In article <vk87a4dkkgfc0d76l5p57oqsi2g09fn94j@4ax.com>,
 William Hamblen <wrhamblen@comcast.net> wrote:

On Wed, 13 Aug 2008 19:01:27 -0700, Timberwoof
<timberwoof.spam@inferNOnoSPAMsoft.com> wrote:

In article <W7Lok.19952$uE5.19027@flpi144.ffdc.sbc.com>,
John Harshman <jharshman.diespamdie@pacbell.net> wrote:

Who says water is an indicator of life? It's only claimed to be 
necessary for life. Methane, as far as I know, is never mentioned. 
Oxygen is the indicator of life, and if you want to suggest an inorganic 
process that can make a lot of free oxygen in an atmosphere, feel free.

Only oxygen? 

Yeah... it's common and it does some handy chemical reactions. But 
similar arguments can be made for water.

Oxygen is reactive enough that oxygen in the atmosphere would be
depleted unless restored from some source.  The only likely source is
photosynthesis.  Where you have atmospheric oxygen you have living
plants.

Yes, that makes sense. I had it in my head that other chemical bases for 
live were being discussed, and perhaps some other element or compound 
could fulfill a similar role. 

But I agree: If oxygen is present in an atmosphere, that would be a 
really really probable sign of life. :-)

-- 
Timberwoof <me at timberwoof dot com> http://www.timberwoof.com
People who can't spell get kicked out of Hogwarts.