We are “the stuff of stars,” as connected to the cosmos as the Galaxy that spawned us. This stuff includes the light elements of hydrogen and helium that emerged from the Hot Big Bang some 14 billion years ago, along with the heavier elements of carbon, nitrogen, oxygen, and silicon that were forged in the bellies of giant stars, and the even heavier elements of iron, lead, cobalt, uranium, and other weighty isotopes that were created and dispersed by supernova explosions.
Thanks to the especially promiscuous ways of the carbon atom, interstellar space positively reeks with organic compounds. These include polycyclic aromatic hydrocarbons (PAHs), alcohols, sugars, and perhaps even amino acids. Water abounds – as vapors within star-forming clouds, as ices in pre-planetary disks, and in the flamboyant comets that have been found in orbit around nearby stars.
Artist’s rendering of the disk that surrounds the star Beta Pictoris. The disk is known to contain both exoplanets and exocomets. Image courtesy of Lynette Cook, the Far Ultraviolet Spectroscopic Explorer (FUSE), and NASA (see http://www.nasa.gov/vision/universe/starsgalaxies/betapic.html).
Within our own Solar System, we have found clear evidence for amino acids in the tarry makeup of carbonaceous chondrites – a type of meteorite. On Earth, the flourishing of extremophilic organisms in the most hostile of environments attests to the incredible hardiness of microbial life. What awaits our further investigations?
Chemical pathways in the Milky Way begin with sites of molecule formation in the atmospheres of red giant stars. Further processing occurs inside star-forming nebulae, within pre-planetary disks, and on the surfaces of planets. Image by R. Ruitterkamp (see P. Ehrenfreund and S. B. Charnley 2000, Annual Review of Astronomy and Astrophysics, vol. 38, pp. 427-483 <http://www.gb.nrao.edu/astrobiology/gbtModel/astroBioARAA.38.1.427.pdf>).
Will we find fossil evidence for life on Mars? Could the watery subsurfaces of Europa and Enceladus be hosting life today? And what about the nitrogen and hydrocarbon-rich chemistries that may be slowly evolving in the lakes and atmospheres of Titan? Beyond the Solar System, what are the prospects for detecting a planet whose atmosphere contains free oxygen – the clarion call of photosynthesizing life?
These and many other questions have motivated cosmochemists to consider what kinds of chemical pathways might ensue within interstellar clouds, upon the surfaces of exoplanets, and within the interiors of these myriad worlds. Meanwhile, many astrobiologists have pursued a policy to “follow the water” within the Solar System and beyond in hopes of assessing the prospects for life beyond Earth.
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Having just finished the complete text of “Astrobiology “ authored by Kevin Plaxco.. Michael Gross and published by Johns Hopkins University Press, I would like to offer some thoughts which hopefully might promote a series of comments extending beyond the strict considerations of a biochemical and evolutionary perspective when considering the possibility of
extra – terrestrial life in the universe.
An understanding of what might be out there in terms of simple and or complex life organisms is hampered by our contextual limitations, since we only can relate to what constitutes life in all its myriad forms as what we observe as manifest here on earth. To be sure , we are granted that the principles and dynamics of physic law and chemical processes remain uniform in the universe. The periodic table does not present a random and chaotic governance when it comes to running the cosmos.
The three essential elements are hydrogen, oxygen and carbon. Hydrogen, being the most abundant element and serving as the fuel of thermonuclear fusion is essential and worthy of being considered the prime ingredient for all life to exist as we understand it. Simply put, no hydrogen means no stars and without the birth and death of stars, there would be no super novae with the subsequent forging of many other requisite elements needed to provide the building blocks of solar systems, rocky and water based planets and ultimately thermodynamic utilizing and species replicating life forms. Without these building – block foundations in place, a carbon based organism such as we could never have come into being.
Water is the universal solvent. Carbon, with its unique bonding capacities and versatility promotes a biochemical complexity which is unrivaled by any other element. In the absence of carbon, our entire systemic and life inducing metabolic processes of multiple pathways would never take place. When we consider other elements such as phosphorus, magnesium, iron, calcium et al, one is forced to inevitably wonder at such complexity and astounding synchronicity.
That sublime moment again appears !
Astrobiology, the successor to the once slighted science of exobiology encompasses a vast and perplexing endeavor. Either nature has indeed an intelligence of unsurpassed proportions approaching a sense of a divining guided hand ….. or whatever we can wrap our minds around as to our being a conscious and uniquely structured life form inevitably causes a stir and mysterious contemplation in our understanding of it all.
Readers interested in cosmochemistry and astrobiology may be interested in perusing Harvard’s Origins of Life Initiative (see http://origins.harvard.edu/). They have recently redone their website, so that a variety of video profiles and talks can be accessed. Having gone to several of their monthly Forum talks, I can attest to the wide range of topics that are addressed by these experts — from finding and characterizing exoplanets, to delineating pre-biotic chemical pathways here and elsewhere, spelunking early life on Earth, and synthesizing new forms of cellular life in the laboratory. This really is the New Frontier.