There's something strange about the near-Earth asteroid 3200 Phaethon: It brightens up as it approaches the Sun, despite not having any reserves of ice that would normally cause this effect as they evaporate and scatter sunlight.

It is ice-laden comets that get brighter as they're heated, not rocky asteroids, which is why Phaethon has long puzzled astronomers. Now a new study suggests that one chemical element in particular might be behind this bizarre behavior.

"Phaethon is a curious object that gets active as it approaches the Sun," says astronomer Joseph Masiero from the California Institute of Technology.

"We know it's an asteroid and the source of the Geminids [ meteor shower]. 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."

Phaethon takes 524 days to complete a full orbit, during which time the Sun heats it up to a maximum of 1,050 Kelvin (777 degrees Celsius or 1,430 degrees Fahrenheit). Any ice on the asteroid would have burned away long ago, but the researchers used computer models to show that sodium could still be present, fizzing away under the surface.

This heating and fizzing might not only explain the brightening of the asteroid, as the sodium escapes through cracks and fissures in the crust, but also the ejection of the rocks that can be seen from Earth as the Geminids meteor shower every December. Phaethon's weak gravitational pull would make it even easier for debris to be cast off.

We know that the Geminid meteors are relatively low in sodium because of the light they give off as they burn up in Earth's atmosphere, and again this can be explained by the modeling done by the research team.

Experiments were then run on fragments of the Allende meteorite, which landed in Mexico in 1969 and likely came from an asteroid like Phaethon. When heated up, the behavior of the fragments confirmed that sodium could indeed turn into vapor and be released from an asteroid, at the sort of temperatures Phaethon is likely to experience.

"This temperature happens to be around the point that sodium escapes from its rocky components," says planetary scientist Yang Liu, from NASA's Jet Propulsion Laboratory (JPL). "So we simulated this heating effect over the course of a 'day' on Phaethon – its three-hour rotation period."

"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."

As well as offering some fascinating insights into what's happening on Phaethon, the research also suggests that the distinction between rocky asteroids and icy comets perhaps isn't as clear as has been previously thought.

The results of the modeling and the experiments here could well give astronomers some useful data that applies to other low-perihelion asteroids – ones that fly close to the Sun.

"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," says Masiero.

The research has been published in the Planetary Science Journal.