In 2004, when NASA’s Cassini probe arrived at Saturn and began studying the planet and its moons, one tiny moon in particular stunned scientists: Enceladus. Cassini sent back spectacular pictures of geysers of water emerging through cracks in its icy surface near the moon’s south pole, hinting at the possibility of an ocean of liquid water underneath—a habitat suitable for life.
The geysers also create a wispy plume of ice that syphons material from the moon to Saturn, injecting Saturn’s magnetic field with plasma and sending electrical currents coursing through space. For more than a decade, scientists have struggled to understand conflicting measurements of those currents from Cassini. Now a new study from Farrell et al. may solve this mystery.
Here’s what scientists expect should be happening: Although the emitted water molecules are typically electrically neutral when emerging from the surface, the Sun’s ultraviolet rays and interaction with fast-moving plasma in Saturn’s magnetosphere immediately begin to knock electrons off of them, creating new water ions. Now carrying a positive charge, these ions experience electric forces associated with Saturn’s plasma in its magnetic field, which is whipping past in rigid lockstep with the planet’s rotation. Like stepping onto a carousel, every newly created ion is suddenly swept away by this electric force, accelerated to speeds reaching 30 kilometers per second to then get “picked up” and merge with the corotating plasma. These hurtling ions generate a large electric current and also create magnetic disturbances that Cassini’s instruments can detect.
At least that’s how it should have worked in theory. However, when Cassini flew through the icy plume, its instruments sent back vastly different readings. According to the craft’s Langmuir probe, which measures particle density, there were enough ions present to generate a current of 10 million amps. The magnetometer indicated a current more than 20 times weaker.
This discrepancy could be explained if the new ions in the plume are shielded from these pickup electric fields by charged dust particles, which trap the charged molecules. Scientists have already confirmed that the plume should also contain a substantial number of dust particles less than a micrometer in diameter, and most of them would be negatively charged and attract the positive water ions.
In the new study, the authors used computer simulations to model this “plasma sheathing” at Enceladus for the first time. They found that if the ions spewing from the geysers are moving slowly enough and wander close to a dust grain, the dust can effectively trap them. If enough ions get trapped, the negative dust and surrounding ion sheath effectively cancel each other out, reducing both the ion and dust currents in the pickup process.
This would resolve the dilemma neatly. If only a small fraction—maybe a few percent—of the ions is moving fast enough to evade the dust grains and get picked up by the magnetic field, that would explain the weaker current readings from the magnetometer. Meanwhile, the slow-moving ion-dust pairings would hang around a lot longer, explaining the high particle density measured by the Langmuir probe.
Cassini’s mission is coming to a close; any future mission will be passing through one of the most complex dusty plasma environments ever visited by spacecraft, the authors note. (Journal of Geophysical Research: Planets, https://doi.org/10.1002/2016JE005235, 2017)
—Mark Zastrow, Freelance Writer