In the ongoing search for extraterrestrial life, it seems to me that so far, most of the probes/rovers have relied on indirect means. They try to determine whether a given environment is/was hospitable to life rather than whether there actually is/was life there. For example, we look for things like evidence of liquid water, atmospheric composition, temperature, pressure, etc.

I am interested in how we would directly decect extra-terrestrial life. Keeping in mind that life might have evolved much differently in a different environment, what we currently know about life on earth may not be applicable to alien life. However, IMO, there would be a few generic traits that the alien life would need to posess to be called life. These would include:

1. Metabolism - The ability to convert available energy into a form usable to the life-form.
   
2. Tropism - The ability to respond to external stimuli.
   
3. Reproduction - The ability to propagate the species.

I am not a biologist, so I am probably oversimplifying here, but what exactly would we use to detect these functions when they might take forms that are completely unpredicted? Also, could we ever definitively conclude that there is/was no "life" on a given planet? Sure, if we detected something that is somewhat familiar, we could prove the positive, but could we ever prove the negative?

well, i'm no export so i'm just giving my 2 cent

I would keep it at seeing if there is a chance of life on a moon/planet/object

if there is a chance,... we can send a more advance probo to posibly find out what.

I see no point in trying to look for life lets say on the moon, i would probebly not find anything becouse it is just not posible.

hmmz doesn't seem to make much sens, hope you get waht i'm trying to say

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~Jorge Schrauwen

The Viking biology experiments were the only ones to look for life on another planet (so far). They appeared to looking for products of metabolism, for the most part.

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Everything I need to know I learned through Googling.

I just though of something, wouldn't it be easest to look for the wast productes of life? stuff life leave behind?

__________________
GCS/S d(+) s+:+ a--- C++(+++)>$ W+++>$ L>+ M+>++ w++ P+>++ tv@ PS b+ DI+ G e-> h! r-- !z+

~Jorge Schrauwen

Thanks for the Viking link To Seek. I had forgotten about the bio experiments on Viking. I was a kid at the time of Viking (born in 1967), but I did follow them fairly closely. In looking at the experiments, it appears that they were strictly looking for a carbon-based life form that metabolizes in ways similar to life on Earth.

While carbon is an ideal base for life because of its bonding characteristics, IMO, it is not the only option. Also, what if there is some metabolization process unique to life on Mars, Titan, or wherever? What would a life form that "fed" on kinetic energy (wind, fluid motion) be like?