The Mystery Of Pluto’s X-Ray Glow

At left we see Pluto in regular light photographed by the New Horizons probe during its July 2015 flyby. The blue glow at at right is Pluto in X-ray light as seen by the Chandra telescope orbiting Earth at the same time. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Center/Chandra X-Ray Center
At left we see Pluto in regular light photographed by the New Horizons probe during its July 2015 flyby. The blue glow at right is Pluto in X-ray light taken by the Earth-orbiting Chandra telescope at around the same time. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Center/Chandra X-Ray Center

If you’ve been to a dentist, you’re probably familiar with X-rays. Like the colors purple and red, X-rays are a “color” of light but one invisible to the human eye. If there were X-ray-sensing life forms, who knows what hue they’d see, but it would have a color. Even though X-rays are invisible, they have many practical uses. Dentists use them to see into teeth and gums to diagnose our oral health.

While dental machines produce X-rays using electricity, nature often cooks them up under conditions of extreme heat or through interactions of tiny subatomic particles with a planet’s magnetic field. For instance,  the sun’s outer atmosphere, called the solar corona, sizzles at several million degrees and produces copious X-rays. You wouldn’t think something as cold as Pluto would register in X-rays, but that’s not what Chandra found. NASA’s Chanda X-Ray Observatory, designed to see the cosmos with X-ray vision, made observations of the dwarf planet in conjunction with the New Horizons flyby last summer and discovered it glowing in X-ray light.

X-rays occupy the high-energy end of the electromagnetic spectrum. What we see as the rainbow of colors comprises just a small slice of light's extensive range of wavelengths. Credit: Univ. of Oregon
X-rays occupy the high-energy end of the electromagnetic spectrum. The rainbow of colors our eyes can see comprises just a small slice of light’s extensive range of wavelengths. Credit: Univ. of Oregon

When looking at the photo panel above, keep in mind there’s a huge difference in scale between the optical and X-ray images. New Horizons made a close flyby of Pluto, but Chandra orbits the Earth, so the level of detail visible in the two images is very different. The Chandra image is 180,000 miles across at the distance of Pluto, but because the planet is only 1,500 miles across, the level of detail is very coarse.

Detecting X-rays from Pluto is surprising given that Pluto is not only cold and rocky but has no magnetic field. Unlike the sun or a dental machine, it can’t produce its own X-rays. But there may be another way. From previous observations of comets we know that the interaction between the gases that boil off a comet’s icy surface and the solar wind — the stream of particles flowing from the sun that spreads across the solar system — can spark X-rays.

 simulation of plasma interactions between Comet 67P/C-G and the solar wind around perihelion. Click for full animation and detailed caption. Credit: Modelling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin
This is a simulation of the interactions between Comet 67P/C-G (right) and the solar wind. Interactions between the wind and a comet’s temporary atmosphere can sometimes spark  X-rays. Credit: Modeling and simulation: Technische Universität Braunschweig and Deutsches Zentrum für Luft- und Raumfahrt; Visualisation: Zuse-Institut Berlin

The New Horizon spacecraft carried an instrument designed to measure that activity up-close – Solar Wind Around Pluto (SWAP). After examining the data, scientists have proposed that this little world contains a very mild, close-in bow shock, where the solar wind first “meets” Pluto, similar to the wave that forms at the bow of a ship when it moves through water. Could gases escaping from Pluto’s atmosphere and interacting with the solar wind be the source of X-rays? Maybe.

Strangely, Chandra found that the amount of X-rays coming from the dwarf planet is much higher than expected from the solar wind interacting with its atmosphere. And unlike a comet, Pluto’s atmosphere is much more stable — it sticks around. While Pluto is releasing enough gas from its atmosphere to create the observed X-rays, there isn’t enough solar wind flowing directly at Pluto at its great distance from the sun to make them, according to computer models devised to explain their origin.

To explain the the anomaly, scientists have proposed other possibilities. Pluto may have a much wider and longer tail of gases trailing it that SWAP couldn’t detect but that Chandra can see. Another is that magnetic field going along for the ride in the solar wind is focusing more particles than expected from the wind into the region around Pluto.

It will take deeper and higher resolution images of X-rays from Pluto’s environment before we arrive at a more satisfying answer. For now it’s something of a mystery.