[BLANK_AUDIO] We've looked at the building blocks of
life, and we've looked At some of the environments in which life
might have evolved. The 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 have said that life is
based on carbon. And what that means is that the molecules
of Life have carbon as their backbone.
Here are two examples of molecules. Methane, which has a single carbon atom
and four hydrogen Atoms, a simple produced by life, but
still based on carbon. And on the right hand 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 phosphorous 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 for forming 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 Is 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 the result 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 silicon. And silicon-based lifeforms have been a
favorite 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 Toward 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 Was so above the temperature of a blast
furnace? But silicon has some major disadvantages. For example it forms a very stable
chemical compounds with oxygen. In fact you can see these compounds by Looking out your window and looking at
rocks. Rocks are silicates where essentially
these are compounds Where silicon has bound with oxygen to
produce very stable silicate minerals. One of the best known examples is quartz. 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 to astrobiologists as
plausible building blocks for life. Of course, We could imagine planets with very, very
low concentrations of oxygen that free up Silicon to get involved in other types of
reactions, but it seems that silicon Has too great a propensity to form
reactions with tiny amounts of oxygen and Form these stable silicate compounds for
it to be useful in the origin of life. Another problem with some silicon
compounds is they're very reactive. For example, here are two simple
molecules,
One of which is the carbon-based molecule
methane. And we know that we can ignite methane. We use it in our gas ovens. And on the right is the corresponding
silicon Compound silane, which spontaneously
ignites at room temperature. It's a very reactive compound, and 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. But 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 fare 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 The complexity of compounds that we
associate with carbon. And elements like magnesium, calcium,
potassium, Sodium tend to form ionic bonds. It's difficult to form bonds with many
elements In the same way that 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. And for life on Earth that's liquid water.
Water has many advantages as a solvent For life.
For example, it readily dissolves many
chemicals, Making it possible to carry out chemical
reactions. Another intriguing advantage of water,
which some people have said is Necessary for life, is that 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 used a
solvent that sunk 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 is 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. We 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 has been
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, to 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 minus 78 to minus
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 they Become solid, they would freeze through
completely. But 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 types Of solvents that have been proposed for
life as well, such as hydrogen fluoride. Hydrogen fluoride, when it's a liquid, has
a very wide temperature range from minus 83 to plus 20 degrees Centigrade, and it
also dissolves a wide range of substances. The problem with hydrogen fluoride is that
fluorine is quite rare in the universe. It's 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
fluoride Also doesn't look like a particularly good
solvent for life. It's interesting to observe that our
apparently narrow view of Life is actually 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,
that'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 prejudices 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 biospace. Are there biospaces using for example Silicon-based life forms and liquid
ammonia?
Are there biospaces 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 learnt in this lecture? Well, we have learned the 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 have also learned that water is the Most versatile and useful substance for
doing chemical reactions. We've learnt that there are other
alternatives such as silicon As a basis for molecules and ammonia
instead of water. And we also learnt the fact that silicon,
ammonia and other compounds seem less Favorable than carbon and water as a basis
for life 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 rarer than the Earth In which these alternative chemistries are
being Experimented with as the basis of life. [BLANK_AUDIO]
