Hi, I'm Sarah Maurer,
and today we're going to talk about some of the stranger ideas of what life could be.
Keep in mind that none of these ideas have ever been proven experimentally,
and they're really just kind of hypotheses or thought experiments
as to some of the stranger types of life that we might see within the universe.
So we talked a lot about diffusion systems,
and how simple chemicals can form into these complex patterns,
and so, one of the weirder types of life that might be hard to identify
is a type of life with no cells.
Now, if you played around with the diffusion system application that you see here,
you may have noticed that there are a very limited set of values of "F" and "k"
that actually give us interesting results; most just go to a non-pattern system.
And so one of the problems with having a cell-free chemical network is that
it's very restrained in terms of the metabolic processes that can go on,
and so it's hard to imagine, really, a true cell-free system
where chemical gradients just form complex patterns that are self-sustaining
that might be called life,
but, there are certain parameters that you can use to get these things,
and so that might be one of the harder-to-identify but possible options
for strange life.
The other thing that we could think about are phase-separated systems
that aren't necessarily membranes like we see in cells today.
And so, we talked about this in some of the earlier videos,
like the protocell video,
where we talked about how we could organize chemical gradients
on things like small mineral grains, or in coacervate systems.
And so, we don't really think about seeing any of these in real life,
except for coacervates.
So the image that's shown here is an image of a small bacterium,
and when it breaks apart, all the cytocell goes into the extra cellular space,
and those -- the material within the cell can actually aggregate into these
small phase-separated systems, that maybe you would call a coacervate.
They're predominantly polysaccharides, and they serve to help the cell rearrange
before it can kind of regrow its cell membrane.
And so, we do see in extant life that we have membrane-less cells,
at least for short periods of time,
as a means of a really fast division process.
So we could imagine that we would have cells that didn't have a membrane,
and were just aerosols or oil droplets or coacervates.
We can also think about even stranger types of life:
so, not having water in the cytocell, but instead replacing it with a polar solvent.
And in this case, you might still have a membrane,
but the interior of the cell would be something like ammonia or sulfuric acid,
and these polar solvents would also serve as a solvent of life.
So everything the cells would also be in a sea of ammonia, if you will.
And the reason that ammonia and sulfuric acid are highly probable,
1 is that we see ammonia -- liquid ammonia within our solar system,
in the icy moons and gas giants,
and additionally we see liquid sulfuric acid on Venus.
And so, if we were looking for weird life within our solar system
that we could reach with a rover, for example,
we might be able to look for weird life on Venus or on icy moons.
And the reason that ammonia and sulfuric acid are good alternative solvents to water,
is that one of the great things that water does is it has two protonation states
which conserve to drive chemical reactions, also in hydrogen bonds.
And so, both ammonia and sulfuric acid have these properties,
where they can serve to catalyze carbon-carbon bond formation,
like you see in the reaction shown here,
and they can hydrogen bond and do a lot of the other things that water does,
like dissolve polar molecules.
And so, while the chemistry would be slightly different,
we would expect that a lot of the rules to life with no water and polar solvents
would be very similar.
We could think of even weirder no-water situations where we have non-polar solvents.
Two places that we see this in our solar system are moons of our main planets.
So, Titan is a moon of Saturn, and Io is a moon of Jupiter,
and on Titan, we know -- or, we are pretty sure, that there are liquid ethane
and methane lakes, and these liquids could serve to house some strange carbon chemistry
that we see in the hazes of the atmosphere of Titan, like you see in this blue haze here.
Additionally, Io has liquid sulfur, although this is a very different type of liquid,
it's not a liquid lake, it tends to be liquid magma,
which shoots out of Io's very hot volcanoes.
And so, you kind of have these two very different non-polar liquids,
where Titan is very cold, and Io is very -- it's hotter than Earth;
it's not very hot, it's, you know, above our 100º C, above the boiling point of water,
and so, you would expect that the types of chemistries that you would see
in these two environments are very different.
One scientist has been working to ask:
can non-polar solvents actually lead to life?
So, in the video that you're watching here, you see that there is an oil droplet
that is made of decanol, so it's phase-separated from water.
the interior of the droplet is hydrophobic; it's not hydrogen-bonding,
and in the external solution, we are going to add a salt solution;
that's that blue solution that you see being added.
And the droplet can actually move towards the salt -- the high concentration of salt,
where there is a lot of blue color.
And then if you, on the other side, add more salt, it will again move towards
that higher concentration of salt.
So, you get chemotaxis within these oil droplets,
which we would consider a very lifelike property,
and I think that these are really unique experiments that show that
very simple non-polar chemical systems can display things that we think of as
a property primarily associated with life.
What about a system that has no liquid at all?
These ideas of having either solid phase or gas phase life
will be even harder to detect than the strange life of a different liquid.
So, one of the problems with both solids and gasses is the size of the cells.
We actually don't know what sizes of cells we would get with a solid or a gas,
but with gasses, they are much more dispersed in space,
so you would expect that you'd need more space to house the chemicals
that you would need to maintain life.
Also, gasses are at a much higher temperature,
and at higher temperatures, molecules tend to be less stable.
So while reactions would go much more quickly,
the possible chemistries that you would have may be really constrained
by the stability of those compounds.
If we think about solids,
here we're not talking about a cell that's living deep in the crust of the Earth
that still has a liquid cytocell.
Here we're talking about the interior of the cell actually being in the solid phase.
And in solid phase, things move very slowly,
and so we would expect reactions to be very slow,
and that would mean that even a cell's "cycle," if you will,
would be on a much longer time scale, even perhaps longer than a human lifespan,
for example.
And some of the solids that have been suggested
are things very similar to liquid water, just ice water;
it's very abundant in our solar system,
but you could also imagine very slow reactions in minerals
that could be conceived as living systems.
and so, I think this brings us to the big open question of
"What other substrates could you imagine life existing in,
and if life did exist in those environments, would we be able to recognize it?"