The Lecture of the Week for April 3, 2006 is now available at:
April 3, 2006
Part III: Could We Tell Life If We Saw It?
Life in Ice on Mars
Christopher P. McKay
NASA Ames Research Center
28 min. (requires QCShow Player)
In the coming century, we are going to seek out life, if not in the
stars, at least on the moons and planets of our solar system.
Water is the sine qua non of life on Earth. Where water is absent, life
doesn't exist on this planet. But the presence of water isn't sufficient
either. There are broad regions of the planet where life hasn't been
able to make a go of it, and the icecaps of Greenland are one of those
Chris McKay points out something that would shock any ecologist: "It's
often said that life covers everywhere on this planet. It's not true.
There are large places on this planet where no life forms have figured
out how to make a go of it. And this is one them: the ice environment."
Is the a priori demand for liquid water a good model for life elsewhere?
At the moment we don't know. "Following the water" is the most
conservative approach we can take, simply because we know it works.
Where liquid water is absent on this planet, life "checks out," and in
this talk, McKay talks about three regions of the planet in which he is
conducting his investigations: in the dry valleys of Antarctica, in the
Atacama desert of Chile, and the tops of the high tropical mountains in
the African rift.
None of these places are perfect analogs of Mars, but they are in many
ways similar. Although current evidence suggests that the surface of
Mars is inimical to terrestrial life, there remain plausible scenarios
for extant microbial life on Mars, but only of very modest plausibility.
The surface of Mars today is far more inhospitable to life than any of
these areas on Earth. It's cold, dry, and chemically oxidizing and is
exposed to an intense flux of solar ultraviolet radiation. Temperature
is of interest, not only because of its controlling influence on
metabolic rates but also because of its influence on the stability of
Liquid water is essential for life. All known terrestrial life is built
on aqueous chemistry. That statement is not based on any theory. It's
merely an observation, but given our current state of knowledge of
chemistry and biology, it is hard for us to imagine the existence of
life independent of liquid water.
In the last few slides of his talk, McKay presents another of his ideas:
No matter what biochemical pathways life may adopt in alien
environments, it will almost certainly not exhibit a broad, continuous
range of molecular constructions. Almost certainly, it will appear as if
it were a Lego-like toolkit, composed of only a few dozen fundamental
molecules. Recognizing this fact provides us the opportunity to
potentially recognize the biomarkers of life, even if its adopted
biochemistry is wholly foreign to us.