Mineralogical composition and geological features of c-type asteroids inferred from meteorites and space missions data

We investigate the composition of C-type asteroids and comets providing insights with the combined petrographic and spectroscopic study of micrometeorites (MMs) (TAM5.29, TAM5.30, TAM18c.11 and TAM18c.13) and laboratory experiments on carbonaceous chondrites (CCs): FRO95002, FRO99040 and FRO90006 (CO3 CCs), MCY14001 (CM2) and DaG521 (CV3). The composition of C-type asteroids is similar to that of CCs. The IR reflectance spectra of C-type asteroids show some unresolved features, in particular the 3 µm band is difficult to attribute to a specific compound and the origin of the organic matter indicated by the 3.4-3.5 µm bands is still debated. The main processes suggested to create these features are low-T aqueous alteration, hydrothermalism and cryovolcanism. TAM5.29 is dominated by a matrix of Fe-olivines and clasts of andradite surrounded by diopside-jarosite derived from hydrothermal alteration at T~250°C. A second episode of alteration at T<100°C resulted in the formation of iddingsite. TAM5.29 mineralogy is unique among the MMs and is also different from the known CV CCs representing a newly described alteration environment on the CV parent body. TAM18c.11 and TAM18c.13 have a high Antarctic alteration with widespread replacement with jarosite. Phyllosilicates are recognisable along with small metal alloys and CH, SH and NH functional groups are ubiquitous. TAM5.30 is a CO-like MM and records a processing history characterised by an initial episode of low-temperature CM-like aqueous alteration, which later transitioned into higher temperature (~300°C) CV-like metasomatic alteration. The 3.15 µm band of TAM5.29 is found scattered around the matrix, thus related to Fe-rich hydrous phases. An isolated spot gave spectra characterised by a 3.04 µm band that always appears together with the 3.4 µm and 3.5 µm bands of the aliphatic organics, indicating a relation between the 3.04 µm band and the organic matter. In TAM18c.11 the 3 µm is found at 3.05-3.06 µm in close relationship with ammonium-jarosite. Spectra of MCY14001 after the experiment exhibit two bands at 2.9 and 3.05 µm, not present in the original spectra. The 3.05 µm band is due to ammoniation of serpentine. Original powders of MCY14001 also had Na-carbonates, which after the experiment were replaced by ammoniated analcime-natrolite. In addition Fe-enstatite, diopside, magnetite and sulfides are also formed during the hydrothermal alteration. Cryovolcanism experiments gave insights on the hydrocarbon formation. The ejected outgassed powders FRO99040, FRO90006, FRO95002 and DaG521 show absorption bands at 3.4 µm and 3.5 µm while in the original unprocessed powders this feature is not found. Gas-chromatography analyses show that these features are given by aliphatic hydrocarbons. The 3.15 µm band in TAM 5.29 is given by Fe-OH bonds, proving that the Europa-like asteroids are Fe-rich hydrous worlds involved by hydrothermal alteration. The 3.07 µm band in TAM18c.11 is related to organic matter, in particular CH and NH in aromatic or aliphatic hydrocarbons, in phyllosilicates structure, sulphates and salts. Hydrothermal experiments on MCY14001 confirm that the 3.05 µm band is related to NH and indicates that Ceres-like asteroids with the 3.05 ± 0.01 µm band are enriched in organic matter. Aliphatic hydrocarbons are created through sublimation-outbursting at T≤-40°C. This is also evidence that organics can be created by Cometary activity itself. TAM5.29 and TAM5.30 give new perspectives in the hydrothermal alteration of CCs parent bodies. They are products of unique accretion windows, occurring on a more heterogeneous CVs parent body in the case of TAM5.29 and in a mixed environment in-between the CO and CM chondrites parent bodies in case of TAM5.30.