Meteoroid ablation spheres from deep-sea sediments

— Depending on their velocity, entry angle and mass, micrometeorites suffer different degrees of heating during their deceleration in the Earth's atmosphere, leading, in most cases, to significant textural, mineralogical and chemical modifications One of these modifications is the formation of a magnetite shell around most micrometeorites, which until now could not be reproduced, neither theoretically nor experimentally The present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell Fragments of the Murchison and Orgueil meteorites were used as analogue material in flash-heating experiments performed in a high-temperature furnace; effects of temperature, heating duration, and oxygen fugacity were investigated



These experiments were able to reproduce most of the micrometeorites textures, from the vesicular fine-grained micrometeorites to the totally melted cosmic spherules For the first time, the formation of a magnetite shell could be observed on micrometeorite analogues We suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite Furthermore, with this study, it is possible to estimate the atmospheric entry conditions of micrometeorites, such as the peak temperature and the duration of flash-heating

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2001.tb01831.x

Experimental simulation of atmospheric entry of micrometeorites

A previously unrecognized property of iron cosmic spheres is reported. The most common spheres larger than 300 microns do not, in fact, contain FeNi metal cores, but instead contain a micrometer-sized nugget composed almost entirely of platinum group elements. These elements appear to have been concentrated by the oxidation of molten meteoritic metal during atmospheric entry. This process is critically dependent on the relative abundance of oxygen in the atmosphere, and the first appearance of the nuggets in the geological record may provide a marker indicating when the oxygen abundance attained half of its present level.

Extraterrestrial platinum group nuggets in deep-sea sediments

Silicate spherules produced by atmospheric melting of meteoric bodies are probably the most common form of extraterrestrial material on the earth. It has never been possible to positively identify such particles although it has been known for more than a century that silicate spherules of suspected extraterrestrial origin are present in deep-sea sediments. One such spherule has been identified as definitely extraterrestrial since its abundances of nonvolatile trace elements closely match those of primitive solar system material.

https://science.sciencemag.org/content/sci/201/4361/1119.full.pdf

Silicate spherules from deep-sea sediments: confirmation of extraterrestrial origin.

Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica - Structure, mineralogy, and chemical composition

Artificial ablation studies were performed on iron and nickel-iron samples using an arc-heated plasma of ionized air. Experiment conditions simulated a meteoroid traveling about 12 km/sec at an altitude of 70 km. The artificially produced fusion crusts and ablation debris show features very similar to natural fusion crusts of the iron meteorites Boguslavka, Norfork, and N'Kandhla and to magnetic spherules recovered from Mn nodules. X-ray diffraction, electron microprobe, optical, and scanning electron microscope analyses reveal that important mineralogical, elemental, and textural changes occur during ablation. Some metal is melted and ablated. The outer margin of the melted rind is oxidized and recrystallizes as a discontinuous crust of magnetite and wustite. Adjacent to the oxidized metallic ablation zone is an unoxidized metallic ablation zone in which structures such as Widmannstatten bands are obliterated as the metal is transformed to unequilibrated alpha 2 nickel-iron. Volatile elements are vaporized and less volatile elements undergo fractionation.

Analysis of ablation debris from natural and artificial iron meteorites


 Multiple particle ejection and redeposition events on the surface of asteroid 101955 Bennu were observed frequently by NASA’s OSIRIS-REx mission and advanced our understanding of active asteroids. This finding motivated us to search for relicts of such activity in a Bennu analog – the carbonaceous chondrite Aguas Zarcas (AZ). Here we discovered compact fragments that were strongly shocked, redistributed on the parent body, and deposited into an unshocked lithology. While breccias in meteorites are common, this type of lithology has been rarely observed. However, it is consistent with the observed particle redeposition process on Bennu. In addition, we observed a match in the size distribution of the compact fragments with that of the Bennu ejecta, indicating that the same process operated on the AZ parent body. Our findings suggest that such mixing may have been unnoticed in most meteorites and that asteroid activity may be common but overlooked.

A. S. Davis

Papers

Analysis of ablation debris from natural and artificial iron meteorites

A record from an active asteroid preserved in a carbonaceous chondrite