Re: life test for planets

From: Mike Lorrey (
Date: Sun Jan 27 2002 - 10:29:02 MST

Doug Jones wrote:
> wrote:
> >
> >
> >
> > <<Planet reveals telltale signature of plant life
> >
> > Astronomers have identified a telltale signature of plant life in
> > light reflected from a planet for the first time - but the planet is
> > Earth. Researchers at the Haute Provence Observatory in France
> > collected light from Earth that had bounced back down from the
> > shadowed side of the Moon.
> >
> > "It's curious that no-one thought of doing this before," says Jean
> > Schneider, of the Paris Observatory, who helped interpret the data.
> > The signature of plants in the light is clear, he says: "There is a
> > lack of reflection below 725 nanometres."
> Curious but quite possibly pointless- that chlorophyll is the dominant
> photosynthetic compound on the surface of the earth today is an accident
> of history. If the green slime hadn't poisoned the atmosphere two
> billion years ago, rhodopsin would be *the* signature of a living
> planet. To expect that independently evolved life in another solar
> system would use the exact same chemical pathways as on earth is
> provincial thinking in the extreme.

Not really, Doug. The question to ask is WHY chlorophyll replaced
rhodopsin. The reason is that the early terran atmosphere density was 52
atmospheres, mostly made up of CO2. Thus, the type of light that early
life forms experienced was distinctly different from that of today.
Definitely much redder, ergo rhodopsin was a more efficient chemical for
transforming sunlight into chemical energy than chlorophyll, given that
spectral band. As the CO2 density in the atmosphere decreased as CO2 was
sequestered into limestone by those life forms, the spectrum that life
forms saw chaged to a much more yellow/blue dominated spectrum,
whereupon chlorophyll became the more efficient chemical.

Chemistry and the laws of physics to not change from planet to planet,
so to expect that the most effective and efficient and easiest chemical
pathways to NOT be used in any environment given similar circumstances
is specious thinking in the extreme.

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