Sparkling sodium could explain the cometary activity of the asteroid Phaethon

Models and laboratory tests suggest the asteroid could release sodium vapor as it orbits near the Sun, explaining its increased luminosity.

When a comet passes through the inner solar system, the Sun heats it, causing ice to vaporize below the surface into space. The exhaust vapor dislodges dust and rock, and the gas creates a glowing tail that can stretch millions of miles from the core like an ethereal veil.

While comets contain many different ices, asteroids are primarily rocks and are not known to produce such majestic displays. But a new study examines how the near-Earth asteroid Phaethon may actually exhibit comet-like activity, despite the absence of significant amounts of ice.

Known to be the source of the annual Geminid meteor shower, the 3.6-mile-wide (5.8-kilometer-wide) asteroid lights up as it approaches the Sun. Comets generally behave as follows: As they heat up, their icy surfaces vaporize, making them more active and brighter as vent gases and dust further scatter sunlight. But what makes Phaethon light up if not vaporizing ice cream?

The culprit could be sodium. As the authors of the new study explain, Phaethon’s 524-day elongated orbit places the object well in Mercury’s orbit, during which the Sun heats the asteroid’s surface to about 1390 degrees. Fahrenheit (750 degrees Celsius). With such a hot orbit, any water ice, carbon dioxide, or carbon monoxide near the asteroid’s surface would have been cooked a long time ago. But at this temperature, sodium can fizzle out of the asteroid’s rock and into space.

“Phaethon is a curious object that becomes active as it approaches the Sun,” said Joseph Masiero, study leader, scientist at IPAC, a Caltech research organization. “We know it’s an asteroid and the source of Geminids. But it contains little or no ice, so we were intrigued by the possibility that sodium, which is relatively abundant in asteroids, could be the driving force. of this activity. “

Asteroid-meteor connection

Masiero and his team were inspired by the observations of the Geminids. When meteorites, small pieces of rocky debris from outer space, pass through the Earth’s atmosphere as meteors, they disintegrate. But before that, friction with the atmosphere causes the air surrounding the meteorites to reach thousands of degrees, generating light. The color of this light represents the elements they contain. Sodium, for example, creates an orange tint. Geminides are known to be low in sodium.

Until now, it was assumed that these little pieces of rock somehow lose their sodium after leaving the asteroid. This new study suggests that sodium may actually play a key role in ejecting Geminid meteorites from Phaethon’s surface.

Researchers believe that as the asteroid approaches the Sun, its sodium heats up and vaporizes. This process would have depleted the surface of sodium a long time ago, but the sodium inside the asteroid still heats up, vaporizes and fizzes in space through cracks and crevices in the outermost crust of Phaethon. These jets would provide enough punch to eject rocky debris from its surface. So, the sparkling sodium could explain not only the comet-like lightening of the asteroid, but also how the Geminid meteorites would be ejected from the asteroid and why they contain little sodium.

“Asteroids like Phaethon have very low gravity, so it doesn’t take a lot of force to push debris off the surface or dislodge rock from a fracture,” said Björn Davidsson, a scientist at NASA’s Jet Propulsion Laboratory in Southern California and a study co-author. “Our models suggest that very small amounts of sodium is all that’s needed to do this – nothing explosive, like steam erupting from the surface of an icy comet; it’s more of a constant fizz.”

Laboratory tests required

To find out if the sodium turns into vapor and escapes from the rock of an asteroid, the researchers tested samples from the Allende meteorite, which fell on Mexico in 1969, in a JPL laboratory. The meteorite may have originated from an asteroid comparable to Phaethon and belongs to a class of meteorites, called carbonaceous chondrites, which formed during the early days of the solar system. The researchers then heated the meteorite shavings to the highest temperature Phaethon would experience as it approached the Sun.

“This temperature is around the point where sodium escapes from its rock components,” said Yang Liu, a JPL scientist and co-author of the study. “So we simulated this heating effect over the course of a ‘day’ on Phaethon – his three hour rotation period – and, comparing the minerals in the samples before and after our lab tests, sodium was lost, while the other elements were This suggests that the same can happen on Phaethon and seems to be in agreement with the results of our models. “

The new study supports a growing body of evidence that categorizing small objects in our solar system as “asteroids” and “comets” is oversimplified, based not only on how much ice they contain, but also how much ice they contain. elements that vaporize at higher temperatures.

“Our latest finding is that if the conditions are right, sodium can explain the nature of some active asteroids, making the spectrum between asteroids and comets even more complex than we previously thought,” Masiero said.

The study, titled “Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent with Low-Perihelia Asteroids”, was published in The Journal of Planetary Sciences August 16, 2021.