Composition of the Volatile Material in Halley's Coma from In Situ Measurements

The investigation of the volatile material in the coma of comets is a key to understanding the origin of cometary material, the physical and chemical conditions in the early solar system, the process of comet formation, and the changes that comets have undergone during the last 4.6 billion years. So far, in situ investigations of the volatile constituents have been confined to a single comet, namely P/Halley in 1986. Although, the Giotto mission gave only a few hours of data from the coma, it has yielded a surprising amount of new data and has advanced cometary science by a large step. In the present article the most important results of the measurements of the volatile material of Halley's comet are summarized and an overview of the identified molecules is given. Furthermore, a list of identified radicals and unstable molecules is presented for the first time. At least one of the radicals, namely CH₂, seems to be present as such in the cometary ice. As an outlook to the future we present a list of open questions concerning cometary volatiles and a short preview on the next generation of mass spectrometers that are being built for the International Rosetta Mission to explore the coma of Comet Wirtanen. This revised version was published online in June 2006 with corrections to the Cover Date.     

THE CLUSTER ION SPECTROMETRY (CIS) EXPERIMENT

The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion COmposition and DIstribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H⁺, He⁺, He⁺⁺, and O⁺) with energies from ~0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements.     

Rosina – Rosetta Orbiter Spectrometer for Ion and Neutral Analysis

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will answer important questions posed by the mission’s main objectives. After Giotto, this will be the first time the volatile part of a comet will be analyzed in situ. This is a very important investigation, as comets, in contrast to meteorites, have maintained most of the volatiles of the solar nebula. To accomplish the very demanding objectives through all the different phases of the comet’s activity, ROSINA has unprecedented capabilities including very wide mass range (1 to >300 amu), very high mass resolution (m/Δ m > 3000, i.e. the ability to resolve CO from N₂ and ¹³C from ¹²CH), very wide dynamic range and high sensitivity, as well as the ability to determine cometary gas velocities, and temperature. ROSINA consists of two mass spectrometers for neutrals and primary ions with complementary capabilities and a pressure sensor. To ensure that absolute gas densities can be determined, each mass spectrometer carries a reservoir of a calibrated gas mixture allowing in-flight calibration. Furthermore, identical flight-spares of all three sensors will serve for detailed analysis of all relevant parameters, in particular the sensitivities for complex organic molecules and their fragmentation patterns in our electron bombardment ion sources.     

Dynamics of magnetospheric ion Composition as observed by the GEOS mass spectrometer

After one year of operation the GEOS-1 Ion Composition Experiment has surveyed plasma composition at all local times in the L range 3 8 and the energy per charge range from thermal to 16 keV/e. From measurements made in the keV range during eleven magnetic storms we find that the percentage of heavy (M/Q > 1) ions present in the outer magnetosphere increases by a factor of 3 to 10 during disturbances. We conclude that two independent sources (solar wind, characterized by ⁴He²⁺, and ionosphere, characterized by O⁺) give on the average comparable contributions to injected populations, although in a single event one or the other source may dominate. However, in magnetically quiet periods protons are the dominant species with a few percent of heavy ions. With the help of special satellite manoeuvres magnetic field aligned fluxes of 0.05-3 keV/e H⁺, He⁺, O⁺ with traces of O²⁺ have been observed which may be related to ion beams found previously at lower altitudes in the auroral zone. At still lower energies ( 1 eV/e) the thermal plasma population is found to be made up of six ion species, three of which, D⁺, He²⁺ and O²⁺, were unknown in the magnetosphere prior to the GEOS-1 measurements. We present here a study of the evolution of doubly charged ions and their parent populations over four consecutive days. Various production mechanisms for doubly charged ions are discussed. We argue that ionization of singly charged ions by UV and energetic electrons and protons is the dominant process for plasmasphere production. Furthermore, the observed high concentrations of O²⁺ at high altitudes are a result of production in the upper ionosphere and plasmasphere combined with upward transport by thermal diffusion. Throughout the 1 year lifetime of GEOS-1 the ICE functioned perfectly and, because of its novel design, a short review of technical performance is included here.     

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the polar mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e^–1 to greater than 32 AMU e^–1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4 solid angle image in a half spin period. The energy per charge range from 15 eV e^–1 to 32 keV e^–1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.     

Rare Atoms,Molecules and Radicals in the Coma of P/Halley

We have searched for rare molecules and radicals in the coma of P/Halley using the ion data obtained by IMS-Giotto. Whereas our established methods were used in the ionosphere, a new model was developed for the interpretation of the ion data in the outer coma. Ne/H₂O < 1.5 × 10^-3 was determined in the coma of the comet. Upper limits for the production of Na were derived from the very low abundance of Na⁺. Methyl cyanide and (probably) ethyl cyanide were identified with abundances of CH₃CN/H₂O = (1.4 ± .6) × 10^-3 and C₂H₅CN/H₂O = (2.8 ± 1.6) × 10^-4. These results and upper limits for other N-bearing species confirm that nitrogen is depleted in the Halley material. C₄H was identified and a point source strength of C₄H/H₂O = (2.3 ± .8) × 10^-3 was derived. Our upper limit for C₃H is lower than the abundance of C₄H. This is in agreement with the enhanced abundances of C_nH species with even numbers of C-atoms found in interstellar molecular clouds, suggesting that the C₄H in Halley was synthesized under molecular cloud conditions. Thus, C₄H and other organics with unpaired electrons may turn out to be indicators for a molecular cloud origin of cometary constituents. This revised version was published online in June 2006 with corrections to the Cover Date.     

The solar WIND and suprathermal ion composition investigation on the WIND spacecraft

The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms^–1 (protons) to 1280 kms^–1 (Fe⁺⁸), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He⁺, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.     

Recent developments in ion mass spectrometers in the energy range below 100 keV

The recognition that heavy ions often have similar or higher abundance than protons in space plasmas and the fact that ions with masses and/or charges larger than unity are ideally suited to test theories on origin, transport, acceleration, and loss of these plasmas led to the development of new and more sophisticated mass spectrometers during the last decade. The characteristics of several state-of-the-art instruments which have been flown or selected for missions will be discussed and aims for future developments will be given.