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Pluto’s red regions may have been created by Charon-forming impact

Pluto map annotated

Annotated view of the informally named Cthulhu Regio, as well as other features, on Pluto. Image Credit: NASA / JHUAPL / SwRI

The dark red regions around Pluto’s equator may have their origins in the giant impact that formed its moon Charon four billion years ago, according to a team of Japanese researchers.

Dark reddish spots extend along Pluto’s equator, the largest being Cthulhu Regio – a whale-shaped area stretching more than 700,000 square miles (1.8 million km2), approximately 1,850 miles (3,000 km) long and 450 miles (750 km) wide.

Charon in enhanced color

Enhanced-color view of Pluto’s moon Charon showing its northern red polar cap. (Click to enlarge) Image Credit: NASA / JHUAPL / SwRI

Similarly colored regions have been spotted on other Kuiper Belt Objects (KBOs) and dwarf planets, including a large area in Charon’s north polar region.

Scientists have identified the material in these reddish areas, which may contain organic matter, as tholins, possibly the product of an interaction between solar radiation and Pluto’s surface.

Alternatively, the tholins could have been brought to the Pluto by comets, scattered when the comets impacted the dwarf planet.

However, neither explanation accounts for the bright bedrock of water-ice seen in the equatorial region.

To confirm their theory that the tholins were created during a giant impact, the Japanese research team conducted both a laboratory experiment and a computer simulation.

Like the Earth’s moon, Charon was created in a giant impact when a KBO about one-third of Pluto’s mass slammed into proto-Pluto.

The impactor likely contained organic compounds such as formaldehyde, commonly found in comets, which originate in the Kuiper Belt.

Heat produced by the impact likely melted much of Pluto’s surface, melting ice into warm liquid water. Formaldehyde from the impactor then flowed into that water.

To simulate that process, the researchers mixed water and simple organic compounds with concentrations similar to those found in comets, then heated the mixture for several hours.

As time passed, complex organic compounds formed, and the mixture became darker and redder.

Heating the liquid at a minimum of 122 degrees Fahrenheit (50 degrees Celsius) for 1,000 hours produced material that looked significantly like the dark regions on Pluto’s equator.

Computer simulations confirmed a collision between proto-Pluto and an object of about one third current Pluto’s mass yielded “warm, liquid-water pools near the equatorial regions of the Pluto-sized object” and could have produced a moon of Charon’s size, said Yasuhito Sekine of the University of Tokyo and study leader.

Although the warm pools of liquid water generated by the impact were only temporary, they lasted long enough for the simple organic compounds from either the impactor or proto-Pluto to form complex organic compounds like tholins.

High-speed impacts likely were common in the ancient outer Solar System and could be the reason for similar dark red regions, as well as a variety of brightnesses, colors, and densities, in other large KBOs.

Findings of the study have been published in the January 30th issue of the journal Nature Astronomy.

Pluto in Natural Color

Pluto’s Cthulhu region are clearly visible as a dark band at lower left in this image from NASA’s New Horizons spacecraft, which made a close flyby of the dwarf planet in July 2015. Image Credit: NASA / JHUAPL / SwRI



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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