Each December, meteors streak across the sky as part of the massive Geminid meteor shower, with up to about 120 meteors visible per hour at the shower’s peak.
This reliable yearly show is a popular sight for stargazers, but the origin of the Geminids is shrouded in a bit of mystery.
Meteor showers like the Perseids, which show up each August, happen when Earth passes through a cloud of dust in its orbit that was left by a passing comet. The dust grains caught in the pull of Earth’s atmosphere burn up, causing the bright streaks we call meteors, or “shooting stars.”
Astronomers know that the Geminids probably came from an asteroid called Phaethon, or from a rocky object that broke up into the present-day Phaethon and a couple of other nearby asteroids. The mystery is how, exactly, the asteroid would have ejected those dust grains.
“There are many possible mechanisms for ejecting material from Phaethon,” David Jewitt, a University of California, Los Angeles planetary scientist who has studied Phaethon, tells Inverse. For instance, Jewitt says temperature changes on the asteroid’s surface could lead to the rock cracking and Phaethon’s fast rotation might help fling material off its surface.
In a study published Monday in The Planetary Science Journal, a team of researchers proposes another mechanism that might help kick dust grains off of Phaethon’s surface.
Here’s the background — Phaeton is what’s called an “active asteroid,” which expels material like a comet. While some active asteroids can be explained by ices near the surfaces vaporizing, Phaeton moves too close to the Sun to have any ices available.
“There's a lot of really mysterious objects in the solar system.”
But Joe Masiero, a solar system scientist at Caltech, and colleagues looked to a different culprit: sodium, one of the most volatile elements in iceless rocks. Masiero is the first author of the new study.
WHAT’S NEW — The team investigated what might happen to the sodium in asteroids that get pretty close to the Sun over the course of their orbits, like Phaethon. They found that sodium can vaporize in the conditions found on these asteroids, and they suggest that bursts of vaporizing sodium might be able to lift dust off an asteroid’s surface.
The motivation for looking into sodium was understanding the “problem object of Phaethon,” Masiero explains. “We knew that it was weird.”
HOW THEY DID IT — One of the team members had previously developed a theoretical model to study how water ice behaved on comets. The researchers adapted this model to probe other volatiles, such as sodium.
The models showed that sodium could indeed sublimate under the conditions that Phaethon experiences, depending on what mineral form the sodium is in.
The researchers also ran experiments on physical meteorite samples. It’s not known which meteorites on Earth, if any, came from the Geminids or from Phaethon, so the researchers used a piece of the Allende meteorite, which they think might be similar enough, for their tests.
The researchers exposed the pieces of the meteorite to various “heating events” similar to what rocks on asteroids that get pretty close to the Sun, like Phaethon, would experience over the course of a “day” (only a couple of hours on fast-rotating Phaethon).
The heating events mimicking conditions on Phaethon made significant amounts of sodium in the meteorite samples disappear — most likely from vaporization, the researchers say.
WHY IT MATTERS — Because sodium is so volatile, most sodium at or a little below the asteroid’s surface would have vaporized before Phaethon reached the hotter parts of its orbit, near the Sun, where astronomers have observed Phaethon’s bursts of activity.
But sodium below that topmost layer could stay intact until this very hot part of the orbit, Masiero explains:
- At that point, this more deeply buried sodium gets hot enough to sublimate, and bursts of that vaporizing sodium could push dust grains off the asteroid’s surface and into surrounding space
- Over time, these dust grains could drift into the “meteor stream” of dust grains that cause the Geminids to streak through our skies each December
Strangely, the amount of activity astronomers see on Phaethon isn’t enough to explain the total mass of dust grains in the Geminid meteor stream, Masiero says.
Recent work suggests that Phaethon used to get even closer to the Sun and reach even higher temperatures in the past, so it’s possible that Phaethon was ejecting much more dust from its surface in the past.
“While we can't make the Geminids with today's Phaethon, it could have been yesterday's Phaethon that was making it,” Masiero says.
WHAT’S NEXT — Now, the study team wants to repeat their meteorite-heating experiments with more realistic conditions — particularly, in vacuum conditions. Heating objects in a vacuum is tricky, so these initial experiments were done at sea-level air pressure.
Masiero says that they are also looking forward to data from the planned DESTINY+ mission, a spacecraft that will make a flyby of Phaethon, to learn more about the asteroid’s composition and perhaps better study what role sodium vaporization might play in the asteroid’s activity.
“There's a lot of really mysterious objects in the solar system,” Masiero says. “Trying to find out what's going on with all of these, I think, will open up some pretty interesting new avenues of research and new understandings about how our solar system works.”
Abstract: Solar system bodies with surface and subsurface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. This is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. However, some bodies (notably the asteroid (3200) Phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. We investigate whether the sodium present in the mineral matrix could act as the volatile element responsible for this activity. We conduct theoretical modeling which indicates that sodium has the potential to sublimate in the conditions that Phaethon experiences, depending on the mineral phase it is held in. To test this, we then exposed samples of the carbonaceous chondrite Allende to varying heating events, similar to what would be experienced by low-perihelion asteroids. We measured the change in sodium present in each sample and find that the highest temperature samples show a significant loss of sodium from specific mineral phases over a single heating event, comparable to a day on the surface of Phaethon. Under specific thermal histories possible for Phaethon, this outgassing could be sufficient to explain this object's observed activity. This effect would also be expected to be observed for other low-perihelion asteroids as well and may act as a critical step in the process of disrupting small low-albedo asteroids.