In early 2007, NASA's Cassini spacecraft observed something extraordinary around Saturn. An unusually strong blast of solar wind sent subatomic particles crashing into the ringed planet's magnetic field, giving rise to perhaps the most tremendous shock wave ever observed emanating from the planet. But newly announced findings reveal the biggest surprise was yet to come.
The artist's impression above depicts the scene just described. Cassini can be seen at left. Emanating from the planet, grey-striped and toroidal, is Saturn's magnetosphere. The shock wave — what is known more formally as the "bow shock" region — is depicted in wispy blue.
Being carried in that shockwave are the solar particles that collided with Saturn's magnetosphere — and they're traveling fast. Observations from Cassini indicate that Saturn's bow shock had managed to accelerate these subatomic particles to nearly the speed of light. That's a feat more typical of shockwaves around distant supernovas, or particle accelerators here on Earth. Massive though it was, Saturn's bow shock would have been necessary, but not sufficient, to produce such high-energy particles — so how did they manage to get moving so fast?
The answer has to do with the orientation of the shock relative to Saturn's magnetic field. Under what are known as "quasi-parallel" conditions, a planet's magnetic field (depicted here with blue lines) will run almost parallel to a vector emanating outward from the edge of the shockwave (depicted in red).
Under "quasi-perpendicular" conditions, the magnetic field lines run more or less perpendicular to the vector. The former describes a scenario wherein the magnetic field lines and the outward direction of the shock wave are more or less aligned, and favorable to particle acceleration. According to NASA, what Cassini observed was, in fact, the first detection of significant acceleration of electrons in a quasi-parallel shock at Saturn.
Shock waves are not rare in and of themselves, but ones capable of accelerating particles to close to the speed of light are typically observed around supernova remnants that exist well beyond our immediate solar neighborhood. "Cassini has essentially given us the capability of studying the nature of a supernova shock in situ in our own solar system," explained astronomer Adam Masters, who led the investigation into Saturn's unprecedented cosmic wave, "bridging the gap to distant high-energy astrophysical phenomena that are usually only studied remotely."
In other words: supernovas are far. Saturn, on a cosmic scale, is right down the street. Its quasi-parallel shocks, however rare, provide us with a valuable means of studying supernova shocks right here in our own solar system; and Cassini gives us front row seats. Just incredible.