[meteorite-list] Mercury Gets a Meteoroid Shower from Comet Encke

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 13 Nov 2015 15:05:54 -0800 (PST)
Message-ID: <201511132305.tADN5tri013488_at_zagami.jpl.nasa.gov>

http://www.nasa.gov/feature/goddard/mercury-gets-meteoroid-shower-from-comet-encke

Mercury Gets a Meteoroid Shower from Comet Encke
November 10, 2015

The planet Mercury is being pelted regularly by bits of dust from an ancient
comet, a new study has concluded. This has a discernible effect in the
planet's tenuous atmosphere and may lead to a new paradigm on how these
airless bodies maintain their ethereal envelopes.

The findings are to be presented at the annual Meeting of the Division
of Planetary Sciences of the American Astronomical Society at National
Harbor, Maryland, this week, by Apostolos Christou at the Armagh Observatory
in Northern Ireland, Rosemary Killen at NASA's Goddard Space Flight Center
in Greenbelt, Maryland, and Matthew Burger of Morgan State University
in Baltimore, working at Goddard.

[Graphic]
Mercury appears to undergo a recurring meteoroid shower when its orbit
crosses the debris trail left by comet Encke. (Artist's concept.)
Credits: NASA/Goddard

Earthlings are no strangers to the effects of cometary dust on a planet
and its environment. On a clear, moonless night we witness the demise
of countless such dust grains as they burn up in the Earth's atmosphere
in the form of meteors or "shooting stars." At certain times of the year,
their numbers increase manyfold, creating a natural fireworks display:
a meteor shower. This is caused by the Earth passing through a stream
of dust particles left behind by certain comets.

One of the most well-known showers, the August Perseids, originates from
comet Swift-Tuttle, which was last seen back in 1992 and won't be back
in the inner solar system for another century. But Earth is not the only
planet in the solar system to sweep up cometary dust in this fashion.
Last year, comet Siding Spring came within 100,000 miles of Mars, loading
its upper atmosphere with several tons of cometary material. The aftermath
was recorded by instruments onboard several Mars-orbiting spacecraft such
as NASA's Mars Atmosphere and Volatile Evolution mission and ESA's Mars
Express.

Bodies such as the moon and Mercury are typically thought of as airless,
yet we have known since the time of the Apollo moon landings that they
are surrounded by clouds of atomic particles either launched from the
surface or brought in by the solar wind. Though tenuous by comparison
to the dense atmospheres of the Earth or Mars, the observational record
has revealed these "surface boundary exospheres" to be complex and dynamic
entities, fascinating to study in their own right.

NASA's MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER),
the first spacecraft to orbit Mercury, measured how certain species in
the exosphere vary with time. Analysis of the data by Burger and colleagues
found a pattern in the variation of the element calcium that repeats from
one Mercury year to the next. To investigate, Killen teamed up with Joe
Hahn of the Space Science Institute, based in Austin, Texas, to understand
what happens when Mercury ploughs through the so-called zodiacal cloud
of interplanetary dust around the sun and its surface is pelted by high-speed
meteoroids.

The researchers found that both the observed amount of calcium and the
pattern in which it varies could be explained in terms of the material
thrown off the planet's surface by the impacts. But one feature in the
data did not make sense: the peak in calcium emission is seen right after
Mercury passes through its perihelion -- the closest point of its orbit
to the Sun -- whereas Killen and Hahn's model predicted the peak to occur
just before perihelion. Something was still missing.

That "something" arrived in the form of a cometary dust stream. Discovered
in the 18th century, comet Encke is named after the German mathematician
who first computed its orbit. It has the shortest period of any comet,
returning to perihelion every 3.3 years at a distance of 31 million miles
(nearly 50 million kilometers) from the sun. Its orbit, and that of any
dust particles thrown off it, is stable enough so, over millennia, a dense
dust stream would have formed. Killen and Hahn proposed that Encke dust
impacting Mercury could kick up more calcium from the surface and explain
what MESSENGER was seeing. The match was not perfect, however. For one
thing, Encke is closest to Mercury?s orbit about a week later than the
calcium peak. The researchers postulated that the evolution of the dust
stream over thousands of years had somehow shifted the stream away from
comet Encke's present orbit.

But what was causing the shift? To find out, Killen and Burger teamed
up with Christou to simulate the evolution of the Encke stream for several
tens of thousands of years -- the likely lifetime of the comet. Christou
had to first compute a 'best guess' of the comet's orbit many thousands
of years before it was first observed. Starting from that point in time,
he followed a cloud of simulated dust grains launched from the comet's
nucleus to find out if -- and, more importantly, where -- their present
orbits would intersect Mercury's. He found that the dust, rather than
shifting away from the comet's orbit, simply spread along it, forming
a stream that encounters Mercury exactly when the comet does.

Then he reran the model, to allow for a subtle interaction between the
dust grains and sunlight called Poynting-Robertson drag. This creates
an extra, though tiny, force on the grains which, over long periods of
time, could amount to a significant change in the orbit. The result was
that the orbit of the stream in the simulations shifted behind the comet's
orbit and toward the location where the peak in calcium emission was observed.
Moreover, the size of the shift depended on the size of the dust grains
-- bigger grains means a smaller drag force -- and on how long ago they
were released from the comet. Christou found that he could reproduce the
timing of the calcium peak for grains a millimeter or so in size, ejected
from Encke between 10,000 and 20,000 years ago. This is consistent with
what we know about cometary dust: droves of millimeter-sized cometary
grains enter the Earth's atmosphere every day, creating visible meteors.
It also agrees with present best estimate of the age of the stream based
on Earth-based meteor studies.

"Finding that we can move the location of stream to match MESSENGER's
observations is gratifying, but the fact that the shift agrees with what
we know about Encke and its stream from independent sources makes us confident
that the cause-and-effect relationship is real," Christou explained.

The work has set an interesting precedent on the importance of the different
dust populations in exosphere production.

"We already knew that impacts were important in producing exospheres,"
Killen said. "What we did not know was the relative importance of comet
streams over zodiacal dust. Apparently, comet streams can have a huge,
but periodic, effect."

Killen looks forward to searching for the signature of the Encke stream
on other exospheric species. "This will be further confirmation of the
relationship," she added.

A paper describing the research appeared in the Sept. 28 issue of Geophysical
Research Letters.

For more information the MESSENGER mission, visit:
http://www.nasa.gov/mission_pages/messenger/main/index.html

Mark Bailey and Apostolos Christou
Armagh Observatory, Northern Ireland

For Goddard inquiries, contact Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Maryland
301-614-5438
elizabeth.a.zubritsky at nasa.gov
Received on Fri 13 Nov 2015 06:05:54 PM PST