Tuesday, April 14, 2015

extraterrestrial life in our solar system?

link to paper: Schulze-Makuche, Grinspoon (Astrobiology vol 5, number 4, page 560-5)


video of Huygens's lander approaching Saturn's mysterious moon Titan:


Is there extraterrestrial life in our solar system?
         Titan, a giant moon of the planet Saturn, formed at about the same time as Earth. Like Earth, Titan was hot at birth due to the kinetic energy (energy due to movement) of the infalling matter that formed it. Heat is a measure of the average velocity of the particles of a body. As planets and moons coalesced from gravitational attraction, a lot of fast moving matter got squished into small volumes, making those bodies quite hot. This is why Earth's core is molten. Thus, even though Titan formed far from the sun, life could have evolved on Titan after it cooled to the point where water and earthlike biochemistry became possible. Plant life would have been possible since Titan lies just on the outer limit of the zone of photosynthesis in our solar system (at ground level on the equator at noon, it is bright enough to read a newspaper on Titan).
         Today, however, Titan is cold (-180 C or -290 deg F) so, as it cooled, life would have had to develop alternative metabolic pathways and discover new liquid biosolvents (such as a saturated mixture of ammonia and water or liquid methane and ethane). Whether such an ad hoc conversion from one biosolvent (water) to another, low temperature biosolvent, is even possible is debatable. We just don't know.

          Titan is interesting for several reasons. It is the only moon in the solar system with a real (dense) atmosphere, weather and seasons. At sea level, Titan's atmospheric pressure is superior to Earth's! Like Earth, nitrogen is the major component of Titan's atmosphere. There are sizeable quantities of methane and hydrogen. There is no oxygen, however, and water exists only as a hard rock ice (or when finely divided, as sand).

         Titan's atmosphere is believed to photosynthesize the biochemical precursors to life (whether this process implicates life is, at this point in time, anyone's guess..) On Earth, those biochemicals led to life. But Titan may be too cold for life (or the life may be marginal and primitive). However, biochemists would like to study Titan's organic chemistry for clues to the origin of life on Earth. They believe that primitive biochemicals have been effectively stored at low temperatures for eons, possibly preserving the earliest stages of the evolution of life. 

            Precipitation, as snows or rains of hydrocarbons like methane, ethane or benzene, occurs. Small shallow seas or lakes of these cold, liquefied gases ring the polar regions. In some places depths of 170 meters (over 500 feet) are attained. The equatorial regions are quite "arid", in contrast.

 Saturn seen from its moon Titan (artist's rendition!)
The clouds and the rivers are of liquid methane and ethane, not water..

               If Titan were to possess life, it would have to meet the Four Requirements:

1- Adequate energy flux. Life requires energy for the biological work of self organization, repair and maintenance of internal structure: "autopoeisis".

2- (at least for "earthlike" life), an adequate biosolvent, available in sufficient quantities. For cellular, "earthlike" life, such solvents provide a medium for mobile biomolecules to achieve sufficient concentrations for effective interactions. Other properties may be important like the capacity to rapidly dissipate the heat of bioreactions to avoid damage to delicate structures or unstable molecules. What really constitutes a good biosolvent, under which conditions, is still in the early stages of investigation..

3- chemical cycling: auto-catalytic reactions in which the products are also employed as inputs to the reaction. The reaction effectively "makes more of itself"! In its globality, life may be seen as a bewilderingly complex self-organizing system of chemical cycling. Thus the human race regenerates its DNA anew each generation. Chemical cycling may be seen as the "first step toward life". It is occuring on Titan, indicating only that life might be there..

4- polymeric chemistry. Polymers string together similar molecular bits like the beads of a rosary or links in a chain. Variable bits - containing information (like the genetic code) - may be attached as side links to the chain. The result is long information bearing molecules which can be used as an "instruction set" to generate essential biochemicals (like proteins) or to fabricate a whole new organism (reproduction). Polymeric chemistry permits the storage and transmission of essential organizing / structuring information from one generation to the next. Thus life overcomes, transcends the mortality of individual organisms.

             If life were to exist on Titan it would need to assure an adequate energy flow. This could come from photosynthesis although using a different biochemistry than that of water based Earth life. 

Terrestrial photosynthesis, generic formula:

6 CO2 + 6 H2O + energy (sunlight) + Enzymes --->  C6H12O6 (glucose, sugar) + 6 O2

Six carbon dioxide molecule are combined with six water molecules using the energy of sunlight to create an energy rich sugar molecule and six oxygen molecules. The sugar, like a battery, stores the energy of the sun for later use. During such "controlled burning" the sugar is recombined with oxygen, releasing energy for biological work, and returning to the "initial point": 6 carbon dioxide and 6 water molecules. 

C6H12O6 + 6 O2 ---> 6 CO2 + 6 H20 + energy (used for biological work)

The cyclic nature of life could not be more clear - chemical cycling!

             In their "hypothesis paper" (pdf in first link above), the authors propose several plausible energy capturing cycles involving photosynthesis. One generic reaction is 

C2H2 + 3H2 --> 2CH4 + energy

This reaction involves in reality several steps, mediated by enzymes: the "generic" formula, shown here, merely shows the end result (which is all that interests us at this stage). In this plausible Titanian "respiration" reaction, one molecule of acetylene is combined with 3 molecules of hydrogen to produce 2 methanes and liberate biologically useful energy. Acetylene is an energy containing molecule created by inorganic (non-living) processes in Titan's upper atmosphere. It is created from the interaction of sunlight and hydrocarbons and, in effect, stores solar energy in the unstable acetylene molecule. At the temperatures prevalent on Titan, acetylene freezes out of the atmosphere after it forms and rains down on the surface, providing an accessible energy source for any enterprising organism to exploit. The above plausible generic "respiration" reaction would liberate the energy stored in acetylene by converting it into methane.

Does such a reaction occur on Titan? Interestingly, methane is unstable under the chemical conditions on the surface of Titan. Theoretically, it should occur only in trace amounts. But, instead, it makes up 5% of the lower atmosphere. Could this excess methane constitute a "biomarker" indicating that life if present? At present, we simply don't have enough information to answer this question. However, living processes tend to separate isotopes of a given element, favoring the lighter isotopes. The methane found on Titan is, in fact, isotopically "light" (elevated carbon 12 to carbon 13 ratio), suggesting that life might be responsible..

             But, once again, we just don't know! Our probes to Titan have not had sufficient resolution to answer the pertinent questions. In fact, we are still at the stage of generating plausible questions to be answered by future space missions..

             For example, would we know what to look for? One of the curious features of Titan's surface is its "freshness". Why is it so smooth? One possibility is that some of the energy flux through Titan's surface is of biological origin. Natural selection may have selected for organisms that liberate a lot of heat to melt ammonia / water mixtures, liberating a biosolvent thus allowing for more biological activity. Such organisms could employ the solar energy trapped in downfalling acetylene to produce heat to melt ammonia / water mixtures. The constant churning of semi-liquified areas by the liberated heat could explain the relatively smooth, fresh surface features of the moon.

               In sum, the probability of life on Titan is low but not demonstrably zero..

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