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.