Organic synthesis associated with serpentinization and carbonation on early Mars

Abiotic formation of organic molecules Mars rovers have found complex organic molecules in the ancient rocks exposed on the planet’s surface and methane in the modern atmosphere. It is unclear what processes produced these organics, with proposals including both biotic and abiotic sources. Steele et al . analyzed the nanoscale mineralogy of the Mars meteorite ALH 84001 and found evidence of organic synthesis driven by serpentinization and carbonation reactions that occurred during the aqueous alteration of basalt rock by hydrothermal fluids. The results demonstrate that abiotic production of organic molecules operated on Mars 4 billion years ago. —KTS

Magnetization of carbonaceous asteroids by nebular fields and the origin of CM chondrites

The solar nebula carried a strong magnetic field that had a stable intensity and direction for periods of a thousand years or more1. The solar nebular field may have produced post-accretional magnetization in at least two groups of meteorites, CM and CV chondrites [1–3], which originated from planetesimals that may have underwent aqueous alteration before gas in the solar nebula dissipated [1,3]. Magnetic minerals produced during aqueous alteration, such as magnetite and pyrrhotite [4], could acquire a chemical remanent magnetization from that nebular field [3]. However, many questions about the size, composition, formation time, and, ultimately, identity of the parent bodies that produced magnetized CM and CV chondrites await answers—including whether a parent body might exhibit a detectable magnetic field today. Here, we use thermal evolution models to show that planetesimals that formed between a few Myr after CAIs and ~1 Myr before the nebular gas dissipated could acquire from the nebular field, and retain until today, a chemical remanent magnetization throughout nearly their entire volume. Hence, in-situ magnetometer measurements of C-type asteroids could help link magnetized asteroids to magnetized meteorites. Specifically, a future mission could search for a magnetic field as part of testing the hypothesis that 2 Pallas is the parent body of the CM chondrites [5]. Overall, large carbonaceous asteroids might record ancient magnetic fields in magnetic remanence that produces strong modern magnetic fields, even without a metallic core that once hosted a dynamo.

Hydrothermal Activities on C-Complex Asteroids Induced by Radioactivity

C-complex asteroids, rich in carbonaceous materials, are potential sources of Earth’s volatile inventories. They are spectrally dark resembling primitive carbonaceous meteorites, and thus, C-complex asteroids are thought to be potential parent bodies of carbonaceous meteorites. However, the substantial number of C-complex asteroids exhibits surface spectra with weaker hydroxyl absorption than water-rich carbonaceous meteorites. Rather, they best correspond to meteorites showing evidence for dehydration, commonly attributed to impact heating. Here, we report an old radiometric age of 4564.7 million years ago for Ca carbonates from the Jbilet Winselwan meteorite analogous to dehydrated C-complex asteroids. The carbonates are enclosed by a high-temperature polymorph of Ca sulfates, suggesting thermal metamorphism at >300°C subsequently after aqueous alteration. This old age indicates the early onset of aqueous alteration and subsequent thermal metamorphism driven by the decay of short-lived radionuclides rather than impact heating. The breakup of original asteroids internally heated by radioactivity should result in asteroid families predominantly consisting of thermally metamorphosed materials. This explains the common occurrence of dehydrated C-complex asteroids.

Constraints on the ice composition of carbonaceous chondrites from their magnetic mineralogy

Carbonaceous chondrites experienced varying degrees of aqueous alteration on their parent asteroids, which influenced their mineralogies, textures, and bulk chemical and isotopic compositions. Although this alteration was a crucial event in the history of these meteorites, their various alteration pathways are not well understood. One phase that formed during this alteration was magnetite, and its morphology and abundance vary between and within chondrite groups, providing a means of investigating chondrite aqueous alteration. We measured bulk magnetic properties and first-order reversal curve (FORC) diagrams of CM, CI, CO, and ungrouped C2 chondrites to identify the morphology and size range of magnetite present in these meteorites. We identify two predominant pathways of aqueous alteration among these meteorites that can be distinguished by the resultant morphology of magnetite. In WIS 91600, Tagish Lake, and CI chondrites, magnetite forms predominantly from Fe-sulfides as framboids and stacked plaquettes. In CM and CO chondrites, <0.1 μm single-domain (SD) magnetite and 0.1–5 μm vortex (V) state magnetite formed predominantly via the direct replacement of metal and Fe-sulfides. After ruling out differences in temperature, water:rock ratios, terrestrial weathering effects, and starting mineralogy, we hypothesise that the primary factor controlling the pathway of aqueous alteration was the composition of the ice accreted into each chondrite group’s parent body. Nebula condensation sequences predict that the most feasible method of appreciably evolving ice concentrations was the condensation of ammonia, which will have formed a more alkaline hydrous fluid upon melting, leading to fundamentally different conditions that may have caused the formation of different magnetite morphologies. As such, we suggest that WIS 91600, Tagish Lake, and the CI chondrites accreted past the ammonia ice line, supporting a more distal or younger accretion of their parent asteroids.

Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.