Comets are renowned for their vast, colorful and stunning tails of gas, ice, rock, and a variety of other materials. These tails occur when the comet's icy core heats up as it approaches the Sun, releasing icy gases during the heating process.
However, the release of gases is not limited to comets. Some moons and moons, such as Jupiter's Ganymede, and other icy bodies in our solar system can heat up enough to release gases.
So when scientists discovered an asteroid that was mostly made of rock and gave off gases, they were completely confused.
Meet Phaethon, a near-Earth asteroid recently discovered to exhibit comet-like activity.
Phaeton lacks a significant amount of ice on its surface; so why does it emit gases from its surface and glow like a comet?
Phaethon is a 5.8 km wide Apollo asteroid that passes closer to the Sun than any other named asteroid, although some smaller, unnamed asteroids have closer perihelions.
The name Phaethon may sound unfamiliar, but it is the parent body of the well-known Geminid meteor shower, which occurs annually in mid-December.
Phaeton's closest approach to the Sun occurs every 524 days, heating the asteroid's surface to about 750 ° C - hot enough to release any water, carbon dioxide or carbon monoxide from the ice on the asteroid's surface.
However, with such a short orbital period, these elements would have completely evaporated long ago. However, the asteroid is still giving off gas.
In a new study led by Joseph Masiero of California Institute of Technology's IPAC (Infrared Processing and Analysis Center) research organization, a team of scientists studied Phaethon's comet-like behavior as it approached the Sun, trying to figure out what the asteroid could be propelled into space.
And they think they have their own answer.
At 750 ° C, sodium can "escape" from the surface of an asteroid into space. In addition, sodium is found in abundance on asteroids and may explain the continuous gas evolution observed on Phaethon during its perihelial passage every 524 days.
That is … if there is enough sodium in the Phaeton.
To find a complex answer to this question, we will return to the Geminid meteor shower that Phaethon creates.
Meteor showers occur when small pieces of rocky material thrown from the surface of their parent bodies enter the Earth's atmosphere and burn, producing different colors and brightness, depending on their composition.
If the meteorite contains sodium, it will glow orange when it enters the atmosphere.
And therein lies the problem. Geminids have a low sodium content. So how can sodium explain Phaethon's comet-like activity?
Prior to the exploration of Masiero and others, it was believed that the rock material ejected from Phaethon lost its sodium shortly after it left the asteroid, which would explain the absence of orange meteorites during the Geminids.
However, Masiero's research suggests that sodium may be the main force pushing rock material out of Phaeton's surface.
The team speculates that as Phaeton heats up as it approaches the Sun, the sodium on the asteroid heats up and evaporates into space.
But, as in the case of ice, if sodium existed on the surface of Phaeton, it would have heated up and evaporated long ago. So, instead, sodium would have to come from the interior of the asteroid, transported to its surface for gas formation through tiny cracks.
As the vaporized sodium “hiss” through space through small cracks and fissures on the asteroid's surface, it will create jets with enough force to propel rocky material off the surface. Thus, creating the Geminids and the persistent comet-like behavior seen today.
"Asteroids like Phaethon have very weak gravity, so it doesn't take much force to throw debris off the surface or knock rock out of a crack," said Bjorn Davidsson, co-author of the study and scientist at NASA's Jet Propulsion Laboratory (JPL).
The ejection of this material would explain the comet-like glow of Phaeton, and the absence of sodium on the outer surface of Phaeton would explain why the Geminids lack sodium.
“Our models assume that this requires very little sodium - nothing explosive like erupting steam from the surface of an icy comet; it's more like a steady hiss."
So how did the team test their hypothesis?
Masiero and his colleagues tested samples of the Allende meteorite at the JPL laboratory, which fell in Mexico in 1969 and belongs to the same class of asteroids, carbonaceous chondrites, as Phaeton.
The team heated the meteorite fragments to the maximum temperature that Phaethon experiences during its approach to the Sun. In addition, the team simulated a day on Phaeton that lasts 3 hours.
“When comparing samples before and after our laboratory tests, sodium was lost while other elements remained. This suggests that the same could be happening on Phaeton and appears to be consistent with the results of our models,”said Yang Liu, JPL scientist and co-author of the study.
The findings may explain how other asteroids continue to be active, as they may undergo the same process as Phaeton.
The results of the study by Maziero and colleagues also support the hypothesis that classifying small objects in the solar system as comets or asteroids is an oversimplification.
Some researchers believe that factors such as the amount of ice and which elements evaporate at certain temperatures should play an important role in classifying small bodies.
A study by Maziero and colleagues, titled Sodium Volatility in Carbonaceous Chondrites at Temperatures Corresponding to Low Perihelion Asteroids, can be found in the August 2021 issue of the Planetary Science Journal.