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Cosmic Chemistry on Pluto and Titan: The Formation of Tholins, and Their Possible Link to Life on Earth

Newly released image of Pluto from the New Horizons probe.
Image from NASA/Johns Hopkins University Applied
Physics Laboratory/Southwest Resarch Institute.

Unless you (and your Facebook news feed) have been living under a rock, you know about the New Horizons mission to explore Pluto, a mission 9 years in the making. The new high-resolution images have shown us both that Pluto loves us (i.e. the heart-shaped region in the southern hemisphere), and it is blushing from all the attention (its red-orange surface). While astronomers have known the color of Pluto’s surface for years, the high-resolution images clearly outline the terrain on the dwarf planet.  But why is Pluto red? The reason may not be the same as you think – and by that, I mean it’s not for the same reason why Mars is red. It’s actually because of some cool chemical reactions between methane & nitrogen with the help of some ultraviolet light and other cosmic rays to produce tholins, long-thought to be crucial to the existence of a “primordial soup”.

Mars is red as a result of the oxidation of iron on the surface of the planet to form iron oxide (rust), primarily in the form of hematite. Hematite is a deep red semiconductor that absorbs in the visible region of the spectrum, and is one of the most abundant materials on the Earth. However, rust is not what causes Pluto’s terrain to be a similar color. This color comes from the formation of a class of chemicals called tholin (Greek for “muddy”). These deposits form as a result of Lyman-alpha waves, a specific range of the ultraviolet spectrum emitted from the Sun. Methane (CH4) and nitrogen gas (N2) in Pluto’s thin atmosphere absorb energy from Lyman-alpha waves and other cosmic rays. They spend that energy in reactions that form a plethora of nitrogen-containing compounds such as nitriles, dinitriles, etc., believed to have been created through photo-generated radicals of methane and nitrogen. These larger molecules undergo subsequent chain reactions, eventually forming high-molecular weight, nitrogen-rich molecules that fall to the surface. Recently, researchers have not only suggested that tholins are capable of hydrolyzing to biomolecules in the presence of water and ammonia, but that these organic molecules are precursors needed to produce organic-based life (a “primordial soup” for those who love buzz words). There is a fantastic review in Chemical Reviews ($) entirely dedicated to the efforts of reproducing tholin synthesis here on Earth, and understanding the mechanisms by which it is formed and consumed on Titan, one of Saturn’s moons.

While life probably doesn’t exist on the surface of Pluto due to the cold temperatures (it’s practically a billion degrees below zero out there… but seriously it’s ~40 K, or -233ºC), it does give scientists some useful insight into these types of reactions that happen or might have happened previously on other planets in our solar system. It also helps determine rock formation processes that might be similar to those found here on Earth. Learning about how higher-order organic molecules are formed through the reaction of N2 and CH4 with cosmic rays can also help us better understand how common this process actually is, and if it really is something from which life could have arisen. However, what lies beneath Pluto’s surface, and other celestial bodies in our solar system like Titan or Mars, is somewhat of a mystery, with evidence to suggest ice or liquid water under their surfaces.

Understanding tholin chemistry is one interesting piece of the larger puzzle of our solar system’s chemistry. Physicists have known for a very long time that the raw materials for matter – carbon, hydrogen, and nitrogen atoms, for example – are formed in dying stars and released by supernovae. However, that’s just the first step in creating organic matter. The processes by which those atoms were pieced together and rearranged into more complicated molecules such as tholins in the early solar system largely remain mysterious.  In a study from 2010, researchers at The Ohio State University in Columbus, Ohio published experiments creating tholins similar to those found on Titan ($) (Saturn’s largest moon) in the lab. The authors hydrolyzed them using ammonia-water mixtures similar to compositions found on Saturn’s moon.  After a year of reaction with water at -20 ºC and +20ºC, they discovered evidence of 6 biomolecules synthesized from Titan’s tholin, listed below.
Other groups in the astrobiology field have seen similar results, concluding that the tholin precursors to more complicated biomolecules can be hydrolyzed under other various conditions to form amino acids like glycine, DL-alanine, and other compounds listed above (Cleaves 2014), particularly when dissolved in water (tholins + water + energy = biomolecules).  Ready for the punch line?  All of these compounds are considered building blocks for DNA and proteins in the organisms found on Earth! In addition, there is evidence to suggest that Titan, a moon with a surface temperature of 94 K, could potentially hold liquid water for up to 104 years before freezing due to impacts of cryovolcanism, which is why Titan has seen so much attention recently.

Although there is absolutely no evidence to suggest life is on Pluto (sad face), we do believe that its color comes from the existence of tholin precipitated from the dwarf planet’s thin atmosphere based on observations from Titan. And while liquid water might not exist on Pluto’s surface to convert tholin to useful biomolecules, it may exist on Titan (along with rivers of methane – cause you know, it’s space).  However, new data collected from New Horizons suggests the existence of oceans inside Pluto surrounding its rocky core! It’s quite fascinating how common tholin is believed to exist just within our solar system. Carl Sagan discussed this in a paper published in 1979 ($), back when he and Bishun Khare first synthesized tholin in the lab from methane and other basic hydrocarbons. In the paper, he and Khare state that “we have discussed this material as a constituent of the Earth’s primitive oceans and therefore as relevant to the origin of life; as a component of red aerosols in the atmospheres of the outer planets and Titan; as present in comets, carbonaceous chondrites, and pre-planetary solar nebulae; and as a major constituent of the interstellar medium…”. To put it simply –  this stuff is everywhere, and it could be an important component in the processes that may have started life here on Earth.


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