How the bluest asteroid in the solar system got its color

An unusual blue-tinted asteroid likely got its signature color from intense sunlight, new research finds, solving a decades-old mystery.

The blue color of the asteroid Phaeton has puzzled scientists since the space rock was discovered in the early 1980s. Now, a new study suggests Phaethon’s unique orbit is responsible for the space rock’s odd hue.

Phaethon walks the Sun in an elliptical orbit that brings it closer to the star than any other asteroid. And although this close approach is only brief and Phaethon then withdraws beyond the orbit of Marchthe intensity of sunlight the asteroid is exposed to during these flybys is enough to gradually strip its surface of the red chemical compounds that astronomers see in asteroids that remain farther from the sun.

Related: Sodium ‘fizz’ gives big asteroid Phaethon its weird comet-like appearance

During its close passes, Phaethon approaches within 13 million miles (21 million kilometers) of the sun. That’s three times closer to the sun than the orbit of Mercurythe solar systemthe most secret planet. As Phaethon passes near the star, the asteroid’s surface heats up to 1,500 degrees Fahrenheit (800 degrees Celsius). The new study suggests that this heat triggers a chain of chemical reactions that vaporize iron-rich substances and red organic compounds.

“You’re basically not blushing the surface,” said Carey Lisse, senior planetary scientist at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Maryland, and lead author of the study. statement. “Although some of the red color re-accumulates as Phaethon orbits beyond Mars, it is lost again as Phaethon approaches the sun. After thousands of revolutions, only materials remain that reflect colors darker and colder.”

Scientists modeled the chemical composition of Phaethon’s surface in detail and calculated what happens to those chemicals as the asteroid’s temperature changes along its orbit.

“I was a bit surprised that the idea actually worked,” said Jordan Steckloff, planetary scientist at the Planetary Science Institute in Tucson, Arizona, and co-author of the paper, in the release. “It just seems crazy to think that maybe Phaethon looks so blue because it’s so hot that it preferentially produces iron gas over rock gas, but apparently that’s not so. crazy after all.”

There are only a few dozen blue-tinted asteroids in the solar system, the researchers said, but Phaethon is the bluest of them all. Interestingly, several comets that dip close to the sun also have a distinct blue tint.

“These kinds of orbits take a very long time to evolve, but that’s exactly what we need: a process that requires the object to be very old and thermally evolved,” Steckloff said. “The story seems to hold.”

Baking carbon into soot at high temperatures near the sun may also contribute to Phaethon’s characteristic blue, Lisse said.

Scientists may learn more about this mysterious space rock in a few years, as the Japan Aerospace Exploration Agency plans to send an orbiter called DESTINY+ to study Phaethon.

The study was published online in the journal Icarus in April.

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Arline J. Mercier