Meteors shower Mercury

OXON HILL, Md. — A meteor shower on Mercury appears to be causing a ruckus on the innermost planet.

A seasonal bloom of calcium atoms that envelops Mercury is caused by the same rain of comet debris that drives the Taurid meteor shower on Earth, planetary scientist Apostolos Christou reported November 9 at a meeting of the American Astronomical Society’s Division for Planetary Sciences.

A mix of interplanetary dust and millimeter-sized pebbles from comet 2P/Encke was already a suspect, but researchers couldn’t get the timing to work. Christou, of the Armagh Observatory in Northern Ireland, ran computer simulations that added a missing piece: sunlight.

Heat radiating unevenly from the sun-drenched comet grains acts “like a tiny braking thruster,” which slows the particles, Christou said. “The thrust is tiny, but it works continuously.” Over several thousand years, it can shift relatively large fragments of comet debris away from the main stream into a slightly separate orbit, creating a parallel trail that pummels Mercury at just the right time and kicks up a cloud of calcium.

“This is a really reasonable explanation,” says Ronald Vervack Jr., a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and a participating scientist with the MESSENGER spacecraft that detected the calcium.

MESSENGER, which orbited Mercury from 2011 to 2015 (SN Online: 4/30/15), also recently detected a surge of manganese, aluminum and ionized calcium around the planet. Unlike the neutral calcium cloud, which appears every Mercury year, these other atoms just showed up in the last three Mercury years of MESSENGER’s visit. “Was it a [meteor] swarm that came by … or did we get lucky?” Vervack asks. “We just don’t know.”

If the comet can explain the recurring uptick of neutral calcium, it might be responsible for these other elements as well. Encke passed within 4 million kilometers of Mercury in 2013, he notes. Perhaps the close encounter dumped a little more dust near Mercury, which subsequently rained down on the planet, though it’s not clear that the comet pieces can get nudged around that quickly. “It is interesting to me that within a few Mercury years of close passage, we started seeing things we didn’t see before,” he says.

Editor’s Note: This story was updated November 10, 2015, to correct the characterization of the clouds of elements appearing around Mercury. Ronald Vervack Jr. was referring to a recent observation of surges of manganese, aluminum and ionized calcium, not the seasonal bloom of neutral calcium.