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?"