The Dymio Ion Mass Spectrometer of the Mars 96 Mission

The orbiter of the Mars 96 mission will carry in the plasma package a low energy ion mass spectrometer named DYMIO. This instrument, which is aimed at studying the thermal plasma and the superthermal ions with energy up to a few hundred of eV in the environment of the planet, is an omnidirectional magnetic ion mass spectrometer which will provide a mass analysis of the ions in the mass range from 1(H + ) to 44(CO 2 + ). The instrument consists of 2 identical sensor heads placed back to back; each comprises an electrostatic optics system which is able to vary the direction of sight of the instrument through 16 sectors each having a FOV of ∼18° half angle for thermal ions The instrument can make an energy analysis of the incoming ions in each sector The mass analysis is achieved using a magnet with a Mattauch-Herzog geometry followed by a detector which can be operated simultaneously in 2 different modes: one is a digitized mass spectrograph with 256 pixels, the second uses four analog integrators to measure the flux of the most likely abundant ions in the foreseen altitude range, namely H + , O + , O 2 + , CO 2 + . In this paper we summarize the science objectives, describe the DYMIO ion optics and electronics and give some snapshots of calibration data.