Astrobiology » Differences in Chemical Compounds

» The Liquid Medium «
Let us now consider if it is possible that the major chemical elements which occur on earth may form be arranged in different ways to give a whole ensemble of different compounds. We shall start with the liquid medium. Most scientists think that life can only exist in the presence of a liquid medium. It is widely considered that the only suitable medium is WATER. Water has so many unusual properties that most scientists consider it essential to life. There are two other compounds which have certain characteristics in common with water namely hydrogen fluoride and ammonia. However of the three chemical elements oxygen is by far the most abundant element not only on Earth but in the universe as a whole. This means that WATER is likely by far to be the most common of the three compounds on all planets. One can hardly imagine seas of liquid hydrogen fluoride!


Hydrogen Bounding

Hydrogen Fluoride - HF

Very strong

Hydrogen Chloride - HCl

Water - H2O

Extremely strong

Hydrogen Sulphide - H2S

Ammonia - NH3

Very strong

Phosphine - PH3

In all three of the compounds hydrogen bonding is strong. This means in practical terms that their boiling points and melting points are very high considering their molecular masses because of the attraction between the molecules. Also they are ionizing solvents and dissolve very large number of other compounds and in addition even a number of metallic salts.



Molecular Mass

Molec. Mass

Melting Point

Melt. Pnt

Boiling Point

Boil. Pnt

























There really is only one compound that could in theory take the place of water. That is ammonia. The unusual properties of water are to a large extent shared by ammonia. At high pressures ammonia could even be a liquid at room temperature and even at the pressure of the earth's atmosphere it boils at -33 Degrees celsius and freezes at -77 Degrees celsius. Thus the question arises could a planet exist at a fairly low temperature in which there were seas of liquid ammonia with a biochemistry based on ammonia instead of water. For this to happen all the oxygen atoms on such a planet would have to be bound up in compounds other than water. Although ammonia based life could just exist it is very highly unlikely. The nearest thing might be oceans of water containing significant amounts of ammonia giving an alkali environment and a different set of biochemical behaviours to most forms of life on Earth. There are in fact some bacteria which do exist in an alkali environment.

» Hydrocarbon Medium «
A recent article in the Scientific American poses the question as to whether it would be possible for the methane/ethane lakes on Saturn's moon to harbour some very low temperature form of life not dependant on liquid water. The title of the article is Could Allow It to Harbor Life 3 March 2015. The recent discoveries made on the climate on Saturn's large moon, Titan, have made some people ask the question – can some form of primitive life exist in the hydrocarbon lakes of Titan. One of the strangest things found on the Cassini -Huygens Mission to the Saturn System is that the climate on Titan resembles that on Earth. Instead of water at 20 degrees Celsius, there is methane /ethane at -180 degrees. These hydrocarbons are presence as vapour in the air and form lakes of liquid and can fall as 'snow' at slightly lower temperatures. There is also a host of other organic compounds in the atmosphere. The big question is - could some form of low temperature chemistry exist on Titan without water which produces some primitive form of 'life'?

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.