The flyby of Comet C/2013 A1, also known as Comet Siding Spring, on planet Mars in October 2014 gave scientists a rare opportunity to see the havoc that the passing comet can cause on the magnetic environment, or magnetosphere, around the Red Planet.
A magnetometer located about 100 miles above the planet's surface showed that the flyby threw Mars's magnetic field into chaos and the effect was profound albeit temporary.
NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft team member Jared Espley said that the close encounter with Comet Siding Spring set off the magnetosphere around Mars into chaos and even blew away a portion of the planet's upper atmosphere.
Unlike Earth that has strong magnetosphere, the magnetic field on Mars is weak and patchy. Comets, meanwhile, produce their own magnetic field when they interact with solar wind, so when Siding Spring flew by Mars at a proximity of 87,000 miles, its magnetism punched the weak magnetic field of the Red Planet, causing a violent turmoil that lasted for several hours.
During the flyby, Siding Spring's coma washed over Mars with the dense inner coma reaching or almost reaching the planet's surface. The powerful cometary magnetic field then temporarily merged with and overwhelmed the weak magnetic field of the planet, which eventually made an impact.
"The main action took place during the comet's closest approach but the planet's magnetosphere began to feel some effects as soon as it entered the outer edge of the comet's coma," said Espley.
The changes were initially subtle but as the magnetosphere of Mars, which is typically draped neatly over the planet, began to react to the approaching comet, some regions started to realign to point in different directions.
As the comet advanced, the effects became more intense with the magnetic field of the planet flapping like a curtain beaten by the wind. At the closest approach when the comet's plasma was at its densest, Mars's magnetic field was in a complete chaos. Hours following the departure of the comet, some disruptions on the planet continued to be detected.
"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," said MAVEN's principal investigator Bruce Jakosky.
The findings were published in Geophysical Research Letters.
