The 5.8-km-wide (3.6-mile) near-Earth asteroid Phaethon, which is the source of the annual Geminid meteor shower, brightens as it gets close to the Sun. The asteroid’s elongated, 524-day orbit takes it well within the orbit of Mercury, during which time the Sun heats the asteroid’s surface up to 750 degrees Celsius (1,390 degrees Fahrenheit).
This illustration depicts the asteroid Phaethon being heated by the Sun. Image credit: NASA / JPL-Caltech / IPAC.
“Phaethon is a curious object that gets active as it approaches the Sun,” said Dr. Joseph Masiero, a researcher at IPAC/Caltech.
“We know it’s an asteroid and the source of the Geminids. But it contains little to no ice, so we were intrigued by the possibility that sodium, which is relatively plentiful in asteroids, could be the element driving this activity.”
Dr. Masiero and his colleagues were inspired by observations of the Geminids.
When meteoroids streak through Earth’s atmosphere as meteors, they disintegrate.
But before they do, friction with the atmosphere causes the air surrounding the meteoroids to reach thousands of degrees, generating light.
The color of this light represents the elements they contain. Sodium, for example, creates an orange tinge. The Geminids are known to be low in sodium.
Until now, it was assumed that these small pieces of rock somehow lost their sodium after leaving the asteroid Phaethon.
The new study suggests that the sodium may actually play a key role in ejecting the Geminid meteoroids from the asteroid’s surface.
The researchers think that as Phaethon approaches the Sun, its sodium heats up and vaporizes.
This process would have depleted the surface of sodium long ago, but sodium within the asteroid still heats up, vaporizes, and fizzes into space through cracks and fissures in Phaethon’s outermost crust.
These jets would provide enough oomph to eject the rocky debris off its surface.
So the fizzing sodium could explain not only the asteroid’s cometlike brightening, but also how the Geminid meteoroids would be ejected from the asteroid and why they contain little sodium.
“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,” said Dr. Björn Davidsson, a researcher at NASA’s Jet Propulsion Laboratory.
“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.”
To find out if sodium turns to vapor and vents from an asteroid’s rock, the researchers tested samples of the Allende meteorite.
The meteorite may have come from an asteroid comparable to Phaethon and belongs to a class of meteorites, called carbonaceous chondrites, that formed during the earliest days of the Solar System.
The scientists then heated chips of the meteorite to the highest temperature Phaethon would experience as it approaches the Sun.
“This temperature happens to be around the point that sodium escapes from its rocky components,” said Dr. Yang Liu, also from NASA’s Jet Propulsion Laboratory.
“So we simulated this heating effect over the course of a ‘day’ on Phaethon — its three-hour rotation period — and, on comparing the samples’ minerals before and after our lab tests, the sodium was lost, while the other elements were left behind.”
“This suggests that the same may be happening on Phaethon and seems to agree with the results of our models.”
The study was published in the Planetary Science Journal.
Joseph R. Masiero et al. 2021. Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent with Low-perihelion Asteroids. Planet. Sci. J 2, 165; doi: 10.3847/PSJ/ac0d02