So the lower limit and
the upper limit of the biosphere are not exactly known at the moment.
And scientists continue to study life in the atmosphere, and
many scientists are studying life in the deep subsurface to try, and
understand the lower depths of the biosphere.
But probably, we can say at the current stage that the biosphere
is something like 10 to 20 kilometers thick.
But even if that number changes quite a lot even if it was to double or
even multiply by 5 times.
It doesn't really change the fact that life on Earth is a very thin veneer
on the surface and near surface environment of our planet.
So you might be thinking, what does all this tell us about the search for
life elsewhere?
Why would I want to know this, if I was searching for life on Mars?
Well, understanding the limits of life on our planet.
Understanding where it lives.
The depths of the Earth in which it might grow and reproduce, and
how high it might live in the atmosphere.
And the physical extremes that it can tolerate tell us something about
the possibilities of life beyond the Earth.
It helps us search for life on planets such as Mars.
If we go back to this conceptual diagram that I had shown you earlier.
I think it's true to say that one of the most unremarkable advances in
astrobiology over the last one or two decades.
Has been the realization that some environments on the Earth,
environments that are very extreme.
But nevertheless, harbor microorganisms have similar physical and
chemical conditions to environments that we find on other planets.
Just one example is very cold and salty environments on the Earth.
For example, if you go to Antarctica and you find a very cold salty lake,
you can find microorganisms growing in that environment.
And we also know the planet Mars has cold salty environments and the question is,
do those environments contain life?
And at the moment we don't know, but the point is this, that some physical and
chemical environments on other planets seem to overlap with the biospace of life.
The space in which life might be able to persist.
And so, by knowing the limits of life on Earth,
by exploring the outer boundaries of our biospace,
we can improve our attempts to search for life on other planets.