something that works. Also DNA is unstable at high temperatures, and doesn't form easily either. Most researchers think that early life went through a stage when it used RNA only.
Or possibly, some other helix structure such as PNA world which has a different backbone from DNA or TNA world, or a molecule that's a hodgepodge mixing different backbones in the same molecule with non heritable variations in backbone structure (or a whole alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA).
So what were the earliest cells like? What are the simplest possible cells? And what were their precursors? Summary on this: The Origins of Cellular life
There are various approaches to this. One is the idea of autopoesis, that a minimal cell might be able to reproduce, by having a simple structure, a vesicle that takes in material from outside the cell wall.
Diagram of an autopoetic cell, from "Chemical Approaches to Synthetic Biology: From Vesicles Self-Reproduction to Semi-Synthetic Minimal Cells" There, L is the cell boundary, lipids in case of Earth life. P and Q are the basic ingredients of cell growth and W, Z the waste materials. E is the genetic and metabolic network, which converts the ingredients into the cell wall as well as the internal components of the cell creating waste products that leave the cell.
Then, inside the cell, there is a network that turns these precursors into the cell wall itself, as well as using them to regenerate itself, and expels waste products.
The vesicle as it gets larger splits to replicate, or alternatively, it creates a daughter cell inside which then leaves the cell. In these primitive protocells, this is regulated, for instance, by the surface area to volume ratio. This could happen without any DNA or RNA to regulate it. This process happens with some fatty acid vesicles for instance.
Some researchers are working with Butschli droplets, a complex mixture of oils and other chemicals such as detergents, that behave rather like cells. Either as droplets of oils in water or of water in oil. These are not likely precursors for us, of course, but are examples that work like protocells which let us explore artificial life scenarios in a different medium.
Or more complex "behaviour"
Researchers exploring this analogy include Rachel Armstrong and Martin Hanczyc:
We have two general basic approaches of metabolism first or replication first. The protocells can "reproduce" in a way but imperfectly, just grow and then split.
Alternatively, the early precursors of life could consist of chemicals that replicate, with no metabolism or cell wall, as for the RNA world idea.
There's also a lot of research into semi-synthetic minimal living cells. Some start with vesicles and try to insert the replication machinery of a modern cell into them. Other approach is to start with a modern cell and insert reduced size DNA or synthetic DNA into it.
Then there's the artificial minimal cell, starting from the bottom up, explored in the Programmable Artificial Cell Evolution project. The very smallest artificial minimal cells are as small as 103 atoms, based on PNA instead of DNA, making it possible to simulate the whole cell as a quantum mechanical system in a computer. These "cells" can be as small as a few nanometers across.
SO WHAT COULD WE FIND IN THE ENCELADUS OCEAN?
Well if we find an ocean just a few tens of millions of years old, as originally suggested for Enceladus, it could be right at the start of the graph. Perhaps protocells or the simplest forms of life, just a few tens of nanometers in diameter.
It could also be like that even with billions of years of "evolution" if it is really hard for life to evolve.
Or it could be an RNA or PNA world in there. Perhaps no metabolism but replicating chemicals. Or a primitive form of life.
In that case, it is right towards the left of our diagram, helping fill in the huge gap in our understanding of abiogenesis.
But then - it could also be way to the right. We could find a billions of years old ecosystem that has evolved through as many stages as our own.
It could even beyond the graph to the right. Evolved further to some future form of life that hasn't yet arisen on the Earth.
Also, there is nothing, actually, to rule out creatures in the ocean of Europa or Enceladus as intelligent as octopuses, dolphins, or even ourselves. A civilization in these oceans would be unable to use fire. Under conditions of high pressure, sealed from the surface by ice, they would probably be unaware of us and us of them, so far, anyway.
However, as well as that, it doesn't need to be on a linear progression with Earth life. Since DNA is so very particular, and "Rube Goldberg" or "Heath Robinson" in the way it works, what if it the life that evolved in the Enceladus ocean works differently?
Enceladus particularly is so far from Earth, and shielded from us by Jupiter, that it may well have had no exchange of life at all with Earth since the very early solar system. So, of all the places we know of so far tolook for life, it's one of the best places to look for life that may be totally unrelated to Earth life.
The interior of every cell of XNA based life might work in a completely different way from DNA based life. I've heard it said that the interior of a cell is so complex, with its million different chemicals, and elaborate structures and processes, that to researchers studying how cells work, it seems as complex as an entire ecosystem. So, what about using actual ecosystems as an analogy here?
Imagine that you have been brought up in the African savannah - with its grasses and trees and elephants and antelopes. You've never seen a marsh or a forest, or a beach. All your life you've lived in a hut in the African Savannah, never traveled more than a few miles from your hut, and that's the only thing you've ever known.
Then one day someone takes you to the sea shore, with its fish, shellfish, seaweeds, and sea anemones, and perhaps they take you on a dive to see a coral reef.
The interior of a cell of XNA based life could be as different from the interior of a cell of DNA based life as the African Savannah is different from a coral reef. And imagine the new perspectives we might get if we can study it.
So - I see all those possibilities as immensely interesting for exobiology. But they have major planetary protection issues.
The life, if so very different from Earth life - could it be hazardous for us, or vice versa? How can we explore Enceladus without risking destroying the very thing we want to find out about?
So that's where the Enceladus geysers come in. But to frame this discussion, first we need the precautionary principle.
PRECAUTIONARY PRINCIPLE OF MODERN INTERNATIONAL LAWThe precautionary principle was developed to help deal with some of the new unprecedented challenges faced by humans.
"We believe there is compelling evidence that damage to humans and the worldwide environment is of such magnitude and seriousness that new principles for conducting human activities are necessary.
"While we realize that human activities may involve hazards, people must proceed more carefully than has been the case in recent history. Corporations, government entities, organizations, communities, scientists and other individuals must adopt a precautionary approach to all human endeavors.
"Therefore, it is necessary to implement the Precautionary Principle: When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.
"In this context the proponent of an activity, rather than the public, should bear the burden of proof.
"The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action."
Wingspread conference on the precautionary principle, 1998
(see also Precautionary Principle - Wikipedia)
This clearly applies to a sample return, if we decide to return a sample to Earth. Nobody has yet done a sample return from Enceladus or Europa. And the affected parties there are everyone on the Earth because it could, potentially, impact on the habitability of the Earth.
The main point there is it has to involve examination of the full range of alternatives including no action.
It's not so clear though in the forward direction. If we contaminate Europa or Enceladus, who is harmed by that?
IDEAS FOR A PRECAUTIONARY PRINCIPLE FOR FORWARD CONTAMINATION
To help motivate this, fast forward in your imagination a few decades, or centuries, or a few thousand years.
Then in your history lessons suppose that they tell you that the XNA life or protocells the Enceladus ocean disappeared when, in their very first exploration of the solar system, humans tried to explore Enceladus with an insuffiicently sterilized ice mole or submarine? That for billions of years, it was an ocean of XNA based life. How you would wish you could go back in a time machine to look at it - but it is all gone now, destroyed as a result of these early clumsy explorations of the oceans.
Do we want to be the early explorers who "didn't know better" like the early explorers who deliberately introduced rabbits to Australia? We don't have to be like that.
There might be some short term gains from exploring the oceans right away, before we are ready. But - what if that comes at expense of those first missions being the only ones that get a chance to study Enceladus or Europa in its original uncontaminated state?
For all we know, Enceladus may be absolutely unique in our solar system. Using our analogy for the interior of a cell. Enceladus life could be the only example of a "coral reef" version of XNA in our solar system. Or the only "Tropical rainforest" style XNA or the only, ...
It might require an interstellar flight to find it's like, and even that might not be enough, for all we know. Maybe our solar system is so special that no other stars with any form of life exist for hundreds of light years in all directions.
Until we know better, then we need to take this attitude to every potential habitat for extra terrestrial life in our solar system.
So, in both directions, I suggest, discussions should always consider "the full range of alternatives including no action."
We already do this in Antarctica, with researchers choosing not to drill into Lake Vostock, or the Blood Falls until they are ready. I suggest we should do the same in the solar system.
It's not an absolute "no action for all time". Rather, it's a case of "no action for now, because we need to know more, or need improved technology".
The aim is to find a way to drill safely and explore the oceans consistent with the precautionary principle.
SUPER POSITIVE OUTCOMES
In the direction of forward contamination we don't have any risks to humans. So we can't motivate this in the same way as the precautionary principle, by looking at negative effects.
But we have something that is like a mirror image of this, what I might call a "super-positive outcome". There might well be discoveries to be made on Enceladus in the field of biology, such as XNA life or using it as a window into the stages between life and non life - that we could only find out otherwise by travelling interstellar distances (and maybe not even then).
The philosopher Nick Bostrom has argued that when you reason about existential risks - risks that impact habitability of the Earth for instance, the probabilities need to be multiplied up to take account of its effect on all present day humans, and also, trillions of future lives.
For instance, if there is only a 1 in a million probability of some adverse outcome, but it potentially has an impact on a billion lives, then the expected number affected is a thousand, even though the chances are high that nothing happens. You would go to a lot of effort to avert a risk that will kill or disable a thousand people. So you should also go to a lot of effort to avert a risk that has a tiny one in a million chance of ever happening, but could affect a billion people in that remote possibility.
In this way he brings philosophical precision to Carl Sagan's intuition that we shouldn't take even a small risk with a billion lives.
I'd like to suggest a positive version of this approach. For instance, a 1 in 10,000 increase in the probability of a super positive outcome for a billion people, due to more care taken over planetary protection measures, means that the expected number of people who benefit is a hundred thousand.
It's well worth going to a fair bit of effort to do something that has an expectation of a positive benefit to a hundred thousand people. My new suggestion is that it is worth doing that, even if it is a tiny probability of a super-positive outcome that could benefit billions of people.
And as with Nick Bostrom's argument, if you take account of future generations as well - all the future generations that would be able to study your contamination free Enceladus ocean - then potentially trillions, and even quadrillions or more people may be affected by a decision about whether or not we take a 1 in 10,000 risk of contaminating the Enceladus ocean right now.
So I see a progression here. Each step requires either a higher level of understanding, and confidence, or more elaborate precautions.
If your understanding is good, and you have high confidence levels in your results, you may not need much by way of precautions, and what precautions you take will be addressed to a known situation and will be proportional and effective. If your understanding is not so good, or your confidence levels for your results are low, you need many precautions and even elaborate precautions may not always be enough.
For Enceladus and Europa I see the stages here as
- Clipper mission, collects at 7 km /sec
- Orbiter, down to 100 meters / sec (this low velocity is why I'm so enthusiastic about the proposal).
- Sample return, sterilized before return
- Surface mission, drilling into the ice
- Sample return unsterilized.
Planetary protection issues seem very low. Main one, to make sure it can't impact into the target body. However, we have never had an instance of an orbiter hitting a moon by mistake. The first orbital insertion burn leading to capture seems quite risky, as those have failed in the past (for Mars). But once the spacecraft is in orbit around another planet, then there is plenty of time for decisions and to correct errors. It seems that these small changes of trajectory for flybys are highly controlled. It may be enough to just require trajectory biasing. May just need to require that the orbiter is never in a situation where if its propulsion system fails, it will impact onto Europa or Enceladus.
Launches Jan 28 2023. (21 day launch window). Flies past Venus twice and Earth twice, reaches Saturn July 29th 2031. Enters Saturn with a six month orbit. Biggest delta v is the insertion into Saturn orbit. Then it uses a sequence of flybys of the inner moons of Saturn to work its way in to Enceladus. First flies past Titan, Then Rhea, then Dione, then Tethys and finally Enceladus. Several close encounters with each one. So a very interesting science mission on its own.
- "The Titan tour would include three flybys of Titan and would end with an encounter with Rhea.
- The Rhea tour would include 15 flybys of Rhea and would end with an encounter with Dione
- The Dione tour would include 10 flybys of Dione and would end with an encounter with Tethys.
- The Tethys tour would include 12 flybys of Tethys and would end with an encounter with Enceladus
- The Enceladus tour would include 12 engineering flybys and 10 science flybys of Enceladus and would end with Enceladus orbit insertion
Sadly a polar orbit around Enceladus is not stable so this orbiter can't pass through the plumes on every orbit. Instead it would explore the equatorial regions mainly, close up, and do an occasional fly throughs of the plumes.
Zero chance of back contamination of Earth and very small chance of contamination of Enceladus to work through. In orbit around Enceladus with 100 meters / second impact of ice particles not just onto the aerogel, but onto the spacecraft itself.
I see that as the main advantage of an orbiter, that it lets you have low impact velocities, so easier to catch microbes and cell structures undamaged.
He reached a top speed of 12.2 metres per second. The impact velocity of 100 meters per second for an Enceladus orbiter is about eight times faster than Usain Bolt, and many microbes, especially in dormant state, would surely survive it intact, especially cushioned by an impact into aerogel. Even with its equatorial orbit, so that it can only fly through the plumes occasionally - this is a major advantage of an orbiter.
That could have planetary protection issues to consider. Could that dislodge microbes that still attached to the outside of the spacecraft - given that the plume itself falls back to Enceladus? What is the chance they could be viable and hit the planetary surface? And then eventually find their way down to the subsurface ocean? A radioresistant spore can be viable after hundreds of thousands of years of cosmic radiation even on surface of Enceladus.
Though the chance of that may be low, with these ideas of a "super positive outcome", this might mean we have to sterilize the orbiter well, if there is a chance of that happening.
There are several ideas for a lander on Enceladus or Europa.
Artist's impression of a probe released by a laser powered cryobot into the Europa ocean. Image: NASA/JPL. See also Robotic tunneler may explore icy moons
Dr. Kris Zacny's Honeybee Robotics Auto-Gopher wireline drill
Many exciting missions. But how can we be sure that we don't just find life that we brought there ourselves?
RECOMMENDATION OF COSPAR
The COSPAR recomendation follows the 1 in 10,000 rule for sterilizing these instruments. Which of course is a major challenge already.
- COSPAR Recomendation 1 in 10,000 chance of contaminating Enceladus per mission.
- Bioload reduction calculation following method of Sagan - Coleman equation.
The 1 in 10,000 figure originates in a calculation by Sagan and Coleman based on the idea of an "exploratory period". The idea was that we need to send humans to Mars within a few decades, so that we just need to keep it free from Earth life until then. And then Sagan and Coleman chose, arbitrarily, a 99.9% chance of keeping Mars free of Earth life as "acceptable". They needed a number to feed in to their calculations, and this is the number they chose.
This is an ethical decision, so individuals can vary in their assessment. It can't be given a scientific basis, as you need to make an ethical decision first, before you can start doing the science. And indeed there are many other suggestions.
Greenberg has suggested, to use the natural contamination standard - that our missions should not have a higher chance of contaminating it than the chance of contamination by meteorites from Earth.
As long as the probability of people infecting other planets with terrestrial microbes is substantially smaller than the probability that such contamination happens naturally, exploration activities would, in our view, be doing no harm. We call this concept the natural contamination standard.
This is something you can apply in some situations, for instance it's used for sample returns from meteorites.
However, there is almost no chance at all of a meteorite transferring life from Earth to Enceladus, and the chance of transfer to Europa is extremely low. And in any case most microbes can't be transferred on meteorites. So it is hard to see how to apply this standard.
Another approach for Europa is the use of binary decision trees which is favoured by the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System under the auspices of the Space Studies Board. This goes through a series of seven steps, leading to a final decision on whether to go ahead with the mission or not. Note that it has the option "or cancel mission" at the end.
Recommendation: Approaches to achieving planetary protection should not rely on the multiplication of bioload estimates and probabilities to calculate the likelihood of contaminating Solar System bodies with terrestrial organisms unless scientific data unequivocally define the values, statistical variation, and mutual independence of every factor used in the equation.So, that's a related idea.
Recommendation: Approaches to achieving planetary protection for missions to icy Solar System bodies should employ a series of binary decisions that consider one factor at a time to determine the appropriate level of planetary protection procedures to use.
- What is the effect of fragments of DNA and proteins, and mitochondria, RNA, and all the machinery of modern life have, if dropped into an ocean of protocells?
Is it enough to remove living cells or should the lander be free from dead life also?
- The individual steps still have to be assessed on the basis of probabilities, as we can't currently achieve certainty. How do we extend this to ideas of a "super positive outcome"?
At present stage of knowledge, Enceladus may be our only chance to find an ocean of protocells, or XNA based life, or whatever is there, without an interstellar journey (and possibly also with one).
EACH TARGET AS "OUR ONLY CHANCE" TO FIND SOMETHING UNIQUE
Until we know more about the targets, and whether they are rare and unusual or commonplace and uninteresting, we should approach each potentially habitable target in our solar system with the view that this may be the only chance we have to find out about something unique.
This is primarily an ethical decision, so it can't be decided by pure scientific methods alone. It requires a moral assessment of acceptable levels of risk.
FINDING A WAY FORWARD
We may never be able to resolve the question of what counts as acceptable levels of risk here. Different people will have different ideas of what is acceptable, and there is no way to use methods of science to decide between these views.
However there are two things we can do to greatly reduce the risk, maybe to the extent it is acceptable to just about everyone.
The first idea is to completely sterilize the exterior of the ice mole so that it is 100% free of organics in any form. If we put it in an enclosure, then seems from the design that only the drill at the front and the nose of the icemole needs to protrude beyond it.
So we may only need to sterilize the enclosure, and the drill bit and any areas of the mole where it is exposed to the exterior.
Is that possible?
DEEP CLEANING WITH SUPER CRITICAL CO2 AND CO2 SNOW
- 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!
If this did work, you still have the question though - what about the interior of the ice mole?
END OF MISSION ICE MOLE IONIZING RADIATION
After the mission is over, the interior and electronics, especially the computer, are likely to contain viable microbes, protected by a protective barrier. At least that is how it is usually done for spacecraft for planetary protection.
Of course you sterilize the computer as well if you can. The liquid CO2 method has been used to sterilize RAM of organics. Can you sterilize an entire computer?
If not, then you have to do something about the interior electronics - and other parts of the internals of the ice mole.
So what do we do about those?
First, the ice mole doesn't penetrate deep into the ice - but - how stable is its situation?
It is a dynamic situation with the geysers widening and narrowing depending on the tides. In the case of Enceladus, with our mole next to a geyser, a new geyser might erupt or the vent shift or widen.
What if, over time, our ice mole works its way down the shaft and ends up in the ocean? Or just parts of it? These could plunge tens of kilometers and rupture under the pressure of the ocean depths.
Can we do anything about this possibility?
I suggest just as a concept for discussion, that we could have a mission end sterilization cobalt60 ionizing radiation source or similar.
Shielding for the radiation source:
The source needs to be 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 and orbiter, 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. Somewhat like the way that the RTG radioactive power sources are already shielded from the main spacecraft.
Then at end of mission, you remove the shielding, and the mole then remains there, but irradiated by several thousand Grays a week or whatever is feasible.
The most radioresistant Earth life can withstand 40,000 Grays. So to be safe, ensure that more than this 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.
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 complex molecules until all you have left are gases.
So, at the end of the mission, you just leave it to sterilize itself for all future time.
Perhaps other solutions can be found. I suggest though that in view of the super positive outcome, and possible effects on future generations also of humans, even centuries, even millennia from now, that we need a long term solution. Not just a solution that works for the next century (say).
And - if the precautionary principle is accepted - we treat Enceladus as we treat the subglacial lakes in Antarctica. If we don't yet have the technology to do it, then the planetary protection requirement assessments should always include the possibility of no action.
Let's make sure that when we do send an ice mole to Enceladus or Europa, that we are technologically ready for it. And in worst case, if we can't do it yet, we may have to leave this for a future generation. In best case, as we work on the technology, we find a way to do it right now.
Next in our decision tree is the possibility of a sample return.
STERILIZED SAMPLE RETURN
This is the safest way to return a sample. After you receive the sample on board the spacecraft, use an ionizing radiation source, to sterilize it.
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 with no planetary protection issues.
UNSTERILIZED SAMPLE RETURN
I'm going to suggest that an unsterilized sample return is so complex and potentially problematical, that it is best to find a way to just bypass all these issues. But first let's see what the issues are.
It would be natural to think that the matter is simple, just to return the samples to a glove box facility in a biohazard 4 laboratory. We have lots of experience containing biologically hazard materials in these facilities, so why would a sample return be any different? And indeed that's what was suggested in papers published up to the 1990s.
But as more studies were undertaken, and as we discovered new forms of life, it turned out that it's not so easy to guarantee to contain life in an extra terrestrial sample. As research continued, the precautions needed got more and more complex. The latest studies recommend a half billion dollar facility with capabilities never tested before, requiring around a decade of work to construct it and develop familiarity with using it.
It is easy to contain a known pathogen, say smallpox, or anthrax, or the Ebola virus etc. That's the general situation for a biohazard laboratory, that you know what pathogens you are handling, and so you know what you have to contain.
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)
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 to contain it.
Also their conclusions have to be reviewed, based on new research, as they conclude at the end of the report.
For instance, perhaps a new report would look into the on going research 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 say in the study, this wasn't in their remit.
So in a full discussion, those issues need to be considered as well.
LIFE BROUGHT TO EARTH ON METEORITES
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.
And of course, there's almost no chance of any life brought from the Enceladus ocean to Earth since the early days of the solar system.
SOUNDS LIKE SCI FI
Most science fiction scenarios are implausible, hard to see them work. E.g. a virus from space able to infect humans. So it can sometimes be hard to take this seriously.
But there are things we should be concerned about, which for some reason are rarely looked at in science fiction movies or stories.
- 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.
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, and be inedible to Earth based life.
- GTAs that tranfer new capabilities to Earth life.
On the first possibility, Joshua Lederberg who took a special interest in this said about Mars life, which might not produce the same peptides and carbohydrates as Earth life:
"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.
As Carl Sagan wrote in his book Cosmic Connection:
…Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high.
…The likelihood that such pathogens exist is probably small, but we cannot take even a small risk with a billion lives.
It's the same for these icy moons.
And - you can't say that because a species is adapted to live in the Enceladus or Europa oceans that it can only cause problems for hydrothermal vent communities and cold areas of the Earth. Life from these oceans might find it easier to live on Earth than in these oceans. Extremophiles can live anywhere. As an example two species of microbes typically found in hydrothermal vents and ice caps, also found living in someone's belly button.
PHOTOSYNTHETIC LIFE FROM THE ICY MOONS
At first sight, seems we probably don't need to worry about from Enceladus or Europa at least from its deep oceans - photosynthetic life. Because there isn't any light under those kilometers of ice.
That is, unless there is life in the geyser vents themselves or on the surface perhaps in a liquid layer below a thin layer of ice. The solar constant is also far less however, so harder to get energy from photosynthesis.
However there's another source of light there also, infrared light from the hydrothermal vents.
Green sulfur bacteria in a Winogradsky column. One species of this type of bacteria is able to use the heat radiation of hydrothermal vents to photosynthesize. So photosynthetic life in the Europa and Enceladus oceans is possible. See Infrared photosynthesis: a potential power source for alien life in sunless places
If any life there does manage to get energy from photosynthesis it would have to be able to make do on minimal levels of light. And this could have major planetary protection implications if returned to Earth, and it is able to adapt to surface sunlight.
Suppose for instance the life uses photosynthesis, but has a third method not yet explored by Earth life?
Salt ponds in San Francisco bay, pink and red with Haloarchaea (salt loving bacteria, the same ones that turn the Red Sea red). Thesephotosynthesize using bacteriarhodopsin, which is what gives them their pink coloration. The light sensitive cells in our eyes use rhodopsin in a similar process. This method of photosynthesis doesn’t generate oxygen or fixate carbon but converts light directly into an electrical potential which the lifeform uses for energy.
We have only three essentially different methods of photosynthesis on Earth. What if Mars life has evolved a fourth form of photosynthesis? What would it to do the Earth to introduce a new lifeform which uses a different method? What if it is slightly better at photosynthesis than any Earth lifeform? And is inedible by Earth life or toxic to it?
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.
ONE WAY TO BYPASS THIS - CHARACTERIZE IT FIRST, THEN RETURN IT
So this whole baroque complexity can be bypassed completely if we simply don't return it to Earth quite yet.
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. Ethically, and practically also, the whole thing will be much easier.
SECOND IDEA RETURN TO HIGH EARTH ORBIT
If we do return an unsterilized sample, and are unsure of its contents, then I think safest return is to High Earth Orbit, above geostationary - or else, just below geostationary orbit.
At 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.
At geostationary orbit, then it could be hit by debris from the satellites at GEO.
In HEO (say at 40,000 km, 5,000 km above GEO) then it's orbit is stable for the indefinite future.
Once we get it back there, 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 because Earth is already protected from it by the vacuum of space.
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. One way would be to return it to L2, and then transfer to a retrograde lunar orbit (prograde around Earth, retrograde around the Moon). This is an orbit stable for periods of order a century or so.
Then we could send a spaceship up from Earth to retrieve it and take it down to its final resting place in HEO.
Once there in a stable orbit, we may keep it enclosed 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 probably launch hundreds of tons of 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 we will 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 above GEO, it obviously can't impact on the Earth (delta v is over 1 km per second to reach L1 and escape from Earth, and a similar amount to get into a Geostationary Transfer Orbit to hit the Earth).
The issues here are simlar to those for a Mars sample return, so for more details about this, see my article Will NASA's Sample Return Answer Mars Life Questions?
QUARANTINE IN EARTH ORBIT
You might think we can return it to human occupied quarantine facilities in Earth orbit. But this has many issues when you think it through.
- Can't take all the organisms in Earth ecosystems into space for testing to see what the XNA life does to them.
- What Carl Sagan caused the "Vexing question of the latency period". Leprosy has a latency period of up to several decades. There was no scientific basis for a short quarantine period for the Apollo astronauts. Any effects of XNA based life on either humans or other organisms might not manifest for decades
- The life might be dormant and need special conditions to wake up before it starts to do anything.
- Microbes evolve quickly. It may start off not well adapted to Earth life, just able to survive, and then at some point it adapts and then becomes a problem, possibly years later
So, I don't think quarantine in Earth orbit is a solution at all. For more on this see the Quarantine section of my article Will NASA's Sample Return Answer Mars Life Questions?
IN SITU INSTRUMENT CAPABILITIES
There are many instruments developed for Mars. Whether they can be used as is for Enceladus, or need more modification, this shows the range of instruments we can send. And I've no idea about the engineering challenge, to examine materials captured in an aerogel "in situ". Anyway here are some of the instruments we can send, and some are exquisitely sensitive and would surely detect life if it is there.
Rapid non destructive preliminary sampling
- Raman spectrometry - analyses scattered light emitted by a laser on the sample. Non destructive sampling able to identify organics and signatures for life. It's sensitive, can measure the distribution of the organics and other compounds by pointing the laser at different points on the surface - and is non destructive so it can be applied first before any of the other tests.
Detection of trace levels of organics and of chirality
- Gas chromatography - this is the idea used for the MOMA (Mars Organic Molecule Analyser). Used to analyse volatiles evolved from the soil samples in small ovens. Some of the ovens are filled with a "derivatisation agent" which can transform the chemical compounds into similar ones suitable for chiral analysis. They are then ionized and analysed with the mass spectrometer
- Urey, originally for ExoMars, with liquid extraction using high temperature, high pressure, sub critical water as a solvent (in case of Urey, between 100°C and 300°C at 20 MPa, or about 200 atmospheres, for several minutes) - under those conditions water has chemical properties similar to organic solvents. Urey is able to study the organics relatively unmodified
- Astrobionibbler - similar idea to UREY, smaller, later development. Able to detect a single amino acid in a gram of soil.[
- Planetary In-situ Capillary Electrophoresis System “lab on a chip” – separate the organics by ionic mobility, by electrokinetic methods in sub millimeter capilliaries with the fluid manipulations done within the chip itself.
- LDChip and Solid3, using monoclonal antibodies to detect organics. These detect a wide range of organics not specific to DNA based life. This instrument was tested in the Atacama desert and was able to detect a layer of previously undiscovered life at a depth of 2 meters below the surface in the hyper-arid core of the desert
Direct search for DNA
These can detect life on Mars if it is DNA based so related to Earth life. As DNA sequencers, they can sequence the entire genome of any lifeform found.
- Miniaturized DNA sequencer could work if we had a common ancestor right back to the very early solar system whenever DNA first evolved. This is in a reasonably advanced state. They say it could be ready to fly by 2018.
- Miniaturized scanning electron microscope. This can’t detect whether it is life or not, but is useful along with the others.
Search for life directly by checking for metabolic reactions
These can detect life even if it doesn't use any recognized form of conventional life chemistry. But requires the life to be "cultivable" in vitro when it meets appropriate conditions for growth.
- Microbial fuel cells, where you check for redox reactions directly by measuring the electrons and protons they liberate. This is sensitive to small numbers of microbes and has the advantage it could detect life even if not based on carbon or any form of conventional chemistry we know of.
- Levin’s idea of chiral labelled release, where he has refined it so you feed the medium with a chiral solution with only one isomer of each amino acid. If the CO2 is given off when you feed it one isomer and not with the other, that would be a reasonably strong indication of life.This has the advantage that the life just needs to metabolize amino acids, and to produce a waste gas that contains carbon (such as methane).
There are many instruments like this we could send, and several of them are already space qualified but never flown.
I also wonder about an optical microscope. Why not send, not just a "geologist's hand lens" but a diffraction limited optical microscope? With resolution of 200 nm. It could tell us things about the behaviour and structure of micro-organisms or protocells we might not be able to find out by other methods.
Ideally you want to see the structure of protocells if they exist, and other sub-optical limit structures, so I do wonder also about the microscopes that go beyond the diffraction limit, but I'd have thought they are probably too complex to send into space? Probably won't verify life or protolife unless it is actually still viable and active. But could give interesting data in combination with the other instruments.
SEARCH IN SITU IN AS MANY WAYS AS ARE FEASIBLE
I suggest we send as many different ways of examining the potential life as is practical and feasible. So then they can cross-check with each other and produce a complete picture. In the case of the Viking labelled release, there were only two other instruments to compare it with. If we had several instruments, that would surely increase the chance of getting collaborated observations with two or more instruments observing the same thing in different ways. And if we have a duplicate orbiter for Europa, can examine both the same way. Main aim is to detect life or interesting biochemistry.
INSIGHTS FOR SAMPLE RETURN
We can do a much better job of the sample return, if we know more about the sample.
- Protocells or imperfectly reproducing early life - may be fragile and need to be kept in water with correct pressure and pH and salinity to stay intact.
- Microbes able to enter a dormant state may survive the journey back better if it is kept in its initial UV resistant dormant state.
- Other lifeforms may be able to survive if kept in the right conditions for the journey back.
LONG DURATION "IN SITU"
With a long term mission we can continue to find out more about Enceladus indefinitely, like Cassini. With a long baseline, with multiple flybys, we can monitor changes in the plumes.
It might be that some plumes have subtle differences in biochemistry from others. May be that we capture intact cells, but only on rare occasions. There may be different species we can examine as time goes on.
It might be that there are times in the orbit, or altitudes above the surface that are optimal for finding life.
OUR FIRST CHANCE TO SEND THESE INSTRUMENTS TO SEARCH FOR LIFE
Whole generations of astrobiologists have had a multitude of ingenious and carefully designed and thought out ideas for instruments to send to space, but never seen any of these instruments fly. So far, nobody, world wide, has had a life detection instrument fly to another place outside the Earth since the Viking landers in the 1970s. The closest we got to this is with the Philae lander, which is searching for pre-biotic chemistry on Comet 67p.
ExoMars will be the first true opportunity, and its capabilities are, though interesting, by no means exploring the full range of what can now be done by in situ exploration.
To be involved in the process of design, and to have their instruments fly on an interesting mission like this would surely invigorate the field.
THINK TANK ON SEARCH FOR LIFE IN THE ICY MOONS OF JUPITER AND ENCELADUS
Many speakers discussed this topic in the think tank this summer on 11th July 2015 at St Hugh's college, Oxford.
Amongst the highlights are presentations by top astrobiologists (including Chris McKay and Charles Cockell), and by scientists from all four of the ice mole project teams. Alex Roger, oceanographer will talk about his investigations of species diversity at hydrothermal vents and their relevance to the icy moon oceans. Also Jill Mikucki, geomicrobiologist will be reporting on the results of the DLR ice mole test in Blood falls in Antarctica last February. It will also have researchers reporting on their research live by skype from a volcano in Iceland. There are many other scientists and engineers from NASA, ESA etc. For the confirmed speakers so far see the conference program (expect more names to be added to the list).
I've been invited also, as you'll see, and will use my fifteen minutes to speak on the value to science and implications of exploration of Enceladus and Europa for signs of life. This article started as my draft for that talk, so I'll touch on some of the things I've said here.This was the poster:
It was a privately funded conference, organized by William Brooks (a space enthusiast, scuba diver and electronic Engineer, working with sensors and data acquisition systems), who has been in communication with Prof. Alex Rogers of Somerville College, Oxford, who suggested the idea of a "think tank".
The goal was to generate public interest, support for anyone working in this field, and to discuss possible future NASA/ESA/Private Company/Public Missions to these moons.