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We've looked at the building blocks of
life and we've looked at some of the

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environments in which life might have
evolved. But one thing we've assumed

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throughout this. Is that life is based on
carbon, and it uses liquid water as a

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solvent. Let's revisit that assumption and
think about whether there are other,

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plausible alternatives that have been
considered by astrobiologists. First of

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all, we said that life is based on carbon,
and what that means is that the molecules

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of life have carbon as their backbones.
Here are two examples of molecules.

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Methane, which has a single carbon atom
and four hydrogen atoms, a simple Compound

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produced by life, but still based on
carbon. And on the right hand side there,

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the much more complex molecule of
deoxyribonucleic acid, or DNA. which is

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also carbon based. The molecule is
constructed around carbon atoms, with

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other atoms attached to it. Such as
hydrogen and nitrogen and phosphorus and

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oxygen. These are carbon based molecules.
And all of our molecules are built up in

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the same way, using carbon. As the basic,
building block of these molecules. What

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are the reasons why carbon is such a good
atom performing molecules? Well one reason

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is it tends to form bonds with other atoms
that are similar in energy. For example it

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can form bonds with hydrogen nitrogen
oxygen and phosphorus. And the reason why

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it's important that those bond energies
between carbon and those other atoms are

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relatively similar. It means that carbon
can move around those different atoms

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essentially exchanging them and forming
different chemical compounds. It can break

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a bond with one atom, and form a bond with
another type of atom without needing much

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energy or without giving up much energy.
And that creates a great versatility in

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the sort of chemical compounds that you
can form. Another reason why carbon is so

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favored is because once it does form a
bond with these other atoms, those

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chemical compounds are quite stable. So
results of this versatility of carbon

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being able to bond with other types of
atoms and the stability of the resulting

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bonds and molecules is that we can produce
quite complex organic compounds such as

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this DNA molecule here. One alternative
that has been discussed by astrobiologists

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is silcon, silicon based lifeforms have
been a, a favorable alternative for

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science fiction writers and also in films
as well. Here's a rather interesting

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speculation from H.G. Wells. In 1894. And
he says, one is startled towards fantastic

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imaginings. Visions of silicon-aluminium
organisms why not silicon-aluminium men at

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once? Wandering through an atmosphere of
gaseous sulphur, let us say, by the shores

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of a sea of liquid iron some thousand
degrees or so above the temperature of a

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blast furnace. But, silicon has some major
disadvantages. For example, it forms very

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stable Chemical compounds with oxygen. In
fact, you can see these compounds by

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looking out your window and looking at
rocks. Rocks are silicate, where

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essentially these are compounds where
silicons bound with oxygen to produce very

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stable silicate minerals. One of the best
known examples is caught. So, on any

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planet where there's oxygen lying around,
it will tend to react with silicon, and

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form these silicate rocks. These very
stable compounds that really prevent

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silicon from engaging in other chemical
compounds that might be of interest.

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Astrobiologists as plausible building
blocks for life. Of course we could

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imagine planets with very, very low
concentrations of oxygen would free up

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silicon to get involved with other types
of reactions, but it seems that silicon

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has too great a propensity to form
reactions with tiny amounts of oxygen to

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form these stable silicate compounds for
it to be useful. In the origin of life.

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Another problem with some silican
compounds is they're very reactive. For

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example, here are two simple molecules,
one of which is a carbon-based molecule,

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methane. We know that we can ignite
methane, we use it in our gas ovens. And

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on the right is the corresponding silican
compound silane, which spontaneously

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ignites at room temperature. It's a ve ry
reactive compound. Now that doesn't mean

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to say that life could not use these
compounds on a planet where temperatures

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are much cooler. Chemical reactions might
occur much more slowly and deal with some

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of the problems with these highly reactive
silicon compounds. Nevertheless, this

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might be one disadvantage of silicon, some
silicon compounds on a planet like earth.

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What about elements other than silicon?
Well if we look through the periodic table

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at other possible elements that we might
use as building blocks for life,

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unfortunately they don't fair much better
than silicon. For example, gases like

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helium and neon are too inactive to be the
basis of chemical building blocks for

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life. Oxygen, nitrogen, boron and other
types of atoms have a limited number of

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bonds to other atoms and therefore are not
going to form complexity of compound that

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we associate with carbon. And elements
like magnesium, calcium, potassium, sodium

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tend to form ironic bonds is difficult to
form bonds with many elements in the same

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way that we. We can do with carbon
compounds. And so, as we look across the

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periodic table and we look at the
advantages and disadvantages of different

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elements, what we find is that carbon
really comes out on top as the best

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element to form stable, complex, and
highly diverse molecules. Life also needs

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a solvent in which to carry out its
chemical reactions So for life on earth

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that's liquid water. Water has many
advantages as a solvent for life for eg,

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it readily dissolves many chemicals making
it possible to carry out chemical

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reactions. Another intriguing advantage of
water, some people has said it's necessary

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for life. Cause when it freezes and
becomes ice, it floats rather than sinks.

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And, as a result, life, like these fish,
can live under a lake that has a frozen

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surface. It's speculated that if life is
the solvent that salt when it became

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solid, then lakes and ponds would simply
freeze through completely and prevent life

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from inhabiting the surface of a planet.
Well, of course, we can argue about

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whether that's true or not, but it is an
intriguing property of water that is very

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beneficial for life living in cold
environments. Water also has a very wide

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temperature range. You find liquid water
in the polar regions and we find liquid

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water in boiling hot volcanic springs and
this wide temperature range allows life to

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carry out chemical reactions in many
diverse environments on the Earth. What

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about other alternatives? Well, one
alternative that's being favored by

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science fiction writers and even
scientists is ammonia. Ammonia is

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interesting because at low temperatures it
also readily dissolves many chemicals and

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it could potentially allow for ammonia
based bonds between chemicals similar for

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example, for amino acids and some people
have proposed Primitive protein chains

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made from ammonia-based chemical
reactions. Problems with ammonia are that

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it has a very narrow temperature range.
It's only liquid from -78 to -34. But

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perhaps that's not a problem on a planet
where there are large surfaces in which

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these temperature ranges are met. And
also, it sinks when it freezes. Meaning

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that in bodies of liquid ammonia when they
become very cold and the become solid,

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they would freeze through completely. And
how do we know that there aren't life

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forms that adapt to surviving for long
periods of time in frozen ammonia, and

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thawing out when the ammonia is melted
again. Of course, these are speculations

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that we can't Address. There are other
types of solvents that are being proposed

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for life as well, such as hydrogen
fluoride. Hydrogen fluoride when it's a

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liquid has a very wide temperature range,
from -83 To +twenty C and it also

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dissolves a wide range of substances. The
problem with Hydrogen flouride is that

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fluorine is quite rare in the universe.
It's a 100,000 times less abundant than

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oxygen, necessary to form water. And, it's
rather aggressive at destroying organic

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compounds. So on the face of it, liquid
Hydrogen flouride also doesn't look like a

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particularly good solvent for life. It's
interesting to observe that our apparen

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tly narrow view of life is consistent with
what we see in the Universe. Some people

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have said that our idea of Carbon based
life forms, using liquid water, is just a

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very narrow. Earth centered view of life
and that on other planets we would expect

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to find life with entirely novel
biochemistries, almost unimaginable to us

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but, in fact when we go to other planets
in our solar system, we don't observe

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unusual life forms. For example, on the
surface of Venus, there's 464 degrees

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centigrade. We don't observe strange
silicon based life forms. Using some

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unknown solvents, we observe what appears
to be a lifeless surface. And so it seems

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that our predjudices about carbon-based
life using liquid water may not be so far

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from the truth, as a plausible view, of
how life might be constructed. Through the

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rest of the universe. But a question that
does face astrobiologists, nevertheless,

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is are there other biospaces? Our
biospace, based on life using carbon as a

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building block for its molecules and
liquid water as a solvent Might just be

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one by space. Are there by spaces using,
for example, silicon based lifeforms in

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liquid ammonia? Are there by spaces that
use carbon based molecules in liquid

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ammonia? A future challenge in
astrobiology is to really determine

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whether these speculations about other
types of chemical compounds, silicon based

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compounds, other types of solvents, like
ammonia and hydrogen fluoride are really

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plausible and whether they really do occur
on other planets or whether they are just

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fanciful ideas of science fiction. So what
have we learned in this lecture? Well,

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we've learned that carbon Is the most
versatile element for building molecules.

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At least as far as we can understand based
on our current knowledge of chemistry.

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We've also learned water is the most
versatile and useful substance for doing

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chemical reactions. We've learned that
there are other alternatives such as

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silicon as a basis for molecules and
ammonia instead of water. We also learnt

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the fact that silicon ammonia and other
compounds seem l ess favorable than

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carbon-water as a basis for life. This
doesn't rule them out. The carbon-water

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biospace seems to be the most plausible or
likely to be the most common architecture

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for other life but as astrobiologists we
should keep an open mind. There may well

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be other planets out there. Perhaps rare
in the, in which these alternative

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chemistries are being experimented with,
as the basis of life.