Does Enceladus’ alkaline ocean make it friendly to life?

Recent data sent back to us by the Cassini space probe as it samples the geyser water being shot into space at Saturn’s moon Enceladus, has determined that the moon’s subsurface ocean has a very high pH.  The pH is estimated to be around 11 or 12.  This would be considered extremely alkaline, but the team analyzing the data concludes that this might improve the odds of supporting life.  They point to the alkaline hydrothermal vents, such as The Lost City, on the ocean floor of earth, where warm alkaline fluids flow out into the cold salty deep.  There is some thought in the astrobiology community that life on earth may have originated in a similar alkaline vent environment 4.5 billion years ago.  The difference, however, is that on early earth the alkaline vent fluid was flowing into an acidic ocean, with a thin mineral wall separating the fluids and allowing a proton gradient to form.  It was this proton gradient that generated the energy necessary to transport electrons from molecule to molecule.  This is exactly what living organisms do to generate energy – they pump protons across a cell membrane, transport electrons to an ultimate electron acceptor (oxygen in our case), and use the proton gradient to generate ATP (the energy currency of the cell).  Cells do the biological equivalent of what the alkaline vents are doing geochemically.  For that reason I wonder if the high pH of Enceladus’ ocean really would support the origin of life since it doesn’t necessarily imply situation where a proton gradient would occur.

 

Reference:

How Friendly is Enceladus’ Ocean to Life?  Astrobiology magazine.  Feb. 4, 2016

Musings on the biochemistry on Saturn’s moon Titan. Part I

Part I: what makes potential life on Titan such a challenge?
By Rich Feldenberg

As we have been repeatedly surprised by the discoveries made by our robotic explorers over the last few decades, it is certainly clear that our solar system is much more interesting and diverse than we ever previously imagined. There appears to be at least several objects among our cosmic neighbors that show promise as potential habits for life outside of Earth. Some of the prime real estate in this respect would be Mars, Jupiter’s moon Europa, and Saturn’s moon Enceladus. Both Europa and Enceladus show evidence of having liquid water, vast oceans in fact, beneath their frozen crust, and Mars most likely had liquid water on its surface billions of years ago, and perhaps still has liquid water somewhere below the surface. Even Venus may have been covered in water in it’s early history only to evaporate it’s oceans away due to an out of control green house effect. In addition to these, there are other candidates moons and even dwarf planets, that may be home to liquid water beneath a frozen surface, and therefore also a potential abode for living things.

Everything we know about life on earth points to the importance of water in the origin and maintenance of life. If we find life on any of these worlds it will probably be “life as we know it”, in the sense of having a similar biochemistry to earth life with a dependance on liquid water. In fact, it would probably be a consequence of being water based that would contribute to the close similarities in molecules like DNA and proteins, that would most likely be present on a watery world. There may certainly be some important and interesting differences, such as using a different genetic code to specify amino acids, perhaps having a different chirality (i.e. Right handed amino acids instead of left handed ones), and possibly having a different set and number of amino acids, but the basics of DNA to code and store information and proteins made of amino acids to carry out metabolism, will likely look like a odd version of what we have here on earth. But, there may be one place in our solar system where the potential for “life as we don’t know it”, also has the potential to exist. That place is Saturn’s moon Titan.

Titan is unique in the solar system in a number of important respects. For a moon its quite big, being the second largest moon in the solar system – only Jupiter’s moon Ganymede is larger. Titan has twice the diameter of earth’s moon. If it was in orbit around the sun instead of Saturn, it might be considered a planet unto itself. It is also the only moon with a thick atmosphere, as dense as the atmosphere down here on earth. Like Earth, the atmosphere on Titan is primarily nitrogen, but unlike earth there is no oxygen to breath. But perhaps the most remarkable thing about Titan is that there is liquid on it’s surface. Besides Titan, only the earth is known to have a large volume of surface liquid today. As we said before, Mars probably had liquid water on it’s surface billions of years ago that has since evaporated away. Of course, the earth has vast oceans of liquid water, but the liquid on Titan is not water, it’s liquid methane. There are lakes of methane, and evidence for rain and rivers of the stuff! We think that liquid is necessary for the chemistry of life to take place. In solids the molecules are too fixed to react much and gases are usually to dispersed for reactions to have a high chance of occurring, and are difficult to contain. A major difference between Earth and Titan, however, is that Titan is cold – really cold. The surface temperature on Titan is -179.2 degrees celsius or -291 degrees fahrenheit. Now that’s pretty darn cold, and Titan would be even colder if not for the greenhouse effect of methane in its atmosphere. It’s both the extreme cold and the fact that methane is the liquid we have to deal with, that makes any type of biochemistry onTitan so challenging to consider.

Water has properties that make it an ideal solvent to dissolve the molecules of life into solution. Could liquid methane serve a similar role on the frozen moon Titan? What would it take for life to find a way to use methane as a solvent for the molecules of “life as we don’t know it”? Let’s look at some of the possibilities for a methane dependent life form that lives in the conditions present on Titan. We will stick, as much as possible, to the rules of organic chemistry, but Warning, wild speculation ahead.

Let’s first look at the major differences between water and methane as your choice of solvent. Every school child knows that water is H2O – made up of two hydrogen atoms covalently bonded to an oxygen atom. It is not a linear molecule, however, but has a bent shape. This creates an electric dipole moment in the molecule, meaning that one side of the molecule is partially negatively charged (the oxygen in this case), and the other end, facing the hydrogens, has a partial positive charge. The electric dipole moment can be measured and it’s value is placed in units of the debye (D), with water having an electric dipole moment of 1.85D. It is this dipole, combined with the fact that oxygen and hydrogen are extremely common throughout the cosmos, that makes water such and interesting, useful, and seemingly indispensable substance. Most organic molecules are electrically neutral – they have no formal charge – but many of the functional groups in the organic molecules do have an uneven distribution of electron sharing, creating a partial charge separation in the chemical bonds between certain atoms. For example, in the carboxyl group there is a partial negative charge on the oxygen and a partial positive charge on the carbon. This means that the carbon can act like an electrophile (attracted to electron rich atoms) and reacts with a partially negative charged molecule, and a nucleophile (attracted to an electron poor atom) will tend to react with the carbon. The reactions due to these kinds of attractive forces are called polar interactions, and it is these types of interactions that create interesting chemistry, and life is all about interesting chemistry.

Water can form hydrogen bonds with other water molecules, as in the figure above. Because of waters dipole properties it will interact strongly with other molecules that contain polar covalent bonds, and this allows them to be soluble in a watery solution. This is energetically favorable for polar molecules, because even though they cause the hydrogen bonds of the water matrix to break as they take up space in the solution (energetically unfavorable), they then form polar bonds with the water to replace the broken bonds (energetically stable)- so in a sense, no harm done. This allows the polar molecules to be soluble in water solution, and this is what you want for your biomolecules if you’re a life form – soluble molecules, not ones that precipitate out of solution. Water helps to stabilize polar molecules and is very important for maintaining the proper shape and structure of important biomolecules like DNA and proteins.

Non-polar molecules, on the other hand, really don’t do well in water and they will not go into solution. The non-polar molecules also break the waters hydrogen bonds since they have to fit in the water matrix, but since they are non-polar they can not replace those bonds, and so remain energetically unfavorable. They will tend to form a separate layer from water and stick together to minimize their interaction with water molecules. For this reason, the non-polar molecules are called hydrophobic, which means water fearing or water hating.

Water is also great at dissolving important salts in solution, like sodium chloride (NaCl) for example. The polar nature of water will break the ionic bonds between the sodium atom and the chloride atom apart, and the sodium will be surrounded by a sphere of water with it’s partially negative oxygen atoms facing it, while the chloride will be surrounded by a sphere of water with it’s partially positive hydrogen atoms facing it. Inorganic ions from salts like sodium chloride, potassium chloride, calcium phosphate, magnesium sulfate, and others are vital to the workings of enzymes and stabilizing biomolecules. Would these ions be able to exist in a non-polar methane sea?

Unlike most organic molecules, one group that is non-polar and water hating are the hydrocarbons. Methane is the simplest hydrocarbon, having just one carbon atom covalently bonded to four hydrogens. There is no dipole moment in the methane molecule. Carbon and hydrogen both have approximately the same amount of pull for the electrons. Even if that weren’t so, the symmetrical tetrahedral shape of methane would still cause the polar bonds to cancel each other out. Hydrocarbons with long chains of carbons are also non-polar. Propane is a hydrocarbon with 3 carbons and 8 hydrogens, and octane is one with 8 carbons and 18 hydrogens. Methane is the major liquid on Titan, with some ethane (2 carbons) thrown in for good measure. Hydrocarbons don’t undergo polar reactions. That’s one reason that they can remain liquid at such low temperatures where other organics would freeze solid – they interact so weakly that it is difficult to get them to interact enough that they become solid. These non-polar molecules do have their own very weak interactions, however. They are subject to non-polar forces called Van der Waals forces. These results from a weak induced dipole, as one molecule gets close to another. Say you have a mixture of methane molecules. There is no permanent dipole, but when the hydrogen of one methane gets too close to the hydrogen of another methane the electron of the first will repel the electron of the second one, and that creates a tiny fleeting attraction between the two molecules. It becomes more pronounced at very low temperatures where the heat energy of motion is not too great. At temperatures that we find comfortable, these minuscule interactions are easily overwhelmed by all the thermal motion. If you have very long hydrocarbons, or other non-polar molecules, then the combined Van der Waals forces acting along the different parts of the molecule can make the effect more significant. This would possibly be a very important force in any Titan life form. On Earth, Van der Waals forces certainly have their important places, and can be important in many DNA-protein interactions and some protein-protein interactions, but many of the interactions between biomolecules rely on polar chemistry to create covalent bonds, and this might be difficult to achieve for a living thing on Titan.

In part II of this article we will examine two possible solutions for Titan biochemistry – What I’m calling The Non-Polar liquid solution (NPLS) versus the Polar-Non-Aqueous liquid (PNAL) solution, or the NPLS vs PNAL. By solution, the meaning can equally double as referring to either a mixture of a liquid with another substance or an answer to a problem – either works fine.

References:
1. Titan: Wikipedia; https://en.m.wikipedia.org/wiki/Titan_(moon)

2. Freezing and Melting Points of some common liquids:
http://www.engineeringtoolbox.com/freezing-points-liquids-d_1261.html

3. “Selected values for electric dipole moments for molecules in the gas phase”, United States Department of Commerce; http://www.nist.gov/data/nsrds/NSRDS-NBS-10.pdf

4. Organic Chemistry; John E. McMurry, 8th edition. ISBN-13: 978-0840054449

5. Also see my article on Silicon Based Life forms (another variety of life as we don’t know it):
In this case it would presumably be water dependent but not based on carbon.
“Why the Horta would not have looked like a rock monster”;
http://darwinskidneys-science.com/2015/06/18/why-the-horta-would-not-have-looked-like-a-rock-monster/

Atomic Tuesday: The Pauling Scale

The Pauling scale is a convenient way to compare the electronegativity of two or more atoms.  An atom’s electronegativity is the attractive force it has for an electron.  The Pauling scale gives a relative magnitude for this attraction.  Linus Pauling, the Nobel winning chemist, contributed enormously to areas of chemistry such as molecular bonding theory.  He devised this scale with the maximum electronegativity set to 4.0.  Fluorine is the most electronegative atom and has a Pauling value of 4.0.  Francium, on the opposite corner of the periodic table from Fluorine, as the lowest Pauling value for any of the elements, and is 0.7.

If two atoms have a large difference in their Pauling values, then they are more likely to form ionic bonds since one of the pair will have a much greater attraction for an electron then the other.  If on the other hand, the pair of atoms have very similar values, then they are likely to form covalent bonds, since both atoms have equal or close to equal attraction and therefore share the electron between them.    Even in covalent bonds, unless the two atoms involved are of identical type there will still be a subtle difference in electronegativity that will create an imbalance in the location of electrons in that bond.  This is a polar bond, and accounts for a lot of interesting chemistry to occur due to placing a partial negative charge on one atom and a partial positive charge on the other.

The atoms on the periodic table are not sitting randomly with respect to their electronegativity .  One of the many valuable features of the periodic table is that you can predict, based on location in the table, which elements will have higher or lower Pauling values than other elements found at other locations on the table.

Why the Horta would not have looked like a rock monster

Why the Horta would not have looked like a rock monster.

By Rich Feldenberg

 

 

In the original Star Trek series the U.S.S. Enterprise, on its heroic five year mission to explore strange new worlds, to seek out new life and new civilizations…, comes across many fascinating and unusual alien species.    In one episode in particular, “The Devil in the Dark”, they make contact with a creature with an entirely different kind of biochemistry from the typical carbon based biochemistry seen elsewhere on earth and throughout the galaxy.  Captain Kirk and the crew of the StarShip Enterprise first encounter the alien after they respond to a distress call sent from a mining colony on the planet Janis VI.  When the Enterprise arrives, Kirk learns that routine mining operations have been disrupted by a strange life form native to the planet.  The creature doesn’t register as a life form on the tricorder, lives deep in the darkness of the mines, and can eat through solid rock due to it’s highly corrosive nature.  Once Spock learns that the mines contain a multitude of spherical shaped silicon nuggets, he has a hunch.  As usual, Spock’s hunch turns out to be correct.  The round silicon nuggets are the eggs of a silicon based life form.  Spock is able to use his mind melding abilities to communicate briefly with the creature, and learns that it is called a Horta, and that the miners have been destroying its eggs, causing it to face total extinction.

While I applaud the episode for thinking outside the box when it comes to attempting to imagine “life as we don’t know it”, I suggest that making the silicon based Horta, essentially a living rock, showed a poor understanding of both organic and inorganic chemistry.  So first let me say what was outstanding about the idea before I lay into it with my chemistry degree equivalent of a phaser set on kill.  

Ok, it was great that there was any attempt at all to speculate on the astrobiology of an organism that was not carbon based back in a 1960s TV show.  By non-carbon based I’m not talking about energy beings here, but legitimate speculation about an alternative chemical system, as we’ll discuss below.  This certainly went well above the usual laziness and poverty of imagination of portraying an alien as a human with pointy ears, or a green girl, or, as in the case of the klingons, with angry eyebrows (they hadn’t evolved there forehead ridges yet!).  Now don’t get me wrong, I don’t have anything against a green blooded pointy eared Vulcan.  In fact, Mr. Spock may be my all time favorite fictional character, but it really isn’t terribly creative when you consider how life forms on different worlds would likely have evolved down completely different pathways leading to completely unique body plans.  In other words, even if most life in the universe is carbon based, like us (probably still a good guess), intelligent aliens wouldn’t necessarily appear anything like humans.  Ok, I get it, there were budget issues!

Now, all life on earth is carbon based.  Carbon chains form the backbone of all the major classes of biomolecules, such as DNA, protein, carbohydrate, and lipids.  Because carbon can form so many complex molecules, it has it’s own special branch of chemistry devoted to it – organic chemistry.  The chemistry of every other element falls under the heading of inorganic chemistry.   That seems really unfair, right?  I mean one element out of the 92 naturally occurring elements, and it gets it’s own branch of science!  Well it actually is fair when you consider that the number of possible organic molecules far exceeded the number of inorganic molecules combined by orders of magnitude.  There are an estimated 10 million carbon compounds in existence.  There are a lot of ways that carbon can react depending on what other atoms are near it on a molecule.  Carbon is element number 6.  It has 6 electrons in orbitals around a nucleus containing 6 protons and 6 neutrons (at least for carbon12, the most common isotope).  Because carbon is in group 14 of the periodic table, its electron configuration is 1S², 2S², 2P².  Each S orbital can only hold two electrons but the P orbital can hold up to 6 electrons so there are 4 vacant spots in carbon’s P orbital.  There is a tendency for an element to move towards a noble gas configuration, meaning that for reasons related to finding the lowest energy state, and hence the greatest stability, the element will either shed electrons or gain them to mimic the electron configuration of the closest noble gas atom.  This is known as the Octet Rule.  

Metals typically lose an electron to this end, and non-metals typically gain them.  In the case of the non-metal carbon, if it fills its four vacant orbitals with electrons it will have the electronic configuration of the noble gas neon (Ne) with 10 electrons, and, ah that feels so good!   Well, how is poor carbon to get four more electrons?  That’s where covalent bonding comes in.  It shares electrons with other atoms, so carbon will try to form 4 bonds with up to four other atoms.  These could be 4 single bonds, in which case carbon is attached to 4 different atoms, or it can also form a combination of single, double, or triple bonds.  Both single and double bonds are very common in organic chemistry.  The reactive double bonds in carbon based molecules account for a lot of the action in organic chemistry.  The really special thing about carbon compounds are that carbon can bond to another carbon, which can bond to another carbon, and so on.  In fact, it is very common to see very long chains of carbon bound together making something like a huge protein or DNA molecule, for instance.  This would seem to make carbon the best suited element in the periodic kingdom for producing life forms.  I’m surprised nature never thought of that, oh wait it did!!

So what about the poor Horta?  The Horta is silicon based.  Silicon (Si) is in group 14 of the periodic table.  Hold on, thats the same column as Carbon (C)!  It also means that like carbon, silicon will also want to gain 4 electrons to fill it’s orbitals.  By the way, this is why the periodic table is so useful.  You can tell a lot about an atom by where it sits on the periodic table.  The reason you can go around the table to the next row down and find another element like the one in the row above it is why it is periodic!  There are repeating patterns.  Anyway, silicon is element number 14, and its electron configuration is 1S², 2S², 2P⁶, 3S², 3P².  If it can just gain four more electrons it will have the stable electron configuration of the noble gas Argon (Ar).  It is for this reason that people have speculated that if there is any other element in the periodic table that could cast a little carbon-like magic, it would have to be silicon.  The other great thing about silicon is that it is common, really common.  It is the 8^th most common element in the universe (carbon is the 4^th most common element in the universe), and is second only to oxygen, as the most common in the earth’s crust.  It is found in rock and sand all over the earth.  You certainly wouldn’t want to have a life form based on a very rare element like Lanthanum (element 57), for instance, where there isn’t enough of it around to make anything  useful.  You want your building materials to be lying around everywhere.

                                                                                           

Unfortunately, the problems faced by silicon based life forms would be many.  First of all, while silicon is capable of forming a variety of silicon compounds, and even forming chains of silicon (silanes) that resemble the long chain alkanes of carbon, the number of actual silicon compounds is far lower than for that of carbon.  The Si-Si single bond is weaker than a C-C single bond due to the silicon atom being larger, and thus the silicon atoms are further apart.  This makes silicon bonds unstable, and you don’t see very long chains of Si show much permanence.  They break apart quickly, and in fact, are so reactive that they will spontaneously combust in an oxygen atmosphere.  Really very bad for any oxygen breathing lifeforms.  Also, whereas, carbon readily forms double bonds, silicon is less likely to form many double bonds, which again limits the kind of chemistry it can undergo.  Double bonds expose electrons to “attack” by other molecules – this is the beauty of organic chemistry – so fewer double bonds in Si lead to fewer potential chemical reactions.  Again, boring chemistry equals inert substances, not vibrant living materials.

Carbon also loves to form carbon rings, even rings that include double bonds that provide a special stability due to a property called resonance associated with the electrons in the ring system.  This is a bit like a little electric circuit in the molecule where the electron can have room to move around and make the bonds between atoms even stronger.  These kind of compounds are called aromatic compounds and are extremely common in living things on earth.  Si doesn’t form rings very readily so the equivalent sorts of aromatic Si compounds would be unlikely to exist.    

Remember, life is all about lots of interesting chemistry.  Interesting chemistry may not explain the purpose of life, but it’s one way to sum up what life is.  It seems most likely that life would only thrive and evolve if it’s chemistry allowed a lot of diversity and potential to form many varieties of stable compounds.  Limitation in the number and stability of compounds and number of reactions is like a death blow to the odds of life.  But what about the Jurassic Park law of biology that says, “life finds a way”?  Yeah, that may be so, but it found carbon chemistry out of all the other useless junk on the periodic table.   

In addition to these problems, there is the problem of how silicon would be recycled throughout it’s biosphere.  Carbon is well suited for recycling due to it’s ability to form the simple gas CO2 (carbon dioxide).  As we all know, CO2 is taken up by plants, and using the energy of the sun, uses the carbon to make glucose (a carbon compound in a ring shape) and releases the waste product molecular oxygen (O2).  Animals eat the plants, thereby gaining the glucose for energy and delivery of a carbon source to make more kinds of carbon compounds.  For both animals and plants the glucose can be oxidized – burned in the presence of oxygen in the tiny midi-chlorians in the cell (sorry, I meant mitochondria)- to release the energy stored in the carbon bonds.  This is the process of chemical respiration.  Once the animal dies, its body decomposes (oxidizes!), and CO2 is released back into the atmosphere where plants can take it in again.  This is the carbon cycle, and it makes life on earth sustainable for billions of years.

For silicon life forms there would need to be some sort of silicon cycle.  This is problematic since SiO₂, while common, is solid and not a gas.  It is also not soluble in water, so how it would circulate through the biosphere to become accessible to the silicon life forms that need it may create an insurmountable dilemma.  With no silicon cycle, even if silicon based life could somehow get a foothold with it’s weak and limited bonding capabilities, it would quickly shut down once the available silicon became trapped in all the dead silicon organisms.  

Ok, so I’ve tried to illustrate why silicon based life forms probably won’t exist to begin with, but let’s say, for the sake of argument, the Horta really was based on Si chemistry.  Why wouldn’t look like a big rock?  This is the thing that really irks me!!  The whole point of invoking the possibility of Si life is because silicon has some chemical similarity to carbon due to its similar electron configuration.  So our imagining a Si based creature to be essentially a living rock is analogous to an intelligent silicon based species speculating that carbon based life forms would be like living lumps of coal or dangerous diamond creatures that can scratch glass.  It might make for a good episode of space Sci-Fi for our silicon based friends to watch on TV, but you can see that this is obviously an incorrect interpretation.  There is a huge diversity of life on earth but we don’t have living coal or diamond creatures.  The reason, of course is that carbon forms very complex compounds with itself and with lots of other kinds of atoms.  Coal and diamond are basically crystallized forms of carbon.  Crystals are not suitable for the basis of life because they don’t allow a lot of rapidly changing chemistry to happen within them.  

If the Horta was silicon based, the whole point would be that Si would be mimicking the complexity and diversity of carbon, and its tissues should be no more rock-like than earth creatures are diamond-like, but instead have soft squishy muscles, nerves, intestines, kidneys, blood, and so on.  It could be conceivable that the Horta could have a hard outer shell made of silicon dioxide (SiO₂) but it’s whole body would not be silicon based rock.  This misinterpretation was brought home when Dr. McCoy was asked to treat the phaser wounded Horta and replied, “I’m a doctor, not a bricklayer”.  Captain Kirk appropriately reprimands McCoy telling him, “Your a healer, there’s a patient, that’s an order”.  

At the end of the Star Trek episode, Kirk realizes that the Horta was only a protective mother and didn’t wish to harm the miners or their equipment.  The crew of the Enterprise is able to make the miners understand that the Horta means them no harm if they leave her eggs alone.  In fact, the natural mining abilities of the Horta could make a collaboration between the miners and the Horta very profitable.  As the Enterprise leaves orbit, the many Horta eggs are getting ready to hatch.  Too bad that due to their silicon chemistry they are likely to spontaneously combust when they are exposed to the oxygen rich atmosphere that the humans are breathing!    

References:

1. Wikipedia entry on silicon:  http://en.wikipedia.org/wiki/Silicon

2. General Chemistry, Principles and Modern Applications, third edition

By Ralph H. Petucci.

3. Inorganic Chemistry; third edition,  by James E. Huheey.