(Submitted April 23, 1998)
Imagine you were an alien roaming around in Space and you were interested
in finding a planet that has life. What is the evidence that Earth has
human life? What are the unambiguous indications of life on Earth?
There are a combination of features of the Earth that you can detect
from space that would be very hard to explain without invoking
the presence of life. These are: the abundance of molecular
oxygen; the presence of chlorophyll, the pigment used by plants
in combining carbon dioxide and water to form sugar; and a trace
amount of methane (which is hard to maintain with all that oxygen
It so happens that the Galileo spacecraft (see, for example
) reached its ultimate destination, Jupiter, via a circuitous route
(for technical reasons), including two flybys of Earth. NASA
attempted, and succeeded, in detecting the abovementioned signs of
life on Earth during one of them.
(Also if you are talking about intelligent life forms, radio and TV
broadcast signals would be a conclusive evidence.)
Of course, there is no guarantee that every planet with living beings
will look like the Earth (it's hard to be sure when you have only one
example!). In fact, Europa, a moon of Jupiter, seems to have a liquid
ocean underneath its icy crust, and some scientists speculate that
there could be life on Europa. In this case, the ice on the surface
makes it difficult to prove or disprove this idea from a distance.
(The JPL Galileo pages above have a lot of information about Europa.)
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