[Lewis Thomson (OP)]

Patrick asks:

Scientists have proposed a mission to send a probe to one of Saturn’s moons, Enceladus, to see if there is life there. The reason for this is that Enceladus has a large, subsurface ocean made of liquid water. However, liquid water in itself isn’t evidence of life. So why do scientists think there could be life on Enceladus? Is it that the water might have come from meteors, which could have also carried amino acids to the planet?

What do you think?

[chiralSPO]

As it stands now, there is no evidence for life on Enceladus. But after decades of study, including recent direct sampling of the plumes of a geyser, there is a lot of evidence indicating that Enceladus has everything needed to generate and sustain life:

In addition to having a liquid water ocean, we know that it has a salty ocean, which indicates that the liquid water is in direct contact with the rocky bottom, and we know from the composition of the geyser plume that the water contains simple organic and inorganic "prebiotic" compounds (NH₃, CH₂O, HCN, H₂S etc.), which would be useful building blocks for making life. Measurements of the plume also indicate significant concentrations of H₂ and significantly alkaline water (pH ~10), which together strongly indicate the presence of hydrothermal vents (at least associated with the geysers.)

All together, these conditions are very similar to ancient oceans on earth, and provide all of the basic ingredients for life: liquid water, minerals, organic matter, and energy (H₂ and/or H₂S from hydrothermal vents--on Earth, hydrothermal vents support massive ecosystems at the bottom of the sea by providing energy-rich foods to bacteria, which are then the base of a large food chain.

Scientists are not necessarily looking for life there--strong evidence that there is NOT life there would also be very interesting and scientifically useful information. Right now the only life we know is that on Earth. We don't know what the actual requirements are, or precisely what processes are involved, so any more data points we can collect will shed light on this current mystery. So if there is life, then clearly there will be much to learn about it (how similar is it to our own? are we actually related? etc.), and even if there isn't, we still get clues about what is needed to make/sustain life-we just have to look more closely at what Earth has that Enceladus doesn't.

[chiralSPO]

This recently-uploaded seminar is quite relevant (12 minutes):

[Bored chemist]

It's not so much that they think "water = life" as that they think "drought = death".

[alancalverd]

Everything we currently consider to be "life" depends on water, so it's a necessary but not sufficient indicator

However (a) life can proceed with remarkably little water and (b) there's no reason why a process that doesn't involve water shouldn't be called life if it has any of the other characteristics of living things.

[glot]

Water may be toxic to most other forms of life. Maybe searching for water first is a negative. Maybe they should be looking for a water free planet.

[evan_au]

Does water on another planet or moon indicate life?

No. But if we see liquid water, that's a sensible place to start looking.

It is hard to imagine at least two of the basic functions of life as we understand it (growth & reproduction) occurring without some fluid flow to allow movement and reorganisation of the components of life.

I find it hard to imagine a lifeform where this fluid is purely gas (or plasma), so looking at liquids is a good place to start.

We do know that liquid water can support life on Earth, so it makes sense to look for life in liquid water elsewhere in the Solar System. We have a fair knowledge of the type of biological molecules and reactions that occur in water.
- Mars is on the outer edge of the temperate zone that might host liquid water on the surface due to heat from the Sun. It might harbour underground water in thin, salty films suitable for bacteria (like the permafrost regions on Earth), or due to geothermal ("ariethermal"?) heat.
- Enceladus and Europa are thought to harbour water under the surface, kept liquid by pressure and tidal heating
- The geysers on Enceladus provides a way for us to study the deep interior without shipping Bruce Willis and a drilling team there. It also provides a way to obtain samples of the interior with minimal risk of contaminating the interior with Earth bacteria or Earth biochemicals carried by a subsurface drill.

Titan has liquid lakes on the surface, but they appear to be a mix of methane and ethane. We have no idea what kind of biochemical reactions to expect there, but the organic molecules we see on Earth would probably be frozen solid unless they were dissolved in some low-temperature solvent like methane. (Plus, our Earth-temperature scientific instruments would probably boil such a lake and any biological contents).

See: https://en.wikipedia.org/wiki/Life

Maybe they should be looking for a water free planet.

Water is one of the more common molecules in space - it would only be missing in environments which are currently hot enough to decompose water, reactive enough to react with the water (like Venus), or formed close enough to the Sun to drive it away as a vapor (like Mercury).

The Philae space probe landed on a comet which had no liquid water - it was so cold that the water was a hard solid. The surface is exposed to the vacuum of space; if the surface got warm enough to melt water, the water would go straight to water vapor. However, Philae did go looking for organic chemicals, and found 16.
See: https://en.wikipedia.org/wiki/Philae_(spacecraft)#Instrument_results

Future probes to Mars are expected to carry sensitive instruments capable of detecting and identifying organic molecules.
See: https://en.wikipedia.org/wiki/Mars_Organic_Molecule_Analyzer

[puppypower]

Water is key to life. The most important reason is, water forms a unique hydrogen bonding environment that is needed for life. Other molecules, like ammonia, can form hydrogen bonds, but only water molecules can form a vast 3-D continuum of four hydrogen bonds, allowing it to mimic carbon, but via secondary bonding.

Diamond, for example, is based on each carbon atom forming four covalent bonds with other carbon atoms, to form a vast 3-D continuum, Water sort of does the same thing, with its four hydrogen bonds. However, water has a few tricks up its sleeve, so this looser 3-D water structure is very adaptable to dissolved surfaces.

The dynamics of life is based on secondary bonds, like hydrogen bonding. For example, the primary covalent structure of DNA is a very sturdy molecular structure, This does not undergo change, except under strong conditions. The activity of the DNA is based on forming and breaking hydrogen bonds. The tetrahedral natural of the hydrogen bonds of water, in the aqueous continuum, allows water to mimic the surface of the primary covalent structure of DNA, and act as a proxy. Ammonia is not symmetrical with respect to hydrogen bonds such that its hydrogen bonding network is limited in scope.  You cannot dissolve DNA in ammonia and expect life, since the hydrogen bond network is not vest enough to communicate properly.

The second reason is water is at the bottom of the chemical energy well, with respect to most of the other proposed molecules for alternate life. In other words, if you burn ammonia, methane, alcohols, etc., water is one of the final products. Say you were to form life in ammonia, for the sake of argument, what prevents this life from evolving to where it can burn or metabolize it own solvent; ammonia, for energy. That solvent can be burned since is is not at minimum energy. If that life form found a way, it would burst into flames, and make water. Once life reaches water, it has reached the floor and stops trying to burnt itself.

Say hypothetically, that ammonia based life developed a fail safe so it did not spontaneously combust. The energy bandwidth, for ammonia based life, would always be much smaller than that of water based life, due to the water having the lowest energy floor. Water life would be more vigorous and could extract extra more energy from the same food, due to the higher energy potential between the water floor and all organic food materials. If formed side by side, life in water would have big selective advantages, due to its lowest energy floor.

The last key reason for water, which is critical to the formation of life, in the first place, can be seen with the analogy of water and oil. If you mix water and oil, instead of a randomization due to oil and water dissolving, you will get order, as the system forms two layers. Water's self hydrogen bonding environment is so vast and strong, that organics tend to be excluded and often need to segregate, resulting in a reverse of entropy, needed to the order within life.

Other solvents tend to dissolve organics. Ammonia is a good degreaser, which means it will discourage organic clustering and will attempt to dissolve and randomize/dissolve any organic structures that try to form. Water, on the other hand,  will self hydrogen bond and induce some organics to segregate and cluster into organelles and others, like lipids to cluster into membrane, allowing life to form.

The search for water, is about the search for the solvent with all the natural selective advantages needed for the fastest forming and most energy vigorous forms of life. We are looking for the A-team, that will not self combust by eating its own solvent in hard times.

Note: Hydrogen bonding is required for life. A much deeper reason is, hydrogen bonds are very unique in that these bonds show both covalent and polar characteristics. The DNA is covalently bonded together, while ions like sodium and potassium interact with polar interactions. Water, via its hydrogen bonds, can bridge this gap, since it can go both ways.

The binary nature of hydrogen bonds also allows water to communicate with anything in the cell, using the hydrogen bonding as a binary switch. The clustering structures of water can flip hydrogen bonding switches, in various sequences, between the two states. This type of binary communication, goes way beyond semi-conductor switches. This is not as simple as on-off, since each state of each binary switch has distinct physical values in terms of enthalpy, entropy and volume/pressure. What that means is information, is more than just data, it also has physical force and energy behind it.

For example, water clusters have been shown in the lab to have both high and low density configurations. This loosely analogous to going between liquid water and ice; contract and expand. Each state is based on the proportion of the hydrogen bonds; polar or covalent. The cluster stays stable, but can alter shape based on the distribution of polar or covalent switches.This means the cluster can get skinny or get fatter. These physics changes, to information, can muscle adjacent things, based on how the information received translates into the cluster. Not only is a pressure being exerted; plus or minus, but with that pressure comes changes in enthalpy and entropy, since each state of the binary is different. The water cluster can push or pull an enzyme, and then give or take away free energy, This is shown in the link below.

As an example, if you flipped the hydrogen bonding water switches to the covalent sides, all the water clusters in the cell will expand and the cell's water will gel. The cell gets very viscous. If we flip the switches the other way, all the clusters contract and the viscosity goes down and the cell is loose. This type of communication would be global, but it can also be induced locally; gel one gene.

http://www1.lsbu.ac.uk/water/images/cluster_equilibrium_2.gif

[Bored chemist]

Say you were to form life in ammonia, for the sake of argument, what prevents this life from evolving to where it can burn or metabolize it own solvent; ammonia, for energy. That solvent can be burned since is is not at minimum energy. If that life form found a way, it would burst into flames,

There are plenty of organism on Earth that can't survive in oxygen, for essentially this reason.
It just means they are not found where oxygen is.

Your "ammonia would always burn" argument also applies to proteins, fats etc...

The other arguments aren't great either- ammonia forms hydrogen bonds in much the same way as water.



"Other solvents tend to dissolve organics. Ammonia is a good degreaser, .."
Aqueous ammonia is a good degreaser, but it seems not to have occurred to you that we use water for washing- because it's a very good solvent.
It's true that ammonia is a better solvent for hydrocarbons than water is, but that might be seen as a benefit- you could use them as "food" more easily.

My best guess is that, if there's life elsewhere in the universe, it's water based, but I'm not so "geocentrist" as to think it's guaranteed.

[evan_au]

Science communicator Dr Karl has speculated that life can only exist where there are solid, liquid and gas phases present. (Perhaps he would allow dissolved gases in the case of deep oceans on Earth, or Enceladus?)

For a cell to exist as we understand it, we need a somewhat solid (but permeable) barrier to separate the inside from the outside, and concentrate the material of life on the inside.

Water is key to life. The most important reason is, water forms a unique hydrogen bonding environment...

On Earth, no argument.

But for most of the volume of the Solar system, water (or any polar solvent with significant hydrogen bonding) will be frozen solid, and useless as a solvent for life.

In a smaller volume of the Solar System, water will be a vapour, and useless as a solvent for life.

The "Goldilocks Zone" excludes most of the volume of space. But it is a good starting point because we (sort of) know what we might be looking for.
See: https://en.wikipedia.org/wiki/Streetlight_effect

water is at the bottom of the chemical energy well, with respect to most of the other proposed molecules for alternate life.

"bottom of the well" compared to what?
Chemical potential energy is a relative thing.
It's true that methane is very flammable in oxygen, but if (speculative) life on Titan is not based on oxygen, then there is no risk.

If reactions are driven by some other cycle than the respiration cycle on Earth with which we are familiar, it is possible that the reactions of life may proceed at a very different pace than what happens on Earth.

In general, reactions occur much more slowly in colder environments (with a lot of caveats...)