Why Phaethon Turned Blue — and How Other Small Sun-Diving Bodies Might, Too

The bizarre blue color of asteroid (3200) Phaethon has been a puzzle for researchers since its discovery in 1983, but a recent study may have finally found the reason why Phaethon got the blues.

Using a model to simulate the physical and chemical processes happening on Phaethon as it orbits the Sun, a pair of researchers showed how extreme heat and the preferential removal of certain molecules from Phaethon’s surface could leave it blue — and could probably do it to any other asteroid, too.

Their findings were published online in April in the journal Icarus.

Scientists know of only a few dozen asteroids in the solar system that have a bluish hue, but Phaethon still stands out, even from that small crowd.

“It is incredibly blue,” said Carey Lisse, a senior planetary scientist at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and study author. “In fact, Phaethon’s close to being the bluest object there is in terms of asteroids.”

Scientists have assumed extreme heat had something to do with Phaethon’s hue. Its unusually comet-like orbit, after all, takes it beyond Mars before plunging it just 13 million miles (20.9 million kilometers) from the Sun — about three times closer than Mercury — spiking temperatures to a roasting 1,500 degrees Fahrenheit (800 degrees Celsius). In fact, its closest laboratory analogs are meteorites that have been exposed to extreme heat, most of them rich in clay minerals and inorganic carbon species such as carbon black.

But what heat did to make Phaethon turn blue hasn’t been clear.

Lisse, however, had an idea after reflecting on work he did for NASA’s New Horizons mission’s close encounter with Kuiper Belt object Arrokoth in 2019. Many airless rocky bodies, including Arrokoth, appear dull gray to rusty red thanks to cosmic and ultraviolet rays that hit the surface and “toast” any organic, carbon-based materials, “much like the stuff you burn in your kitchen,” he explained. The rays also smelt the rock, forcing up tiny dark-red iron crystals just a few billionths of a meter long.

But what happens if those materials are exposed to the punishing temperatures near the Sun? Because molecular species freeze, melt and vaporize at different temperatures and pressures, it’s possible Phaethon was once red like other rocky bodies but lost those materials as they were vaporized by the Sun’s heat.

Lisse and his colleague Jordan Steckloff, a planetary scientist at the Planetary Science Institute in Tucson, Arizona, created a model to estimate the temperature of Phaethon’s surface at every point along its orbit and calculate how much of each material on Phaethon’s surface —carbon-rich organics, water, iron, rocky minerals such as pyroxene and olivine — vaporized along the way.

They found that at closest approach to the Sun, red organics and tiny bits of iron on the surface boil off before the hardier, rocky materials. “You’re essentially un-reddening the surface,” Lisse said. Although some red color re-accumulates as Phaethon orbits out beyond Mars, it’s lost again as Phaethon approaches the Sun. After thousands of revolutions, all that remains are materials that reflect darker, cooler colors.

“I was a little surprised the idea actually worked,” Steckloff said. He initially wasn’t sure the iron, specifically, would vaporize fast enough to make a difference. “It just seems wild to think that maybe Phaethon looks so blue because it gets so hot that it preferentially produces iron gas versus rock gas, but apparently that isn’t so crazy after all.”

Lisse suspects there’s even more to Phaethon’s blue hue. “It’s possible that you’re maybe even leaving behind carbon residue that has been baked to soot,” which has a blue hue to it, he said. “But can you actually burn the stuff cleanly and leave behind a sooty residue, or will it all just vaporize and disappear?”

Regardless, he and Steckloff argue that this de-reddening process means potentially any small body could turn blue if it fell into an orbit like Phaethon’s. Comet 96P/Machholz, for example, grazes even closer to the Sun than Phaethon — just 11.5 million miles (18.6 million kilometers); it’s also depleted of carbon species and appears abnormally blue. Comet 322P, another Sun-grazing comet, also has an unusually blue nucleus.

“These types 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 together.”

He notes there are some limitations. The body’s surface, for instance, has to be mostly stable and relatively flat. “It couldn’t have a lot of topography,” he explained, because stresses from extreme heat would cause landslides, or the equivalent, that would rebury whatever surface had been changed.

There also remain many lingering questions. Clay, for example, requires water, but all of that should have boiled off long ago. “Is it somehow related to the thermal shock Phaethon experienced when it first was kicked into its now extreme orbit? We don’t know,” Lisse said. “We just need to go to Phaethon to find out.”

That wish will soon be granted. The Japan Aerospace Exploration Agency’s DESTINY+ mission (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon flyby and dust Science), scheduled to launch in 2024, will take direct compositional measurements of Phaethon’s surface and the coma around it when it’s closest to the Sun, providing a real litmus test of the pair’s idea.

Closer to home, the pair suggested researchers could sample pieces of the annual Geminid meteor shower, which Phaethon sources and Earth passes through each December. In fact, NASA did this in 2020 for the first time using its stratospheric particle sample collector, collecting six particles that it soon expects to release publicly.

“This may be the first paper we’ve written that will be quickly disproven by observations,” Steckloff said, laughing. “But it could be that we see some of these interesting effects.” Either way, he said, he’s excited to see what they find.