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Chandra detects Pluto X-rays; Charon’s red polar cap mystery solved

Composite Image of Pluto from Chandra and New Horizons

The first detection of Pluto in X-rays has been made using NASA’s Chandra X-ray Observatory in conjunction with observations from NASA’s New Horizons spacecraft as it approached and then flew by the dwarf planet in 2015. Image Credits: X-ray – NASA/CXC/JHU-APL/R.McNutt et al.; Optical – NASA/JHU-APL/SwRI

Observations conducted with NASA’s Chandra X-ray Observatory for comparison with data from NASA’s New Horizons spacecraft surprised scientists by discovering X-rays coming from Pluto.

Collected on four separate occasions between February 2014 and August 2015, the Chandra data shows a much higher level of X-rays coming from the dwarf planet than does that taken by the New Horizons Solar Wind Around Pluto (SWAP) instrument.

Chandra discovered low-energy X-rays coming from Pluto, which scientists speculate that it results from the interaction of gases in its tenuous atmosphere with the solar wind.

SWAP, designed to measure such activity up close, revealed that contrary to many scientists’ expectations, Pluto’s atmosphere is not rapidly escaping like that of a comet and that interaction between Pluto and the solar wind is much more like that between the solar wind and Mars.

“We’ve detected, for the first time, X-rays coming from an object in our Kuiper Belt, and learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion,” said Chandra observation team leader Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. “We can expect large Kuiper Belt Objects to be doing the same.”

Prior to the Chandra study, the farthest Solar System objects from which X-rays were detected were Saturn and its rings.

According to Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, and co-author of a study on the Chandra findings published in the journal Icarus, skepticism about the likelihood of finding any X-rays coming from Pluto caused debate as to whether observing the small world with Chandra was worth the effort.

While Pluto’s atmosphere is emitting enough gases to produce X-rays, the solar wind at that distance is insufficient to do so, especially at the level of brightness detected. The cold, rocky world has no known magnetic field from which X-rays could be emitted.

Approximately 20 years ago, Lisse led a research team that found X-rays coming from a comet, so he knew that X-rays could be produced through constant interaction between the solar wind and gases from a small object.

Working together with SWAP co-investigators David McComas of Princeton University and Heather Elliott of the Southwest Research Institute, Lisse and his team members proposed several possible explanations for the higher-than-expected amount of X-rays coming from Pluto.

One possibility is Pluto has a longer and wider tail of gases trailing it than SWAP detected. Another suggests more particles from the solar wind than expected are being pushed into the region around Pluto by interplanetary magnetic fields. A third explanation is that the solar wind, which has a low density in the outer Solar System, is somehow forming a doughnut-shaped ring of neutral gas around the dwarf planet’s orbit.

Based on SWAP’s findings, scientists think that Pluto has a gentle, close bowshock or region where it meets the solar wind as well as a small tail behind it.

“When you have a chance at a once in a lifetime flyby like New Horizons at Pluto, you want to point every piece of glass – every telescope on and around Earth – at the target,” said New Horizons co-investigator Ralph McNutt, also of APL, who was also part of the Chandra team. “The measurements come together and give you a much more complete picture you couldn’t get at any other time, from anywhere else.”

Scientists do not expect to detect X-rays coming from New Horizons’ second target, KBO 2014 MU69, but consider that Chandra might find them coming from larger KBOs the spacecraft will observe from a distance.

In addition to interacting with the solar wind, Pluto’s atmosphere is responsible for the red color of Charon’s north polar region, according to New Horizons scientists.

Pluto's moon Charon in enhanced color

NASA’s New Horizons spacecraft captured this high-resolution, enhanced color view of Pluto’s largest moon, Charon, just before closest approach on July 14, 2015. Image & Caption Credit: NASA/JHU-APL/SwRI

Methane gas escaping from Pluto’s atmosphere becomes “trapped” by Charon’s gravity and freezes to ice on the surface of its north pole, the mission team noted in an article published in the journal Nature.

The interaction between the Sun’s ultraviolet light and the frozen methane produces heavier hydrocarbons that when exposed to solar radiation become the reddish organic materials, known as tholins, seen on both Pluto and Charon.

Using both computer models and New Horizons images, mission scientists sought to determine whether the 753-mile (1,212-km) wide Charon is capable of catching and processing methane gas.

The computer models showed Charon to have extreme weather at its poles, primarily because Pluto’s 248-year elliptical orbit causes each pole to alternatively experience 100 years of sunlight and 100 years of darkness. During the poles’ dark winters, temperatures plunge as low as minus 430 degrees Fahrenheit (minus 257 degrees Celsius), which is cold enough for methane gas to freeze and become solid.

After escaping Pluto’s atmosphere, “The methane molecules bounce around on Charon’s surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring,” explained mission co-investigator Will Grundy of the Lowell Observatory in Flagstaff, Arizona.

While the frozen methane evaporates as each pole moves into daylight, the heavier hydrocarbons stay on Charon’s surface. Solar radiation transforms these hydrocarbons into red tholins, which have accumulated on both of the moon’s poles over millions of years.

Even though Charon’s south pole is now in complete darkness, mission scientists were able to study it using light reflected from Pluto and confirm the presence of tholins there.

A similar process of atmospheric transfer could also be occurring on other Kuiper Belt planets that have moons, principal investigator Alan Stern noted.



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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