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First observation of a rocky meteoroid from the Oort cloud

Presentation #305.02 in the session Meteoroids and Meteors.

Published onOct 20, 2022
First observation of a rocky meteoroid from the Oort cloud

Conventional models of Solar System formation posit the Oort cloud as populated with icy planetesimals ejected from the giant planet region of the solar system. This icy population can only be observed indirectly when objects enter the inner Solar System and become long-period comets (LPCs). The presence of ice-free refractory material in the Oort cloud is much harder to explain. Measuring the fraction of rocky material in the Oort cloud can differentiate between the “massive” and “depleted” proto-asteroid belt scenarios among competing Solar System formation models. Several Manx comets, which are objects on LPC orbits showing asteroidal properties based on reflectance spectra and the lack of visible tail have been discovered to date. However, strength properties of these objects have yet to be directly measured.

Here we present the first direct observation of a decimeter-sized rocky meteoroid on a retrograde LPC orbit (e ≈ 1.0, i = 121°). This ~2 kg object entered the atmosphere at 62 km/s and the associated fireball terminated at a height of 46.5 km. This is 40 km deeper than cometary objects of similar mass and speed. The fireball was observed optically by three cameras with high accuracy (trajectory fit errors ~50 m) and by satellite sensors providing a calibrated lightcurve. During its flight, the meteoroid experienced dynamic pressures of several MPa, on par with meteorite-dropping fireballs which produce ordinary chondrites. The fragmentation profile shows two discrete phases of fragmentation at ~0.1 and ~1 MPa, similar to that followed by fireballs produced by ordinary chondrites. In contrast, as previously measured by fireball surveys and in situ by Rosetta, cometary material has compressive strengths on the order of 1 kPa. The earliest detectable fragmentation of this fireball occurred at >100 kPa, consistent with a minimum global strength significantly higher than cometary material. Ablation model fitting to the photometric and dynamic measurements produces bulk density and ablation properties consistent with refractory chondritic meteoroids.

Using this event together with historical fireball surveys, we estimate the flux of rocky decimeter objects impacting Earth from the Oort cloud to be 1.08+2.81-0.95 meteoroids/106 km2/yr to a mass limit of 10 g. This is ~6+13-5% of the total flux of objects originating in the Oort cloud and impacting Earth to these masses. Our result gives support to migration-based dynamical models of the formation of the Solar System and a massive proto-asteroid belt, conditions which predict that significant rocky material is implanted in the Oort cloud.

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