H2O ≈ Life
Why?
FYOS 1001
Various Alien Life Forms
Because of our limited perception, we may be quite biased as
… a person in a small local Chinese village may convince that only the
Chinese language exists in the world (since it is the only one spoken
by all the people around!)…
Life can take place in a wide variety of environments very different from Earth
 their life forms can be quite different from us, in harmony with their local
conditions.
 a life can arise from repeated interactions among its constituent subunits…
(e.g., cells in our bodies)
# of interactions of a human over a lifetime : A human (170cm) with a walking speed
of about 1m/sec over 70 years of lifetime  interact with the environment that is 1
billion times of its body size…
Need about “1 billion” interactions
Non-chemical Life?
• Entire galaxy as a living organism
• Life based on gravitational force
• Stars are the subunit as atoms in our body
• Stars interact on a time scale of many
millions of years.
• If we think that a life can be arisen from
repeated interactions among stars…
 millions of such interactions are needed for
life to begin at the smallest (e.g., molecular
equivalent) level.
 Universe is far too young to have such life
forms!
Life on Neutron Stars
Non-chemical Life?
• Strong force based life…
• Surface temperature ~ 1million degrees,
enormous gravity…  no molecules (not
even atom) can survive.
• Nuclei would last for only 10-15 second,
however this can be equivalent to a lifetime!
evolution (or change in life) happens faster
Entire civilizations could rise and fall a
million times while a human eye can wink.
If true, no meaningful way to communicate
with them.
•Nuclei of size 10-13 cm with speed of 1000 km/sec and lifetime of 10-15 second 
travel distance = 10-15 sec*1000 km/sec = 10-12 km =
Let’s Focus on life forms that we can perceive
and possibly communicate with!
 Chemical Life Forms
Earth Life is Carbon-Based
• Life on Earth is made from more
than 20 chemical elements, but
four elements make up about 96%
of the mass.
Oxygen  mostly in H2O
Cell structures and their functions
are due to Carbon  Carbonbased life!
Any chemical compounds contain C
 “organic compound”
chemical composition of
the human body by weight.
There are 103 chemical elements known to exist in the Nature.
A life form with an iron skeleton and using methanol as solvent?
Origin of the Elements
After Big Bang, lightest elements (H, He, Li,
Be) were created, but only for a limited
time period (3 to 20 minutes).
92% H + 8% He and almost no other
elements.
• Later stages of Nuclear fusions in Stars
Cosmic abundance
• Not much Li (H+He), Be,
B (Be+H or He+Li)…
Cosmic Abundance  Big Bang, Stellar & SN nucleosynthesis
Chemical Reactions  filling the valence shell
Valence shell :
Outermost shell of
an atom  Wants to fill
this valence shell with 8
electrons
Chemical bonding
• Elements want to have electrons in certain numbers!
2, 10, 18, etc.
• Thus an element with 11 electrons wants to get rid of one electron while an
element with 7 electrons wants to accept one more electron from
somewhere
 Chemical reactions
covalent bond : sharing an electron (s)
Ionic bond : Example of a chemical
bonding that makes up salt
Examples of Good pairings are
H + F  Hydrogen Floride
Na + Cl  Sodium Chloride (salt)
or 2H + O  H2O
Why Carbon-based?
Any other base
element?
• C60 (Bucky-balls)
Entirely made of Carbon.
(superconductor material, etc.)
Requirements for the base element:
o Abundant element
o Combine easily with itself and others
• Advantages of Carbon!
Let’s consider all different ways of combining with
Hydrogen…
1. Oxygen : H2O and H2O2
2. Nitrogen : NH3 (ammonia) and N2H2
(hydrazine)
3. Carbon : so many different ways…
E.g., C90H84 or C167H336, …
# of possible isomers for C167H336 = 9×1083
? Can this may be due to the difference in the
maximum number of bondings for C (No=4),
O (No=2), N (No=3)?
• Then why not Si-based life?
Silicon-based Life?
• Si has four “bonds”
similar to Carbon.
For example, CH4 (methane), SiH4 (silane).
o Si-Si bond strength is ½ of the C-C bond
o C-C, C-H, and C-O bond strengths are ~same,
but Si-H and S-O bonds are stronger than Si-Si
 hard to make a large complex structures
(chain, rings, etc.)
o At low temperature, nearly all Si atoms will
form SiO2 (quartz!) and this rock-forming
material are very difficult to dissociate!
Although life based on silicon may be possible,
it will be extremely uncommon at best!
Life based on carbon seems favored as the
dominant kind of life in the Universe!
Need for good solvent in chemical life
Solvent = “the matrix of life”
= dissolve other chemical compounds
= carrying molecules to/from cells
In any chemical life form, an efficient carrier of molecules is necessary.
Among solid, liquid, gas solvents, liquid ones may be the most efficient.
If so, what are possible solvents for life in the universe?
Good Solvents
H2O
NH3
CH3OH
CH4
C2H6
• Requirements for a good solvent:
1. A solvent must be abundant!
2. Remain liquid for a wide range of Temp!
3. Dissolve a wide variety of chemical
compounds!
4. T-range for liquid is high!
Solvent
T range for liquid
Total range of T
water
0 to 100 C
100 C
ammonia
-78 to -33 C
45 C
methyl alcohol
-94 to +65 C
159 C
methane
-182 to -164 C
18 C
ethane
-183 to -89 C
94 C
At low temperature, chemical reactions are slower hence slower metabolism.
H2O good choice for Life
• Liquid Water as the best solvent for life
1. A wider, higher range of temperature for being
liquid
2. Solid water floats over liquid water
3. Polar molecule  dissolve certain types (polar)
of molecules only : cell membrane
4. Water is the most common liquid (H is the most
abundant element and Oxygen is the 3rd most
common element in the Universe)
5. Heat capacity and Heat of Vaporization 
regulate temperature
6. Highest surface tension
Environmental Requirements for Life
1.
2.
3.
Must have a source of molecules
Must have a source of energy
Must have a liquid medium
Any worlds meet the #3 requirement, likely meet the first two requirements
also.  Why?
Conservatively…
A habitable world = only if it has a liquid water!
Habitability of Planets
Goldilocks planets
C and H2O for alien life!
Searching for Life ≅ Searching for liquid Water!