Mercury Gets A Meteoroid Shower From Comet Encke

Armagh Observatory, 10th November 2015: The results of a new study of the planet Mercury have been announced this week at the Annual Meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland, USA, by astronomers Apostolos Christou (Armagh Observatory), Rosemary Killen (NASA´s Goddard Space Flight Center in Greenbelt, Maryland, USA) and Matthew Burger (Morgan State University in Baltimore, USA). The researchers together with Joe Hahn (Space Science Institute, Austin, Texas) have been analyzing data on Mercury´s dynamic atmosphere, obtained over a period of more than four years by NASA´s Messenger spacecraft.

Messenger had been orbiting Mercury from March 2011 until — at the end of its mission on 30 April 2015 — it finally crashed into the planet´s surface. Among the important measurements obtained during this spacecraft´s mission was evidence of a puzzling peak in the amount of Calcium observed in the planet´s dynamic atmosphere at a particular phase or “season” of Mercury´s 88-day period of revolution, or year, around the Sun. Christou, Killen and Burger have now concluded that this feature can be explained by high-velocity impacts of small, millimetre-size particles, known as meteoroids, on to Mercury´s cratered surface. But what is most exciting about this result is that they have proved that the source of the impacting meteoroids was an exceptional short-period comet, a remnant of which is now known as Comet Encke, orbiting between 10 and 20 thousand years ago.

In short, the planet Mercury is being pelted regularly by bits of dust from an ancient comet. This affects the planet´s tenuous atmosphere and may lead to a new understanding not just of Comet Encke´s evolution but how airless bodies such as Mercury maintain their ethereal dynamic atmospheres.

At certain times of the year, our planet passes through streams of dust left behind by comets, leading to a natural fireworks display: a meteor shower. One of the best known, the August Perseids, originates from comet Swift-Tuttle, last seen in the inner solar system back in 1992 and not to return for more than another century. Another, the Taurids, originates from Comet Encke, and bombards Earth around the end of October and early November, while another part of Comet Encke´s dust trail runs into Earth during daylight hours around the end of June.

But Earth is not the only planet to sweep up cometary dust in this fashion. Last year, a comet called Siding Spring passed exceptionally close to the planet Mars, loading its upper atmosphere with several tons of cometary material. The aftermath was recorded by instruments onboard Mars-orbiting probes like NASA´s MAVEN and ESA´s Mars Express.

For airless bodies like the Moon and Mercury, we have known since the Apollo landings that they are surrounded by clouds of atomic particles either launched from the surface or brought in by the solar wind. Though tenuous compared to the dense atmospheres of the Earth or Mars, these dynamic atmospheres or “Surface Boundary Exospheres” are now recognized to be complex and dynamic entities in themselves, fascinating to study in their own right.

In the case of Mercury, Burger and colleagues analyzed the Messenger data and found a pattern for the element Calcium that repeats from one of Mercury´s 88-day long years to the next. To investigate, Killen worked with Hahn to understand what happens when Mercury ploughs through the so-called zodiacal cloud of interplanetary dust around the Sun and its surface is bombarded by high-speed meteoroids.

They found that both the amount and the pattern of calcium 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 Comet Encke, named after the German mathematician who computed its orbit. This comet has the shortest period of any known active comet, returning to perihelion every 3.3 years at a distance of approximately 31 million miles (nearly 50 million kilometres) from the Sun. Its orbit is stable enough so, over millennia, a dense dust stream would have formed. In their paper, Killen and Hahn proposed that dust from Comet Encke, impacting Mercury, could kick up more calcium from the surface and explain the Messenger data. The match was not perfect, however, because Encke encounters Mercury´s orbit about a week after the calcium peak. The researchers postulated that the stream´s evolution over thousands of years had somehow shifted it away from the comet´s present orbit.

But what was causing the shift? To find out, Killen and Burger teamed up with Christou, at Armagh Observatory, to simulate the evolution of the Encke stream for several tens of thousands of years, the likely lifetime of the comet. Christou modeled a cloud of simulated dust grains launched from the comet´s nucleus to find out if — and more importantly where — their present orbits could intersect that of Mercury. He found that the dust, rather than shift away from the comet´s orbit, simply spreads along it, forming a stream that encounters Mercury exactly when the comet does.

Then he re-ran the model, to allow for a subtle interaction between the dust grains and sunlight called Poynting-Robertson drag. This exerts 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 of the observed calcium peak. Moreover, the size of the shift depended on the size of the dust grains, as there is a relatively smaller drag force, compared to gravity, on larger grains. The size of the effect also depended 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 millimetre or so in size, provided that they were ejected from Comet Encke between ten and twenty thousand years ago. This is consistent with what we know about cometary dust, as droves of millimetre-size cometary grains enter the Earth´s atmosphere every day, creating visible meteors. It also agrees with the age estimate of the stream based on Earth-based meteor studies.

“Finding that we can move the location of the 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 on the Messenger mission, see: http://www.nasa.gov/mission_pages/messenger/main/index.html

FOR FURTHER INFORMATION PLEASE CONTACT: Mark Bailey at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; meb@arm.ac.uk; URL: http://star.arm.ac.uk/, and Apostolos Christou, E-mail:aac@arm.ac.uk.

For Goddard Space Flight Center inquiries, contact Elizabeth Zubritsky, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; Tel: 001-301-614-5438; Elizabeth.a.zubritsky@nasa.gov

Poster

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Figure Caption: Artist´s conception of the planet Mercury orbiting through a stream of debris from Comet Encke. This causes a shower of meteoroids on the planet on each one of its 88-day long years. Image credit Jay Friedlander, courtesy NASA Goddard Space Flight Center (NASA/GSFC)

Last Revised: 2015 November 10th