QUESTION:
If we got rained on by ammonia and methane on Earth, would anything be
able to survive?

ANSWER from Meg Urry on April 29, 1996:
My guess is yes, even if the atmosphere were replaced by ammonia and
methane, some forms of life would survive, although not people or most
animals.

Ammonia and methane are two of the simple substances that were thought 
to form, with water (H2O) and hydrogen, the basic proteins from which 
life is made. (This was first demonstrated in a famous experiment by 
scientist Stanley Miller at the University of Chicago half a century 
ago.) And nitrogen,  a major component of ammonia, is 20% of our 
atmosphere already and is an important constituent in our bodies as 
well.

But life as we know it today has evolved so that most animals need 
oxygen as "fuel." If ammonia and methane replaced the oxygen in the 
atmosphere (so the Earth had a "reducing" rather than "oxidizing" 
atmosphere), many life forms (like us) would probably not survive 
(unless we developed an elaborate survival technology).

The life that would survive would probably be bacteria that can use 
ammonia as fuel, although the ones we know about today usually need 
carbon dioxide (CO2) as well. (These kinds of bacteria are important 
in agriculture.) The biological history of the earth shows that when 
there are drastic changes in the climate or chemistry of the Earth, 
life that profits from the new regime will flourish, while life that 
relies on the old regime dies out.
No doubt, the same would be true were there ammonia and methane 
raining down on the Earth today.