PBS Space Time

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Life as we know it is carbon-based, but does it have to be that way?

There is another element on the periodic table that shares some of the key properties of carbon but is far more abundant on most planets.

Im talking about silicon.

So could there silicon-based life out there?

All of the chemical machinery of life - all biochemistry - is based around the carboan atom.

At least as far as we know.

If we want to understand the possible diversity and abundance of life in the universe, we need to ask whether this is the only option Perhaps the most popular alternative - at least in science fiction - is silicon-based life.

We have the xenomorphs of the Alien series to the giant space worm in Star Wars and many, many others.

So why are scifi writers so into silicon-based life?

Because it really is the most promising non-carbon prospect due to its chemical similarity.

Its also more abundant on Earth than carbon.

So, can life be based onsilicon?

And if so, why arent we silicon-based?

When we say life is carbon-based, we dont just mean that life uses carbon a lot - we mean that literally every aspect of the molecular machinery that makes up life is based around the chemistry of carbon.

Carbon chains and rings form the scaffolding for an enormous variety of molecules which enable the raw machinery of life - from DNA & RNA to amino acids & proteins to all the structures built by these.

Biochemistry is by definition the chemistry of Carbon.

In fact, biochemists don't even write Carbon in their formulas, they just draw bent lines and they know that's supposed to be a Carbon atom.

So why is carbon so favored by life?

Is it just that the first life form happened to be carbon based, so everything else got stuck with that atom?

Or is carbon-based chemistry reallythe only way life can happen anywhere, ever?

Ultimately, carbons incredible utility comes down to the way it bonds with other elements, which enable just the right balance of reactivity and stability.

To understand this, we need to think about energy.

Nature will always try to get to the lowest energy state that it possibly can.

A stick of dynamite for example, is in a high energy statedue to all the energy stored in the chemical bonds of its nitroglycerine.

If the atoms find a way to a lower energy state theyll go for it, explosively releasing that energy in the process of converting the nitro to smaller molecules.

Nitroglycerine is unstable because it only takes a little energy to break its tenuous bonds, which in turn releases a lot more energy than went in, initiating a chain reaction.

That reaction only goes in one direction, because its not energetically favourable to reassemble all the smaller molecules back into nitro.

Life is basically an ongoing chemical reaction.

You dont want the chemistry of life to be unidirectional.

You dont want all the raw ingredients to end up in the same overly-stable molecules, with no way back.

That means the reaction is over, which for life means well, death.

At the same time, you want the molecules doing the work of life to be stable enough - they need to last long enough to do their jobs.

In other words, you want a balance of reactivity and stability.

Lets figure out what elements might be good for reaching this balance.

And to start were going to open up to the entire periodic table and see what we can rule out.

The key is in what types of chemical bonds a given element can make.

There are three main bond types: ionic, metallic, and covalent.

Ionic bonds are way too unstable, while metallic bonds result in repeating structures - crystals - which dont have the chemical variety needed for life.

Covalent bonds are really the only option.

In these, atoms share electrons with one or more neighbors in order to fill up their outer or valence shells.

For example, carbon has 4 valence electrons.

It wants 8 for maximum stability.

It achieves that stability by connecting to 4 hydrogen atoms and sharing their single electron.

This also gives each of those hydrogen atoms one more electron to fillup its own valence shells.

The result of this arrangement is methane.

The cool thing about covalent bonds is that there are countless ways to do this electron sharing, enabling enormous diversity and modularity.

For example, if we remove a hydrogen from two methane molecules and stick them together you have ethane.

Now replace one of the hydrogens with an oxygen and we have ethanol.

Covalent bonds are the key to the complex and resilient molecular structures needed for life.

With this in mind we can look at the Periodic Table and rule out all the elements that form mostly ionic or metallic bonds, which is actually most elements.

Were keeping things that are good at forming covalent bonds, even if they also sometimes bond in those other ways.

We can also rule out Noble Gasses since they are barely reactive at all, and the Halogens, because theyre just too much trouble.

It's hard to get them to form covalent bonds with more than one element at the same time, and when they react with something it's usually in an explosive manner.

Finally, were going to scrap elements in rows 4 and below.

Although these may form covalent bonds, these atoms are so large that the nucleus doesn't hold the electrons with enough strength, leading to dangerously weak molecules By the way, this is the reason Arsenic is toxic.

Cells can accidentally build molecules using Arsenic in place of Phosphorus due to them having the same number of valence electrons.

When that happens the resulting molecules will break, destroying important structures in the cell, or interrupting important processes, and if this happens too much the cell will die.

In the end we are left with only seven elements: Hydrogen, Carbon, Nitrogen, Oxygen, Silicon, Phosphorus, and Sulfur.

Of these, all but silicon are considered fundamental building blocks of life on Earth.

Other elements are used by life for very specific functions, but hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur are the most important.

And of these, carbon is by far the more critical because it provides the scaffolding of all biochemistry.

The reason its good for this is that it forms strong bonds with itself, and does so in a variety of ways.

In fact, of the 6 building blocks of life, only carbon can form both 1-D and 2-D structures with itself.

In our process of identifying elements that may be useful for life we got to the 6 main building blocks plus silicon.

But why did silicon get dropped by evolution?

This seems weird because silicon is chemically similar to carbon, also having 4 valence electrons.

Silicon can imitate Carbon and form long chains, and rings, and the resulting molecules often have similar properties to molecules made with Carbon.

To give you just one example we can make Silicon oils, and they feel and look just like other kinds of oil.

Its the only other element capable of forming complex covalent scaffolds and supporting a large diversity of molecules.

Despite all of this, Earth life makes less use of silicon than it does of many other trace elements like iron.

This seems extra strange when you remember that silicon accounts for more than 28% of the mass of Earths crust, in all them silicate rocks.

On the other hand, Carbon makes up only around 0.02% of the crust.

This tells us that carbon must have substantial advantages over silicon.

So lets talk about someof the issues with silicon as a basis for life.

One huge one is that silicon-based molecules tend to be far more reactive to water than carbon-based.

In fact, although silicon chemistry is extremely diverse, most complex silicon molecules are unstable in water.

You can see how this would be a problem for silicon-based life on our very watery Earth.

In general, life needs a solvent - a liquid medium that allows molecular machinery to move around and interact.

Water is incredible for this.

Its thermal stability, power as a solvent, and abundance through the universe make it the best, and perhaps even the only decent choice of the base solvent for life.

The instability of most silicon molecules in water is actually a huge blow to the viability of silicon-based life.

However water is not the only option.

A much greater variety of silicon-based molecules are stable in liquid hydrocarbons - which are present on the moons of the gas giants, and even in sulphuric acid, which is abundant in Venuss atmosphere.

Both cases have their issues - liquid hydrocarbons are cold, which among other things means they have trouble holding in solution the large, complex molecules that known life relies on.

Sulphuric acid is chemically extremely aggressive, perhaps too much so for life to get started.

On the other hand, xenomorphs are silicon-based and they do have acidic blood - makes sense that theyre silicon -in-sulphuric-acid-based.

If thats what Venusian life is like, lets cancel the cloud city construction program before the space horror begins.

There is one way to potentially get silicon-based life in water - and thats by coupling silicon with either carbon or oxygen for our scaffolding molecule.

Chains of these pairs of atoms can produce some chemical diversity.

However neither case seems to provide significant advantage over just using carbon chains.

Another reason that silicon seems implausible as a scaffolding element for life is that the stuff really, really loves forming bonds with oxygen.

Silicon forms stronger bonds with oxygen than with any of the other building blocks of life, and certainly more so than with other silicon atoms.

Silicon-based molecules are easily broken by oxygen, and silicon-oxygen bonds are hard to break in turn.

That means reactions tend to go in one direction when theres oxygen available.

And the strength of the silicon-oxygen bond is perhaps the main reason that silicon is rarely used by life on Earth.

Although theres an awful lot of it, Earths silicon is locked up in rocks and is inaccessible to life.

On the other hand, most of Earths carbon is in CO2 in the atmosphere, which is relatively easily accessible through photosynthesis.

There is one sense in which the strength of the silicon-oxygen bond could be useful for life, and thats in energy generation.

Lets think about where our energy comes from.

Its from the conversion of oxygen molecules - which contain a ton of energy in their bonds - into CO2 molecules - which contain very little.

The simplest way to get that energy is to set something on fire, which works great for steam engines but not so great for cells.

The mitochondria in our eukariot cells liberate that energy with a few more steps, finally storing it in the ATP molecule.

So could silicon-based aliens get energy by converting O2 into silicon-dioxide?

On the surface this looks good, because SIO2 is even more stable than CO2, so theres more energy to liberate.

The problem is that while CO2 is a gas that can be expelled easily, silicon-dioxide is silica its sand.

Its coarse and rough and gets everywhere, I hate it.

Its not something you want building up in your cells or your lungs.

And this wouldn't be a small amount of sand.

A regular person exhales about a kilogram of Carbon Dioxide throughout the day, meaning Silicon aliens would need to cough out a good amount of sand.

Or they poop glass beads or something - probably theres a way around the issue.

But its certainly not as convenient and energy-cheap as exhaling CO2.

And with life, any little saving in energy is a big deal.

Lets summarize.

In certain conditions silicon potentially has the chemical diversity for an alien biochemistry, but those conditions are pretty specialized, and are far less ideal that the ones that support carbon-based life.

It seems that if life has the option of using carbon then it should do so.

But its not impossible that inhighly unusual environments silicon-based life But what would that life even look like?

For decades science fiction authors have imagined silicon based life as variations of living crystals or rocks, that's because thats the common form of silicon on Earth is just that.

But we carbon-based creaturesdont look like walking diamonds or pencil leads, so theres no reason to think of silicon-based aliens would look particularly crystally.

Well, actually.

There is a reason.

And thats because living silicon crystals already exist on Earth.

Let me present you to Diatoms.

These are single celled organism that have incorporated Silicon into their cell walls, making them rigid, and basically just Silica crystals that encapsulate all the parts of the cell inside them.

These beautiful living crystals live simple lives.

They just float around in the ocean in long chains photosynthesizing, producing half of the oxygen that we breath.

Nowto be clear, Diatoms are not Silicon based life, it's just their cell walls.

Everything inside them is based on Carbon.

But they are at least a proof of concept that one important component of life - the cell wall - can be built from silicon.

OK, so what have we learned?

Well, silicon-based aliens are probably slightly over-represented in fiction compared to their likely frequency out there in the universe.

In fact, carbon-in-water is such an astoundingly good option for biochemistry that there may be no other game in town.

Even Carl Sagan called himself a carbon-chauvinist for this reason.

Well probably be more similar than different to the alien species of our first-contact.

But lets not ignore the hydrocarbon lakes of Titan or the acid cloud-scapes of Venus, where silicon-based critters may expand our notion of what is possible in the variety of life across space time.

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To learn more about Brilliant, go to brilliant.org/spacetime Today were doing comment responses for the last two episodes - the one about the new simulations that show how our moon formed from a giant impact with higher resolution than ever before.

And the one on Max Tegmarks mathematical universe hypothesis which proposes that the most fundamental reality is a platonic realm of mathematics.

Well start with the latter.

onebronx opines that the basic math of our Universe is likely not the SM lagrangian and the GR equation, but the principle of a stationary action and a specific set of mutually-consistent symmetries.

I absolutely agree on the core point - which is that IF the mathematical universe hypothesis is true, a given universe probably shouldnt defined by the platonic existence of a complicated set of equations, but rather a very simple set of rules.

In the mathematical universe hypothesis, the physical world is emergent from a more fundamental mathematical world.

But that doesnt mean that complex mathematical structures cant also be emergent from simpler, more fundamental mathematical structures.

S. Lee Odegard mentions that we should have included Sir Rodger Penrose's three worlds ontology, in which reality is described as three realms of existence, each merging from the other.

I agree, and this was totally going to be in the episode but we ran out of space for it.

To give an overview - Penrose proposes this way of thinking about the circular emergence of reality.

We have the material world as emerging from a platonic mathematical world - in the sense that without the possibility of mathematical regularities, the material world couldnt exist; and then the mental world emerges from the material - in the sense that material brains generate mind; and finally the platonic world emerges from the mental - in the sense that minds are able to conceive of the full language of mathematics even beyond its manifestation in the physical.

This is pretty different from Tegmarks idea that the platonic math world is the ontological precursor to everything else.

Penrose doesnt grant primacy to the platonic - which is why I ultimately decided to leave that out - perhaps for a future episode.

If anyone wants to read about this, get Penroses book Road to Reality - but fair warning, reading it is a full semester intensive physics course.

Which is, by the way, a recommendation.

OK, and now on to the far less abstract idea that the moon formed when a planet that we name Theia smashed into the newly formed Earth markcentral mentioned an idea Id never heard before.

That lumps of relatively intact Theia material may still be present in the earths mantle.

There are these high density blobs of material that sit at the boundary between Earths mantle and core called the Large low-shear-velocity provinces.

Some scientists have suggested that this is material from the planet that collided with Earth in the Moons formation.

There is some loose evidence that isotopic ratios in basalt thought to be from lava plumes rising from the regions above the LLSVPs have primitive isotopic ratios consistent with material that wasnt mixed in the giant impact.

Very cool possibility, though obviously a lot more work is needed.

Alice asks: If the moon is a very unlikely event, and an unusually strong iron core is necessary to protect the development of life, does this provide a sufficiently hard filter to resolve the Fermi paradox?

The answer is: we dont know, but people have speculated that this might be the case.

My personal intuition is that the extremely hard filter needed to explain the Fermi paradox is actually a chain of somewhat hard filters, and the large moon may be one of these.

Others include having a planetary composition like ours - large protective gas giants in the outer solar system but only small terrestrial planets in close, a star of a very particular quality, and so on.

Check our video on the rare earth hypothesis for more on this.

Id like to address a comment about my comment response from the supercritical fluids episode where I joked that there are animals with hands large enough to swim in gas - theyre called birds.

Several people noted that bats would be a better example here because, as Xovvo notes, and I quote Bat wings are formed from membranous extensions from palm to fingertip between digits two through five.

Bird hands are actually very small and have completely fused together into a structure called the carpometaparpus, tipped by greatly reduced digits II and III (digit I bearing the alula, anterior to the rest of the carpometacarpus).

Birds just have very long feathers to form their flight surface.

So Ive learned a lot from my mistake - both regarding the size of flighted animal hands, and that even casual jokes should be subject to rigorous research and fact-checking.

My sincere apologies to everyone Several people noticed that I flubbed when naming Neptunes moon Triton.

I misspoke and said Titan, which is of course Saturns moon.

Thanks to those in the comments who wondered if it might just be my Australian accent - like, we dont pronounce rs after ts or something.

Yeah, thats it - just remember any time you hear me misspeak its actually just how Aussies pronounce the actual correct word.

Also, all Australian birds have gigantic hands, unlike everywhere else in the world.