The just reported finding that the Mars Curiosity Rover used its laser-induced breakdown spectrometer to detect the key elements needed for life (nitrogen, oxygen, carbon, sulfur and phosphorus) was encouraging to be sure and has been described as “a major coup for the Mars Science Laboratory Mission”. This has been operated out of the Jet Propulsion Laboratory in Pasadena.But let’s be sure we don’t confuse life’s building blocks (we are assuming carbon based life) with the precursors of life.These precursors, in the form of complex carbon compounds (the basis for what we call Organic chemistry) have yet to be revealed or detected. JPL researchers do say that a key priority is to search for locations where “organics” might be preserved.
Interestingly, previous missions, e.g. Viking, have disclosed that Mars could have supported microbes billions of years ago, when water was perhaps more available – as opposed to being in frozen form now.
How would such primitive microbes have come to be? The famous Miller and Urey experiment provides a clue. They basically applied an electrical discharge to a chemical brew resembling the Earth’s primitive reducing atmosphere. This brew included ammonia and methane, as well as hydrogen and water vapor. The effect of the discharge transformed the mix into a diverse yield of organic compounds. These included amino acids, as well as substances such as formic acid and urea that normally occur in living organisms.
The very fact so many organic compounds could arise is remarkable in itself, given the vast number of possible compounds that might have emerged. And while it is true that the discharge didn’t produce actual living cells, there is no reason – given enough time, that a primitive pre-biotic cell in the distant past could not have emerged given the building blocks left behind.
The consensus of current research is already fairly clear about the nature or form of the first primitive organisms. They were prokaryotic autotrophs . More specifically, they were suspended colloidal micro-spheres capable of exchanging energy with their surroundings. To get energy, these self-sustaining coacervate droplets could use one or two basic reactions involving adenosine triphosphate (ATP) and adenosine diphosphate:
L*M + R + ADP + P -> R + L + M + ATP
ATP + X + Y + X*Y -> ADP + X*Y + X*Y + P
In the above, L*M is some large, indeterminate, energy-rich compound that could serve as ‘food’. Whatever the specific form, it’s conceived here to have two major parts capable of being broken to liberate energy. Compound R is perhaps a protenoid, but in any case able to act on L*M to decompose it. Concurrent with the first reaction is the possibility of a second, entailing autocatalytic molecules X*Y. These molecules could accelerate their own formation, using ATP.
On the basis of the chemical reactions, the hypothetical coacervate would consist of the combination: X*Y + R. Now, what properties ought we expect for any such primitive life form? These include: simple organization, ability to increase in size, and ability to maintain itself over extended intervals. Does the coacervate meet these conditions?
Well, it has a simple organization, consisting of the molecules X*Y and R. It can increase its size by synthesizing more of X*Y, growing until hydrodynamically unstable. Finally, it can maintain itself over indefinite intervals, so long as it can extract the chemical components it needs. What about replication? We expect that this is feasible when it splits into ‘daughters’ after growing too large. Then, so long as each has some of the protenoid R there is the capacity for replication.
From all the evidence thus far uncovered, it does appear that ancient Martian organisms in the form of coacervates roamed that planet billions of years ago. How or why Mars changed so radically we may never know - but perhaps Curiosity can also help us to find clues. Among the reigning theories is that Mars suffered a collision with a giant asteroid billions of years ago which caused it to catastrophically lose its atmosphere. With the atmosphere gone, temperatures became too extreme to support liquid water, or life. Another theory is that in the distant past Mars lost it protective magnetic field (in a primitive magnetosphere) perhaps owing to a monster solar flare or coronal mass ejection. With the loss of magnetosphere the planet would have been blasted by high frequency radiation that no life could have surivived. This, even if it did have a residual atmosphere.
Meanwhile, in basic astronomy courses, students learn that Mars never really had a chance to keep an atmosphere because of its low surface gravity. In this case typical atmospheric molecules, say like oxygen (O2) would easily have reached escape velocity. Readers can learn more about such factors here:
http://en.wikipedia.org/wiki/Atmospheric_escape
