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The Cometary and Interstellar Dust Analyzer at Comet 81P/Wild 2

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Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1774-1776
DOI: 10.1126/science.1098836

Abstract

The CIDA (Cometary and Interstellar Dust Analyzer) instrument on the Stardust spacecraft is a time-of-flight mass spectrometer used to analyze ions formed when fast dust particles strike the instrument's target. In the spectra of 45 presumably interstellar particles, quinone derivates were identified as constituents in the organic component. The 29 spectra obtained during the flyby of Comet 81P/Wild 2 confirm the predominance of organic matter. In moving from interstellar to cometary dust, the organic material seems to lose most of its hydrogen and oxygen as water and carbon monoxide. These are now present in the comet as gas phases, whereas the dust is rich in nitrogen-containing species. No traces of amino acids were found. We detected sulfur ions in one spectrum, which suggests that sulfur species are important in cometary organics.

The CIDA (Cometary and Interstellar Dust Analyzer) instrument on the Stardust spacecraft builds on the tradition and experience gained from the PUMA 1 and 2 instruments on the Vega 1 and 2 spacecrafts and from PIA on Giotto (1, 2). CIDA is a time-of-flight mass spectrometer based on the fact that dust grains striking a silver target at high enough velocities give rise to the emission of atomic and molecular ions. These ions can be detected, and their flight time, from the moment of impact until detection, is a direct measure of their mass. The relation between ion mass and time of flight is t = a × sqrt(m) + b, where t is the measured flight time of the ion of mass m, and a is an instrumental parameter, which varies by about 1% as a function of the dust impact location, with the actual values of a growing from the lower target (away from the spacecraft) to the upper end. It is constant within one spectrum. b is the time offset of the data set. The dust grain decomposition depends on the impact speed (3). At the relatively low speed of 6.1 km/s, complex molecular ions can be formed.

The CIDA instrument (4) records the signal from the ion detector and of the charge released from (or introduced to) the target using a transient recorder. Samples are taken every 25 ns. A dust grain impact causes a signal at the ion detector and at the target. A step-like signal from the target occurs at the time of impact. The signals are compressed quasilogarithmically and fed to 8-bit analog–to-digital converters. Calibration signals are injected into the front ends of the amplifiers at the detector after every event to provide calibration of the actual signal levels.

The instrument target is a 100-μm-thick silver foil, divided into a 32-mm-diameter inner and a 130-mm-diameter outer concentric part. The purpose of this design is to handle large dust fluxes. If more than 200 counts/s occur, only the inner target section is used to analyze the impacts, until the rate drops below 4 counts/s. Each event causes a dead time of 9 ms, after which new impacts can be detected with 500 ns of dead time. During the flyby of Comet 81P/Wild 2, in no case was more than one event detected within the instrument's 1-s readout intervals. The instrument setup was alternated every 2 min between the positive- and the negative-ion mode, with a forced 5-min period of positiveion mode around the nominal time of closest approach, which is the reason why only two spectra were taken in the negative-ion mode. A total of 29 complete events were recorded and they produced 29 spectra. This number corresponds to the event rate counter, showing that no extra, unseen events had occurred.

As recent laboratory impact mass spectra from organics (5, 6) and the CIDA cruise phase spectra (7) have shown, the negative-ion mass spectra carry much more chemical information than the positive-ion spectra. Although positive ions can carry high internal energy and hence decay during their acceleration and drift, negative ions that survive are not internally excited and show sharp mass lines.

The only mass spectrometric analysis of cometary material done before this was done at Comet 1P/Halley at a relative speed of >68 km/s and showed that the comet's total content of organic material was higher than the mineral parts by a factor of 3 to 10. Although at high speeds, impact ions come from the entire dust particle, at lower speeds (such as here) the ions seem to come from the uppermost few hundred monolayers (3) These surface layers should contain organic phases, which is consistent with modeling (8) and with the CIDA spectra being dominated by organic phases. Although the nucleus of the comet is thought to be made up of ices (such as water and carbon monoxide) and solid grains with mineral kernels coated with volatile and refractory organic molecules, most of the ice and volatile fraction was lost in the 20 to 60 min it took for a dust particle to reach CIDA. Organic components can be identified by characteristic mass lines (7), but ions from the mineral phase are usually identified by their isotopic patterns. Only one mass spectrum recorded at Wild 2 (Fig. 1) shows lines that could be interpreted as atomic ions associated with minerals. Taking our results at Halley as a guideline, we could not achieve a satisfactory match; this spectrum matches better with a homologous series of organic ions consistent with the other ion types within that spectrum.

Fig. 1.

A positive-ion spectrum, converted from time of flight into a linear mass scale. The amplitude scale is logarithmic. The spectrum is typical for nitrogen organic chemistry. The m/z = 107, 109 doublet is due to the Ag+ from the target.

For the two negative-ion mass spectra we measured from Wild 2, the dominating ion is CN (except for the trivial ones H and e, which stem from the Ag target surface), which points to a nitrogen-rich chemistry. Although negative ionization is sensitive to oxygen and sulfur, the oxygen content (judging by the abundance of O and OH) is quite low. Thus, nitriles and polymerization products of hydrocyanic acid are probable components of the cometary dust that struck and ionized on our targets. The presence of water ice in the striking grains can be excluded, because it should produce ions of the type OH*(H2O)x and H3O+*(H2O)x, with x = 0,1,2,.., and these ions are not seen in any of our spectra. In addition, the presence of polyoxymethylene (POM), which easily forms negative ions because of its electronegativity, can be excluded at a detection level of >3%. Moreover, we can exclude the presence of free amino acids, because at least the quasimolecular anions of glycine and alanine would be detectable by CIDA, and they are not seen in our spectra. Free amino acids are also absent from the positive-ion mass spectra, where they would also be expected to produce characteristic patterns. Aminonitriles have also not been identified in our spectra but would fall within the type of chemistry reported here. These and other oxygen-poor precursor polymers hydrolyze to amino acids in acidic liquid water (9). Because no amino acids were detected, it is possible that the dust grains never came into contact with liquid water.

Another peculiarity is the presence of SH [mass-to-charge ratio (m/z) = 33 and 35]in one negative spectrum (Fig. 2). SH was not observed in interstellar dust during the Stardust cruise phase (7) (Fig. 3). This may be a hint that there is also some sulfur organic chemistry in the cometary grains. Sulfur atoms may have been transferred from the mineral into the organic phase by radiation processes during the several billion years that the dust spent in interstellar space and later in the comet's nucleus. We did not detect S, which would be expected from sulfide mineral phases such as troilite or pyrite, which we saw as positive ions in Halley (10).

Fig. 2.

A negative-ion spectrum, converted like Fig. 1. The dominating ion is CN, as is typical for nitrogen organic chemistry. The oxygen region (m/z = 16, 17) is surprisingly low; however, the SH (m/z = 33, 35) is high. The shift of the H line is a known instrumental effect.

Fig. 3.

Three negative-ion spectra, converted like Fig. 2, are shown for comparison with Fig. 2. The spectra are from three interstellar dust particles. The carbon-oxygen region is dominant, and the major peak in the mass range from 20 to 30 daltons is clearly 25, not 26.

We found three different types of positiveion mass spectra. One type has three spectra that show lines starting with the CH+ ion (m/z = 13+); traces of N+,NH+,O+,OH+; and mainly lines of unsaturated organic species, which also contain some nitrogen (Fig. 1). The primary nitrogen species are N-heterocyclic fragment ions similar to the nitrogen species seen in Halley (11). However, N is electropositive, and thus those species containing N are favored in the production of positive ions. The presence of complex O-containing species as postulated by (11) cannot be confirmed. One spectrum (Fig. 1) shows lines where Cr, Fe, and Ni (m/z = 52, 54, 56, and 58) would be; however, it can also be interpreted as C3NH +x (x = 2, 4, 6, 8) because of other homologous species in this spectrum. Moreover, the presence of Fe without at least some Mg, Si, S, or Ca seems very unlikely. The intensities at m/z = 24+, 25+, 26+, 32+, 34+, 40+, 44+, and 48+ do not match any reasonably assumed isotopic ratio, so they more likely to be part of the organic fraction. The second set of positive-ion spectra start with Na+ (m/z = 23+) and contain K+ (m/z = 39+, 41+), the two known contaminants of the Ag target. In the spectra from Wild 2, C6NH +4 (m/z = 90+) is the most prominent ion, and it forms as a contribution of N-containing alicyclics. With N-free and O-free alicyclic polymers, as tested with polystyrene (5), with an impact velocity of 6 km/s, only the tropylium ion C7H +7 (m/z = 91+) was detected as a prominent organic line in laboratory spectra as an expected nonradical ion by rearrangement of the 7C ring structure due to Debye-Hückel stability (12).

If we started with the methylpyridine rearrangement, the 6C-1N ring (C6NH +6) would be unstable, because the localized charge at N will cause H 2 loss during its formation. This second set of positive ions shows an abundance of heterocyclic, probably annealed, rings as backbones of most of the organic structure. The analysis of the third set of spectra is ongoing. They are similar to the second set but have a higher background because of the decay of excited ions.

There is a difference in the number and flux of impacts seen by the unshielded DFMI [Dust Flux Monitor Instrument (13)] as compared to CIDA. However, during the flyby of Wild 2, the ratio of the number of CIDA events to that of DFMI events remained nearly constant. CIDA is sensitive to particles >0.1 pg. The difference in the number of measured impacts between the two instruments may be due to the sensitivity of DFMI (>1 pg), the dust shield geometry, and the position of the instrument on the spacecraft, which is favorable for DFMI. Because several scenarios may explain the difference, this issue needs further investigation.

The CIDA mass spectra at Wild 2 are similar to the spectra obtained from Halley, indicating that the two comets have similar chemistry even though they are of different ages, were sampled in different regions of the solar system, and were sampled during different levels of cometary activity. Our spectra from the flyby of Wild 2 at 1.86 astronomical units (AU) from the Sun suggest that all of the water and most of the carbon monoxide were no longer in the dust particles, because we did not see their characteristic mass lines. At Halley, spectra dominated by C+ and O+ (atomic ions at the very high impact speed) were found within 5000 km from the nucleus, coinciding with the distance range needed to explain the extended gas source for CO (14). Nitrogen chemistry was predominant, whereas C-, N-, and O-containing moieties, such as amino acids, are unlikely to be present, based on our spectra. Sulfur, commonly attributed to mineral phases of cometary dust, may also be an important component of the organic phases, because we see sulfur ions in one of our spectra of negative ions, and this spectrum is associated with organic phases.

References and Notes

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