Silica-Normative Mineralogy and Abundant Oxygen-Rich Organic Matter in Dust of Comets 1P and 67P

51st Lunar and Planetary Science Conference (2020) 1260.pdf

SILICA-NORMATIVE MINERALOGY AND ABUNDANT OXYGEN-RICH ORGANIC MATTER IN DUST OF COMETS 1P AND 67P. M. Yu. Zolotov, School of Earth and Space Exploration, Arizona State Univer- sity, Tempe, Arizona 85287-1404, e-mail: [email protected].

Introduction: Cometary dust [1] has been sampled Fig. 1. Mass fractions of refractory organic matter by mass spectrometers onboard Giotto, Vega 1 (comet (IOM) in cometary dust constrained by the C/N/Si 1P/Halley [2-4]) and Rosetta (comet 67P/Churyumov- ratios in comets 1P and 67P. The curves correspond Gerasimenko [5-7]). Concentrations of rock-forming el- to mixtures of C100H80O20N3.5S3 with H-free re- ements (Si, O, Mg, Al, Na, etc.) in the dust of comet 1P duced mineralogy of the comets and CI-type chon- and especially comet 67P reveal significant deviations drites, and with dry oxidized CI-type compositions. from the composition of CI carbonaceous chondrites [2,5,6]. There are no signs for oxidized (e.g. Fe silicates 70 67P and/or oxides) and hydrated minerals in 1P and 67P dust 60 [2,5]. Compositions of 1P and 67P dust indicate above 50 1P chondritic abundances of H, C, N and O that are at- CI 40 CI-oxid tributed to high molecular weight organic compounds CI chondrites 30 similar to chondritic insoluble organic matter (IOM 20 [8,9]) [2-7]. In 67P dust, the organic fraction may com- 67P 10 Sun pose ~45 wt.% [5]. This report presents further evalua- 1P tions of mass fractions and phase compositions of inor- 0 0 1 2 3 4 5 6 7 8 9 10 ganic (mineral) and organic constituents of the dust. C/Si atomic ratio Models: After [2,3,5], we model cometary dust as a 70 67P mixture of minerals and organic matter similar to IOM. 60 1P H, C and N are ascribed to the organic matter with the 50 CI CI-oxid composition of C100H80O20-60N3.5S3 in which the H/C ra- 40

CI chondrites tio corresponds to H-rich chondritic IOM [8,9] and 1P 30

data [2,3], the N/C ratio reflects IOM [8,9], 1P [3], 67P wt% mixture, mineral-IOM in fraction IOM

IOM fractionIOM in mineral-IOM mixture, wt% 20 [5,6] and IDP data, and the C/S ratio agrees with IOM 67P [8] and 1P data [2,3]. In our nominal organic composi- 10 1P tion, the O/C ratio of 20 at.% reflects 1P data [3] and a 0 0.0 0.1 0.2 0.3 0.4 typical O-rich IOM [8]. Variable elevated O/C ratios are N/Si atomic ratio used to account for the high O content in the dust. In the mineral part, the assessed O content corresponds to sup- (Table 1). All measured S is attributed to sulfides. Be- posed silicates and oxides of rock-forming elements cause some S is in the organic fraction, the listed sulfide contents are upper limits. In addition to mineral parts of Table 1. The normative mineral composition of 1P and 67P, O-poor (reduced) and O-rich (oxidized, Fe 2+ cometary dust, moles a. is in Fe ) H-free CI-type compositions [10] are consid- ered for comparison and to represent anhydrous dust Mineral/solid phase 1P 67P

Silica, SiO2 1.47 2.78 Enstatite, MgSiO3 1.66 0.381 Table 2. The abundance of refractory organic mat- Diopside, MgCaSi2O6 - 0.019 ter (IOM, C100H80O20-30N3.5S3) in cometary dust Albite, NaAlSi3O8 0.127 0.0536 constrained by compositions of 1P and 67P dust. K feldspar, KAlSi3O8 0.00384 0.00740 Ratios in the dust IOM fraction in mineral- Na silicate, Na2SiO3 0.0327 0.113 [2,5,6] IOM mixtures, wt.% Chromite, FeCr O 0.00865 0.00465 2 4 1P (H-free reduced minerals) – IOM mixture Fe0 metal - 0.511 C/Si in 1P 41 (34 – 47) Ni0 metal 0.0788 (0.0557)b N/Si in 1P 50 (38 – 59) Troilite, FeS <0.991 (<0.484)b Alabandite, MnS <0.00962 (<0.0150)b 67P (H-free reduced minerals) – IOM mixture Mg sulfide, MgS <0.262 - C/N/Si in 67P 52 (42 – 60) Ca sulfide, CaS <0.121 - CI-chondrite (H-free reduced) – IOM mixture a Based on atomic compositions [2,5,6] normalized C/Si as in 1P 31 (25 – 37) to Fe abundance. b Estimated from S/Fe and Ni/Fe N/Si as in 1P 39 (28 – 48) ratios in CI carbonaceous chondrites [10]. C/N/Si as in 67P 36 (27 – 43)

51st Lunar and Planetary Science Conference (2020) 1260.pdf

Fig. 2. The mass fraction and composition of refrac- with CI abundances. A comparison of calculated compositions of mineral-organic mixtures with C/N/Si ra- tory organic matter (IOM, C100H80OxN3.5S3) in mineral-IOM mixtures constrained by the elemental com- tios constrains mass fractions of organic and mineral position of cometary dust. The dotted curves and ar- parts. O/Si ratios assess O contents in the organic part. rows show uncertainties. A better match of C/Si, N/Si Results: Abundant Si in 1P [2] and 67P [5] dust sug- and O/Si ratios occurs at O-rich IOM compositions gests silica-normative compositions (Table 1) that agree (Table 3). with elevated O contents in the dust. Elevated Na/Al ratios imply a presence of Al-free Na silicate(s). In 1P 0 70 dust, the high S/Fe ratio suggests no Fe metal and abun- Comet 1P dust dant sulfides of Mg and Ca. In 67P, below CI chondritic contents of refractory Ca, Al, Mg and Cr correspond to 60 low abundances of corresponding phases relatively to O/Si N/Si silica that composes 53 wt.% of the mineral fraction. At C/O of 20–30 at.% in the organic phase, C/N/Si 50 ratios correspond to 34–59 wt.% and 42–60 wt.% of organic matter in 1P and 67P dust, respectively (Fig. 1, C/Si 40 Table 2). A consistency with O/Si ratios requires similar mass fractions of the organic matter but elevated O/C ratios in the organic phase (Fig. 2, Table 3) that are re- 30 20 30 40 50 60 ported for some anhydrous IPDs [11] and chondritic 75 IOM [8]. For CI-IOM mixtures, especially for oxidized O/C in IOM, Cometatomic %67P dust 70 O/Si mineral compositions, cometary C/N/O/Si ratios [2,5,6] suggest lesser organic fractions than in 1P and 67P dust. 65 Conclusion: Elemental compositions of dust of comets 1P and 67P suggest the mass fraction of refrac- 60

IOM fraction in 67P dust, wt% 67Pin dust, fraction IOM tory organic matter of 34–64 %. Compared to usual chondritic IOM [8,9], the refractory organic matter 55 N/Si C/Si could be enriched in H [2,3,5] and O (O/C > ~30 at.%),

IOM fraction in mineral-IOM mixture, wt% mixture, fractionIOM mineral-IOM in 50 consistent with its primitively. The elemental contents imply an abundant silica phase, Na silicate(s) and low 45 contents of Fe-bearing minerals relatively to Si-bearing 20 30 40 50 60 phases. These inferences could constrain the composi- O/C ratio in IOM, atomic % tion and physical properties (grain density, viscosity, thermal conductivity, etc.) of non-icy Table 3. The abundance and composition of refractory organic mat- constituents of TNOs, some icy moons, ter (IOM, C100H80OxN3.5S3) in cometary dust constrained by ele- Ceres and Ceres-like asteroids. mental compositions of comets 1P, 67P and CI chondrites. References: [1] Levasseur-Regourd, Atomic ratios in the IOM fraction in mineral- O/C ratio in A-C. et al. (2018) Space Sci. Rev., 214:64. dust [2,5,6] IOM mixtures, wt.% IOM, at.% [2] Jessberger E. K. et al. (1988) Nature, 1P (H-free reduced minerals) – IOM mixture 332, 691–695. [3] Kissel J. and Krueger C/Si/O in 1P 46 (38 – 52) > 28 F. R. (1987) Nature, 326, 755–760. [4] N/Si/O in 1P 52 (42 – 60) – Fomenkova N. M. et al. (1994) Geochim. C/N/Si/O in 1P 45 – 52 28 – 55 Cosmochim. Acta, 58, 4508–4512. [5] Bardyn A. et al. (2017) MNRAS, 469, 67P (H-free reduced minerals) – IOM mixture S712–S722 [6] Rubin M. et al. (2019) C/N/Si/O in 67P 59 (53 – 64) > 39 MNRAS, 489, 594–607. [7] Fray N. et al. CI-chondrite (H-free reduced minerals) – IOM mixture (2016) Nature, 538, 72–74. [8] Alexander C/Si/O as in 1P 33 (25 – 39) > 14 C.M.O’D. et al. (2007) Geochim. Cosmo- N/Si/O as in 1P 39 (28 – 47) < 55 chim. Acta, 71, 4380–4403. [9] Alexander C/N/Si/O as in 1P 28 – 39 14 – 55 C.M.O’D. et al. (2018) Chemie der Erde, C/N/Si/O as in 67P 39 (33 – 44) 38 (24 – 65) 77, 227–256. [10] Palme H. et al. (2014) CI-chondrite (H-free oxidized minerals) – IOM mixture Treatise on Geochemistry, 2nd Ed., Vol. C/Si/O as in 1P 31 (< 37) 24 (< 52) 2, 15–36. [11] Flynn G. et al. (2008) Proc. N/Si/O as in 1P 37 (< 46) < 44 IAU Symp., 251, 267–276. C/N/Si/O as in 1P 28 – 37 < 44 This work is supported by NASA C/N/Si/O as in 67P 37 (30 – 42) 29 (16 – 48) SSW program grant 80NSSC19K0786.