A sparkling approach
Aptly named after the son of the Sun god in Greek mythology, Phaethon has an orbit of 524 days which brings him down to just 0.14 astronomical units – where 1 AU is the average distance between Earth and the Sun – from our star. , well in the orbit of Mercury. At this distance, the Sun heats the asteroid’s surface to about 1390 degrees Fahrenheit (750 degrees Celsius). While any water ice, carbon dioxide, or carbon monoxide just below the surface would have evaporated long ago, sodium – an element abundant in asteroids – could sparkle just below its surface.
A constant boiling of sodium would explain why Phaethon lights up as he approaches the Sun, as the resulting gas and dust would scatter the sunlight further. It could also explain how the fuel of the Geminids is detached from Phaethon.
“Asteroids like Phaethon have very low gravity, so it doesn’t take a lot of force to knock debris off the surface or dislodge rock from a fracture,” study co-author Björn Davidsson said. JPL in a press release. “Our models suggest that very small amounts of sodium is all that is needed to do this.”
To determine if sodium could indeed be the cause, the team heated chipped samples of the Allende meteorite – an object that fell to Earth in 1969 and may have come from an asteroid like Phaethon – to temperatures hotter than Phaethon experiments with the approach of the Sun. After heating it for three hours, the equivalent of a day on a fast-spinning Phaethon, the researchers found that although other elements remained, the sodium in the chips had evaporated.
More data is needed to confirm this as the reason for Phaethon’s comet-like behavior, including repeating the vacuum test to better simulate Phaethon’s environment. And although the researchers point out that this scenario depends a lot on the minerals present in a given object, they suspect that it could be applied to other active asteroids that have approaches close to the Sun. This study supports a growing body of evidence that the classification between comet and asteroid may be too straightforward. As the lead author of the study, Joseph Masiero of Caltech, put it: “The spectrum between asteroids and comets [is] even more complex than we previously thought.