Often in science, things get presented as facts, rules, and definitive statements. But part of the joy of science is that “facts” can be in flux, and rules are made to be broken.
The Geminid meteor shower, which peaks each year in December, is one such rule-breaker. Unlike every other known meteor shower, the Geminids are *not* caused by debris left behind from an active comet.
A Geminid meteor cuts across star trails over Georgia in 1985.
—Image courtesy Jimmy Westlake via NASA
Instead, scientists have traced the Geminids to an object called 3200 Phaethon. This bizarre body has all the mineral makings of an asteroid, but, like a comet, it left behind a stream of dust that follows along in its orbit.
Thanks to gravitational interactions, this dusty trail regularly crosses Earth’s orbit. The particles then burn up in our atmosphere, creating the Geminids meteor shower.
Until recently, the favored view of Phaethon was that it’s a dead comet—the rocky core of a “dirty snowball” that lost its ices after too many close encounters with the sun.
What we see as the Geminids is debris left behind many many many years ago, since the body has shown no recent signs of losing material.
Seems plausible. Phaethon gets unusually close to the sun, making its closest approach at a mere 13 million miles (almost 21 million kilometers). That’s even closer in toward the star than Mercury gets!
At such temperatures, you’d expect a comet to lose plenty of surface ice.
But then Phaethon got even weirder. In June 2009 astronomers using the STEREO sun-watching probe suddenly saw the rocky body flare to life as it neared the sun, brightening by a factor of two.
According to a paper published online September 14, 2010, the scientists interpreted this brightening “as an impulsive release of dust particles from Phaethon.”
So, not so dead after all. But that brings us back to figuring out what exactly Phaethon is, and why it’s spewing material like a comet.
Luckily, not all scientists were always convinced that Phaethon is a dead comet, and some were already hard at work on alternate explanations.
Last week, during the 42nd AAS/DPS meeting in Pasadena, researcher Humberto Campins of the University of Central Florida presented his latest work linking Phaethon to the so-called Pallas family of asteroids.
One of the reasons Campins had pegged Phaethon as an asteroid is that he’d done a study in 2007 showing that the orbiting body has hydrated minerals, aka clays, on its surface. So far, no known comets contain clays.
Based on its spectrum, “to me Phaethon looks like Pallas,” Campins said.
A more detailed analysis showed that the body does in fact share certain chemical signatures with Pallas, a roughly 326-mile-wide (525-kilometer-wide) primitive asteroid in the main belt between the orbits of Mars and Jupiter. (See an interactive solar system.)
“We found a very nice spectral match with Phaethon and the Pallas family, the small objects created by a cratering event on Pallas,” Campins said. At about three miles (five kilometers) wide, Phaethon is about the right size, too, to be a Pallas-family asteroid.
Artist’s concept of an impact event on Pallas.
—Artwork created using the 3D-shape model published by Schmidt, et al., 2009 in the journal Science. (B. E. Schmidt and S. C. Radcliffe)
The spectral data was complemented by models showing the debris pattern from an impact on Pallas could in fact put a body into an orbit like Phaethon’s.
The scientists working this part of the study “found a dynamical path, even though their first reaction was that this is impossible,” Campins said.
The match is exciting, if correct, because one of the other distinguishing features of the Geminids is that it’s a low-velocity shower.
The pieces move relatively slowly through Earth’s atmosphere, which means there’s a higher chance bits of Geminid debris survive the fall and end up as meteorites on Earth.
If the Geminids are the spawn of Pallas, their meteorites could teach us a lot about that huge and hard-to-reach parent body.
“Pallas is second largest asteroid [by volume], and we don’t know much about it,” Campins said.
“So if we actually have pieces of it that come to the Earth and could be recovered, we would have a free sample-return mission to the asteroid.”
Now, even if we accept Phaethon as a Pallas family member, there’s still the question of why it’s an oddly active asteroid.
The most favored theory, according to Campins, is that the rocky body is occasionally hit by other rocky bodies, so that the impact sends dust flying, creating the debris trail.
“There is also the possibility that some objects could retain water caverns inside that are more or less shielded by insulation until the object gets really close to sun,” as Phaethon does, he said.
Yet another idea is that the activity is driven by carbonate clays, which are formed by carbon-rich rocks interacting with water.
(See related: “Life on Mars? ‘Missing Mineral’ Find Boosts Chances.”)
Heating carbonates to very high temperatures, like those reached on the surface of Phaethon as it nears the sun, can make such minerals explode.
“So it could be an activity similar to a comet, but instead of water ice, it’s minerals” sending out bursts of debris, Campins said.
This theory would fit nicely with the notion of Phaethon as a Pallas-family object, Campins added, since “the spectrum of Pallas has very clear hydration bands. … There’s no question there are clays in there.”