Glassy surface of iron-rich asteroids tricks telescopes

M-type asteroids that are composed of nickel and iron were formed from metallic-core fragments of larger asteroids pulverized by collisions over our solar system’s history. But only 30–40% of the M types observed by telescopes in the visible and near-IR match the expected metallic-asteroid composition. Guy Libourel from the Côte d’Azur Observatory in France and his colleagues may have determined why some asteroids are missing. Their spectral analysis shows that the iron-rich bodies can be obscured by a glass that forms when projectiles slam into the metal at asteroid-impact speeds.

Three of the coauthors—Akiko Nakamura, Ryo Ogawa, and Sunao Hasegawa—completed impact experiments at the Institute of Space and Astronautical Science in Japan. Millimeter-sized chunks of basalt and dunite rock, which are similar in composition to the materials that make up more common stony asteroids, were fired out of a gun at speeds of 3–7 km/s and smashed into centimeter-sized steel and iron meteorites. (A basalt projectile colliding with a steel target can be seen in the video below.) The collisions produced craters a few millimeters in size. The impacts partially melted the silicate projectile and coated the asteroid’s pockmarked surface with a highly reflective glass layer. The electron-backscattered image above shows the glass (pink) that camouflages a substantial part of the iron-rich surface (green).

The laboratory results are consistent with measurements made at the Arecibo Observatory in Puerto Rico of the metallic asteroid 16 Psyche: Variations in radar reflectivity of the asteroid’s surface suggest a patchwork of metal- and silicate-rich regions. NASA’s Psyche orbiter, which is scheduled to launch in August 2022, will make measurements of Psyche’s surface. With those data, scientists can test whether the initial laboratory results agree with in situ observations. (G. Libourel et al., Sci. Adv. 5, eaav3971, 2019 .)