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Mars magnetic field thrown into chaos due to close comet flyby

Mars magnetic field thrown into chaos due to close comet flyby

A one-of-a-kind opportunity given to scientists to see an intimate view of the havoc, which was created by a comet's passing on the magnetic environment, or magnetosphere, around Mars, was indeed unique. The effect was temporary but profound.

Just weeks before the historic encounter of comet C/2013 A1 (Siding Spring) with Mars in October 2014, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft entered orbit around the Red Planet. To protect sensitive equipment aboard MAVEN from possible harm, some instruments were turned off during the flyby; the same was done for other Mars orbiters. But a few instruments, including MAVEN's magnetometer, remained on, conducting observations from a front-row seat during the comet's remarkably close flyby.

Comet Siding Spring plunged the magnetic field around Mars into chaos, said Jared Espley, a MAVEN science team member at NASA's Goddard Space Flight Center in Greenbelt, Maryland. We think the encounter blew away part of Mars' upper atmosphere, much like a strong solar storm would.

Comet Siding Spring is also surrounded by a magnetic field. This results from the solar wind interacting with the plasma generated in the coma - the envelope of gas flowing from a comet's nucleus as it is heated by the sun. Comet Siding Spring's nucleus - a nugget of ice and rock measuring no more than half a kilometer (about 1/3 mile) -- is small, but the coma is expansive, stretching out a million kilometers (more than 600,000 miles) in every direction. The densest part of the coma -- the inner region near the nucleus -- is the part of a comet that's visible to telescopes and cameras as a big fuzzy ball.

When comet Siding Spring passed Mars, the two bodies came within about 140,000 kilometers (roughly 87,000 miles) of each other. The comet's coma washed over the planet for several hours, with the dense inner coma reaching, or nearly reaching, the surface. Mars was flooded with an invisible tide of charged particles from the coma, and the powerful magnetic field around the comet temporarily merged with - and overwhelmed - the planet's own weak one.

The main action took place during the comet's closest approach, said Espley, but the planet's magnetosphere began to feel some effects as soon as it entered the outer edge of the comet's coma.

At first, the changes were subtle. As Mars' magnetosphere, which is normally draped neatly over the planet, started to react to the comet's approach, some regions began to realign to point in different directions. With the comet's advance, these effects built in intensity, almost making the planet's magnetic field flap like a curtain in the wind. Espley and colleagues think the effects of the plasma tide were similar to those of a strong but short-lived solar storm.

With MAVEN, we're trying to understand how the sun and solar wind interact with Mars, said Bruce Jakosky, MAVEN's principal investigator from the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder. By looking at how the magnetospheres of the comet and of Mars interact with each other, we're getting a better understanding of the detailed processes that control each one.

This research was published in Geophysical Research Letters.

Mars magnetic field thrown into chaos due to close comet flyby

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