Discovery of suprathermal Fe⁺ in Saturn's magnetosphere

Abstract

Measurements in Saturn's equatorial magnetosphere from mid-2004 through 2013 made by Cassini's charge-energy-mass ion spectrometer indicate the presence of a rare, suprathermal (83-167 keV/e) ion species at Saturn with mass ~56 amu that is likely Fe⁺. The abundance of Fe⁺ is only ~10^-4 relative to that of W⁺ (O⁺, OH⁺, H₂O⁺, and H₃O⁺), the water group ions which dominate Saturn's suprathermal and thermal ions along with H⁺ and H₂⁺. The radial variation of the Fe⁺ partial number density (PND) is distinctly different from that of W⁺ and most ions that comprise Saturn's suprathermal ion populations which, unlike thermal energy plasma ions, typically have a prominent PND peak at ~8-9 Rs (1 Saturn radius, Rs = 60,268 km). In contrast, the Fe⁺ PND decreases more or less exponentially from ~4 to ~20 Rs, our study's inner and outer limits. Fe⁺ may originate from metal layers produced by meteoric ablation near Saturn's mesosphere-ionosphere boundary and/or possibly impacted interplanetary dust particles or the Saturn system's dark material in the main rings.