On July 14, 2015, after nearly 10 years and three billion miles traveled though our solar system, NASA’s New Horizons space probe made history when it performed the first flyby of Pluto and captured the most detailed pictures of the dwarf planet to date.
Since then, New Horizons has been transmitting data back to Earth, providing planetary scientists with incredible discoveries. Among these discoveries is Pluto’s colour.
New Horizons’ view of Pluto, courtesy of NASA (link)
Seen clearly above, Pluto is reddish brown. This is remarkable considering that when picturing Pluto, we often imagine an icy-blue, rock world.
This marks Pluto as the second red planet in our solar system, next to Mars. However, unlike Mars, Pluto’s colour is not the result of iron oxide. Instead, Pluto is red due to the formation of complex organic molecules called tholins.
Tholins are composed of carbon, hydrogen and nitrogen, and while they do not form naturally here on Earth, they have been found on many icy bodies in the outer Solar System, including Neptune’s moon Triton, and Saturn’s moon Titan.
The formation of tholins is believed to be the product of ultraviolet light driving a reaction between methane (CH4) and nitrogen gas (N2) in the planet’s thin atmosphere. The resulting tholins then fall to Pluto’s surface, giving it a reddish appearance.
The Hörst Laboratory has synthesised tholins. Credit to Chao He, Xinting Yu, Sydney Riemer, and Sarah Hörst of Johns Hopkins University.
The structure of tholins is undefined. The term was coined by Dr. Sagan and Dr. Khare in their 1979 paper when a new term was required to describe the varying organic products created by subjecting gases abundant on Titan to ultraviolet light [3]. In their paper, they define tholin to be “complex organic solids formed by the interaction of energy”. [3].
To give a more ralatable explanation, planetary Scientist Dr Sarah Hörst – who has made studying tholins part of her research – compares the word ‘tholin’ to ‘salad’ as both are nonspecific and describe “a mixture of a number of different compounds and spans a fairly broad range of materials” [4].
As for the New Horizons space craft, it will continue to push further into the Kuiper Belt where its next scheduled destination is 2014 MU69 in January, 2019.
– Kevin O’Connor
References
- NASA’s Three-Billion-Mile Journey to Pluto Reaches Historic Encounter. https://www.nasa.gov/press-release/nasas-three-billion-mile-journey-to-pluto-reaches-historic-encounter (access October 22, 2016).
- Pluto: The ‘Other’ Red Planet. http://www.nasa.gov/nh/pluto-the-other-red-planet (access October 22, 2016).
- Sagan, C.; Khare, B. Tholins: organic chemistry of interstellar grains and gas. Nature, 1979, 277
- What in the world(s) are tholins? http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html (access October 22, 2016).
