We've looked at the building blocks of life and we've looked at some of the
environments in which life might have evolved. But one thing we've assumed
throughout this. Is that life is based on carbon, and it uses liquid water as a
solvent. Let's revisit that assumption and think about whether there are other,
plausible alternatives that have been considered by astrobiologists. First of
all, we said that life is based on carbon, and what that means is that the molecules
of life have carbon as their backbones. Here are two examples of molecules.
Methane, which has a single carbon atom and four hydrogen atoms, a simple Compound
produced by life, but still based on carbon. And on the right hand side there,
the much more complex molecule of deoxyribonucleic acid, or DNA. which is
also carbon based. The molecule is constructed around carbon atoms, with
other atoms attached to it. Such as hydrogen and nitrogen and phosphorus and
oxygen. These are carbon based molecules. And all of our molecules are built up in
the same way, using carbon. As the basic, building block of these molecules. What
are the reasons why carbon is such a good atom performing molecules? Well one reason
is it tends to form bonds with other atoms that are similar in energy. For example it
can form bonds with hydrogen nitrogen oxygen and phosphorus. And the reason why
it's important that those bond energies between carbon and those other atoms are
relatively similar. It means that carbon can move around those different atoms
essentially exchanging them and forming different chemical compounds. It can break
a bond with one atom, and form a bond with another type of atom without needing much
energy or without giving up much energy. And that creates a great versatility in
the sort of chemical compounds that you can form. Another reason why carbon is so
favored is because once it does form a bond with these other atoms, those
chemical compounds are quite stable. So results of this versatility of carbon
being able to bond with other types of atoms and the stability of the resulting
bonds and molecules is that we can produce quite complex organic compounds such as
this DNA molecule here. One alternative that has been discussed by astrobiologists
is silcon, silicon based lifeforms have been a, a favorable alternative for
science fiction writers and also in films as well. Here's a rather interesting
speculation from H.G. Wells. In 1894. And he says, one is startled towards fantastic
imaginings. Visions of silicon-aluminium organisms why not silicon-aluminium men at
once? Wandering through an atmosphere of gaseous sulphur, let us say, by the shores
of a sea of liquid iron some thousand degrees or so above the temperature of a
blast furnace. But, silicon has some major disadvantages. For example, it forms very
stable Chemical compounds with oxygen. In fact, you can see these compounds by
looking out your window and looking at rocks. Rocks are silicate, where
essentially these are compounds where silicons bound with oxygen to produce very
stable silicate minerals. One of the best known examples is caught. So, on any
planet where there's oxygen lying around, it will tend to react with silicon, and
form these silicate rocks. These very stable compounds that really prevent
silicon from engaging in other chemical compounds that might be of interest.
Astrobiologists as plausible building blocks for life. Of course we could
imagine planets with very, very low concentrations of oxygen would free up
silicon to get involved with other types of reactions, but it seems that silicon
has too great a propensity to form reactions with tiny amounts of oxygen to
form these stable silicate compounds for it to be useful. In the origin of life.
Another problem with some silican compounds is they're very reactive. For
example, here are two simple molecules, one of which is a carbon-based molecule,
methane. We know that we can ignite methane, we use it in our gas ovens. And
on the right is the corresponding silican compound silane, which spontaneously
ignites at room temperature. It's a ve ry reactive compound. Now that doesn't mean
to say that life could not use these compounds on a planet where temperatures
are much cooler. Chemical reactions might occur much more slowly and deal with some
of the problems with these highly reactive silicon compounds. Nevertheless, this
might be one disadvantage of silicon, some silicon compounds on a planet like earth.
What about elements other than silicon? Well if we look through the periodic table
at other possible elements that we might use as building blocks for life,
unfortunately they don't fair much better than silicon. For example, gases like
helium and neon are too inactive to be the basis of chemical building blocks for
life. Oxygen, nitrogen, boron and other types of atoms have a limited number of
bonds to other atoms and therefore are not going to form complexity of compound that
we associate with carbon. And elements like magnesium, calcium, potassium, sodium
tend to form ironic bonds is difficult to form bonds with many elements in the same
way that we. We can do with carbon compounds. And so, as we look across the
periodic table and we look at the advantages and disadvantages of different
elements, what we find is that carbon really comes out on top as the best
element to form stable, complex, and highly diverse molecules. Life also needs
a solvent in which to carry out its chemical reactions So for life on earth
that's liquid water. Water has many advantages as a solvent for life for eg,
it readily dissolves many chemicals making it possible to carry out chemical
reactions. Another intriguing advantage of water, some people has said it's necessary
for life. Cause when it freezes and becomes ice, it floats rather than sinks.
And, as a result, life, like these fish, can live under a lake that has a frozen
surface. It's speculated that if life is the solvent that salt when it became
solid, then lakes and ponds would simply freeze through completely and prevent life
from inhabiting the surface of a planet. Well, of course, we can argue about
whether that's true or not, but it is an intriguing property of water that is very
beneficial for life living in cold environments. Water also has a very wide
temperature range. You find liquid water in the polar regions and we find liquid
water in boiling hot volcanic springs and this wide temperature range allows life to
carry out chemical reactions in many diverse environments on the Earth. What
about other alternatives? Well, one alternative that's being favored by
science fiction writers and even scientists is ammonia. Ammonia is
interesting because at low temperatures it also readily dissolves many chemicals and
it could potentially allow for ammonia based bonds between chemicals similar for
example, for amino acids and some people have proposed Primitive protein chains
made from ammonia-based chemical reactions. Problems with ammonia are that
it has a very narrow temperature range. It's only liquid from -78 to -34. But
perhaps that's not a problem on a planet where there are large surfaces in which
these temperature ranges are met. And also, it sinks when it freezes. Meaning
that in bodies of liquid ammonia when they become very cold and the become solid,
they would freeze through completely. And how do we know that there aren't life
forms that adapt to surviving for long periods of time in frozen ammonia, and
thawing out when the ammonia is melted again. Of course, these are speculations
that we can't Address. There are other types of solvents that are being proposed
for life as well, such as hydrogen fluoride. Hydrogen fluoride when it's a
liquid has a very wide temperature range, from -83 To +twenty C and it also
dissolves a wide range of substances. The problem with Hydrogen flouride is that
fluorine is quite rare in the universe. It's a 100,000 times less abundant than
oxygen, necessary to form water. And, it's rather aggressive at destroying organic
compounds. So on the face of it, liquid Hydrogen flouride also doesn't look like a
particularly good solvent for life. It's interesting to observe that our apparen
tly narrow view of life is consistent with what we see in the Universe. Some people
have said that our idea of Carbon based life forms, using liquid water, is just a
very narrow. Earth centered view of life and that on other planets we would expect
to find life with entirely novel biochemistries, almost unimaginable to us
but, in fact when we go to other planets in our solar system, we don't observe
unusual life forms. For example, on the surface of Venus, there's 464 degrees
centigrade. We don't observe strange silicon based life forms. Using some
unknown solvents, we observe what appears to be a lifeless surface. And so it seems
that our predjudices about carbon-based life using liquid water may not be so far
from the truth, as a plausible view, of how life might be constructed. Through the
rest of the universe. But a question that does face astrobiologists, nevertheless,
is are there other biospaces? Our biospace, based on life using carbon as a
building block for its molecules and liquid water as a solvent Might just be
one by space. Are there by spaces using, for example, silicon based lifeforms in
liquid ammonia? Are there by spaces that use carbon based molecules in liquid
ammonia? A future challenge in astrobiology is to really determine
whether these speculations about other types of chemical compounds, silicon based
compounds, other types of solvents, like ammonia and hydrogen fluoride are really
plausible and whether they really do occur on other planets or whether they are just
fanciful ideas of science fiction. So what have we learned in this lecture? Well,
we've learned that carbon Is the most versatile element for building molecules.
At least as far as we can understand based on our current knowledge of chemistry.
We've also learned water is the most versatile and useful substance for doing
chemical reactions. We've learned that there are other alternatives such as
silicon as a basis for molecules and ammonia instead of water. We also learnt
the fact that silicon ammonia and other compounds seem l ess favorable than
carbon-water as a basis for life. This doesn't rule them out. The carbon-water
biospace seems to be the most plausible or likely to be the most common architecture
for other life but as astrobiologists we should keep an open mind. There may well
be other planets out there. Perhaps rare in the, in which these alternative
chemistries are being experimented with, as the basis of life.