Why is Pluto Red?

The short answer to the question posed in the title of this post is the surface of Pluto is covered by tholin, not to be confused with Star Trek’s Tholians.  The word was first coined by Carl Sagan in 1979.  It is derived from the Greek word tholos for muddy, due to the texture of this substance that Sagan was able to produce in laboratory experiments.  Tholin is not produced naturally on Earth as oxygen would break it apart in the atmosphere soon after its formation.  However, it is possible for tholin to have been produced early in Earth’s history and may have played a key role in the formation of life.

Tholin is found on some bodies located in the outer Solar System, notably Saturn’s moon Titan, Neptune’s moon Triton, and Pluto.  As tholin no longer occurs naturally on Earth, the study of these objects may allow us to acquire key information on the chemical processes that occurred early in our planet’s history.

The chemical process that produces tholin is fairly complex.  Ultraviolet light and negative ions (electrons) combine to breakdown methane (CH4) and nitrogen (N2) molecules in the atmosphere.  Natural gas that we use to heat our homes during the winter is mostly methane.  Through a series a chemical processes, tholin is formed and falls onto the surface as a reddish, gooey type substance.  Below is how tholin appears when produced in a laboratory:

Credit: Chao He, Xinting Yu, Sydney Riemer, and Sarah Hörst, Johns Hopkins University

The reddish tinge in Titan’s atmosphere is thought to be a result of the presence of tholin.  Below is an image of Titan taken by the Huygens probe as it descended towards its surface in 2005.  Huygens was originally part of the Cassini spacecraft that has been exploring the Saturn system for the past 11 years now.  As a side note, this is still the most distant landing from Earth successfully attempted by any space mission.

Credit: NASA/JPL

So why is this important and why did Carl Sagan spend several years researching this stuff?  When tholin is on the surface of a body, and there is water present on the surface, it dissolves and forms amino acids that are the building blocks of life.

The atmosphere of the young Earth was not anything like we enjoy today.  A half billion years after Earth’s formation, the atmosphere was dominated by the outgassing of volcanic activity and the Earth was covered with a blanket of carbon dioxide and methane.  At the same time, water began to form on the surface.  These conditions would have been ripe for the formation of tholins on Earth and the eventual breakdown into amino acids in the oceans.  How exactly this could have led to life on Earth is not completely understood at this point.  And that is why astrobiologists are very keen to study places where tholins are naturally present.

To learn more about the search for life in space and how it was formed on Earth, NASA’s Astrobiology website is a good start.  At Cornell University, the recently formed Carl Sagan Institute’s purpose is to continue Sagan’s quest to find life beyond Earth.  Their website can be accessed here.

*Image of Pluto at top of post.  Credit:  Credits: NASA/JHUAPL/SWRI

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