In 2014, comet C/2013 A1, known as sliding spring, came within 140,000 Km of the planet Mars. This is a bit more than a third of the distance from the Earth to the Moon. Comets are small, so gravitationally this interaction was insignificant, but from an electromagnetic point of view, things were shaken up big time!
Comets are small, relatively speaking. A typical comet is a few kilometers across, about the size of a big city. But with sunlight melting ices and liberating gases and dust from the comet’s interior, the part of the comet we see in the sky, known as the Coma, can be hundreds of thousands of kilometers across. Comet sliding spring has a coma that spans 600,000 Km, bringing Mars directly into it’s heart during the fly-by
Comets are responsible for most meteor showers. The trail of dust and ice they leave behind continues to follow their orbit for hundreds of thousands of years. If the path of the comet crosses a planetary orbit, that planet passes through the debris, triggering the shower. Most of the meteor showers that occur here on Earth have a related comet.
This means that sliding spring triggered a massive meteor shower on Mars, so intense it could be called a meteor storm. Most showers on Earth see a few dozen meteors per hour, but rates on Mars during the flyby would have easily exceeded 1000/hr. But there is more to the story.
Because the Sun releases charged particles, the gases and dust in the coma can become charged, ionized, etc, forming a plasma. You end up with moving charges, which create a comet with a magnetic field. Mars’ atmosphere at high altitudes is also a plasma thanks to solar radiation, giving Mars a magnetic field as well. The problem with Mars is that it no longer has a strong magnetic dynamo in it’s core, and so it has a relatively weak magnetic field generated by atmospheric plasma.
So sliding spring’s magnetic field actually merged with, and overpowered, Mars’s field. It started gradually as the comet approached, but like waves washing over the shore, when the comet reached closest approach, Mars’ field was in chaos. This resulted in an increase in the amount of Mars’ atmosphere that was lost to space, and it left the Martian atmosphere and magnetosphere in turmoil for hours after closest approach.
We wouldn’t have been able to see any of this if it wasn’t for NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which had just entered Martian orbit weeks before the comet’s approach. Most of the craft’s instruments were turned off to protect them during the fly-by, but a few remained on, including the magnetometer, giving us the incredible science we are now seeing.