2024-04-13 06:00:05
By Guy Libourel, Cosmochemist, Côte d’Azur University
NASA’s Psyche space mission, successfully launched on October 13, 2023 at 10:19 a.m., is en route to the asteroid Psyche, located in the asteroid belt between Mars and Jupiter approximately 500 million kilometers away. L’asteroidwith a diameter of 250 kilometers, initially considered almost entirely metallic, arouses curiosity scientist. New data indeed suggests that it might be a mixed world of metal and silicate.
NASA/JPL-Caltech, CC BY
The best analyzes more accurately indicate that Psyche is likely a mixture of rock and metal, with metal making up between 30 and 60 percent of its volume. At present, it is still difficult to say whether Psyche is all or part of the iron-rich core of a planetary embryo, or a piece of another iron-rich body formed from metal-rich material in THE solar system.
In the first case, Psyche would be the metallic core of a planet that never saw the light of day, while in the second, it would correspond to other types of materials rich in iron, such as pallasites, formed during impacts. or by iron volcanoes.
Trace the history of the formation of telluric planets
Visiting the metallic asteroid (16) Psyche will therefore be a first which will teach us more regarding the collisional history of the formation of telluric planets (Mercury, Venus, Earth and Mars), and regarding their planetary interior.
Although it is difficult to give it an age, we know that the formation of iron meteorites is very old, and well before the formation of our planet. The formation of a celestial body, like the Earth, occurs by accretion, namely by increase in volume by the addition of external matter. In planetary accretion, these are planetesimals, or planetary embryos (like Psyche) which accrete (agglomerate) to each other little by little form the planetary body.
The Psyche space probe is expected to arrive in orbit around Psyche in July 2029. Until then, many research teams around the world are working to better understand this celestial body. Ours in Nice led by the Côte d’Azur Observatory (Nice), in collaboration with colleagues from theUniversité of Kobe/ISAS (Japan) and the University of Grenoble Alpes/IPAG (Grenoble), aims to better describe the evolution of the surface of iron asteroids under the effect of impacts and violent collisions during their time of life in the main asteroid belt.
Hypervelocity impact experience. Video of a basalt glass projectile 3 mm in diameter impacting a metal cylinder at 7 km/s. Note following the impact (light flash) the shock wave and the ejecta of molten material.
Credit: Akiko M. Nakamura, Guy Libourel, Patrick Michel.
In the case of the Psyche mission, the nature, appearance and topography from the surface of this metallic world when the probe arrives in 2029 are real unknowns. To answer these questions, we are carrying out experiments on hypervelocity impacts of rock projectiles on steel targets and iron-rich meteorites using double-stage gas cannons. These experiments make it possible to reach dizzying impact speeds, up to nearly 8 km/s, or 30,000 km/h, simulating impacts occurring between asteroids in the main belt. For comparison, for a handgun the speed of the bullet is on average around 300m/s.
As seen in the video, these experiments make it possible to study the behavior of materials during impact or the state of the surface following impact.
Understand what the surface of Psyche looks like
The main results highlight very irregular impacted surfaces where metal and silicate alternate. The edges of the craters are characterized by straightened metal blades giving particular “landscapes” resembling deformed metal sheets. We also showed that the crater floors were systematically coated by silicate liquids/glasses produced during high-speed impacts. In some cases, spectacular glassy and bubbled carapaces line the bottom of the craters to form strange reliefs, giving a glimpse of what the surface of Psyche might look like.
In general, the analysis of electromagnetic radiation in the visible andinfrared (spectral observation) and microwave reflectivity (radar albedo observation) of the surfaces of main-belt asteroids indicate that there are fewer iron-rich objects than one would expect to find , given the relative abundance of iron meteorites.
However, the spectral study of our targets following impact shows that this deficit is only apparent. It would simply be caused by the camouflage of metallic surfaces by molten silicates, now glassy, produced during meteorite projectile impacts at impact speeds typical of those encountered in the asteroid belt (≈ 5 km/s).
These results are important because they highlight two essential points. Firstly, Psyche’s metal-silicate mixture might be of collisional origin and secondly that spectral observations from the ground (telescope) can be misleading in identifying iron-rich asteroids.
Scanning electron microscope image of backscattered electrons of a crater resulting of a hyper-velocity impact (5.08 km/s) of a basaltic projectile on a piece of Gibeon, a meteorite of iron used as a target and simulating the surface of an iron-rich asteroid. The dark appearance is linked to the coating of the crater floor with silicate impact glass, initially liquid at the time of the impact.
Provided by the author
To obtain more reliable results, our study shows that it is necessary to combine spectral observations with radar measurements. Microwaves, used for radar being more penetrative in the surface materials of asteroids than visible or infrared radiation waves, make it possible to distinguish iron-rich asteroids from other types of asteroids, even if they are covered with vitreous silicates.
We are continuing our efforts to describe, understand and model the evolution of the surface of iron-rich asteroids subjected to violent impacts typical of the asteroid belt. We recently performed hypervelocity impact experiments on mesosiderites and pallasites, two classes of rare (and therefore expensive!) meteorites made up of metallic parts and silicates. These iron meteorites are frequently proposed as possible constituents of iron-rich asteroids, particularly Psyche.
To carry out these studies, we joined our usual partners with colleagues from INSA Lyon who are helping us to manufacture “fake pallasites” using a flash sintering technique, or spark plasma sintering (SPS). ). The objective in mixing different proportions of silicate and metal is to evaluate the role of the ductile/metal – brittle/silicate transition in collisions between asteroids and possibly in the formation of their regolith (soil).
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