Phaethon: after 40 years, they decipher the enigma of the mysterious blue asteroid

(3200) Phaeton, or simply Phaeton, is an asteroid that is part of the so-called Apollo asteroids, discovered on October 11, 1983. It has an unusual orbit that brings it closer to the Sun, but its most striking feature is perhaps its intense blue color, unusual for this type of object heavenly.

Now, and using a model to simulate the physical and chemical processes that occur in the asteroid orbiting the sun, Researchers have shown how extreme heat and the preferential elimination of certain molecules from its surface could explain its particular blue color.

Scientists only know of a few dozen asteroids in the solar system that have a bluish tint, but Phaethon always stands out, even in this small crowd. Their findings were published online in April in the journal Icarus.

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

Scientists speculated that the extreme heat had something to do with Phaethon’s pitch. After all, its unusually comet-like orbit takes it past Mars before sinking it. just 20.9 million kilometers from the Sun, about three times closer than Mercury, raising temperatures to 800 degrees Celsius. In fact, its closest laboratory analogues are meteorites that have been exposed to extreme heat, most rich in clay minerals and inorganic carbonaceous species such as carbon black. Although it’s still unclear what he did the heat so that Phaethon turned blue.

Smooth, however, came up with an idea after reflecting on the work she did for the close encounter of the mission New Horizons from NASA with the Kuiper Belt object Arrokoth in 2019. Many airless rocky bodies, including Arrokoth, appear dull gray to rusty red thanks to ultraviolet and cosmic rays hitting the surface and “grilling” any organic material carbon-based, “just like the things you burn in your kitchen”, explained in a press release. The lightning also melted the rockcausing tiny dark red iron crystals a few billionths of a meter long to emerge,” he says.

Because molecular species freeze, melt and vaporize at different temperatures and pressures, Phaethon may have been red like other rocky bodies, but lost those materials when vaporized by the heat of the sun, says the ‘study.

Lisse and Jordan Steckloff, a planetary scientist at the Planetary Science Institute in Tucson, Arizona, created a model to estimate Phaethon’s surface temperature at each point in its orbit and calculate the amount of each material on Phaeton’s surface. Phaeton, rich in organic carbon, water, iron, rock minerals such as pyroxene and olivine, vaporized along the way.

They discovered that at the closest approach to the Sun, the red organic compounds and the small pieces of iron on the surface evaporate before the more resistant rocky materials. “You basically remove the redness from the surface”Lisa explained. Although some of the red color reaccumulates as Phaethon orbits Mars, it is lost again as it approaches the Sun. After thousands of revolutions, only materials remain that reflect darker, cooler colors.

Image of the movement of the asteroid. Credit: Marco Langbroek.

“I was a bit surprised that the idea actually worked,” Steckloff said. I was initially unsure if iron, in particular, would vaporize quickly enough to make a difference. “It seems crazy to think that maybe Phaethon looks so blue because it’s so hot that it preferentially produces iron gas instead of rock gas, but apparently that’s not so crazy. after all,” he said.

The scientist suspects there is even more to Phaethon’s blue hue. “It may even leave carbon residue that has turned into soot. But, can you really burn things clean and leave a soot residue, or will it just vaporize and disappear? » to add.

This process of removing blushing means that any tiny body could turn blue if it fell into an orbit like Phaethon’s. Comet 96P/Machholz, for example, is even closer to the Sun than Phaethon, just 18.6 million km away; it is also depleted in carbonaceous species and appears abnormally blue. Comet 322P, another solar-skimming comet, also has an unusually blue core.

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

He noted that there were certain limitations. The surface of the body, for example, should be mostly stable and relatively flat. “There couldn’t be a lot of topography,” he explained, because the stresses of the extreme heat would cause landslides, or the equivalent, which would bury any modified surface.

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