The Curiosity
rover is preparing to drill a little hole in a slab of Martian sedimentary rock
to extract material for testing in the ongoing search for life on the red
planet. What can we expect to find? What was the environment like for the origin
and evolution of life forms on Mars?
We know
far more about the present physical and chemical conditions on the surface of
Mars than we know about the distant, presumably warmer and wetter, past. Since Mars has a thin, very dry atmosphere of
97% pure carbon dioxide, ultraviolet (UV) light from the Sun readily penetrates
to the surface. In the absence of more
than a tiny trace of oxygen, ozone cannot be made in quantity, and cannot
provide an ozone layer similar to Earth’s to protect the surface from killing UV
radiation. In fact, UV light can
dissociate carbon dioxide into carbon monoxide (CO) and atomic oxygen (O) even
at the surface of the planet. Even a
tiny trace of atomic oxygen is very bad news for organic matter: O is a very
powerful oxidizing agent. Any organic matter
exposed at the surface of Mars, whether exposed by weathering of ancient
organic-bearing sediments or dropped onto Mars by impacts of carbonaceous
meteorites, would quickly ”burn” into carbon dioxide and water vapor. It is only in the interiors of ancient
sedimentary rocks, where O cannot penetrate, that organic matter might
survive.
The CO2
content of the atmosphere of Mars is sufficient to provide an average pressure
of about 0.006 atmospheres at the surface, although this number is very variable
from place to place because of the wide range of elevations spanning a deep
basin (Hellas) and several towering volcanoes.
The CO2 famous for maintaining Earth’s surface temperature
above the freezing point (via the greenhouse effect) has a surface pressure of
less than 0.0004 atmospheres. So why is
Mars so cold? Several reasons: the
greenhouse effect on Earth is dominated by water vapor, which is very rare on
Mars; Mars experiences about half the intensity of sunlight that Earth
receives. So an earlier, warmer Mars requires that it was also a wetter
Mars. You need water vapor to make Mars
warm enough to have water vapor! Given
favorable early conditions on Mars, with liquid water present and a strong
greenhouse effect at work, life may indeed have originated there. But what evidence of that former life would
we be able to find today? There are two
obvious possibilities: well-protected organic matter deep inside ancient sedimentary
rocks, or fossils of simple life forms.
But evidence of ancient life would not necessarily prove an independent
origin for life off Earth: large impact events can launch surface rocks from
both Earth and Mars into orbits around the Sun, from which they can collide
with and land on either planet. Martian
life, if any, may be expatriate Earth life--- and vice
versa.
1 comment:
Very interesting ! We can be Martians! :)
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