Comet Siding Spring
© NASA/JPL-CaltechThis artist's impression depicts Comet Siding Spring narrowly miss Mars in 2014.
When Comet C/2013 A1 (Siding Spring) swung past the Red Planet in October 2014, it was an unprecedented opportunity for an armada of Mars robots to have a ringside seat of the interplanetary spectacle. But as dazzling as the flyby was, the real drama wasn't seen by the cameras of Mars orbiters or rovers; it was detected by a magnetometer. And that magnetometer, located 100 miles above the Martian surface, detected chaos.

"Comet Siding Spring plunged the magnetic field around Mars into chaos," said Jared Espley, of NASA's Goddard Space Flight Center in Greenbelt, Md. and science team member of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, in a NASA press release. "We think the encounter blew away part of Mars' upper atmosphere, much like a strong solar storm would."

Although Mars' magnetic field is weak and patchy (unlike Earth's strong, global magnetosphere), MAVEN's sensitive magnetometer detected a huge upheaval in orbit as Siding Spring's own magnetism rattled the planet's magnetic field.

The comet's nucleus may only be a third of a mile wide, but the atmosphere surrounding the nucleus (known as the coma) was as wide as 600,000 miles when it encountered Mars. (The coma is formed through solar heating — the ices contained within a comet's nucleus sublimate into space, pumping the coma with gas.) Through interactions with the solar wind, comets also generate their own magnetic fields that loop their way through the coma. So when Siding Spring buzzed Mars, coming as close as 87,000 miles, the cometary magnetism punched Mars' weak magnetic field, sending it into violent turmoil for several hours.

MAVEN scientists likened the effect as a magnetic curtain flapping in the wind.

"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."

These magnetic observations were very lucky as MAVEN had only just arrived in Mars orbit weeks before the close encounter. Although most of the spacecraft's instruments were powered down at that time (to protect them from possible damage by comet dust), the magnetometer remained on for the duration, carrying out unique observations of two magnetic fields slamming into one another.

"With MAVEN, we're trying to understand how the sun and solar wind interact with Mars," added Bruce Jakosky, MAVEN's principal investigator from the University of Colorado 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."

We now know that Mars' atmosphere is slowly leaking into space as its magnetic field isn't strong enough to completely protect it from solar wind erosion. And, from these MAVEN measurement, it looks like this magnetic "hit and run" by Siding Spring also ripped a chunk out of the Red Planet's atmosphere, giving us a profound look at what happens when comets and planets (almost) collide.
Siding Spring and Mars
© NASA/Goddard The close encounter between comet Siding Spring and Mars flooded the planet with an invisible tide of charged particles from the comet’s coma. The dense inner coma reached the surface of the planet, or nearly so. The comet’s powerful magnetic field temporarily merged with, and overwhelmed, the planet’s weak field, as shown in this artist’s depiction.