[meteorite-list] MESSENGER Data Suggest Recurring Meteor Shower on Mercury

[Ron Baalke]

http://www.nasa.gov/press/goddard/2014/december/messenger-data-suggest-recurring-meteor-shower-on-mercury/

December 12, 2014
RELEASE 14-041

MESSENGER Data Suggest Recurring Meteor Shower on Mercury

The closest planet to the sun appears to get hit by a periodic meteor 
shower, possibly associated with a comet that produces multiple events 
annually on Earth.

The clues pointing to Mercury's shower were discovered in the very thin 
halo of gases that make up the planet's exosphere, which is under study 
by NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, 
and Ranging) spacecraft.

"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 in Greenbelt, Maryland, and lead 
author of the study, available online in Icarus.

A meteor shower occurs when a planet passes through a swath of debris 
shed by a comet, or sometimes an asteroid. The smallest bits of dust, 
rock and ice feel the force of solar radiation, which pushes them away 
from the sun, creating the comet's sometimes-dazzling tail. The larger 
chunks get deposited like a trail of breadcrumbs along the comet's orbit 
- a field of tiny meteoroids in the making.

Earth experiences multiple meteor showers each year, including northern 
summer's Perseids, the calling card of comet Swift-Tuttle, and December's 
reliable Geminids, one of the few events associated with an asteroid. 
Comet Encke has left several debris fields in the inner solar system, 
giving rise to the Southern and Northern Taurids, meteor showers that 
peak in October and November, and the Beta Taurids in June and July.

The suggested hallmark of a meteor shower on Mercury is a regular surge 
of calcium in the exosphere. Measurements taken by MESSENGER's Mercury 
Atmospheric and Surface Composition Spectrometer have revealed seasonal 
surges of calcium that occurred regularly over the first nine Mercury 
years since MESSENGER began orbiting the planet in March 2011.

The suspected cause of these spiking calcium levels is 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 as interplanetary dust and meteoroids 
rain down on the planet. However, 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, 
for example, a cometary debris field. Examination of the handful of comets 
in orbits that would permit their debris to cross Mercury's orbit indicated 
that the likely source of the planet's event is Encke.

"If our scenario is correct, Mercury is a giant dust collector," said 
Joseph Hahn, a planetary dynamist in the Austin, Texas, office of the 
Space Science Institute and coauthor of the study. "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 researchers created detailed computer simulations to test the comet 
Encke hypothesis. However, the calcium spikes found in the MESSENGER data 
were offset a bit from the expected results. This shift is 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 in New York. "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."

The Johns Hopkins University Applied Physics Laboratory built and operates 
the MESSENGER spacecraft and manages this Discovery-class mission for 
NASA.


Nancy Neal-Jones
NASA's Goddard Space Flight Center, Greenbelt, Maryland
301-286-0039
nancy.n.jones at nasa.gov

Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Maryland
301-614-5438
elizabeth.a.zubritsky at nasa.gov