Mercury is not alone in its orbit, and scientists still don’t know why.
A cloud of cosmic dust traces the innermost planet’s path around the Sun. And although other planets, like Earth and Venus, also have dusty companions on their circumsolar journeys, Mercury really should not.
In a new study published in the Planetary Science Journal, scientists tried to trace the genesis of Mercury’s dust. And although they still don’t know how this improbable cloud formed, they do know that it probably has a different origin story than the one escorting our own planet.
Dusty Orbital Traffic
Earth’s dust ring formed from the immense cloud of dust, called a zodiacal or interplanetary dust cloud, that pervades the space between planets.
Interplanetary dust comes from different sources including asteroid collisions, cometary activity, and cometary breakups. Dispersed throughout the cloud are more tightly packed regions that host comet trails, dust bands in the asteroid belt, and dust rings near planets. Unlike Saturn’s famous rings, however, even the densest regions in a dust cloud are detectable using only very sensitive equipment.
The dust particles traverse the solar system, pulled toward the Sun. But, like travelers on a road trip, they make pit stops along the way.
For example, as grains created in the asteroid belt migrate toward the Sun, they cross paths with the inner planets. Planets like Mars have low mass and less gravitational pull, so the dust sails by with no problem, but more massive planets like Earth can ensnare the debris in their orbit.
Once trapped, the planets and the dust move in lockstep in a phenomenon called orbital resonance. Like musical rhythms, the interlocked planets and dust move together in whole-number ratios: in perfect synchrony, two dust revolutions per Earth revolution, three dust revolutions per two Earth revolutions, and so on.
Eventually, the dust particles escape their cosmic dance and continue with their migration—until they encounter the next planet. “It’s like [they are] in a traffic jam on a highway,” said David Nesvorný, a researcher at the Southwest Research Institute in Boulder, Colo., who was not involved in the recent study. “You need to stop, and then you start moving again until the next jam.”
Impacts to the Rescue
Earth and Venus are massive enough to stall migrating dust, but Mercury is not. That close to the Sun, phenomena including solar winds, solar light, and strong magnetic fields should, quite literally, kick the dust up, and Mercury isn’t large enough to trap the celestial debris in its gravitational pull.
Those conditions, the scientists argued in the new study, mean that Mercury’s ring couldn’t have formed like Earth’s did. The ring probably wasn’t fed by an active comet or asteroid that breezed by the planet, either.
Having exhausted the obvious sources of dust, the team turned its focus to Mercury itself. What if an asteroid or a comet crashed into the planet? The impact of a body large enough would propel material out of the planet’s gravitational well and into orbit to form the dust ring we see today.
Plausible, but Not Proven
The ring near Mercury is young, so to test the theory, the team needed to find evidence of collisions that are relatively recent. “We concluded that nothing could survive there for longer than 20 million years,” said Petr Pokorný of Catholic University of America and NASA Goddard Space Flight Center, who led the study.
Using data from NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft, the team identified two craters larger than 40 kilometers in diameter on Mercury’s surface that might be younger than 50 million years. Perhaps collateral debris from these strikes billowed out into space.
To test whether that was plausible, the authors constructed a simplified model of an impact and showed that in principle, this scenario could work. But, still, a closer inspection revealed that the details don’t add up. “At the moment, the story doesn’t hold that well,” Pokorný said.
For example, scientists estimated that the ring’s mass is equivalent to an asteroid that is 1–1.5 kilometers wide. These two impacts alone could not account for all of the dust in orbit.
Perhaps, the authors wrote, several smaller asteroids could have struck Mercury and added to the cosmic debris. This theory is plausible, but it’s hard to test because small, young craters are challenging to date. Older craters get dinged as they’re struck by more recent asteroids, so scientists can date a crater based on how pockmarked it is. “As you reduce the counting area, the number of superposed craters is also reduced,” said Mallory Kinczyk, a postdoctoral researcher at the Johns Hopkins University Applied Physics Laboratory, who wasn’t part of the new study. Inferring age based on just a few superposed craters would yield uncertain estimates, Kinczyk said.
Nesvorný doesn’t believe that smaller asteroid impacts would solve the problem, anyway. The model presented in the recent study predicted that only a sliver of ejecta from the planet’s surface would transfer into orbit. A more sophisticated model might favor a more efficient transfer, he said. If so, then perhaps the two massive impacts identified by the team are sufficient to explain the ring. For now, though, the case remains open.
—Jure Japelj, Science Writer