MESSENGER discovers a periodic meteor shower on Mercury
Sen—NASA’s MESSENGER spacecraft has found hints in Mercury's exceptionally thin atmosphere that suggest it experiences its own periodic meteor shower, possibly produced by a comet that is responsible for similar events on Earth.
Mercury is surrounded by such a thin halo of gases that it is termed an exosphere rather than an atmosphere. Its composition has been analysed by an instrument aboard MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging.
“The possible discovery of a meteor shower at Mercury is really exciting and especially important because the plasma and dust environment around Mercury is relatively unexplored,” said Rosemary Killen, a planetary scientist at NASA’s Goddard Space Flight Center and lead author of the study.
A meteor shower occurs when a planet passes through debris shed by a comet, or sometimes an asteroid. The smallest bits of dust, rock and ice get pushed away from the Sun by the force of solar radiation, creating the comet’s tail. The larger chunks get deposited along the comet’s orbit.
Comet Encke has left several debris fields in the inner Solar System, giving rise to meteor showers on Earth, including the Southern and Northern Taurids, meteor showers that peak in October and November, and the Beta Taurids in June and July.
The hallmark of a meteor shower on Mercury is a surge of calcium in the exosphere. Launched in August 2004, measurements taken by MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer revealed seasonal surges of calcium that occurred regularly over the first nine Mercury years since the spacecraft began orbiting the planet in March 2011.
This artist's impression shows NASA's Mercury-bound MESSENGER spacecraft from the instrument side. Image credit: Johns Hopkins University Applied Physics Laboratory
The cause of these surges in calcium levels is thought to be a shower of small dust particles hitting the planet and knocking calcium-bearing molecules free from the surface. This process, called impact vaporization, continually renews the gases in Mercury’s exosphere.
The general background of interplanetary dust in the inner solar system cannot, by itself, account for the periodic spikes in calcium. This suggests a periodic source of additional dust from a cometary debris field. Only a handful of comets have debris trails that would cross Mercury’s orbit suggests the likely source of the planet’s event is comet Encke.
“If our scenario is correct, Mercury is a giant dust collector,” said Joseph Hahn, a planetary dynamist at the Space Science Institute and co-author of the study, published online in the journal Icarus. “The planet is under steady siege from interplanetary dust and then regularly passes through this other dust storm, which we think is from comet Encke.”
The team created detailed computer simulations to test the comet Encke hypothesis. The calcium spikes found in the MESSENGER data were offset a bit from the expected results, probably due to changes in the comet’s orbit over time, due to the gravitational pull of Jupiter and other planets.
“The variation of Mercury’s calcium exosphere with the planet’s position in its orbit has been known for several years from MESSENGER observations, but the proposal that the source of this variation is a meteor shower associated with a specific comet is novel,” added MESSENGER Principal Investigator Sean Solomon, of the Lamont-Doherty Earth Observatory at Columbia University.
“This study should provide a basis for searches for further evidence of the influence of meteor showers on the interaction of Mercury with its Solar-System environment.”