Asteroid 3200 Phaethon is the archetypal sungrazer, and yet shows some very odd behavior on the hottest part of its orbit. Comets famously vaporize ice the closer they get to the Sun, leaving a bright trail of dust and rock and gas behind them. Asteroids, being mainly rock not ice, famously do not, and yet Phaethon gets more active the closer it gets to the Sun. A new theory suggests this is a result of sodium fizzing from inside like a sherbet bomb in your mouth.
Since its discovery, the oddly blue Phaethon has been found to undergo strange comet-like behavior as it approaches the Sun, brightening as material streams off it. However, the volatile substances in a comet's tail would have been lost from Phaethon's surface many millions of years ago, if it ever had them. A paper in the Planetary Science Journal has given astronomers an explanation.
In ancient Greek mythology, Phaethon was the son of Helios who tried to drive his father's Sun-bearing chariot across the sky with disastrous consequences. The name was thought appropriate for the 6-kilometer (3.6-mile) wide rock that comes closest to the Sun of any named asteroid (a few smaller unnamed bodies have been detected getting closer still).
When Phaethon gets closes to the Sun – a distance half that of Mercury – its daytime temperature reaches 750ºC (1,400ºF). Exposure to that kind of heat has serious consequences. Most meteor showers are the result of dust left behind by comets hitting the Earth's atmosphere after the ice binding it together melted. The Geminids, one of the year's best meteor showers, is produced by material lost from Phaethon, raising the question of how the dust escaped.
"Phaethon is a curious object that gets active as it approaches the Sun," lead author Dr Joseph Masiero of Caltech said in a statement. "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."
The Geminids gave the hint; they lack the distinctive orange tint of burning sodium some other meteor showers display. Since Phaethon should once have been rich in sodium, its separation from the dust that causes the annual pre-Christmas fireworks display requires explaining.
In a vacuum, sodium turns to gas at temperatures low enough any on Phaethon's surface would have boiled off long ago. However, Masiero and co-authors wondered what would happen to sodium locked in materials like feldspar further in.
They propose it vaporizes and then finds its way through cracks in the rock – an inevitable product of the extreme heating and cooling the asteroid undergoes in an orbit that takes it out past Mars.
“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, co-author Dr Björn Davidsson from NASA's Jet Propulsion Laboratory said. “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.”
If the authors are right, the sodium-depleted dust on Phaethon's surface, and the sodium from within that gives it a push, part company long before they reach Earth.
To test their theory, Masiero and co-authors heated samples from the Allende meteorite, considered closer to Phaethon's composition than most and containing 0.5 percent sodium, to temperatures up to 800ºC. In the three hours that match a Phaethon day the sodium poured out, but Allende's other elements stayed put, supporting their case.
Astronomers have started to question the simple binary of comets and asteroids. As Masiero said: “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.”