PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2008-12-17, shorted version, Jouni Ryno, FMI 2012-09-17 SBN:T.Barnes Fixed to KISSELETAL2007 reference" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "RO" INSTRUMENT_ID = "COSIMA" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COMETARY SECONDARY ION MASS ANALYZER" INSTRUMENT_TYPE = "MASS SPECTROMETER" INSTRUMENT_DESC = " NOTE: This text is a summary of the COSIMAPAPER.ASC located in the DOCUMENT subdirectory. The reader is suggested to use the more readable pdf version of this document, COSIMAPAPER.PDF. This document has an additional chapter 3, which simply lists the properties of each substrate. TABLE OF CONTENTS - 1. Introduction - 2. Functional Description - 3. Collection Substrates - 4. In-Flight Operation - 5. The Ground Laboratory Program - 6. Anticipated Results - 7. References COSIMA - High Resolution Time-of-Flight Secondary Ion Mass Spectrometer for the Analysis of Cometary Dust Particles onboard ROSETTA 1 Introduction 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/dm @ 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. The in situ chemical analysis of solids in space is among the tasks that are technically most difficult. There are two main reasons for that: With a few exceptions, solids in space are not abundant, and, secondly, it is not easy to remove small samples from the solid into the vacuum for the analysis in a mass spectrometer. For COSIMA, the objects of interest are cometary dust particles, which are abundant, indeed, in the neighborhood of the comet nucleus. It remains, however, to collect and bring the particles to the entrance of the spectrometer. Most mass spectrometers need the parts of the sample to be analyzed, to carry an electric charge. The process of removing an ion from the specimen is then the critical feature of the method to be chosen. The only mass spectrometric data on cometary dust particles available to date come from the dust impact mass spectrometers PIA, PUMA, and CIDA on the GIOTTO, VEGA, and Stardust spacecrafts, respectively. (Far more and more detailed mass spectra of IDPs of supposed cometary origin are available from laboratory measurements, and soon there will be results from the cometary dust particles retrieved from Stardust.) While other, remote, or indirect methods allow measurements of collective properties of the cometary dust, the mass spectrometers allowed the analysis of individual particles (cf. Kissel et al. 1986a, b). Since then, we know unambiguously that each particle is an intimate mixture of a mineral component and simple as well as complex organic molecules. Since the impact velocity was large (>60 km/s), mostly atomic ions were formed and analyzed in the Halley case. In a first attempt, however, Kissel and Krueger (1987) found evidence for the chemical nature of the organic cometary material. It is clear that not only a few well known molecules constitute the cometary organics, but rather several chemical classes, with each being represented by a large number of individual substances. Indeed, it seems possible that all stable molecules compatible with the chemical environment are formed. COSIMA therefore needed to be based on a method, which is readily available in laboratory, and which allows for tracing the ion directly to the molecular and structural form in which it was present in the solid. Since the size distribution of the dust particles is known (cf. Mazets et al. 1987, McDonnell et al. 1989), a reasonable ionizing beam focus should be achieved under the limitations of space instrumentation. Even though the method would be destructive, its sensitivity should be high enough to allow several analyses at different depths for one individual ~20 um particle. To satisfy all these requirements, we choose the method of Secondary Ion Mass Spectrometry (SIMS). A fast primary ion, in this case 115In+ at 8 keV, impacts the sample and releases by desorption atoms and molecules of the sample, of which typically 0.1 to 10 % are ionized, the so-called secondary ions. For sensitivity reasons, the analysis of a rather large mass range should be achieved simultaneously, which in turn leads to the type of a time-of-flight mass spectrometer. The mass resolution must be high enough to resolve isobaric ions, at least atomic from molecular ions. The mass range should at least cover 3500 amu. In total, the COSIMA instrument has the following main functional hardware elements: * dust collector and target manipulator, * COSISCOPE, a microscope for target inspection * primary ion source, and * mass spectrometer including the ion extraction optics and the ion detector. Of course, for autonomous operation, the entire instrument is under microprocessor based software control. The overall parameters and resources describing COSIMA are: * Atomic mass range 1 . . . 3500 amu * Rel. atomic mass resolution m/dm (at m = 100 amu) > 2000 at 50% peak width * Indium ion pulse duration about 3 ns * Indium ion energy 8 keV * Ion beam width about 50 um FWHM * Telemetry rate about 500 bit/sec * Mass 19.8 kg * Power consumption from 28 V DC 20.4 W 2 Functional Description COSIMA is a time-of-flight secondary ion mass spectrometer. Like all such instruments, primary ions generate secondary ions from a target, which they hit. Those secondary ions are then accelerated to the same, constant kinetic energy, and their flight time to the ion detector is measured. This flight time follows the equation: (1) time-of-flight = a * (mass/abs(q))0.5 + b; with a - is an instrumental constant depending on the length of the flight path and the electrical voltages applied, for COSIMA it is about 3.1 us/(mass)0.5 b - is a constant for the offset of the time measurement mass - is the ion mass in amu q - is the ion charge state, in most cases = -1 or +1 COSIMA works pretty much like a laboratory instrument, with a somewhat reduced performance due to the limitations for a space instrument, but remotely operated in a 'distant lab'. The limitations come first of all from the mass and power available, but more pronounced from the limited capacity of data transfer to and from the instrument, combined with a reaction time (round trip time delay) of 32 minutes during the main operation time. Consequently the individual steps for the analysis of a dust particle are: 1) exposure of a collection substrate for a predetermined time 2) optical search of the substrate for dust particles by the camera system COSISCOPE 3a) moving a dust particle in front of the spectrometer 3b) either first clean the particle and surrounding by sputtering or 3c) start the particle characterization 6) perform onboard chemistry 7) do another detailed analysis in both, the positive and the negative, ion modes 8) move to the next particle COSIMA has dedicated surfaces for the collection of cometary dust particles. These may impact with their release speed of the order of 100 m/s. Since SIMS is a very sensitive method, the collection surfaces need to be a material, which does not interfere with the type of materials expected in the dust. This excluded any kind of organic material. A study performed identified metal blacks as suitable materials from the structural, and silver, palladium, gold, or platinum from the material properties point of view. The choice has turned out to be very sensible, as these blacks are also efficient pumps for contaminants unavoidable for a space instrument. Since the cometary particles can have a wide range of speeds depending on their size and on the gas activity of the nucleus, each unit of targets has three 1*1 cm2 collection areas (substrates) and a separate 0.3*3 cm2 strip as an unexposed reference area. Always an entire target assembly is exposed at a time. Manipulation of the target units is achieved with the Target Manipulator Unit (TMU), which was designed and added at a late state. It is a semiautonomous system, which provides access to the positions 'COLLECT', 'STORE', 'COSISCOPE', 'CLEAN', 'ANALYZE', and 'CHEMISTRY' upon simple (internal) commands. The first inspection of the sample is done by COSISCOPE, a COSIMA internal camera. One of the 3 collection substrates is checked at a time. Light beams from two sets of LEDs left and right of the sample illuminate it. The images are checked for features like, e.g., shadows and interpreted as sites, the coordinates of which are transferred to the main instrument. The next step is the decision if the target, or a part of it, needs cleaning by ion-sputtering. If so, it is moved in front of a DC ion beam at the 'CLEAN' position, else, it will be presented to the mass spectrometer. The primary ion source (PIBS) has a separate beam for cleaning, providing 10-100 uA on a 100 um diameter spot equivalent to removing some 100 monolayers per second. The next step is to analyze a site, i.e., to take a mass spectrum. The target is put in front of the extraction lens of the spectrometer, with the selected site within 100 um of the lens' center. Operational values are taken from the initial adjustment of the primary ion source. Trains of ion packages of 3 ns width at a rate of 1 kHz release secondary ions into the spectrometer. Its second order focusing design provides an inherent mass resolution of m/dm @ 50 % peak height above 3000 (i.e., with 0-time spread of the primary beam). A typical primary ion package has 500 ions and releases 1 to 10 non-hydrogen secondary ions, depending on the material irradiated. All secondary ions of the appropriate polarity are picked up by the electric field in front of the ion extraction lens. This field of about 1 kV/mm minimizes initial time-of-flight differences due to the acceleration process, which cannot be compensated later. The lens field also guides the ions into the time-of-flight section of the instrument reducing their drift energy to nominally 1 keV and focuses their arrival location on the detector into a 15 mm diameter area. As the ions travel down the drift tube, they pass the secondary deflection plates, which are needed to reposition the secondary ion package, should the analysis site be off center. It is also used to compensate the small changes in ion trajectories between the positive and negative secondary ions modes. At the end of the drift tube the ions enter the ion reflector, the device, which improves the intrinsic mass resolution of the spectrometer part by more than a factor of 100. State-of-the-art would be a gridless reflector ( which we have studied) but which is very sensitive to the position of the ion entry and thereby on the perfect function of the secondary deflection plates. The actual design we have chosen uses several grids separating the drift section, the retarding section, and the reflecting section. As the ions enter under a small angle of 1deg, they exit at a slightly different location but again into the same physical drift tube. At the end of this tube the ions hit the ion detector. The position and the orientation of the detector were carefully chosen, as both are part of the system, which determines the mass resolution of COSIMA. Since we expect some of the organic ions to have high masses (well above 350 amu) the detector can be biased for a post-acceleration of the secondary ions by almost 9 kV (with the appropriate polarity for the respective ion type). The active element is a microsphere plate, which for its thickness of 1.4 mm provides a gain value well above 107. In order to decouple the signal from its high electrical potential (up to 14 kV in the case of negative secondary ions), the microsphere output is directed to an anode, which is one side of a capacitor, while the other side is connected to the input of a pulse amplifier. All arrival times and shot numbers are stored at 2 ns resolution. The differences between the modes for positive or negative secondary ions are reflected in the respective operational voltages. Besides the reversal of polarity, minor adjustments need to be applied to the voltages for the deflection plates of both, the primary beam and the secondary ion path. The safe switch-over from one ion type to the other will take a couple of minutes to allow the high voltages to decay, before the grounding of the power supplies is switched. The secondary ion arrival times are accumulated into the time-bins of a time-of-flight spectrum. 3. Collection Substrates The 24 target holders are numbered with hexadecimal numbers from #C1 to #D8. The top substrate is marked with the number #100, the middle with #200 and the low with #300. The combination of these numbers give the substrate identification number used in the instrument commanding and data handling. The list bellow gives the substrates ID, the surface layer properties and a possible comment #1C1 'Palladium, black' #2C1 'Platinum, deep black' #3C1 'Platinum, deep black' #1C2 'Silver, 73 micrometer thickness, blank with rectangular hole 3.5x3.5mm' #2C2 'Silver, 69 micrometer thickness, blank with AgTe spot of about 3 mm size at center' #3C2 'Gold, 17 micrometer thickness, olivine particles' #1C3 'Gold, 8 micrometer thickness' #2C3 'Gold, 15 micrometer thickness' #3C3 'Gold, 20-30 micrometer thickness' #1C4 'Palladium, black' #2C4 'Silver, 14 micrometer thickness' #3C4 'Gold, 12 micrometer thickness' #1C5 'Platinum, light black,' #2C5 'Platinum, deep black' #3C5 'Gold, 13 micrometer thickness' #1C6 'Platinum, deep black' #2C6 'Platinum, deep black' #3C6 'Gold, 8 micrometer thickness' #1C7 'Silver, blank' #2C7 'Silver, 21 micrometer thickness' #3C7 'Gold, 15 micrometer thickness' #1C8 'Platinum, deep black' #2C8 'Platinum, deep black' #3C8 'Gold, 20-30 micrometer thickness' #1C9 'Gold, 5-8 micrometer thickness' #2C9 'Gold, 5-8 micrometer thickness' #3C9 'Gold, 11 micrometer thickness' #1CA 'Gold, 5-8 micrometer thickness' #2CA 'Gold, 16 micrometer thickness' #3CA 'Silver, 10 micrometer thickness' #1CB 'Gold, 17 micrometer thickness' #2CB 'Gold, 14 micrometer thickness' #3CB 'Gold, 20-30 micrometer thickness' #1CC 'Silver, 21 micrometer thickness' #2CC 'Silver, 21 micrometer thickness' #3CC 'Silver, 24 micrometer thickness' #1CD 'Gold, 5-8 micrometer thickness' #2CD 'Gold, 14 micrometer thickness' #3CD 'Gold, 20-30 micrometer thickness' #1CE 'Gold, 5-8 micrometer thickness, Ag particles' #2CE 'Gold, 11 micrometer thickness' #3CE 'Gold, 20-30 micrometer thickness' #1CF 'Gold, 8 micrometer thickness' #2CF 'Gold, 12 micrometer thickness, Ag particles' #3CF 'Gold, 20-30 micrometer thickness' #1D0 'Gold, 20-30 micrometer thickness' #2D0 'Gold, 20-30 micrometer thickness' #3D0 'Gold, 20-30 micrometer thickness, Ag particles' #1D1 'Silver, blank' #2D1 'Gold, 13 micrometer thickness' #3D1 'Gold, 13 micrometer thickness' #1D2 'Gold, 8 micrometer thickness' #2D2 'Gold, 8 micrometer thickness' #3D2 'Silver, 30 micrometer thickness' #1D3 'Silver, 10 micrometer thickness' #2D3 'Silver, 10 micrometer thickness' #3D3 'Silver, 32 micrometer thickness' #1D4 'Platinum, sintered' #2D4 'Platinum, deep black' #3D4 'Platinum, deep black' #1D5 'Platinum, deep black' #2D5 'Silver, 22 micrometer thickness' #3D5 'Silver, 21 micrometer thickness' #1D6 'Platinum, deep black' #2D6 'Palladium, black' #3D6 'Platinum, deep black' #1D7 'Silver, blank' #2D7 'Platinum, sintered' #3D7 'Platinum, sintered' #1D8 'Silver, blank, square hole 3.5x3.5mm at center' #2D8 'Silver, blank' #3D8 'Gold, 8 micrometer thickness' 4 In-Flight Operation Rosetta commissioning was carried out in 2004, the COSIMA instrument was operated in-flight after nearly a decade of development and tests on the ground. Positive and negative SIMS ion mass spectra were taken of one of the Ag metal targets, the resulting spectra are shown in Fig. 18a and Fig. 18b, respectively. The inorganic and organic ions and molecules show up in separable and well resolved mass peaks, such as Si and C2H4 or Fe and C4H8. Small amounts of organic molecules being known to be present in cometary dust such as CN are adsorbed on the blank metal target and reveal themselves in the SIMS mass spectra. Since COSIMA carries also a heating station for the target, the level of volatile contamination can be further decreased before exposure of the target to the cometary dust. Comparison with mass spectra taken of the same target in 2002 before launch already shows a slight decrease of volatiles such as hydrocarbons absorbed on the target surface after residing only about 6 months in space. This contamination by organic molecules was, in part, released from the insulation material of the electrical harness and from boxes used for ground transportation. Note that the Ag peaks are caused by ions released from the target and the In peak is due to primary ions from the ion source. For high secondary ion rates, the mass spectra show saturation effects due to dead-times inherent in the ion detector counting electronics, best visible adjacent to each of the intense Ag isotope peaks. 5 The Ground Laboratory Program During the cruise phase to the comet as well as while the probe is actually analyzing cometary dust, an active laboratory program will be maintained. While the SIMS method is routinely applied for relevant geological and planetological studies, the TOF-SIMS technique will benefit from this terrestrial work with the COSIMA RM reference model. Additionally, growing experimental and methodological expertise comes from the day-to-day work with relevant samples at the TOF-SIMS laboratory instrument currently in use at the University of Munster. The complex mixture of organic and inorganic material, expected to make up the comet, will result in equally complex mass spectra. They have to be disentangled to provide information on the chemical, isotopic, and molecular composition of the particles that reflect the origin, history, and present state of cometary matter. The particle-to-particle variation will testify to its diversity, i.e., to the extent of the pristine nature of these materials, and to its stellar sources. Interpreting the mass spectra requires calibration. Calibration here is not meant to mimic the experiment in a one-to-one scale in the laboratory, but rather to understand the processes, which control ion formation with complex samples, as well as the instrumental parameters of mass separation and ion detection. To that end, state-of-the-art laboratory TOF-SIMS analyses with utmost lateral and mass resolution of various materials available on Earth will be performed, which include, but are not limited to: Interplanetary dust particles (IDPs): Some of these small and difficult-to-analyze particles actually may stem from comets. It is an active research topic to sort these out and distinguish them from asteroidal IDPs. IDPs of a certain class are mixtures of inorganic and organic matter and as such represent the best analog material for the COSIMA case. Carbonaceous chondrites: They are mechanical mixtures of fine-grained low-temperature matrix and carbon-rich phases (kerogen) with high- temperature clasts, single minerals, and chondrules. Their overall chemical composition is solar as far as the condensable elements are concerned. They most closely represent and witness the Early Solar System materials and processes. They are regarded as the base line material in planetology. Certain IDPs and chondrites contain particles with non-standard isotopic composition of a number of elements that point towards their nucleosynthetic stellar sources. These particles (stardust or presolar particles) will serve to learn how to obtain the relevant information from cometary matter. The extraterrestrial materials mentioned above are not as pristine as we expect cometary dust to be. IDPs are altered to an unknown extent during passage through the atmosphere, deceleration processes and residence in the stratosphere; carbonaceous chondrites were subject to metamorphism and aqueous alteration on their parent bodies. Consequently, for the analysis of organic (and icy) components anticipated in cometary particles artificial analogs are needed. They are siliceous vaporization deposits contaminated with organic substances. Certainly not all possible organic substances can be analyzed, but rather we will perform TOF-SIMS measurements of relevant substance classes. As mentioned above, these analyses are performed with the highest possible mass and (where appropriate) lateral resolution. In addition, the temporal evolution of the mass spectra is evaluated by using the data from all primary ion pulses. This will give hints to a possible layering within the material and / or validate influences, which the TOF-SIMS method may cause in the material (by, e.g., 'radiation damage'). New statistical and chemometric methods and procedures, now under test, will be further developed and applied. The laboratory program will benefit from and be adjusted to new insights, which will come from the analysis of the samples returned by Stardust. 6 Anticipated Results 6.1 The Mineral Phase The anticipated results of the investigation of the mineral phase of the cometary dust are manifold: 1. First, we shall establish the mean composition of the main elements, and then check whether this varies from grain to grain, either within the statistical variance or beyond that. In the latter case we'll be able to identify various classes of minerals. 2. Second, the distribution of molecular ions should be sensitive to the order structure of the minerals, may they be in an amorphous or in one of several crystalline states. This would give an important clue to the thermal and radiation history of the dust. 3. Third, once an elemental and molecular distribution of the main isotopic contribution is established, the contribution of the minor isotopes can be measured from grain to grain, needed for the question whether various stellar sources have played a role to form the dust. 4. Fourth (last but not least), a cross-correlation of the types of organics sitting upon the same grain will give insight into aspects of the real grain forming process as well as to possible catalytic interferences between each phase. This is important for the 'Origin of Life' topic and its relation to nano-systems. 6.2 The Organic Phase The refractory organic phase will most probably be an intimate mixture of a lot of individual species. But still, the history of grain formation and the incorporation of them into cometary nuclei is reflected - at least partially - in the distribution of the organic species. Substance class analysis will provide clues to whether the organic phase is mainly produced by condensation of vapors and subsequent radiation processing with polycondensation and oligomerization, etc., or, if it is mainly produced by condensation of larger molecules and aggregation of small refractory particles. Another important question is, whether the organic material in comets is suitable to start the onset of life on earth. In this case, the analysis will show that the precursors of such molecules needed for life's self-organization by exenthalpic reactions with liquid water are present. As water usually shows up in different ways in the mass-spectra, clues to the source of water in the comet may be found, too: is it just a condensation of water ice in the grains, or, is it the result of long-term radiation induced processes, such as polycondensation or Born-Haber-type processes, or is perhaps a fraction of the water in a comet produced by thermally induced reactions, when the comet approaches the sun? COSIMA results thus address the main questions concerning the origin of the hydrosphere, and further on, the biosphere on earth. 7 References Kissel, J. et al., COSIMA - High resolution time-of-flight secondary ion mass spectrometer for the analysis of cometary dust particles onboard Rosetta, Space Sci. Rev., 128(1-4), 823-867, doi:10.1007/s11214-006-9083-0, 2007. This document is available in the DOCUMENT directory. ----------------------- " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KISSELETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END