Cosima – High Resolution Time-of-Flight Secondary Ion Mass Spectrometer for the Analysis of Cometary Dust Particles onboard Rosetta

Abstract

The ESA mission Rosetta, launched on March 2nd, 2004, carries an instrument suite to the comet 67P/Churyumov-Gerasimenko. The COmetary Secondary Ion Mass Anaylzer -COSIMA -is one of three cometary dust analyzing instruments onboard Rosetta. COSIMA is based on the analytic measurement method of secondary ion mass spectrometry (SIMS). The experiment's goal is in-situ analysis of the elemental composition (and isotopic composition of key elements) of cometary grains. The chemical characterization will include the main organic components, present homologous and functional groups, as well as the mineralogical and petrographical classification of the inorganic phases. All this analysis is closely related to the chemistry and history of the early solar system. COSIMA covers a mass range from 1 to 3500 amu with a mass resolution m/ m @ 50% of 2000 at mass 100 amu. Cometary dust is collected on special, metal covered, targets, which are handled by a target manipulation unit. Once exposed to the cometary dust environment, the collected dust grains are located on the target by a microscopic camera. A pulsed primary indium ion beam (among other entities) releases secondary ions from the dust grains. These ions, either positive or negative, are selected and accelerated by electrical fields and travel a well-defined distance through a drift tube and an ion reflector. A microsphere plate with dedicated amplifier is used to detect the ions. The arrival times of the ions are digitized, and the mass spectra of the secondary ions are calculated from these time-of-flight spectra. Through the instrument commissioning, COSIMA took the very first SIMS spectra of the targets in space. COSIMA will be the first instrument applying the SIMS technique in-situ to cometary grain analysis as Rosetta approaches the comet 67P/Churyumov-Gerasimenko, after a long journey of 10 years, in 2014.

Dust particle flux and size distribution in the coma of 67P/Churyumov-Gerasimenko measured in situ by the COSIMA instrument on board Rosetta

“…Once these showers are put back together, the number of compact particles dominates the GIADA collection by a factor comprised between 2.6 and 14, which is a factor of two to seven higher than the COSIMA observations. The classes defined as compact and porous aggregates can be slightly different for the two instruments and one cannot exclude that the COSIMA compact or aggregates are different from the GIADA compact and fluffy classifications, since both instruments measure different physical parameters (Kissel et al 2007;Colangeli et al 2007). On one hand, the COSIMA particles are classified from the morphology derived from optical images.…”

“…Each target has an area of 1 cm × 1 cm and the majority of the targets are coated with a highly porous metallic surface layer appropriate for the adhesion of cometary particles (Kissel et al 2007;Hornung et al 2014). For dust collection, the targets are placed within the instrument at the end of a dust funnel with an aperture of 15°× 23°oriented on the spacecraft in the same direction as GIADA and the camera system Optical Spectroscopic and Infrared Remote Imaging System (OSIRIS).…”

“…New features on a target are detected by blinking the images taken before and after the exposure to the cometary particle flux. A detailed description of the COSISCOPE sub-system can be found in Kissel et al (2007) and in Langevin et al (2016).…”

“…The COmetary Secondary Ion Mass Analyzer (COSIMA) is a mass spectrometer on board Rosetta which allows the collection, imaging, and compositional analysis of dust particles in the coma of 67P/Churyumov-Gerasimenko (Kissel et al 2007). The in situ collection of dust during the entire mission allows the study of the dust particle properties and their evolution along the orbit of the comet.…”

Giada: Its Status After the Rosetta Cruise Phase and on-Ground Activity in Support of the Encounter With Comet 67p/Churyumov-Gerasimenko

“…Similar dust bursts were detected at the Stardust NExT encounter of comet 9P/Tempel 1 in February 2011 (Economou et al, 2012). The new GIADA instrument will measure the physical properties of dust in the coma of short-period comet 67P/Churyumov-Gerasimenko and when used in tandem with on-board particle analyzers COSIMA (Kissel et al, 2007) and MIDAS (Riedler et al, 2007), the ejected dust compositions can be assessed albeit not of the same grains detected by GIADA. In any case, standalone GIADA data coupled with calibration curves obtained in laboratory on comet dust analogs using a spare flight GIADA model will offer constraints on the mineral compositions of individual comet grains detected by GIADA.…”