And yet, when Phaethon approaches the sun, it sprouts a tail and wraps itself in a comet's disguise. Scientists have long suspected that Phaethon's tail came from the asteroid shedding dust. Now, researchers are pointing to a different culprit: sodium. As Phaethon approaches the sun, the theory goes, the sodium deep inside the asteroid heats up, turns into vapor and seeps through cracks in the asteroid's surface. In the process, the sodium kicks up a storm of dust and tiny rocks that sweeps out behind it, brightening Phaethon like a comet and forming its characteristic tail.
"Asteroids like Phaethon have very weak gravity, so it doesn't take a lot of force to kick debris from the surface or dislodge rock from a fracture," Björn Davidsson, a scientist at NASA's Jet Propulsion Laboratory in Southern California and a co-author of the study, said in a statement. "Our models suggest that very small quantities of sodium are all that's needed to do this — nothing explosive, like the erupting vapor from an icy comet's surface; it’s more of a steady fizz."
Phaethon may not be what astronomers consider a comet, but it gives a pretty good go at pretending to be one. It whirls around the sun on an elongated orbit like that of a comet: Over 524 days, it swings from beyond the orbit of Mars, plunges into the searing space nearer the sun than Mercury, then shoots back out again.
The heat Phaethon experiences along the way means that any ice — water, carbon dioxide or carbon monoxide, the chemicals that typically boil away into a comet's tail — once on Phaethon's surface would have long ago boiled off Phaethon's surface.
But scientists know that asteroids often contain sodium, and if Phaethon does, its sodium would be more resistant to heat than ices. But researchers saw only faint traces of sodium in December's annual Geminid meteor shower, which is caused by debris from Phaethon burning up in Earth's atmosphere. Researchers wondered where that sodium went.
So the scientists behind the new research tested samples from a meteorite that landed in Mexico in 1969 — a meteorite that experts think came from an asteroid with a similar composition to that of Phaethon. When the researchers heated those samples to temperatures comparable to what Phaethon experiences during its solar system journey, they found that the sodium did indeed "fizz" out.
"Our latest finding is that if the conditions are right, sodium may explain the nature of some active asteroids, making the spectrum between asteroids and comets even more complex than we previously realized," Joe Masiero, a solar system scientist at Caltech, said in the statement.
The research is described in a paper published Monday (Aug. 16) in The Planetary Science Journal.
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Rahul Rao is a graduate of New York University's SHERP and a freelance science writer, regularly covering physics, space, and infrastructure. His work has appeared in Gizmodo, Popular Science, Inverse, IEEE Spectrum, and Continuum. He enjoys riding trains for fun, and he has seen every surviving episode of Doctor Who. He holds a masters degree in science writing from New York University's Science, Health and Environmental Reporting Program (SHERP) and earned a bachelors degree from Vanderbilt University, where he studied English and physics.